Research

The dribbling agility test as a potential tool for evaluating the dribbling skill in young soccer players.

Res Sports Med. 2018 Jun 28;:1-11

Authors: Bekris E, Gissis I, Kounalakis S

Abstract
The present study examined the construct validity and reliability of a new dribbling agility test (DAT) that incorporates reactive agility and multiple change of direction. To check its’ validity, (a) DAT was performed by four groups (under 10, under 12, under 14 and under 16 yrs) of young soccer players (n = 125 in each group) and (b) a regression analysis was conducted to define the best DAT predictors. The reliability of DAT was assessed with repeated measurements. This test can differentiate the dribbling skill between groups (p < 0.01). Furthermore, 68% of the observed variance in DAT was explained by zigzag dribbling test, Illinois agility test, reaction time and running speed. The test-retest reliability was high in all groups (ICC = 0.77 – 0.90, p < 0.01). It was concluded that DAT can be a potential tool to evaluate the dribbling performance in young soccer players.

PMID: 29952671 [PubMed – as supplied by publisher]

Neuromuscular electrical stimulation can improve mobility in older adults but the time course varies across tasks: Double-blind, randomized trial.

Exp Gerontol. 2018 Jul 15;108:269-275

Authors: Mani D, Almuklass AM, Amiridis IG, Enoka RM

Abstract
Declines in mobility with advancing age are often associated with a reduction in the use of lower leg muscles. We examined the influence of two interventions that involved neuromuscular electrical stimulation (NMES) applied to the triceps surae muscles on the mobility and muscle function of older adults. Thirty healthy older adults (73.5 ± 4.8 yrs) participated in a 6-week intervention comprising 3 weekly sessions of either narrow- or wide-pulse NMES. Motor function was assessed at Weeks 0, 4, 7, and 10. There were no statistically significant differences in the changes in mobility for the two groups of participants, so the data for the two groups were combined to examine changes across time. Time to walk 400 m decreased and maximal walking speed increased after 3 wks of NMES (Week 4) but did not change further at Weeks 7 and 10. In contrast, time to complete the chair-rise and rapid-step tests decreased progressively up to Week 7 but did not change further at Week 10. Moreover, the increase in plantar flexor strength was only observed at Week 7. NMES can elicit improvements in the motor function of older adults, but the time course of the adaptations differs across the mobility tests.

PMID: 29738790 [PubMed – in process]

Effects of hip flexion angle on surface electromyographic activity of the biceps femoris and semitendinosus during isokinetic knee flexion.

Muscles Ligaments Tendons J. 2017 Apr-Jun;7(2):286-292

Authors: Kellis E, Galanis N, Kofotolis N, Hatzi A

Abstract
Background: Identifying combinations of the hip and knee joint angles which can selectively recruit specific hamstring muscles may be beneficial for injury prevention or rehabilitation. The purpose of this study was to examine the joint torque and electromyographic (EMG) activity of the semitendinosus (ST) and biceps femoris long head (BFlh).
Methods: Twenty subjects performed maximum isokinetic concentric and eccentric knee flexor efforts at 60°·s-1, 120°·s-1 and 150°·s-1 from three different hip joint angles while surface EMG of ST and BFlh was recorded.
Results: Analysis of variance showed that there are no inter-muscular differences in EMG amplitude across testing conditions (p > .05). Peak EMG occurred near full knee extension for the BFLh and at a higher flexion angle for the ST while exercise from a prone position shifted the peak EMG towards higher knee flexion angle (p < 0.05).
Conclusion: Maximal dynamic knee flexion exercises do not induce a higher EMG amplitude of BFlh or ST. Exercising from a higher hip flexion angle near full knee extension may selectively activate the BFlh.
Level of evidence: IIb.

PMID: 29264340 [PubMed]

Biceps femoris fascicle length during passive stretching.

J Electromyogr Kinesiol. 2018 Feb;38:119-125

Authors: Kellis E

Abstract
The purpose of this study was to quantify the relative changes in fascicle (FL) and muscle-tendon unit (LMTU) length of the long head of the biceps femoris (BFlh) at different combinations of hip and knee joint positions. Fourteen participants performed passive knee extension trials from 0°, 45° and 90° of hip flexion. FL, LMTU, pennation angle (PA) and effective FL (FL multiplied by the cosine of the PA) of the BFlh were quantified using ultrasonography (US). Three-way analysis of variance designs indicated that at each hip angle, FL and LMTU increased and PA decreased from 90° to 0° of knee flexion. Increasing hip flexion angle from 0° to 90° led to a higher FL and LMTU and a lower PA (p < .05). The average lengthening of the LMTU and effective FL was 28.00 ± 1.82% and 85.88 ± 21.92%, respectively. The average effective FL change accounted for 51.36 ± 7.39% of LMTU change. The relationship between effective FL and LMTU was almost linear with a slope equal to 0.49 ± 0.06 (r2 = 0.52 to 0.97). To achieve greater lengthening of the fascicles of the BFlh, passive stretch with the hip flexed at least 45° and the knee reaching full extension is necessary.

PMID: 29223017 [PubMed – indexed for MEDLINE]

Combined visual and dribbling performance in young soccer players of different expertise.

Res Sports Med. 2018 Jan-Mar;26(1):43-50

Authors: Bekris E, Gissis I, Ispyrlidis I, Mylonis E, Axeti G

Abstract
We aimed to evaluate dribbling performance in terms of technique and visual skills assessment of both young experienced (EX, n = 24) and novice (NO, n = 24) soccer players. Both groups performed two dribbling tests with four levels of difficulty in visual signals (A1-A4 and B1-B4; B – half distance of A; 1 – no visual signal; 4 – signal with the shorter flashing time). All players performed slower when visual signals were added to the testing process (~2.5 s; p < 0.01). EX completed all tests faster than NO (~3 s, p < 0.01). The average number of visual mistakes was significantly lower for EX than NO in all tests (p < 0.01). These results demonstrated the importance of evaluating dribbling along with visual stimuli in young soccer EX and NO players.

PMID: 29058470 [PubMed – indexed for MEDLINE]

Effects of two proprioceptive training programs on ankle range of motion, pain, functional and balance performance in individuals with ankle sprain.

J Back Musculoskelet Rehabil. 2018;31(3):437-446

Authors: Lazarou L, Kofotolis N, Pafis G, Kellis E

Abstract
BACKGROUND: Following ankle sprain, residual symptoms are often apparent, and proprioceptive training is a treatment approach. Evidence, however, is limited and the optimal program has to be identified.
OBJECTIVE: To investigate the effects of two post-acute supervised proprioceptive training programs in individuals with ankle sprain.
METHODS: Participants were recruited from a physiotherapy center for ankle sprain rehabilitation. In a pre-post treatment, blinded-assessor design, 22 individuals were randomly allocated to a balance or a proprioceptive neuromuscular facilitation (PNF) group. Both groups received 10 rehabilitation sessions, within a six-week period. Dorsiflexion range of motion (ROM), pain, functional and balance performance were assessed at baseline, at the end of training and eight weeks after training.
RESULTS: Follow-up data were provided for 20 individuals. Eight weeks after training, statistically significant (p< 0.017) improvements were found in dorsiflexion ROM and most functional performance measures for both balance and PNF groups. Eight weeks after training, significant (p< 0.017) improvements in the frontal plane balance test and pain were observed for the balance group.
CONCLUSIONS: Balance and PNF programs are recommended in clinical practice for improving ankle ROM and functional performance in individuals with sprain. Balance programs are also recommended for pain relief.

PMID: 28946541 [PubMed – in process]

Semitendinosus muscle architecture during maximum isometric contractions in individuals with anterior cruciate ligament reconstruction and controls.

Muscles Ligaments Tendons J. 2017 Jan-Mar;7(1):147-151

Authors: Karagiannidis E, Kellis E, Galanis N, Vasilios B

Abstract
BACKGROUND: The most widely used graft as a replacement in anterior cruciate ligament (ACL) reconstruction is the semitendinosus (ST) tendon graft. After harvesting for ACL reconstruction, the hamstring tendon regenerates in most people and becomes similar to normal. The effect of graft harvest on muscle morphology and function remains unclear. The present study aimed to examine the morphology of the ST during isometric contraction after harvesting the ST tendon for ACL reconstruction.
METHODS: Maximal isometric contractions of the knee flexors from two angular positions were performed by 8 participants, at least 1 year after ACL reconstruction with an ST tendon graft and 8 matched controls. Ultrasonographic images were used to measure the pennation angle and muscle thickness of the ST muscle.
RESULTS: There was not a statistically significant difference in pennation angle values between the control group and the group who underwent ACL reconstruction (p >0.05). Muscle thickness was significantly higher in the ACLR group compared with controls (p< 0.05).
CONCLUSIONS: Individuals who underwent ACL reconstruction display a higher ST muscle thickness but similar pennation angle compared with controls. This indicates that ACL reconstruction has an effect on ST muscle belly but effect on force generation capacity is rather limited.
LEVEL OF EVIDENCE: IIb.

PMID: 28717622 [PubMed]

Reliability of Sonographic Assessment of Biceps Femoris Distal Tendon Strain during Passive Stretching.

Ultrasound Med Biol. 2017 09;43(9):1769-1779

Authors: Kellis E, Ellinoudis A, Intziegianni K

Abstract
The purpose of this study was to determine the intra-rater, inter-examiner and inter-observer reliability of biceps femoris long head (BFlh) tendon strain using ultrasound imaging. Nineteen patients (age: 20.4 ± 0.35 y) were tested twice with a 1-wk interval. Each session included passive stretching from three different hip positions. Tests were performed independently by two examiners while BFlh tendon displacement (mm) and strain (%) were manually extracted from ultrasound video footages by two observers. Intra-rater comparisons revealed an intra-class correlation coefficient (ICC2,1) range of 0.87 to 0.98 and a variability less than 4.74%. Inter-examiner comparisons revealed an ICC2,1 range of 0.83 to 0.99 and less than 4.69% variability. Inter-observer ICCs ranged from 0.93 to 0.97 with variability less than 4.89%. Using a well-defined scanning protocol, two experienced examiners attained high levels of intra-rater agreement, with similarly excellent results for inter-rater and inter-observer reliability for BFlh tendon displacement and strain.

PMID: 28606648 [PubMed – indexed for MEDLINE]

Motor unit activity in biceps brachii of left-handed humans during sustained contractions with two load types.

J Neurophysiol. 2016 Sep 01;116(3):1358-65

Authors: Gould JR, Cleland BT, Mani D, Amiridis IG, Enoka RM

Abstract
The purpose of the study was to compare the discharge characteristics of single motor units during sustained isometric contractions that required either force or position control in left-handed individuals. The target force for the two sustained contractions (24.9 ± 10.5% maximal force) was identical for each biceps brachii motor unit (n = 32) and set at 4.7 ± 2.0% of maximal voluntary contraction (MVC) force above its recruitment threshold (range: 0.5-41.2% MVC force). The contractions were not sustained to task failure, but the duration (range: 60-330 s) was identical for each motor unit and the decline in MVC force immediately after the sustained contractions was similar for the two tasks (force: 11.1% ± 13.7%; position: 11.6% ± 9.9%). Despite a greater increase in the rating of perceived exertion during the position task (task × time interaction, P < 0.006), the amplitude of the surface-recorded electromyogram for the agonist and antagonist muscles increased similarly during the two tasks. Nonetheless, mean discharge rate of the biceps brachii motor units declined more during the position task (task × time interaction, P < 0.01) and the variability in discharge times (coefficient of variation for interspike interval) increased only during the position task (task × time interaction, P < 0.008). When combined with the results of an identical study on right-handers (Mottram CJ, Jakobi JM, Semmler JG, Enoka RM. J Neurophysiol 93: 1381-1392, 2005), the findings indicate that handedness does not influence the adjustments in biceps brachii motor unit activity during sustained submaximal contractions requiring either force or position control.

PMID: 27334949 [PubMed – indexed for MEDLINE]

Use of Ultrasound to Monitor Biceps Femoris Mechanical Adaptations after Injury in a Professional Soccer Player.

J Sports Sci Med. 2016 Mar;15(1):75-9

Authors: Kellis E, Galanis N, Chrysanthou C, Kofotolis N

Abstract
This study examined the use of ultrasound to monitor changes in the long head of the biceps femoris (BF) architecture of aprofessional soccer player with acute first-time hamstring strain. The player followed a 14 session physiotherapy treatment until return to sport. The pennation angle and aponeurosis strain of the long head of the biceps femoris (BF) were monitored at 6 occasions (up until 1 year) after injury. The size of the scar / hematoma was reduced by 63.56% (length) and 67.9% (width) after the intervention and it was almost non-traceable one year after injury. The pennation angle of the fascicles underneath the scar showed a decline of 51.4% at the end of the intervention while an increase of 109.2% of the fascicles which were closer to deep aponeurosis was observed. In contrast, pennation angle of fascicles located away from the injury site were relatively unaffected. The treatment intervention resulted in a 57.9% to 77.3% decline of maximum strain per unit of MVC moment and remained similar one year after the intervention. This study provided an example of the potential use of ultrasound-based parameters to link the mechanical adaptations of the injured muscle to specific therapeutic intervention. Key pointsChanges in fascicle orientation after biceps femoris mild tear were reduced after a 28 day intervention and remained similar one year after injury.Tendon/aponeurosis strain per unit of moment of force decreased during the course of the therapeutic intervention.Future studies could utilize ultrasonography to monitor mechanical responses after various types of hamstring injury and interventions in order to improve criteria for a safe return to sport.

PMID: 26957929 [PubMed]

Effects of Pilates and trunk strengthening exercises on health-related quality of life in women with chronic low back pain.

J Back Musculoskelet Rehabil. 2016 Nov 21;29(4):649-659

Authors: Kofotolis N, Kellis E, Vlachopoulos SP, Gouitas I, Theodorakis Y

Abstract
BACKGROUND: Pilates programs are widely used as a form of regular exercise in a broad range of populations investigating their effectiveness for chronic low back pain (CLBP) treatment.
OBJECTIVE: The aim of this study was to compare the effects of a Pilates program and a trunk strengthening exercise program on functional disability and health-related quality of life (HRQOL) in women with nonspecific CLBP.
METHODS: A total of 101 volunteer women with CLBP provided data with a 3-month follow-up. They were randomized to either a Pilates (n= 37), trunk strengthening exercise (n= 36) or a control group (n= 28), exercising for a period of 8 weeks, three times a week. Data were collected on HRQOL using the Short-Form 36 Health Survey (SF-36v2), and functional disability using the Roland Morris Disability Questionnaire prior to program initiation, mid-intervention, immediately after program termination, and three months post-intervention.
RESULTS: The Pilates participants reported greater improvements on self-reported functional disability and HRQOL compared with participants in the trunk strengthening exercise and control groups (p < 0.05). The effects were retained for a period of three months after program termination for the Pilates group and to a lesser extent for the trunk strengthening exercise group.
CONCLUSIONS: An 8-week Pilates program improved HRQOL and reduced functional disability more than either a trunk strengthening exercise program or controls among women with CLBP.

PMID: 26922845 [PubMed – indexed for MEDLINE]

Fatigue-induced adjustment in antagonist coactivation by old adults during a steadiness task.

J Appl Physiol (1985). 2016 05 01;120(9):1039-46

Authors: Arellano CJ, Caha D, Hennessey JE, Amiridis IG, Baudry S, Enoka RM

Abstract
The purpose of this study was to determine the adjustments in the level of coactivation during a steadiness task performed by young and old adults after the torque-generating capacity of the antagonist muscles was reduced by a fatiguing contraction. Torque steadiness (coefficient of variation) and electromyographic activity of the extensor and flexor carpi radialis muscles were measured as participants matched a wrist extensor target torque (10% maximum) before and after sustaining an isometric contraction (30% maximum) with wrist flexors to task failure. Time to failure was similar (P = 0.631) for young (417 ± 121 s) and old (452 ± 174 s) adults. The reduction in maximal voluntary contraction torque (%initial) for the wrist flexors after the fatiguing contraction was greater (P = 0.006) for young (32.5 ± 13.7%) than old (21.8 ± 6.6%) adults. Moreover, maximal voluntary contraction torque for the wrist extensors declined for old (-13.7 ± 12.7%; P = 0.030), but not young (-5.4 ± 13.8%; P = 0.167), adults. Torque steadiness during the matching task with the wrist extensors was similar before and after the fatiguing contraction for both groups, but the level of coactivation increased after the fatiguing contraction for old (P = 0.049) but not young (P = 0.137) adults and was twice the amplitude for old adults (P = 0.002). These data reveal that old adults are able to adjust the amount of antagonist muscle activity independent of the agonist muscle during steady submaximal contractions.

PMID: 26846553 [PubMed – indexed for MEDLINE]

Acute Effects of Different Agonist and Antagonist Stretching Arrangements on Static and Dynamic Range of Motion.

Asian J Sports Med. 2015 Dec;6(4):e26844

Authors: Amiri-Khorasani M, Kellis E

Abstract
BACKGROUND: Traditionally, stretching exercises are considered as basic components of warm up aiming to prepare the musculoskeletal system for performance and to prevent injuries.
OBJECTIVES: The purpose of this study was to examine the effects of different agonist and antagonist stretching arrangements within a pre-exercise warm-up on hip static (SROM) and dynamic range of motion (DROM).
MATERIALS AND METHODS: Sixty trained male subjects (Mean ± SD: height, 177.38 ± 6.92 cm; body mass, 68.4 ± 10.22 kg; age, 21.52 ± 1.17 years) volunteered to participate in this study. SROM was measured by V-sit test and DROM captured by a motion analysis system before and after (i) static stretching for both hip flexor and extensor muscles (SFSE), (ii) dynamic stretching for both hip flexor and extensor muscles (DFDE), (iii) static stretching for the hip flexors and dynamic stretching for hip extensors (SFDE), and (iv) dynamic stretching for the hip flexors and static stretching for hip extensors (DFSE).
RESULTS: DFSE showed a significantly higher increase in DROM and SROM than the remainder of the stretching protocols (P < 0.05). There were significant differences between DFDE with SFSE and SFDE (P < 0.05) and SFSE showed significant increase as compared to SFDE (P < 0.05).
CONCLUSIONS: In conclusion, DFSE is probably the best stretching arrangement due to producing more post activation potentiation on agonist muscles and less muscle stiffness in antagonist muscles.

PMID: 26715975 [PubMed]

Biceps femoris and semitendinosus tendon/aponeurosis strain during passive and active (isometric) conditions.

J Electromyogr Kinesiol. 2016 Feb;26:111-9

Authors: Kellis E

Abstract
The purpose of this study was to quantify strain and elongation of the long head of the biceps femoris (BFlh) and the semitendinosus (ST) tendon/aponeurosis. Forty participants performed passive knee extension trials from 90° of knee flexion to full extension (0°) followed by ramp isometric contractions of the knee flexors at 0°, 45° and 90° of knee flexion. Two ultrasound probes were used to visualize the displacement of BFlh and ST tendon/aponeurosis. Three-way analysis of variance designs indicated that: (a) Tendon/aponeurosis (passive) elongation and strain were higher for the BFlh than the ST as the knee was passively extended (p<0.05), (b) contraction at each angular position was accompanied by a smaller BFlh tendon/aponeurosis (active) strain and elongation than the ST at higher levels of effort (p<0.05) and (c) combined (passive and active) strain was significantly higher for the BFlh than ST during ramp contraction at 0° but the opposite was observed for the 45° and 90° flexion angle tests (p<0.05). Passive elongation of tendon/aponeurosis has an important effect on the tendon/aponeurosis behavior of the hamstrings and may contribute to a different loading of muscle fibers and tendinous tissue between BFlh and ST.

PMID: 26710667 [PubMed – indexed for MEDLINE]

Correlation between semitendinosus and gracilis tendon cross-sectional area determined using ultrasound, magnetic resonance imaging and intraoperative tendon measurements.

J Electromyogr Kinesiol. 2016 Feb;26:44-51

Authors: Galanis N, Savvidis M, Tsifountoudis I, Gkouvas G, Alafropatis I, Kirkos J, Kellis E

Abstract
The purpose of this study was to examine the correlation in semitendinosus (ST) and gracilis (GT) tendon cross-sectional area (CSA) evaluated directly during anterior cruciate ligament (ACL) surgery and pre-operatively using ultrasound (US) and magnetic resonance imaging (MRI). A total of 14 patients undergoing ACL reconstruction with a quadruple ST-GT graft by the same orthopaedic surgeon participated in this study. Pre-operative evaluation included determination of ST and GT CSA area using US and MRI. Intraoperative measurement of the diameters of the ST, GT and the final ACL graft using a closed-hole sizing block with 0.5-mm increments was made and this diameter was used to estimate tendon CSA. The correlation between graft diameter and CSA were 0.563 (GT) and 0.807 (ST) for MRI and 0.498 (GT) and 0.612 (ST) for US. The final ACL graft diameter displayed a correlation coefficient of 0.813 with MRI CSA and 0.518 with US CSA. No differences in CSA were observed between intraoperative, MRI and US methods (p>0.05). The intraclass correlation coefficients between the US, MRI and intraoperative graft methods for the ST and GT data ranged from 0.502 to 0.903 with an estimation error ranging from 1.41% to 2.26%. These results indicate that in clinical situations where MRI is contra-indicated or not accessible, US can provide measurable values which could predict sufficient diameter of the ACL graft. In addition, determination of tendon CSA using US displays errors less than 2% which is similar to that observed using MRI. This suggests that the application of US can be applied to in vivo examination of the ST and GT CSA.

PMID: 26708406 [PubMed – indexed for MEDLINE]

Age and gender differences in kinematics of powerful instep kicks in soccer.

Sports Biomech. 2015 Sep;14(3):287-99

Authors: Katis A, Kellis E, Lees A

Abstract
Soccer kicking training should be adjusted to the characteristics of the athletes. Therefore, examination of differences in kicking kinematics of females and pubertal players relative to males is worthwhile. The purpose of the study was to compare kicking kinematics and segmental sequence parameters between male, female, and pubertal players. Ten adult male, ten adult female, and ten male pubertal players participated in the study. Participants performed five consecutive kicking trials of a stationary ball, as powerful as they could. Analysis of variance showed significantly higher ball velocity, higher joint linear velocities for the knee and the hip, and higher angular velocities of the knee and the ankle for males compared to female and pubertal players (p < 0.05). Similarly, the peak joint velocity was achieved significantly closer to ball impact in males compared to other groups (p < 0.05). Males also showed a more plantarflexed ankle immediately before ball impact (p < 0.05). Females and pubertal players may benefit from skill training aiming to increase ankle plantarflexion and hip flexion prior to ball impact, and to adjust thigh and shank motion, such that the shank-foot segment travels through a higher range of motion and with a greater velocity.

PMID: 26211619 [PubMed – indexed for MEDLINE]

Effect of Combined Sensorimotor-Resistance Training on Strength, Balance, and Jumping Performance of Soccer Players.

J Strength Cond Res. 2016 Jan;30(1):53-9

Authors: Manolopoulos K, Gissis I, Galazoulas C, Manolopoulos E, Patikas D, Gollhofer A, Kotzamanidis C

Abstract
The purpose of the study was to investigate the effects of resistance training (RT) and sensorimotor training combined with RT (SM-RT) on balance, 1 repetition maximum (RM), rate of force development (RFD), and squat jump (SJ) height. Twenty amateur soccer players were equally divided into 2 groups assigned as SM-RT group (age: 22 ± 1.7 years, body mass: 79.9 ± 6.3 kg, body height: 1.81 ± 0.06 m) and RT group (age: 21.3 ± 1.3 years, body mass: 77.4 ± 9.3 kg, body height: 1.78 ± 0.04 m). Both groups were trained over a 6-week period with 2 session units per week. SM-RT group performed sensorimotor training (balance on balance board) followed by a high-intensity RT at 8-5RM leg press. The RT group performed the resistance program only. Both groups showed significantly increased 1RM leg press strength, RFD, SJ height, and balance abilities (p ≤ 0.05), whereas no significant between-group differences were observed in any of the outcome variables (p > 0.05). It was concluded that SM-RT was not superior compared with RT for both balance and strength enhancement. These findings have implications in time management during training for soccer players.

PMID: 25992657 [PubMed – indexed for MEDLINE]

Modulation of motor unit activity in biceps brachii by neuromuscular electrical stimulation applied to the contralateral arm.

J Appl Physiol (1985). 2015 Jun 15;118(12):1544-52

Authors: Amiridis IG, Mani D, Almuklass A, Matkowski B, Gould JR, Enoka RM

Abstract
The purpose of the study was to determine the influence of neuromuscular electrical stimulation (NMES) current intensity and pulse width applied to the right elbow flexors on the discharge characteristics of motor units in the left biceps brachii. Three NMES current intensities were applied for 5 s with either narrow (0.2 ms) or wide (1 ms) stimulus pulses: one at 80% of motor threshold and two that evoked contractions at either ∼10% or ∼20% of maximal voluntary contraction (MVC) force. The discharge times of 28 low-threshold (0.4-21.6% MVC force) and 16 high-threshold (31.7-56.3% MVC force) motor units in the short head of biceps brachii were determined before, during, and after NMES. NMES elicited two main effects: one involved transient deflections in the left-arm force at the onset and offset of NMES and the other consisted of nonuniform modulation of motor unit activity. The force deflections, which were influenced by NMES current intensity and pulse width, were observed only when low-threshold motor units were tracked. NMES did not significantly influence the discharge characteristics of tracked single-threshold motor units. However, a qualitative analysis indicated that there was an increase in the number of unique waveforms detected during and after NMES. The findings indicate that activity of motor units in the left elbow flexors can be modulated by NMES current and pulse width applied to right elbow flexors, but the effects are not distributed uniformly to the involved motor units.

PMID: 25930023 [PubMed – indexed for MEDLINE]

Kinematics and knee muscle activation during sit-to-stand movement in women with knee osteoarthritis.

Clin Biomech (Bristol, Avon). 2015 Jul;30(6):599-607

Authors: Bouchouras G, Patsika G, Hatzitaki V, Kellis E

Abstract
BACKROUND: The purpose of this study was to compare joint kinematics, knee and trunk muscle activation and co-activation patterns during a sit-to-stand movement in women with knee osteoarthritis and age-matched controls.
METHODS: Eleven women with knee osteoarthritis (mean and standard deviation, age: 66.90, 4.51 years, height: 1.63, 0.02 m, mass: 77.63, 5.4 kg) and eleven healthy women (mean and standard deviation, age: 61.90, 3.12 years, height: 1.63 m, 0.03, mass: 78.30, 4.91 kg) performed a Sit to Stand movement at a self-selected slow, normal and fast speed. Three-dimensional joint kinematics of the lower limb, vertical ground reaction forces and electromyographic activity of the biceps femoris vastus lateralis and erectus spinae were recorded bilaterally.
FINDINGS: A two-way ANOVA showed that the osteoarhtitis group performed the sit to stand task using a smaller knee and hip range of motion compared with the control group while no differences in temporal kinematics and ground reaction force-related parameters were observed. In addition, women with osteoarhtritis displayed significantly lower vastus lateralis coupled with a higher biceps feomoris electromyographic activity and higher agonist-antagonist co-contraction and co-activation than asymptomatic women. The activation of erectus spinae was not different between groups.
INTERPRETATION: Results indicate that patients with moderate knee osteoarthritis rise from the chair using greater knee muscle co-contraction, earlier and greater activation of the hamstrings which results in reduced hip and knee range of motion. This may be a way to overcome the pain and potential muscle atrophy of knee extensor muscles without compromising overall task duration.

PMID: 25846323 [PubMed – indexed for MEDLINE]

Recovery of powerful kick biomechanics after intense running fatigue in male and female soccer players.

Asian J Sports Med. 2014 Dec;5(4):e24013

Authors: Katis A, Amiridis I, Kellis E, Lees A

Abstract
BACKGROUND: Fatigue seems to have a significant effect on soccer kick performance. However, the duration of these effects has not been previously investigated.
OBJECTIVES: The purpose of the present study was to investigate the duration of the acute effects of fatigue on soccer kick performance in males and females.
PATIENTS AND METHODS: Ten male (age: 26.3 ± 4.9 years, height: 178.1 ± 5.1 cm, mass: 81.3 ± 8.1 kg) and ten female (age: 24.4 ± 4.2 years, height: 169.7 ± 5.7 cm, mass: 61.8 ± 5.1 kg) amateur soccer players performed three instep kicks prior to and after running on a treadmill till exhaustion. Three-dimensional kinematics were collected pre- and post-fatigue.
RESULTS: Analysis of variance indicated a statistically significant decline in ball speed during the first and the second trial after fatigue (P < 0.05), but recovered to pre-fatigue levels during the third post-fatigue kicking trial (P > 0.05). Similarly, maximum ankle, knee and hip linear velocity and ankle angular displacement were significantly lower during the first two trials (P < 0.05), but not during the third trial after fatigue (P > 0.05).
CONCLUSIONS: Soccer kick parameters recovered to pre-fatigue levels approximately within a minute after the end of the protocol. These findings have a practical meaning for players who have to perform set piece kicks under fatigue conditions and for coaches who have to guide their players.

PMID: 25741413 [PubMed]

Reliability of Single-leg and Double-leg Balance Tests in Subjects with Anterior Cruciate Ligament Reconstruction and Controls.

Res Sports Med. 2015;23(2):151-66

Authors: Kouvelioti V, Kellis E, Kofotolis N, Amiridis I

Abstract
The purpose of this study was to assess the test-retest reliability of postural balance in patients with anterior cruciate ligament reconstruction (ACL) and controls. Ten healthy subjects and 15 individuals with ACL reconstruction performed single-leg and double-leg balance tests. The center of pressure (COP) was recorded using a pressure platform. For the total COP path, the intraclass correlation coefficient (ICC) ranged from 0.79 to 0.91. For the COP standard deviation, the ICCs ranged from 0.68 to 0.94. For the COP velocity, the ICCs ranged from 0.72 to 0.91. The sway area and ellipse scores displayed ICCs values of 0.67 to 0.95 and 0.53 to 0.92, respectively. The ICCs were higher for double leg tests compared with single-stance ones. These results indicate that 30 s balance tests in double and single-leg stance are reliable tools to assess static balance. The use of such tests to monitor rehabilitation programs following ACL reconstruction is recommended.

PMID: 25649642 [PubMed – indexed for MEDLINE]

Hamstring Elongation Quantified Using Ultrasonography During the Straight Leg Raise Test in Individuals With Low Back Pain.

PM R. 2015 Jun;7(6):576-83

Authors: Kellis E, Ellinoudis A, Kofotolis N

Abstract
BACKGROUND: Although the straight leg raise (SLR) test frequently is used to assess hamstring extensibility in individuals with low back pain (LBP), evidence relating LBP, SLR, and hamstring extensibility remains unclear. The SLR measures the angle between the lifted leg and the horizontal, however, and, as such, it is not a direct measure of the elongation capacity of the hamstrings.
OBJECTIVE: To examine the differences in hamstring elongation (quantified via ultrasonography) and SLR score between individuals with LBP and asymptomatic controls and to determine the relationship between hamstring elongation, SLR, and functional disability scores.
DESIGN: Cross-sectional study.
SETTING: University laboratory.
PARTICIPANTS: Forty men and women with chronic LBP (mean ± SD, age 43.51 ± 3.71 years and 40 control subjects (age 45.11 ± 4.01 years) participated in this study.
ASSESSMENT: Passive SLR, elongation assessed via ultrasonography, and functional disability.
ASSESSMENT AND OUTCOME MEASUREMENTS: SLR score, elongation of tendinous tissue within the semitendinosus muscle, and Oswestry Disability Index.
RESULTS: Two-way analysis of variance tests indicated a significantly lower SLR score and a greater Oswestry score in LBP group compared with control subjects (P < .05). In contrast, there were no significant group differences in hamstring elongation (P > .05). Gender did not have an effect on all dependent measures (P > .05). Hamstring elongation showed a low correlation with SLR score and a minimal correlation with Oswestry score.
CONCLUSIONS: These results indicate that the SLR score is not determined by hamstring elongation (quantified via ultrasonography).

PMID: 25600625 [PubMed – indexed for MEDLINE]

Post-activation potentiation: The neural effects of post-activation depression.

Muscle Nerve. 2015 Aug;52(2):252-9

Authors: Xenofondos A, Patikas D, Koceja DM, Behdad T, Bassa E, Kellis E, Kotzamanidis C

Abstract
INTRODUCTION: Our knowledge of the neurophysiology of post-activation potentiation (PAP) is limited. The purpose of this study was to examine the effect of PAP on twitch torque and H-reflex amplitude after a 10-s maximal voluntary contraction (MVC).
METHODS: PAP measurements were assessed with the plantarflexors in a relaxed state and during a tonic contraction at 10% MVC.
RESULTS: The H-reflex/maximum M-wave ratio (H/M) decreased significantly (P<0.05) and returned to baseline levels after 1 min. The decrement in H/M was depressed when the plantarflexors were active at 10% MVC, and the depression was more obvious in the lateral gastrocnemius than in the soleus muscle.
CONCLUSIONS: The inhibition induced immediately after contraction could be attributed to post-activation depression. We conclude that PAP after a 10-s MVC cannot be attributed to increased motor neuron excitability through the reflex pathway as assessed by the H-reflex technique.

PMID: 25504211 [PubMed – indexed for MEDLINE]

Changes in specific jumping performance after detraining period.

J Sports Med Phys Fitness. 2015 Oct;55(10):1150-6

Authors: Kannas TM, Amiridis IG, Arabatzi F, Katis A, Kellis E

Abstract
AIM: The purpose of this study was to examine the effects of a detraining period after specific plyometric training on vertical jump performance and mechanical properties of aponeurosis of medial gastrocnemius (MG).
METHODS: Twenty participants were assigned into two groups and performed plyometric training on incline (IP, N.=10) and plane ground (PP, N.=10), respectively. Vertical jumping (squat jump [SJ], countermovement jump [CMJ], slow and fast drop jumps from 20 and 40 cm [DJ20, DJ40, DJf20, DJf40]) and aponeurosis strain of MG have been re-evaluated four weeks after the end of the training period. The electromyographic (EMG) activity of MG and tibialis anterior (TA) muscles and MG architecture were also recorded after a 4 week of detraining period.
RESULTS: After detraining period, jumping height was decreased from 31.20±3.3 cm to 27.5±2 cm and from 31.8±2.9 cm to 29.5±1.9 cm for DJf20 and DJf40, respectively. For the IP group, these changes were accompanied by a significantly lower MG activity during the propulsion phase (from 0.93%±0.09 to 0.77±0.09% and from 0.89±0.09% to 0.60%±0.06 for DJf20 and DJf40, respectively). Similarly, strain was decreased from 22.7% (±0.05) to 16.3% (±0.05) after detraining period.
CONCLUSION: These findings suggest that after four weeks of detraining, ankle muscle tendon complex properties withdraw to the pre-training values with lower performance, possible reflecting a different working length of the muscle.

PMID: 25323480 [PubMed – indexed for MEDLINE]

Can balance trampoline training promote motor coordination and balance performance in children with developmental coordination disorder?

Res Dev Disabil. 2015 Jan;36:13-9

Authors: Giagazoglou P, Sidiropoulou M, Mitsiou M, Arabatzi F, Kellis E

Abstract
The present study aimed to examine movement difficulties among typically developing 8- to 9-year-old elementary students in Greece and to investigate the possible effects of a balance training program to those children assessed with Developmental Coordination Disorder (DCD). The Body Coordination Test for Children (BCTC; Körperkoordinationstest fur Kinder, KTK, Kiphard & Schilling, 1974) was chosen for the purposes of this study and 20 children out of the total number of 200, exhibited motor difficulties indicating a probable DCD disorder. The 20 students diagnosed with DCD were equally separated into two groups where each individual of the experimental group was paired with an individual of the control group. The intervention group attended a 12-week balance training program while students of the second – control group followed the regular school schedule. All participants were tested prior to the start and after the end of the 12-week period by performing static balance control tasks while standing on an EPS pressure platform and structured observation of trampoline exercises while videotaping. The results indicated that after a 12-week balance training circuit including a trampoline station program, the intervention group improved both factors that were examined. In conclusion, balance training with the use of attractive equipment such as trampoline can be an effective intervention for improving functional outcomes and can be recommended as an alternative mode of physical activity.

PMID: 25280002 [PubMed – indexed for MEDLINE]

Neuromuscular interactions around the knee in children, adults and elderly.

World J Orthop. 2014 Sep 18;5(4):469-85

Authors: Kellis E, Mademli L, Patikas D, Kofotolis N

Abstract
Although injury and neuromuscular activation patterns may be common for all individuals, there are certain factors which differentiate neuromuscular activity responses between children, adults and elderly. The purpose of this study is to review recent evidence on age differences in neural activation and muscle balances around the knee when performing single joint movements. Particularly, current evidence indicates that there are some interesting similarities in the neuromuscular mechanisms by which children or the elderly differ compared with adults. Both children and elderly display a lower absolute muscle strength capacity than adults which cannot fully be explained by differences in muscle mass. Quadriceps activation failure is a common symptom of all knee injuries, irrespective of age but it is likely that its effect is more evident in children or adults. While one might expect that antagonist co-activation would differ between age categories, it appears that this is not the case. Although hamstring: quadriceps ratio levels are altered after knee injury, it is not clear whether this is an age specific response. Finally, evidence suggests that both children and the elderly display less stiffness of the quadriceps muscle-tendon unit than adults which affects their knee joint function.

PMID: 25232523 [PubMed]

Multiarticular isokinetic high-load eccentric training induces large increases in eccentric and concentric strength and jumping performance.

J Strength Cond Res. 2014 Sep;28(9):2680-8

Authors: Papadopoulos C, Theodosiou K, Bogdanis GC, Gkantiraga E, Gissis I, Sambanis M, Souglis A, Sotiropoulos A

Abstract
This study investigated the effects of short-term eccentric exercise training using a custom-made isokinetic leg press device, on concentric and eccentric strength and explosiveness as well as jumping performance. Nineteen healthy males were divided into an eccentric (ECC, n = 10) and a control group (CG, n = 9). The ECC group trained twice per week for 8 weeks using an isokinetic hydraulic leg press machine against progressively increasing resistance ranging from 70 to 90% of maximal eccentric force. Jumping performance and maximal force generating capacity were measured before and after eccentric training. In the ECC group, drop jump (DJ) height and maximal power were increased by 13.6 ± 3.2% (p < 0.01) and 25.8 ± 1.2% (p < 0.01), whereas ground contact time was decreased by 17.6 ± 2.6% (p < 0.01). Changes in ankle, knee, and hip joint angles were also reduced by 33.9 ± 1.1%, 31.1 ± 1.0%, and 32.4 ± 1.6% (all p < 0.01), respectively, indicating an increase in muscle stiffness during the DJ. Maximal eccentric and concentric leg press force was increased by 64.9 ± 5.5% (p < 0.01) and 32.2 ± 8.8% (p < 0.01), respectively, and explosiveness, measured as force attained in the first 300 milliseconds, was increased by 49.1 ± 4.8% (p < 0.01) and 77.1 ± 7.7% (p < 0.01), respectively. The CG did not show any statistically significant changes in all parameters measured. The main findings of this study were that maximal concentric and eccentric force, explosiveness, and DJ performance were markedly increased after only 16 training sessions, possibly because of the high eccentric load attained during the bilateral eccentric leg press exercise performed on this custom-made device.

PMID: 24626142 [PubMed – indexed for MEDLINE]

Static vs. Dynamic Acute Stretching Effect on Quadriceps Muscle Activity during Soccer Instep Kicking.

J Hum Kinet. 2013 Dec 18;39:37-47

Authors: Amiri-Khorasani M, Kellis E

Abstract
The purpose of this study was to compare the effects of static and dynamic stretching on quadriceps muscle activation during maximal soccer instep kicking. The kicking motion of twelve male college soccer players (body height: 174.66 ± 5.01 cm; body mass: 72.83 ± 4.83 kg; age: 18.83 ± 0.75 years) was captured using six synchronized high-speed infra-red cameras whilst electromyography (EMG) signals from vastus medialis (VM), lateralis (VL) and rectus femoris (RF) were recorded before and after static or dynamic stretching. Analysis of variance designs showed a higher increase in knee extension angular velocity (9.65% vs. -1.45%, p < 0.001), RF (37.5% vs. -8.33%, p < 0.001), VM (12% vs. -12%, p < 0.018), and VL EMG activity (20% vs. -6.67%, p < 0.001) after dynamic stretching exercises. Based on these results, it could be suggested that dynamic stretching is probably more effective in increasing quadriceps muscle activity and knee extension angular velocity during the final swing phase of a maximal soccer instep kick than static stretching.

PMID: 24511339 [PubMed]

Patellar tendon and hamstring moment-arms and cross-sectional area in patients with anterior cruciate ligament reconstruction and controls.

Comput Methods Biomech Biomed Engin. 2015 Aug;18(10):1083-1089

Authors: Kellis E, Karagiannidis E, Patsika G

Abstract
The purpose of this study was to examine the moment-arm and cross-sectional area (CSA) of the patellar tendon (PT) and the hamstrings after anterior cruciate ligament (ACL) reconstruction. The right knee of five males who underwent ACL reconstruction with a PT graft and five age-matched controls was scanned using magnetic resonance image scans. Based on three-dimensional (3D) solids of the PT, CSAs and moment-arms of semitendinous (ST), biceps femoris (BF) long head and semimembranosus (SM) were estimated. Analysis of variance indicated no significant group differences in muscle moment-arms (p>0.05). 3D moment-arms of PT, ST and BF were significantly lower than the corresponding 2D values (p < 0.05). The ACL group displayed a significantly higher maximum BF CSA, a lower ST CSA (p < 0.05) but similar PT and SM CSAs compared with controls. It is concluded that any alterations in PT properties 1 year after harvesting do not affect knee muscle moment-arms compared with age-matched controls. Moment-arm estimation differed between 3D and 2D data, although it did not affect comparisons between ACL reconstruction group and controls. Design of rehabilitation programmes should take into consideration a potential alteration in hamstring morphology following surgery with a PT graft.

PMID: 24460238 [PubMed – as supplied by publisher]

Postural leaning direction challenges the manifestation of tendon vibration responses at the ankle joint.

Hum Mov Sci. 2014 Feb;33:251-62

Authors: Kanakis I, Hatzitaki V, Patikas D, Amiridis IG

Abstract
In this study, we examined the interaction between central and peripheral proprioceptive afferent pathways by applying ankle tendon vibration during postural leaning in different directions. Twenty young participants stood for 60s over the midline of two adjacent force platforms in (a) neutral stance distributing Body Weight (BW) equally between the platforms, (b) forward leaning transferring 80% of BW to the front platform and (c) backward leaning transferring 80% of BW to the rear platform. Participants controlled the degree of leaning by receiving on-line visual feedback of BW distribution matched to a target line. Vibration (80Hz, 1.5-1.8mm) was applied over the Achilles or tibialis anterior tendon during the middle 20s of standing. This induced a postural shift towards the vibration side and an increase in the variability of the BW distribution that was greater in backward compared to forward leaning. EMG responses to tendon vibration were independent of the leaning direction. Antagonistic activity also increased in response to vibration, the amplitude of this increase however was direction dependent. These results favor the hypothesis about the central co-modulation of the vibration evoked proprioceptive inflow based on postural and visual feedback rather than muscle tension constraints.

PMID: 24289985 [PubMed – indexed for MEDLINE]

Effects of stabilization exercises on health-related quality of life in women with chronic low back pain.

J Phys Act Health. 2014 Sep;11(7):1295-303

Authors: Moussouli M, Vlachopoulos SP, Kofotolis ND, Theodorakis Y, Malliou P, Kellis E

Abstract
BACKGROUND: The study examined the effects of a 4-week intensive isometric and isotonic stabilization exercise program on dimensions of health-related quality of life (HRQL) in women with chronic low back pain (CLBP).
METHODS: A total of 39 women (27-72 years old) provided data in an experimental study with a 9-month follow-up. Random allocation was undertaken for the two treatment groups out of the 3 groups: isometric stabilization (n = 13), isotonic stabilization (n = 13), and a control group (n = 13) that did not participate in any form of exercise. Health-related quality of life measures using the Short-Form 36 Health Survey were assessed before program initiation, immediately after program termination, and 4 times postintervention for a period of 9 months.
RESULTS: The isometric stabilization group displayed large improvements in bodily pain and vitality for women with CLBP attending a 4-week intensive isometric stabilization exercise program. The effects were retained for a period of 9 months after program termination.
CONCLUSIONS: Isometric stabilization exercises reduce pain and enhance vitality as dimensions of HRQL among women with chronic low back pain with such effects lasting for at least 9 months.

PMID: 24184617 [PubMed – indexed for MEDLINE]

In vivo examination of the morphology of the tendinous inscription of the human semitendinosus muscle: gender and joint position effects.

J Morphol. 2014 Jan;275(1):57-64

Authors: Kellis E, Balidou A

Abstract
A tendinous inscription divides the semitendinosus muscle in two parts and it may have an effect on its function. The purpose of this study was to determine the effects of joint position and gender on the tendinous inscription morphology. Ultrasonography scans were taken from 76 young males and females at rest, in nine combinations of hip and knee joint angles. The length of the tendinous inscription arms and the angles formed by the two arms (apex angle), the tendinous inscription with the superficial (surface angle), and deep (deep angle) aponeurosis were determined. The tendinous inscription was clearly visible in 70 (out of 76) subjects. Analysis of variance designs showed that increasing hip flexion angle from 0 to 90° increased the long arm and muscle thickness but decreased the short tendinous inscription arm (P < 0.05). Changing knee flexion angle from 0 to 90° was accompanied by a longer tendinous inscription arm and an increased apex angle (P < 0.05). Long arm length and muscle thickness significantly increased from the shortest (hip 0° – knee 90°) to the longest muscle lengths (hip 0° – knee 90°). Males had a significantly higher surface, apex, and deep angle and a lower normalized tendinous inscription long arm than females (P < 0.05). These results indicate that the effect of the tendinous inscription (if any) on semitendinous muscle function depends on hip and knee joint angle while it may be gender dependent.

PMID: 24127198 [PubMed – indexed for MEDLINE]

Strain and elongation of the human semitendinosus muscle – tendon unit.

J Electromyogr Kinesiol. 2013 Dec;23(6):1384-90

Authors: Kellis E, Patsika G, Karagiannidis E

Abstract
The semitendinosus (ST) consists of a long distal tendon and it is divided in two parts by a tendinous inscription (TI). The purpose of this study was to quantify strain and elongation of the TI and the distal tendon of ST. Fourteen subjects performed ramp isometric contractions of the knee flexors at 0°, 45° and 90° of knee flexion. Two ultrasound probes were used to visualize the displacement of the distal tendon and selected points across the TI and aponeuroses. Three-way analysis of variance designs indicated that: (a) strain and elongation of the ST distal muscle-tendon junction were higher than that of the aponeurosis – TI junction points (p < 0.05) (b) the long arm of the TI reach strain of 49.86 ± 7.77% which was significantly (p < 0.05) higher than that displayed by the short arm (28.35 ± 0.59%) (c) Strain of tendinous and TI-aponeuroses segments significantly increased from 90° to 0° of knee flexion while the inverse was observed for the TI arm length (p < 0.05). (d) Tendon strain was significantly higher than strain of the TI-aponeuroses segments at 45° and 90° of knee flexion while the opposite was observed at 0° of knee flexion. The arrangement of TI along ST length results in differential local strains, indicating that the mechanical properties of the ST muscle are affected by tendon, aponeuroses and tendinous inscription interactions.

PMID: 23992632 [PubMed – indexed for MEDLINE]

Side-alternating vibration training for balance and ankle muscle strength in untrained women.

J Athl Train. 2013 Sep-Oct;48(5):590-600

Authors: Spiliopoulou SI, Amiridis IG, Tsigganos G, Hatzitaki V

Abstract
CONTEXT: Side-alternating vibration (SAV) may help reduce the risk of falling by improving body balance control. Such training has been promoted as a strength-training intervention because it can increase muscle activation through an augmented excitatory input from the muscle spindles.
OBJECTIVE: To determine the effect of SAV training on static balance during 3 postural tasks of increasing difficulty and lower limb strength.
DESIGN: Randomized controlled clinical trial.
SETTING: Laboratory.
PATIENTS OR OTHER PARTICIPANTS: A total of 21 healthy women were divided into training (n = 11; age = 43.35 ± 4.12 years, height = 169 ± 6.60 cm, mass = 68.33 ± 11.90 kg) and control (n = 10; age = 42.31 ± 3.73 years, height = 167 ± 4.32 cm, mass = 66.29 ± 10.74 kg) groups.
INTERVENTION(S): The training group completed a 9-week program during which participants performed 3 sessions per week of ten 15-second isometric contractions with a 30-second active rest of 3 exercises (half-squat, wide-stance squat, 1-legged half-squat) on an SAV plate (acceleration = 0.91-16.3g). The control group did not participate in any form of exercise over the 9-week period.
MAIN OUTCOME MEASURE(S): We evaluated isokinetic and isometric strength of the knee extensors and flexors and ankle plantar flexors, dorsiflexors, and evertors. Static balance was assessed using 3 tasks of increasing difficulty (quiet bipedal stance, tandem stance, 1-legged stance). The electromyographic activity of the vastus lateralis, semitendinosus, medial gastrocnemius, tibialis anterior, and peroneus longus was recorded during postural task performance, baseline and pretraining, immediately posttraining, and 15 days posttraining.
RESULTS: After training in the training group, ankle muscle strength improved (P = .03), whereas knee muscle strength remained unaltered (P = .13). Improved ankle-evertor strength was observed at all angular velocities (P = .001). Postural sway decreased in both directions but was greater in the mediolateral (P < .001) than anteroposterior (P = .02) direction. The electromyographic activity of the peroneus longus increased during the sharpened tandem (P = .001) and 1-legged tasks (P = .007). No changes were seen in the control group for any measures.
CONCLUSIONS: The SAV training could enhance ankle muscle strength and reduce postural sway during static balance performance. The reduction in mediolateral sway could be associated with the greater use of ankle evertors due to their strength improvement.

PMID: 23914911 [PubMed – indexed for MEDLINE]

Muscle reaction function of individuals with intellectual disabilities may be improved through therapeutic use of a horse.

Res Dev Disabil. 2013 Sep;34(9):2442-8

Authors: Giagazoglou P, Arabatzi F, Kellis E, Liga M, Karra C, Amiridis I

Abstract
Reaction time and muscle activation deficits might limit the individual’s autonomy in activities of daily living and in participating in recreational activities. The aim of the present study was to assess the effects of a 14-week hippotherapy exercise program on movement reaction time and muscle activation in adolescents with intellectual disability (ID). Nineteen adolescents with moderate ID were assigned either to an experimental group (n=10) or a control group (n=9). The experimental group attended a hippotherapy exercise program, consisting of two 30-min sessions per week for 14 weeks. Reaction time, time of maximum muscle activity and electromyographic activity (EMG) of rectus femoris and biceps femoris when standing up from a chair under three conditions: in response to audio, visual and audio with closed eyes stimuli were measured. Analysis of variance designs showed that hippotherapy intervention program resulted in significant improvements in reaction time and a reduction in time to maximum muscle activity of the intervention group comparing to the control group in all 3 three conditions that were examined (p<0.05). The present findings suggest that the muscle reaction function of individuals with ID can be improved through hippotherapy training. Hippotherapy probably creates a changing environment with a variety of stimuli that enhance deep proprioception as well as other sensory inputs. In conclusion, this study provides evidence that hippotherapy can improve functional task performance by enhancing reaction time.

PMID: 23747935 [PubMed – indexed for MEDLINE]

Synergetic and antagonist muscle strength and activity in women with knee osteoarthritis.

J Geriatr Phys Ther. 2014 Jan-Mar;37(1):17-23

Authors: Patsika G, Kellis E, Kofotolis N, Salonikidis K, Amiridis IG

Abstract
BACKGROUND AND PURPOSE: People with knee osteoarthritis (OA) display limitations in daily activities and a lower quality of life. The purpose of this study was to investigate the differences in strength balance and activation during maximum strength efforts between women with knee OA and asymptomatic women.
METHODS: Twelve women with knee OA (age 60.33 ± 6.66 years) and 11 controls (age 56.54 ± 5.46 years) performed maximum isokinetic eccentric and concentric knee extension and flexion tests at 60°/s, 120°/s, and 150°/s. Surface electromyography (EMG) was recorded from the biceps femoris (BF), vastus lateralis (VL), and vastus medialis (VM). Hamstrings-to-quadriceps moment ratios (H/Q), the synergetic (VL/VM), and co-contraction (BF/[VM + VL]) EMG ratios were calculated.
RESULTS: Analysis-of-variance designs showed that women experiencing knee OA had significantly higher H/Q moment ratios and VM/VL EMG ratios than controls (P < 0.05). The co-contraction index was significantly lower in the OA group only during knee flexion (P < 0.05).
CONCLUSIONS: Women with knee OA showed a higher H/Q moment ratios probably because of the need for better joint stability or a lower quadriceps capacity. This deficiency was accompanied by a higher VM activation, which probably serves to stabilize the patella upon maximum contraction as well as a higher activation of antagonist muscles.

PMID: 23635991 [PubMed – indexed for MEDLINE]

Effects of a 10-week resistance exercise program on soccer kick biomechanics and muscle strength.

J Strength Cond Res. 2013 Dec;27(12):3391-401

Authors: Manolopoulos E, Katis A, Manolopoulos K, Kalapotharakos V, Kellis E

Abstract
The purpose of the study was to examine the effects of a resistance exercise program on soccer kick biomechanics. Twenty male amateur soccer players were divided in the experimental group (EG) and the control group (CG), each consisting of 10 players. The EG followed a 10-week resistance exercise program mainly for the lower limb muscles. Maximal instep kick kinematics, electromyography, and ground reaction forces (GRFs) as well as maximum isometric leg strength were recorded before and after training. A 2-way analysis of variance showed significantly higher ball speed values only for the EG (26.14 ± 1.17 m·s vs. 27.59 ± 1.49 m·s before and after training, respectively), whereas no significant differences were observed for the CG. The EG showed a decline in joint angular velocities and an increase in biceps femoris electromyography of the swinging leg during the backswing phase followed by a significant increase in segmental and joint velocities and muscle activation of the same leg during the forward swing phase (p < 0.05). The EG also showed significantly higher vertical GRFs and rectus femoris and gastrocnemius activation of the support leg (p < 0.05). Similarly, maximum and explosive isometric force significantly increased after training only for the EG (p < 0.05). These results suggest that increases in soccer kicking performance after a 10-week resistance training program were accompanied by increases in maximum strength and an altered soccer kick movement pattern, characterized by a more explosive backward-forward swinging movement and higher muscle activation during the final kicking phase.

PMID: 23539080 [PubMed – indexed for MEDLINE]

The human knee: gross, microscopic, surgical, and radiological anatomy.

Anat Res Int. 2012;2012:698346

Authors: Natsis K, Anastasopoulos N, Kellis E, Koebke J, Sioga A, Tsitouridis I

PMID: 23304512 [PubMed]

The effect of high vs. low carbohydrate diets on distances covered in soccer.

J Strength Cond Res. 2013 Aug;27(8):2235-47

Authors: Souglis AG, Chryssanthopoulos CI, Travlos AK, Zorzou AE, Gissis IT, Papadopoulos CN, Sotiropoulos AA

Abstract
The purpose of this study was to compare the distances covered during a 11-a-side soccer match after players had consumed either a high carbohydrate (CHO) or a low CHO diet. Twenty-two male professional soccer players formed 2 teams (A and B), of similar age, body characteristics, and training experience. The 2 teams played against each other twice with a week interval between. For 3.5 days before the first match, the players of team A followed a high CHO diet that provided 8 g CHO per kg body mass (BM) (HC), whereas team B players followed a low CHO diet that provided 3 g CHO per kg BM (LC) for the same time period. Before the second match the dietary treatment was reversed and followed for the same time period. Training during the study was controlled, and distances covered were measured using global positioning system technology. Every player covered a greater total distance in HC compared with the distance covered in LC (HC: 9,380 ± 98 m vs. LC: 8,077 ± 109 m; p < 0.01). All distances covered from easy jogging (7.15 km·h-1) to sprinting (24.15 km·h-1) were also higher in HC compared with LC (p < 0.01). When players followed the HC treatment, they won the match (team A vs. team B: 3-1 for the first game and 1-2 for the second game). The HC diet probably helped players to cover a greater distance compared with LC. Soccer players should avoid eating a low (3 g CHO per kg BM) CHO diet 3-4 days before an important soccer match and have a high CHO intake that provides at least 8 g CHO per kg BM.

PMID: 23168373 [PubMed – indexed for MEDLINE]

Mechanisms that influence accuracy of the soccer kick.

J Electromyogr Kinesiol. 2013 Feb;23(1):125-31

Authors: Katis A, Giannadakis E, Kannas T, Amiridis I, Kellis E, Lees A

Abstract
Goal scoring represents the ultimate purpose of soccer and this is achieved when players perform accurate kicks. The purpose of the present study was to compare accurate and inaccurate soccer kicks aiming to top and bottom targets. Twenty-one soccer players performed consecutive kicks against top and bottom targets (0.5m(2)) placed in the center of the goal. The kicking trials were categorized as accurate or inaccurate. The activation of tibialis anterior (TA), rectus femoris (RF), biceps femoris (BF) and gastrocnemius muscle (GAS) of the swinging leg and the ground reaction forces (GRFs) of the support leg were analyzed. The GRFs did not differ between kicking conditions (P > 0.05). There was significantly higher TA and BF and lower GAS EMG activity during accurate kicks to the top target (P < 0.05) compared with inaccurate kicks. Furthermore, there was a significantly lower TA and RF activation during accurate kicks against the bottom target (P < 0.05) compared with inaccurate kicks. Enhancing muscle activation of the TA and BF and reducing GAS activation may assist players to kick accurately against top targets. In contrast, players who display higher TA and RF activation may be less accurate against a bottom target. It was concluded that muscle activation of the kicking leg represents a significant mechanism which largely contributes to soccer kick accuracy.

PMID: 23021602 [PubMed – indexed for MEDLINE]

Effect of a hippotherapy intervention program on static balance and strength in adolescents with intellectual disabilities.

Res Dev Disabil. 2012 Nov-Dec;33(6):2265-70

Authors: Giagazoglou P, Arabatzi F, Dipla K, Liga M, Kellis E

Abstract
The aim of this study was to assess the effects of a hippotherapy program on static balance and strength in adolescents with intellectual disability (ID). Nineteen adolescents with moderate ID were assigned either an experimental group (n=10) or a control group (n=9). The experimental group attended a 10-week hippotherapy program. To assess static balance, three tasks of increasing difficulty (Double-Leg Stance with opened or closed eyes, and One-Leg Stance with opened eyes) were performed while standing on an EPS pressure platform (Loran Engineering S.r.I., Bologna, Italy). The strength measurements consisted of three maximal isometric half-squats from the seating position (knee joint at 90°). The hippotherapy intervention program resulted in significant improvements in strength parameters, and on the more complex balance task (i.e. standing on one leg). In conclusion, this study provides evidence that hippotherapy can be used as an effective intervention for improving balance and strength in individuals with ID, and could thus influence functional activities and quality of life.

PMID: 22853887 [PubMed – indexed for MEDLINE]

Architectural differences between the hamstring muscles.

J Electromyogr Kinesiol. 2012 Aug;22(4):520-6

Authors: Kellis E, Galanis N, Kapetanos G, Natsis K

Abstract
The purpose of this study was to understand the detailed architectural properties of the human hamstring muscles. The long (BFlh) and short (BFsh) head of biceps femoris, semimembranosus (SM) and semitendinosus (ST) muscles were dissected and removed from their origins in eight cadaveric specimens (age 67.8±4.3 years). Mean fiber length, sarcomere length, physiological cross-section area and pennation angle were measured. These data were then used to calculate a similarity index (δ) between pairs of muscles. The results indicated moderate similarity between BFlh and BFsh (δ=0.54) and between BFlh and SM (δ=0.35). In contrast, similarity was low between SM and ST (δ=0.98) and between BFlh and SM (δ=1.17). The fascicle length/muscle length ratio was higher for the ST (0.58) and BFsh (0.50) compared with the BFlh (0.27) and SM (0.22). There were, however, high inter-correlations between individual muscle architecture values, especially for muscle thickness and fascicle length data sets. Prediction of the whole hamstring architecture was achieved by combining data from all four muscles. These data show different designs of the hamstring muscles, especially between the SM and ST (medial) and BFlh and BFsh (lateral) muscles. Modeling the hamstrings as one muscle group by assuming uniform inter-muscular architecture yields less accurate representation of human hamstring muscle function.

PMID: 22564790 [PubMed – indexed for MEDLINE]

Tendon vibration during submaximal isometric strength and postural tasks.

Eur J Appl Physiol. 2012 Nov;112(11):3807-17

Authors: Spiliopoulou SI, Amiridis IG, Hatzitaki V, Patikas D, Kellis E

Abstract
To establish the effect of tendon vibration (TV) on steady submaximal strength and static balance control, 29 women performed isometric plantarflexions at 10, 20, 30 and 50% of MVC and postural tasks of increasing difficulty (Normal Quiet Stance, NQS, Sharpened Tandem Stance and One Leg Stance) with vibrators (80 Hz) applied to the Achilles tendon. Both tasks were performed under four conditions (10 s each): eyes open, eyes closed, eyes closed with TV, eyes open with TV. During the isometric tasks, the application of TV increased the plantarflexion torque at 20 and 30% of MVC, accompanied by a greater (p < 0.001) activation of agonists (Medial Gastrocnemius, MGAS and Soleus, SOL), as well as a greater (p < 0.05) coactivation of the antagonist (tibialis anterior, TA). Non-significant differences were observed at 10 and 50% of MVC. During posture, TV resulted in a backward displacement of the Centre of Pressure only for NQS and an increase (p < 0.01) in EMG activity of the ankle muscles in all balance tasks. The MGAS contraction during TV application was confirmed (by ultrasonography) at ten participants by an increase of the pennation angle and decrease of its fascicle length (p < 0.05). The vibration-induced muscle excitation and accompanying increase in motor output, probably due to excitatory Ia afferent input, was confirmed during strength and postural tasks. However, motor output attenuates when the magnitude of central drive to the ankle muscles increases.

PMID: 22388339 [PubMed – indexed for MEDLINE]

Biomechanical differences between incline and plane hopping.

J Strength Cond Res. 2011 Dec;25(12):3334-41

Authors: Kannas TM, Kellis E, Amiridis IG

Abstract
Kannas, TM, Kellis, E, and Amiridis, IG. Biomechanical differences between incline and plane hopping. J Strength Cond Res 25(12): 3334-3341, 2011-The need for the generation of higher joint power output during performance of dynamic activities led us to investigate the force-length relationship of the plantar flexors during consecutive stretch-shortening cycles of hopping. The hypothesis of this study was that hopping (consecutive jumps with the knee as straight as possible) on an inclined (15°) surface might lead to a better jumping performance compared with hopping on a plane surface (0°). Twelve active men performed 3 sets of 10 consecutive hops on both an incline and plane surface. Ground reaction forces; ankle and knee joint kinematics; electromyographic (EMG) activity from the medial gastrocnemius (MG), soleus (Sol) and tibialis anterior (TA); and architectural data from the MG were recorded. The results showed that participants jumped significantly higher (p < 0.05) when hopping on an inclined surface (30.32 ± 8.18 cm) compared with hopping on a plane surface (27.52 ± 4.97 cm). No differences in temporal characteristics between the 2 types of jumps were observed. Incline hopping induced significantly greater ankle dorsiflexion and knee extension at takeoff compared with plane hopping (p < 0.05). The fascicle length of the MG was greater at initial contact with the ground during incline hopping (p < 0.05). Moreover, the EMG activities of Sol and TA during the propulsion phase were significantly higher during incline compared with that during plane hopping (p < 0.05). It does not seem unreasonable to suggest that, if the aim of hopping plyometrics is to improve plantar flexor explosivity, incline hopping might be a more effective exercise than hopping on a plane surface.

PMID: 22080320 [PubMed – indexed for MEDLINE]

Incline plyometrics-induced improvement of jumping performance.

Eur J Appl Physiol. 2012 Jun;112(6):2353-61

Authors: Kannas TM, Kellis E, Amiridis IG

Abstract
The aim of this study was to examine the effects of incline plyometrics training on muscle activation and architecture during vertical jumping and maximum strength. Twenty male participants were divided in two training groups which followed after a 4 week training program. The incline plyometrics group (n = 10) trained by performing consecutive jumps on an inclined surface (15°) while the plane plyometrics (PP) group (n = 10) performed the same jumps on a plane surface. Both groups trained four times per week and performed 8 sets of 10 jumps in each session. Subjects performed squat jumps, counter movement jumps and drop jumps (DJ) prior to and immediately after the training period, while the electromyographic activity of the medial gastrocnemius (MGAS) and tibialis anterior muscles and the architecture of MGAS were recorded. Maximal isokinetic and isometric strength of the plantar flexors were performed. Analysis of variance showed that only the IP group improved fast DJ height performance by 17.4 and 14.4% (20 and 40 cm, p < 0.05). This was accompanied by a significantly higher MGAS activity during the propulsion phase (24% from 20 cm and 50% from 40 cm, p < 0.05) of the DJ and a longer working fascicle length (5.08%, p < 0.05) compared with the PP group. There were no significant changes in isokinetic and isometric strength of the plantar flexors after training for both groups. The increase of jumping performance, after incline plyometrics should be taken into consideration by coaches, when they apply hopping exercise to improve explosiveness of the plantar flexors.

PMID: 22038146 [PubMed – indexed for MEDLINE]

Olympic weightlifting training causes different knee muscle-coactivation adaptations compared with traditional weight training.

J Strength Cond Res. 2012 Aug;26(8):2192-201

Authors: Arabatzi F, Kellis E

Abstract
The purpose of this study was to compare the effects of an Olympic weightlifting (OL) and traditional weight (TW) training program on muscle coactivation around the knee joint during vertical jump tests. Twenty-six men were assigned randomly to 3 groups: the OL (n = 9), the TW (n = 9), and Control (C) groups (n = 8). The experimental groups trained 3 d · wk(-1) for 8 weeks. Electromyographic (EMG) activity from the rectus femoris and biceps femoris, sagittal kinematics, vertical stiffness, maximum height, and power were collected during the squat jump, countermovement jump (CMJ), and drop jump (DJ), before and after training. Knee muscle coactivation index (CI) was calculated for different phases of each jump by dividing the antagonist EMG activity by the agonist. Analysis of variance showed that the CI recorded during the preactivation and eccentric phases of all the jumps increased in both training groups. The OL group showed a higher stiffness and jump height adaptation than the TW group did (p < 0.05). Further, the OL showed a decrease or maintenance of the CI recorded during the propulsion phase of the CMJ and DJs, which is in contrast to the increase in the CI observed after TW training (p < 0.05). The results indicated that the altered muscle activation patterns about the knee, coupled with changes of leg stiffness, differ between the 2 programs. The OL program improves jump performance via a constant CI, whereas the TW training caused an increased CI, probably to enhance joint stability.

PMID: 21997458 [PubMed – indexed for MEDLINE]

Wrist flexors are steadier than extensors.

Int J Sports Med. 2011 Oct;32(10):754-60

Authors: Salonikidis K, Amiridis IG, Oxyzoglou N, Giagazoglou P, Akrivopoulou G

Abstract
To examine torque variability in 2 antagonistic muscles 20 individuals performed maximal and submaximal (5, 10, 20, 50 and 75% of Maximal Voluntary Contraction, MVC) isometric wrist flexions and extensions (5 s) at 5 different angles (230, 210, 180, 150 and 130°). The EMG activity of Flexor Carpi Ulnaris (FCU) and Extensor Digitorum (ED) was recorded and quantified as the integral of EMG. Participants showed higher maximal isometric torque (32.43±11.17 vs. 17.41±3.84 Nm) and lower coefficient of variability during wrist flexion compared to extension. The normalized agonist EMG increased across higher levels of torque for both wrist muscles. Interestingly, the coactivation of ED during wrist flexion was greater compared to the coactivation of FCU during wrist extension at 50 and 75% of MVC, regardless of wrist angle (f.e.: at 180° and 75% of MVC the normalised EMG of ED as antagonist was 14.84±5.18% vs. 9.33±6.94 of the FCU). It is concluded that a stronger isometric wrist flexion is more steadily produced, with greater coactivation compared to a weaker wrist extension, independently from muscle length and torque level. Even if the relative contribution of antagonists to the resultant torque is to be considered, altered activation patterns responsible for differences in force fluctuations could be suggested.

PMID: 21913160 [PubMed – indexed for MEDLINE]

In vivo and in vitro examination of the tendinous inscription of the human semitendinosus muscle.

Cells Tissues Organs. 2012;195(4):365-76

Authors: Kellis E, Galanis N, Natsis K, Kapetanos G

Abstract
The aim of this study was to examine the tendinous inscription (TI) of the human semitendinosus (ST) muscle using dissection (cadavers) and ultrasound (in vivo). Ultrasonography (US) scans were taken in 18 young males at rest and at maximum voluntary contraction (MVC). Further, the ST was dissected and removed from its origins in 10 cadaveric specimens (5 cadavers). The cadaveric long arm of the TI was 6.67 ± 0.64 cm (6.45 ± 1.21 in US) while the shorter arm was 2.39 ± 0.38 cm (1.99 ± 0.75 in US). The angle formed by the two TI arms ranged from 53.19 (US) to 56.05° (cadavers) while more superficial fascicles intersected the inscription at significantly higher angles (range 31.98 ± 6.15 to 34.69 ± 7.71°) compared with deeper fascicles (p < 0.05). Fascicle length did not differ between compartments, but it was significantly smaller in superficial compared with deeper layers (p < 0.05). With the exception of the angle between the TI arm and the deep aponeurosis, all measured angles as well as the length of the long arm of the TI increased significantly from rest to MVC (p < 0.05). The role of the TI probably lies in the local interconnections with the fascicles of either compartment, which upon contraction is such that the ST muscle contracts as one muscle. However, the TI arm morphology changes from rest to MVC, indicating a nonuniform displacement of the TI, mainly between the superficial and deeper layers of the muscle.

PMID: 21828998 [PubMed – indexed for MEDLINE]

Responses to Achilles tendon vibration during self-paced, visually and auditory-guided periodic sway.

Exp Brain Res. 2011 Sep;213(4):423-33

Authors: Radhakrishnan SM, Hatzitaki V, Patikas D, Amiridis IG

Abstract
Achilles tendon vibration (ATV) alters proprioceptive input of the triceps surae muscles resulting in a posterior postural shift during standing. When this is applied in combination with a more dynamic proprioceptive perturbation, postural responses to ATV are attenuated. In this study, we applied ATV during self-paced, visually and auditory guided voluntary periodic sway in order to examine how the vibration-induced afferent input is processed and reweighted at the presence of inter-sensory guidance stimuli. Seventeen healthy adults (aged 26.7 ± 4.23 years) performed 15 cycles of periodic sway under three sensory guidance conditions: (a) self-paced, (b) auditory paced (0.25 Hz), and (c) visually driven by matching the resultant force vector to a target sine-wave (0.25 Hz). Bilateral ATV (80 Hz, 3 mm) was applied between the 5th and 10th sway cycles. ATV evoked an earlier burst onset and increased activity of the plantarflexors consistent with a reduction in the amplitude and duration of forward sway. This in turn resulted in an increase in dorsiflexors’ activity in order to compensate for the greater backward sway. Postural responses to ATV were augmented when sway was auditory and visually guided. Forward sway variability increased with ATV and remained high while backward sway variability decreased in the post-vibration phase. Our results suggest that sensory context-dependent constraints that determine the degree of active control of posture and associated postural challenge involved in a particular task determine how the vibration-induced Ia afferent input will be registered and further processed by the central nervous system.

PMID: 21748332 [PubMed – indexed for MEDLINE]

Differences in soccer kick kinematics between blind players and controls.

Adapt Phys Activ Q. 2011 Jul;28(3):251-66

Authors: Giagazoglou P, Katis A, Kellis E, Natsikas C

Abstract
The purpose of the current study was to examine the kinematic differences during instep soccer kicks between players who were blind and sighted controls. Eleven male soccer players who were blind and nine male sighted performed instep kicks under static and dynamic conditions. The results indicated significantly higher (p < .05) ball speed velocities (20.81m/sec) and ball/foot speed ratio values (1.35) for soccer players who were blind during the static kick compared with sighted players (16.16m/sec and 1.23, respectively). Significant group effect on shank and foot angular velocity was observed during the static kicking condition (p < .05), while no differences were found during the dynamic kicking condition (p > .05). Despite the absence of vision, systematic training could have beneficial effects on technical skills, allowing athletes who are blind to develop skill levels comparable to sighted athletes.

PMID: 21725117 [PubMed – indexed for MEDLINE]

Neuromuscular efficiency during sit to stand movement in women with knee osteoarthritis.

J Electromyogr Kinesiol. 2011 Oct;21(5):689-94

Authors: Patsika G, Kellis E, Amiridis IG

Abstract
The purpose of this study was to investigate the neuromuscular efficiency of women with knee osteoarthritis (OA) when performing a sit-to-stand movement and during maximum strength efforts. Twelve women with unilateral knee OA (age 60.33±6.66 years, height 1.61±0.05 m, mass 77.08±9.2 kg) and 11 controls (age 56.54±5.46 years, height 1.64±0.05 m, mass 77.36±13.34 kg) participated in this study. Subjects performed a sit-to-stand movement from a chair while position of center of pressure and knee angular speed were recorded. Furthermore, maximal isokinetic knee extension and flexion strength at 60°/s, 120°/s and 150°/s was measured. Surface, electromyography (EMG) from the biceps femoris (BF), vastus lateralis (VL) and vastus medialis (VM) was recorded during all tests. Analysis of variance (ANOVA) showed that during the sit-to-stand OA group demonstrated significantly lower knee angular speed (44.49±9.61°/s vs. 71.68±19.86°/s), a more posterior position of the center of pressure (39.20±7.02% vs. 41.95±2.49%) and a higher antagonist BF activation (57.13±20.55% vs. 32.01±19.5%) compared with controls (p<0.05). Further, women with knee OA demonstrated a lower Moment-to-EMG ratio than controls in extension and eccentric flexion at 60°/s and 150°/s, while the opposite was found for concentric flexion at 60°/s (p<0.05). Among other factors, the slower performance of the sit-to-stand movement in women with OA is due to a less efficient use of the knee extensor muscles (less force per unit of EMG) and, perhaps, a higher BF antagonist co-activation. This may lead subjects with OA to adopt a different movement strategy compared with controls.

PMID: 21689947 [PubMed – indexed for MEDLINE]

Is soccer kick performance better after a “faking” (cutting) maneuver task?

Sports Biomech. 2011 Mar;10(1):35-45

Authors: Katis A, Kellis E

Abstract
Cutting in soccer is a common skill used to avoid the opponent’s pressure but the potential effects of such a skill on instep kicking performance have not been previously investigated. The purpose of this study was to examine the differences in lower limb biomechanics between straight approach soccer kicks and kicks performed following a cutting maneuver task. Ten young amateur soccer players performed, in a random order, instep kicks after a two-step straight approach run and kicks after a double “faking” cutting maneuver task. The results showed that kicking after a cutting maneuver task displayed significantly lower ball speed values compared with the straight approach instep kicking (16.73 vs. 19.78 m/s, respectively; p < 0.05). Moreover, analysis of variance showed significant differences between the two kicking conditions in ankle, knee and hip joint displacements. The present study indicated that performing instep kicks after a double-cutting maneuver reduces ball and foot speed probably due to increasing joint frontal and transverse plane rotations. Improvements in the performance of the cutting maneuver task through training might result in better transfer of energy and speed to the kicking task thus permitting players to perform more powerful kicks under realistic game conditions.

PMID: 21560750 [PubMed – indexed for MEDLINE]

Changes in the limits of stability induced by weight-shifting training in elderly women.

Exp Aging Res. 2011 Jan;37(1):46-62

Authors: Gouglidis V, Nikodelis T, Hatzitaki V, Amiridis IG

Abstract
This study examined the impact of visually guided weight-shifting training on elderly women’s limits of stability during static leaning and dynamic swaying in different directions. Sixty-three elderly women were assigned into a group that practiced weight shifting (30-min sessions, 3 sessions/week, 4 weeks) either in the anterior/posterior or medio/lateral direction and a control group. Training resulted in a reduction of upper body rotation and an increase of shank rotation and center of pressure displacement. It is concluded that weight-shifting practice increases the limits of stability and enhances the inverted pendulum pivoting of the human body during leaning and dynamic swaying balance tasks.

PMID: 21240818 [PubMed – indexed for MEDLINE]

Muscle coactivation before and after the impact phase of running following isokinetic fatigue.

J Athl Train. 2011 Jan-Feb;46(1):11-9

Authors: Kellis E, Zafeiridis A, Amiridis IG

Abstract
CONTEXT: The effects of fatigue on impact loading during running are unclear, with some authors reporting increased impact forces and others reporting decreased forces.
OBJECTIVE: To examine the effects of isokinetic fatigue on muscle cocontraction ratios about the knee and ankle during running.
DESIGN: Cross-sectional study.
SETTING: Neuromechanics laboratory.
PATIENTS OR OTHER PARTICIPANTS: Female middle-distance runners (age  =  21.3 ± 1.93 years) with at least 5 years of training experience.
INTERVENTION(S): Participants ran on the treadmill at 3.61 m/s before and immediately after the fatigue protocol, which consisted of consecutive, concentric knee extension-flexion at 120°/s until they could no longer produce 30% of the maximum knee-extension moment achieved in the familiarization session for 3 consecutive repetitions.
MAIN OUTCOME MEASURE(S): Electromyographic (EMG) amplitude of the vastus medialis (VM), biceps femoris (BF), gastrocnemius (GAS), and tibialis anterior (TA) was recorded using surface electrodes. Agonist∶antagonist EMG ratios for the knee (VM∶BF) and ankle (GAS∶TA) were calculated for the preactivation (PR), initial loading response (LR(1)), and late loading response (LR(2)) phases of running. Hip-, knee-, and ankle-joint angular displacements at initial foot contact were obtained from 3-dimensional kinematic tracings.
RESULTS: Fatigue did not alter the VM∶BF EMG ratio during the PR phase (P > .05), but it increased the ratio during the LR(1) phase (P < .05). The GAS∶TA EMG ratio increased during the LR(1) phase after fatigue (P < .05) but remained unchanged during the PR and LR(2) phrases (P > .05).
CONCLUSIONS: The increased agonist EMG activation, coupled with reduced antagonist EMG activation after impact, indicates that the acute decrease in muscle strength capacity of the knee extensors and flexors results in altered muscle-activation patterns about the knee and ankle before and after foot impact.

PMID: 21214346 [PubMed – indexed for MEDLINE]

On the Evaluation of Postural Stability after ACL Recostruction.

J Sports Sci Med. 2011;10(2):422-3

Authors: Kellis E, Amiridis IG, Kofotolis N

PMID: 24137059 [PubMed]

Three-dimensional kinematics and ground reaction forces during the instep and outstep soccer kicks in pubertal players.

J Sports Sci. 2010 Sep;28(11):1233-41

Authors: Katis A, Kellis E

Abstract
The purpose of the present study was to compare the three-dimensional kinematics of the lower extremities and ground reaction forces between the instep kick and the kick with the outside area of the foot (outstep kick) in pubertal soccer players. Ten pubertal soccer players performed consecutive kicking trials in random order after a two-step angled approach with the instep and the outstep portion of the foot. Three-dimensional data and ground reaction forces were measured during kicking. Paired t-tests indicated significantly higher (P < 0.05) ball speeds and ball/foot speed ratios for the instep kick compared with the outstep kick. Non-significant differences in angular and linear sagittal plane kinematic parameters, temporal characteristics, and ground reaction forces between the instep and outstep soccer kicks were observed (P > 0.05). In contrast, analysis of variance indicated that the outstep kick displayed higher hip internal rotation and abduction, knee internal rotation, and ankle inversion than the instep kick (P < 0.05). Our results suggest that the instep kick is more powerful than the outstep kick and that different types of kick require different types of skill training.

PMID: 20845213 [PubMed – indexed for MEDLINE]

Muscle architecture variations along the human semitendinosus and biceps femoris (long head) length.

J Electromyogr Kinesiol. 2010 Dec;20(6):1237-43

Authors: Kellis E, Galanis N, Natsis K, Kapetanos G

Abstract
The purpose of this study was to examine whether muscle architecture of the long head of biceps femoris (BF) and semitendinosus (ST) muscles varies along their length. The ST and BF muscles were dissected and removed from their origins in eight cadaveric specimens (age range 67.8-73.4 years). One-way analysis of variance designs were used to compare fascicle length (FL), pennation angle (PA) and muscle thickness (MT) between proximal, mid-belly and distal positions. Tendon and muscle length properties were also quantified. For the BF muscle, one-way analysis of variance tests showed a higher PA (23.96±3.82°) and FL (7.12±0.48 cm) proximally than distal positions (PA=17.78±1.95° and FL=6.35±0.89 cm, respectively). For the ST, there was a significantly (p<0.05) lower PA (8.81±1.22°) and FL (13.10±1.54 cm) proximally than distally (PA=14.69±1.09° and FL=15.49±2.30 cm, respectively). Muscle thickness significantly increased from distal to more proximal positions (p<0.05). These data suggest that the ST and BF architecture is not uniform and that measurement of these parameters largely depends on the measurement site. Modeling these muscles by assuming a uniform architecture along muscle length may yield less accurate representation of human hamstring muscle function.

PMID: 20727788 [PubMed – indexed for MEDLINE]

Vertical jump biomechanics after plyometric, weight lifting, and combined (weight lifting + plyometric) training.

J Strength Cond Res. 2010 Sep;24(9):2440-8

Authors: Arabatzi F, Kellis E, Saèz-Saez De Villarreal E

Abstract
The purpose of this study was to compare the effects of an Olympic weight lifting (OL), a plyometric (PL), and combined weight lifting + plyometric (WP) training program on vertical jump (VJ) biomechanics. Thirty-six men were assigned randomly to 4 groups: PL group (n = 9), OL group (n = 9), WP group (), and control (C) group (n = 8). The experimental groups trained 3 d.wk, for 8 weeks. Sagital kinematics, VJ height, power, and electromyographic (EMG) activity from rectus femoris (RF) and medial gastrocnemius (GAS) were collected during squat jumping and countermovement jumping (CMJ) before and after training. The results showed that all experimental groups improved VJ height (p < 0.05). The OL training improved power and muscle activation during the concentric phase of the CMJ while the subjects used a technique with wider hip and knee angles after training (p < 0.05). The PL group subjects did not change their CMJ technique although there was an increase in RF activation and a decrease of GAS activity after training (p < 0.05). The WP group displayed a decline in maximal hip angle and a lower activation during the CMJ after training (p < 0.05). These results indicate that all training programs are adequate for improving VJ performance. However, the mechanisms for these improvements differ between the 3 training protocols. Olympic weight lifting training might be more appropriate to achieve changes in VJ performance and power in the precompetition period of the training season. Emphasis on the PL exercises should be given when the competition period approaches, whereas the combination of OL and PL exercises may be used in the transition phases from precompetition to the competition period.

PMID: 20706157 [PubMed – indexed for MEDLINE]

Vibration effects on static balance and strength.

Int J Sports Med. 2010 Sep;31(9):610-6

Authors: Spiliopoulou SI, Amiridis IG, Tsigganos G, Economides D, Kellis E

Abstract
The purpose of this study was to investigate the effects of a vibration training protocol and a conventional strength training program consisting of similar exercises on knee extensors and flexors strength and postural sway in middle-aged women. 38 women were randomly assigned into a Vibration Group (n=12, static and dynamic exercises on a vibration plate, frequency: 15-25 Hz, amplitude: 2-12.8 cm), a Strength Group (n=16, same exercises without vibration) and a Control Group (n=10). Both experimental groups trained for 12 weeks (3 sessions/w). Static balance was assessed in 3 tasks of increasing difficulty: Normal Quiet Stance, Sharpened Tandem, and One-Legged Stance. Postural sway was evaluated using the Centre of Pressure variations in the Anterior/Posterior and Medio/Lateral direction. Eccentric and concentric strength of knee extensors and flexors was recorded using a Cybex dynamometer. After vibration training, postural sway significantly decreased in both directions for the vibration group in all tasks (p<0.05), whereas no significant differences were observed for the other groups. Isokinetic strength significantly (p<0.05) increased for both experimental groups at selected angular velocities. It was concluded that side-alternating vibration could have beneficial effects on static balance control for middle-aged women. Gains in isokinetic strength were quite similar for both experimental groups.

PMID: 20589590 [PubMed – indexed for MEDLINE]

Medial gastrocnemius architectural properties during isometric contractions in boys and men.

Pediatr Exerc Sci. 2010 Feb;22(1):152-64

Authors: Kannas T, Kellis E, Arampatzi F, de Villarreal ES

Abstract
The aim of this study was to examine the differences in muscle architecture during isometric tests between children and adults. Eight boys (age= 11.2 +/- 0.26 years) and eight men (age= 22.3 +/- 2.01 years) performed plantar flexion isometric efforts at angles of -15 degrees, 0 degrees, 15 degrees at 0%, 40%, 60%, 80% of MVC. Analysis of variance tests indicated that adults showed greater fascicle length from rest to 80% of MVC (p < .05), greater pennation angle at 80% and 100% of MVC (p < .05) and greater aponeuroses displacement at levels of effort greater than 60% of MVC (p < .05). These differences observed in MG would appear to favor better utilization of the force-length and the force-velocity relationships, of the muscle in adults compared with children.

PMID: 20332547 [PubMed – indexed for MEDLINE]

The effects of age and gender on the weight and use of schoolbags.

Pediatr Phys Ther. 2010;22(1):17-25

Authors: Kellis E, Emmanouilidou M

Abstract
PURPOSE: To examine age and gender differences in schoolbag use and to compare findings in Greece with those of other countries.
METHOD: Schoolbag weight and carrying characteristics were assessed in 703 boys and girls aged 6 to 14 years.
RESULTS: Multinomial logistic regression models showed that younger students (6-8 years) were almost 5 more times more likely to carry heavier schoolbags than older students (p < 0.05). Similarly, more girls than boys carried heavier bags, and girls were twice more likely to experience fatigue symptoms when carrying schoolbags compared with boys (p < 0.05). The mean schoolbag weight was 12.4% of the student’s body weight and moderately heavier than that reported for other countries.
CONCLUSIONS: Younger children are in greater need for education about schoolbag weight compared with older ones. There is also evidence that girls might experience more problems when carrying their schoolbag compared with boys.

PMID: 20142701 [PubMed – indexed for MEDLINE]

Muscle fatigue during intermittent exercise in individuals with mental retardation.

Res Dev Disabil. 2010 Mar-Apr;31(2):388-96

Authors: Zafeiridis A, Giagazoglou P, Dipla K, Salonikidis K, Karra C, Kellis E

Abstract
This study examined fatigue profile during intermittent exercise in 10 men with mild to moderate mental retardation (MR) and 10 men without mental retardation (C). They performed 4 x 30s maximal knee extensions and flexions with 1-min rest on an isokinetic dynamometer. Peak torque of flexors (PTFL) and extensors (PTEX), total work (TW), and lactate were measured. Fatigue was calculated as the magnitude of decline (%) in PTFL, PTEX, and TW and as rate of decline (linear slope) in TW from 1st to 4th set. MR had lower PTFL, PTEX, TW, and lactate throughout the protocol than C, while pre-motor time was greater in MR vs. C (p<0.05). MR demonstrated a delayed pattern of reduction in muscular performance. Lower values were observed in MR vs. C in the magnitude of decline for PTEX and TW and the rate of decline for TW. In conclusion, MR exhibit a different fatigue profile during intermittent exercise than C. The lower magnitude and decline rate in neuromuscular performance in MR during intermittent exercise is associated with their lower peak strength, short-term anaerobic capacity, and lactate accumulation. Rehabilitation and sport professionals should consider the differences in fatigue profile when designing intermittent exercise programs for MR.

PMID: 19910157 [PubMed – indexed for MEDLINE]

Force variability during isometric wrist flexion in highly skilled and sedentary individuals.

Eur J Appl Physiol. 2009 Dec;107(6):715-22

Authors: Salonikidis K, Amiridis IG, Oxyzoglou N, de Villareal ES, Zafeiridis A, Kellis E

Abstract
The association of expertness in specific motor activities with a higher ability to sustain a constant application of force, regardless of muscle length, has been hypothesized. Ten highly skilled (HS group) young tennis and handball athletes and 10 sedentary (S group) individuals performed maximal and submaximal (5, 10, 20, 50, and 75% of the MVC) isometric wrist flexions on an isokinetic dynamometer (Kin-Com, Chattanooga). The wrist joint was fixed at five different angles (230, 210, 180, 150, and 1300). For each position the percentages of the maximal isometric force were calculated and participants were asked to maintain the respective force level for 5 s. Electromyographic (EMG) activation of the Flexor Carpi Ulnaris and Extensor Digitorum muscles was recorded using bipolar surface electrodes. No significant differences were observed in maximal isometric strength between HS and S groups. Participants of HS group showed significantly (P < 0.05) smaller force coefficient of variability (CV) and SD values at all submaximal levels of MVC at all wrist angles. The CV and SD values remained unaltered regardless of wrist angle. No difference in normalized agonist and antagonist EMG activity was observed between the two groups. It is concluded that long-term practice could be associated with decreased isometric force variability independently from muscular length and coactivation of the antagonist muscles.

PMID: 19756710 [PubMed – indexed for MEDLINE]

Static balance control and lower limb strength in blind and sighted women.

Eur J Appl Physiol. 2009 Nov;107(5):571-9

Authors: Giagazoglou P, Amiridis IG, Zafeiridis A, Thimara M, Kouvelioti V, Kellis E

Abstract
The aim of the present study was to examine isokinetic and isometric strength of the knee and ankle muscles and to compare center of pressure (CoP) sway between blind and sighted women. A total of 20 women volunteered to participate in this study. Ten severe blind women (age 33.5 +/- 7.9 years; height 163 +/- 5 cm; mass 64.5 +/- 12.2 kg) and 10 women with normal vision (age 33.5 +/- 8.3 years; height 164 +/- 6 cm; mass 61.9 +/- 14.5 kg) performed 3 different tasks of increasing difficulty: Normal Quiet Stance (1 min), Tandem Stance (20 s), and One-Leg Stance (10 s). Participants stood barefoot on two adjacent force platforms and the CoP variations [peak-to-peak amplitude (CoPmax) and SD of the CoP displacement (CoPsd)] were analyzed. Sighted participants performed the tests in eyes open and eyes closed conditions. Torque/angular velocity and torque/angular position relationships were also established using a Cybex dynamometer for knee extensors and flexors as well as for ankle plantar and dorsiflexors. The main finding of this study was that the ability to control balance in both anterior/posterior and medio/lateral directions was inferior in blind than in sighted women. However, when sighted participants performed the tests blindfolded, their CoP sway increased significantly in both directions. There were no differences in most isometric and concentric strength measurements of the lower limb muscles between the blind and sighted individuals. Our results demonstrate that vision is a more prominent indicator of performance during the postural tasks compared to strength of the lower limbs. Despite similar level of strength, blind individuals performed significantly worse in all balance tests compared to sighted individuals.

PMID: 19701648 [PubMed – indexed for MEDLINE]

The effect of a 4-week training regimen on body fat and aerobic capacity of professional soccer players during the transition period.

J Strength Cond Res. 2009 Sep;23(6):1697-703

Authors: Sotiropoulos A, Travlos AK, Gissis I, Souglis AG, Grezios A

Abstract
The purpose of this study was to evaluate the changes in body fat percentage and aerobic capacity in professional soccer players, after the implementation of a specific 4-week training regimen during the transition period. Fifty-eight professional soccer players of the Greek Premier National Division were separated in experimental (n = 38) and control groups (n = 20). Body composition and maximum oxygen intake were evaluated before and after a 4-week training regimen followed during the transition period. The experimental design used for analyzing weight (kg), percent body fat (%) and VO2 max values (ml x kg(-1) x min(-1)) was a 2 x 2 (Groups x Measures), with Groups as a between-subjects factor and Measures as a within-subjects factor. The level of significance was set at p < or = 0.05 for all analyses. Analyses of variances showed that the experimental and the control groups achieved statistically significant (a) increases from pretest to posttest measures in body weight (0.595 kg and 1.425 kg, respectively) and percent body fat (0.25 and 0.82, respectively), and (b) decreases in VO2 max values from pretest to posttest measures (0.81 and 3.56, respectively). The findings of the study revealed that the players who followed the training regimen compared with the players that did not follow any specific training program gained less weight and body fat and exhibited lower reduction in their VO2 max values.

PMID: 19675494 [PubMed – indexed for MEDLINE]

Validity of architectural properties of the hamstring muscles: correlation of ultrasound findings with cadaveric dissection.

J Biomech. 2009 Nov 13;42(15):2549-54

Authors: Kellis E, Galanis N, Natsis K, Kapetanos G

Abstract
The purpose of this study was to compare the architectural parameters of the long head of biceps femoris (BFlh) and semitendinosus (ST) muscles by comparing measurements from ultrasound (US) with those obtained from direct dissection. The BFlh and ST architectures were examined bilaterally in 6 legs from 3 male cadavers. The fascicle length, pennation angle, muscle thickness and muscle and tendon length were obtained from direct measurement and US scans along each muscle. Intraclass correlation coefficients between the two methods ranged from 0.905 to 0.913 for the BFlh variables and from 0.774 to 0.974 for the ST parameters. Compared with the direct measurements, the US method showed a mean typical error of 0.09-0.14 cm for muscle thickness, 1.01-1.31 degrees for the pennation angle, 0.92-1.71 cm for fascicle length and muscle-tendon length measurements. The US method is a valid alternative tool for assessing basic architectural parameters of ST and BFlh components of the hamstring muscles.

PMID: 19646698 [PubMed – indexed for MEDLINE]

Effects of sex and mode of carrying schoolbags on ground reaction forces and temporal characteristics of gait.

J Pediatr Orthop B. 2009 Sep;18(5):275-82

Authors: Kellis E, Arampatzi F

Abstract
The aim of this study was to examine the effects of different modes of carrying schoolbags on children. Ten girls and eight boys walked without a bag (NO-BAG), with a bag placed low at the back (LBACK), with a back placed high at the back (HBACK) and carrying the bag using the handle (HANDBAG). Ground reaction forces and temporal characteristics were recorded. When carrying a schoolbag, children walked with shorter strides, at lower speeds, with higher double support phase and increased vertical ground reaction forces compared with the NO-BAG condition (P<0.01). Compared with normal walking biomechanics, the HANDBAG condition showed the highest alterations, whereas the LBACK the least. Carrying the schoolbag by the handle has the largest effect on gait characteristics. Wearing backpacks by adjusting the shoulder straps to a loose position may reduce schoolbag effects.

PMID: 19491708 [PubMed – indexed for MEDLINE]

Fatigue resistance during high-intensity intermittent exercise from childhood to adulthood in males and females.

Eur J Appl Physiol. 2009 Jul;106(5):645-53

Authors: Dipla K, Tsirini T, Zafeiridis A, Manou V, Dalamitros A, Kellis E, Kellis S

Abstract
This study examined the maturation pattern of fatigue resistance (FR) from childhood to adulthood in females and males during high-intensity intermittent exercise and compared FR between females and males in childhood and adolescence. Thirty males (boys 11.3 +/- 0.5 years, teen-males 14.7 +/- 0.3 years, men 24.0 +/- 2.1 years) and 30 females (girls 10.9 +/- 0.6 years, teen-females 14.4 +/- 0.7 years, women 25.2 +/- 1.4) participated in this study. They performed high-intensity intermittent exercise (4 x 18 maximal knee flexions and extensions with 1-min rest) on an isokinetic dynamometer at 120 degrees s(-1). Peak torque of flexors (PTFL) and extensors (PTEX), and total work (TW) were measured. FR was calculated as % of PTEX, PTFL, and TW in 4th versus 1st set. FR was greater (P < 0.05) in boys versus teen-males and men, and in teen-males versus men. In females, FR was greater (P < 0.05) in girls versus teen-females and women, but not different between teen-females and women. FR was not different in boys versus girls and in teen-males versus teen-females. FR for PTFL, PTEX, and TW correlated negatively (P < 0.001) with the respective peak values (r = -0.68 to -0.84), and FR for TW with peak lactate (r = -0.58 to -0.69). In addition, age correlated (P < 0.01) with FR for males (r = -0.75) and females (r = -0.55). In conclusion, FR during high-intensity intermittent exercise undergoes a gradual decline from childhood to adulthood in males, while in females the adult profile establishes at mid-puberty (14-15 years). The maturation profile of FR in males and females during development appears to reflect the maturation profiles of peak torque, short-term muscle power, and lactate concentration after exercise.

PMID: 19404672 [PubMed – indexed for MEDLINE]

The effect of selective muscle fatigue on sagittal lower limb kinematics and muscle activity during level running.

J Orthop Sports Phys Ther. 2009 Mar;39(3):210-20

Authors: Kellis E, Liassou C

Abstract
STUDY DESIGN: Controlled laboratory study.
OBJECTIVES: To compare the changes in lower limb sagittal kinematics in running after a knee fatigue protocol with those observed after an ankle fatigue protocol.
BACKGROUND: Impaired force-generating ability of specific muscles may affect running mechanics, with negative implications for injury occurrence and performance. Identifying the strategies used to compensate for fatigue of selected muscles may assist in the design of more effective exercise programs for injury prevention and performance enhancement in running.
METHODS AND MEASURES: Sagittal plane kinematic data and the electromyographic (EMG) signal of the vastus medialis (VM), gastrocnemius (GAS), and biceps femoris (BF) muscles were collected from 15 females running at 3.61 m/s on a treadmill prior to, and following, an isokinetic knee extension/flexion and, on a separate day, an ankle plantar flexion/dorsiflexion fatigue protocol performed at 120 degrees/s.
RESULTS: Ankle muscle fatigue caused decreased ankle dorsiflexion, while knee fatigue caused increased knee flexion at initial contact (P<.05). Both protocols increased knee flexion angle at toe-off, as well as the amplitude of GAS and VM EMG signal, with the hip more extended after knee fatigue and the ankle more plantar flexed after ankle muscle fatigue. Ankle muscle fatigue caused a significant increase in hip extensionand ankle plantar flexion angular velocity, and a decline of BF EMG signal during the swing phase (P<.05). Knee muscle fatigue decreased hip and knee flexion angular velocity and increased BF EMG signal during the swing phase (P<.05).
CONCLUSION: Localized muscle fatigue effects on sagittal kinematics differed between the 2 protocols. However, the strategy used to compensate for fatigue was similar for both protocols: to protect the joints at initial impact and to prevent impairments in performance during toeoff and swing phase.

PMID: 19252259 [PubMed – indexed for MEDLINE]

Determining variables of plyometric training for improving vertical jump height performance: a meta-analysis.

J Strength Cond Res. 2009 Mar;23(2):495-506

Authors: de Villarreal ES, Kellis E, Kraemer WJ, Izquierdo M

Abstract
Plyometric training improves vertical jump height (VJH). However, the effectiveness of plyometric training depends on various factors. A meta-analysis of 56 studies with a total of 225 effect sizes (ESs) was carried out to analyze the role of various factors on the effects of plyometrics on VJH performance. The inclusion criteria for the analysis were a) studies using plyometric programs for lower-limb muscles, b) studies employing true experimental designs and valid and reliable measurements, and c) studies including enough data to calculate ESs. Subjects with more experience in sport obtained greater enhancements in VJH performance (p < 0.01). Subjects in either good or bad physical condition benefit equally from plyometric work (p < 0.05), although men tend to obtain better power results than women after plyometric training (p < 0.05). With relation to the variables of performance, training volumes of more than 10 weeks and more than 20 sessions, using high-intensity programs (with more than 50 jumps per session), were the strategies that seemed to maximize the probability of obtaining significantly greater improvements in performance (p < 0.05). To optimize jumping enhancement, the combination of different types of plyometrics (squat jump + countermovement jump + drop jump) is recommended rather than using only 1 form (p < 0.05). However, no extra benefits were found to be gained from doing plyometrics with added weight. The responses identified in this analysis are essential and should be considered by strength and conditioning professionals with regard to the most appropriate dose-response trends for optimizing plyometric-induced gains.

PMID: 19197203 [PubMed – indexed for MEDLINE]

Effects of small-sided games on physical conditioning and performance in young soccer players.

J Sports Sci Med. 2009;8(3):374-80

Authors: Katis A, Kellis E

Abstract
The purpose of this study was to examine, first, the movement actions performed during two different small-sided games and, second, their effects on a series of field endurance and technical tests. Thirty-four young soccer players (age: 13 ± 0.9 yrs; body mass: 62.3 ± 15.1 kg; height: 1.65 ± 0.06 m) participated in the study. Small-sided games included three-a-side (3 versus 3 players) and six-a-side (6 versus 6 players) games consisting of 10 bouts of 4 min duration with 3 min active recovery between bouts. Soccer player performance was evaluated using five field tests: a) 30m sprint, b) throw-in for distance, c) Illinois Agility Test, d) dribbling the ball and e) horizontal jump before, in the middle and after the implementation of both game situations. Heart rate was monitored during the entire testing session. Each game was also filmed to measure soccer movements within the game. The ANOVA analysis indicated that the three-a- side games displayed significantly higher heart rate values compared with the six-a-side games (p < 0.05). The number of short passes, kicks, tackles, dribbles and scoring goals were significantly higher during the three-a-side compared with the six-a-side game condition (p < 0. 05) while players performed more long passes and headed the ball more often during the six-a-side (p < 0.05). After the three-a-side games, there was a significant decline in sprint and agility performance (p < 0.05), while after both game conditions significant alterations in the throw-in and the horizontal jump performance were observed (p < 0.05). The results of the present study indicated that three-a-side games provide higher stimulus for physical conditioning and technical improvement than six-a-side games and their use for training young soccer players is recommended. Key pointsThree-a-side games display higher HR compared with six-a-side games.In the three-a-side games players performed more short passes, kicks, dribbles, tackles and scored more goals compared with the six-a-side games.Impairment in endurance and field test performance was observed mainly after three-a-side games.The use of the three-a-side games to develop physical fitness and technique in young soccer players is recommended.

PMID: 24150000 [PubMed]

Visual feedback training improves postural adjustments associated with moving obstacle avoidance in elderly women.

Gait Posture. 2009 Feb;29(2):296-9

Authors: Hatzitaki V, Voudouris D, Nikodelis T, Amiridis IG

Abstract
The study examined the impact of visually guided weight shifting (WS) practice on the postural adjustments evoked by elderly women when avoiding collision with a moving obstacle while standing. Fifty-six healthy elderly women (70.9+/-5.7 years, 87.5+/-9.6 kg) were randomly assigned into one of three groups: a group that completed 12 sessions (25 min, 3s/week) of WS practice in the Anterior/Posterior direction (A/P group, n=20), a group that performed the same practice in the medio/lateral direction (M/L group, n=20) and a control group (n=16). Pre- and post-training, participants were tested in a moving obstacle avoidance task. As a result of practice, postural response onset shifted closer to the time of collision with the obstacle. Side-to-side WS resulted in a reduction of the M/L sway amplitude and an increase of the trunk’s velocity during avoidance. It is concluded that visually guided WS practice enhances elderly’s ability for on-line visuo-motor processing when avoiding collision eliminating reliance on anticipatory scaling. Specifying the direction of WS seems to be critical for optimizing the transfer of training adaptations.

PMID: 18996012 [PubMed – indexed for MEDLINE]

Reliability of spinal range of motion in healthy boys using a skin-surface device.

J Manipulative Physiol Ther. 2008 Oct;31(8):570-6

Authors: Kellis E, Adamou G, Tzilios G, Emmanouilidou M

Abstract
OBJECTIVE: The purpose of this study was to examine the interrater and intrarater reliability of spinal extension and flexion measurements using a skin-surface (Spinal Mouse; Idiag, Voletswil, Switzerland) instrument.
METHODS: Spinal curvature was measured during standing, full flexion, and full extension in 81 healthy children (10.6 +/- 1.7 years) by 3 raters on 2 separate occasions. Intraclass correlation coefficients (ICCs) and standard errors of measurement were used to examine between-day and interrater reliability for thoracic, lumbar, and hip range of motion.
RESULTS: The intrarater ICCs ranged from 0.61 to 0.96, and the interrater ICCs ranged from 0.70 to 0.93. The standard error of measurement ranged from 0.61 degrees to 13.18 degrees .
CONCLUSION: Evaluation of spinal range of motion measures in children using the Spinal Mouse demonstrates fair to high reliability.

PMID: 18984239 [PubMed – indexed for MEDLINE]

Direction-induced effects of visually guided weight-shifting training on standing balance in the elderly.

Gerontology. 2009;55(2):145-52

Authors: Hatzitaki V, Amiridis IG, Nikodelis T, Spiliopoulou S

Abstract
BACKGROUND: Controversial findings exist in the literature with respect to the efficacy of visually guided weight-shifting (WS) training as a means of improving balance in healthy older adults.
OBJECTIVE: The purpose of this study was to investigate the impact of two direction-specific, visually guided WS training protocols on standing balance of healthy elderly women.
METHODS: Forty-eight community-dwelling elderly women, all free of any neurological or musculoskeletal impairment, were randomly assigned into: a group that practiced WS in the anterior/posterior direction (A/P group, n=19), a group that practiced WS in the medio/lateral direction (M/L group, n=15) and a control group (n=14). Participants performed 12 training sessions of visually guided WS (3 sessions a week for 25 minutes per session). Static balance was measured before and after training in normal (bipedal) quiet stance (NQS) and sharpened-Romberg stance (SRS) by recording center of pressure (CoP) variations and angular segment kinematics.
RESULTS: In NQS, neither of the two training protocols had a significant impact on postural sway measures, although a significant decrease in interlimb asymmetry of CoP displacement was noted for the A/P group. In SRS, A/P training induced a significant reduction of CoP displacement, lower limb pitch and upper trunk roll rotation.
CONCLUSION: The results of the study stress the importance of using direction-specific WS tasks in balance training, particularly in the A/P direction, in order to improve control of static balance in elderly women.

PMID: 18594127 [PubMed – indexed for MEDLINE]

The effects of plyometric, tennis-drills, and combined training on reaction, lateral and linear speed, power, and strength in novice tennis players.

J Strength Cond Res. 2008 Jan;22(1):182-91

Authors: Salonikidis K, Zafeiridis A

Abstract
Reaction time, first-step quickness, lateral (side steps), and forward speed over short distances are important parameters for tennis performance. The aims of this study were: (i) to diagnose the presence of laterality in tennis lateral movements and (ii) to compare the effects of plyometric training (PT), tennis-specific drills training (TDT), and combined training (CT) on performance in tennis-specific movements and power/strength of lower limbs. Sixty-four novice tennis players (21.1 +/- 1.3 years) were equally (n = 16) assigned to a control (C), PT, TDT, or CT. Training was performed 3 times/week for 9 weeks. Testing was conducted before and after training for the evaluation of reaction time (single lateral step), 4-m lateral and forward sprints, 12-m forward sprints with and without turn, reactive ability, power, and strength. There was a significant difference in lateral speed (side-steps) between the 2 sides (P < 0.05). PT, TDT, or CT improved the 4m lateral and forward sprints (P < 0.05). PT and CT improved also the reaction time of the “slow” side (P < 0.05), whereas TDT and CT improved the 12-m sprint performances with and without turn (P < 0.05). Power and strength improved in most tests after PT and CT. Lateral and forward sprints were correlated (r = -0.50 to -0.75; P < 0.05) with power/strength. In conclusion, PT improved fitness characteristics that rely more on reactive strength and powerful push-off of legs such as, lateral reaction time, 4-m lateral and forward sprints, drop jump and maximal force. TDT improved all 4-m and 12-m sprint performances, whereas CT appeared to incorporate the advantage of both programs and improved most tests items. Tennis coaches should be aware that each training regimen may induce more favorable changes to different aspects of fitness.

PMID: 18296973 [PubMed – indexed for MEDLINE]

Sequentially allocated clinical trial of rhythmic stabilization exercises and TENS in women with chronic low back pain.

Clin Rehabil. 2008 Feb;22(2):99-111

Authors: Kofotolis ND, Vlachopoulos SP, Kellis E

Abstract
OBJECTIVE: To examine the effectiveness of rhythmic stabilization exercises and transcutaneous electrical nerve stimulation (TENS) and their combination in treating women with chronic low back pain.
DESIGN: Sequentially allocated, single-blinded and controlled study, with a two-month follow-up.
SETTING: The data were collected in a patient rehabilitation setting.
SUBJECTS: A total of 92 women (34-46 years old) with chronic low back pain were studied.
INTERVENTIONS: Sequential allocation was undertaken into four groups: ;rhythmic stabilization’ (n=23), ;rhythmic stabilization – TENS’ (n=23), TENS (n=23), and a placebo group (n = 23). Each programme lasted for four weeks. All outcome measures were assessed prior to, immediately after, four weeks and eight weeks post intervention.
MAIN MEASURES: Data were obtained on functional disability, pain intensity, trunk extension range of motion, dynamic endurance of trunk flexion and static endurance of trunk extension.
RESULTS: A total of 88 patients provided two-month follow-up data. The ;rhythmic stabilization’ and the ;rhythmic stabilization – TENS’ groups displayed statistically significant (P<0.05) improvements in functional disability and pain intensity (ranging from 21.2 to 42.8%), trunk extension range of motion (ranging from 6.5 to 25.5%), dynamic endurance of trunk flexion and static endurance of trunk extension (ranging from 13.5 to 74.3%) compared with the remaining groups.
CONCLUSIONS: The rhythmic stabilization programmes resulted in more gains in women with chronic low back pain regarding the present outcome variables compared with the other groups; therefore, its application in female chronic low back pain patients aged 34-46 years is recommended.

PMID: 18212032 [PubMed – indexed for MEDLINE]

Quantification of functional knee flexor to extensor moment ratio using isokinetics and electromyography.

J Athl Train. 2007 Oct-Dec;42(4):477-85

Authors: Kellis E, Katis A

Abstract
CONTEXT: Evaluating moment balance around the knee helps athletic trainers set appropriate targets for injury prevention and rehabilitation programs.
OBJECTIVE: To examine the knee flexor (KF) to knee extensor (KE) moment ratios using the moments when each muscle group acts as an agonist and using the moments when the KE acts as an agonist and the KF acts as an antagonist.
DESIGN: Cross-sectional.
SETTING: University research laboratory.
PATIENTS OR OTHER PARTICIPANTS: Seventeen pubertal males (age = 13.7 +/- 0.2 years, height = 1.61 +/- 0.04 m, mass = 51.3 +/- 2.7 kg).
INTERVENTION(S): The subjects performed maximal isokinetic concentric KE (KE(CON)) and eccentric KF (KF(ECC)) efforts and performed eccentric KE (KE(ECC)) and concentric KF efforts at 60 degrees /s and 180 degrees /s while we recorded the bipolar surface electromyographic (EMG) signal of the involved muscles. The KF antagonist moment was estimated from EMG-moment relationships determined during calibration KF efforts. Maximal moments were used to estimate the KF:KE ratios, and EMG-based moments were used to estimate the antagonist to agonist ratios.
MAIN OUTCOME MEASURE(S): We calculated KF:KE ratios for various angular positions, velocities, and movement directions.
RESULTS: The KF(ECC):KE(CON) ratio significantly increased as the knee extended (P < .05) at increased angular velocity (P < .05), reaching a value of 3.14 +/- 1.95 at full extension. The estimated knee flexor antagonist to knee extensor agonist ratio also increased near full extension (0.32 +/- 0.21).
CONCLUSIONS: Although the KFs have a higher capacity to produce maximal moment near knee extension and at increased angular velocities, knee joint movement is achieved through a balanced coactivation of the 2 antagonistic muscle groups to maintain joint stability and movement efficiency. The combined use of moment and EMG data can provide additional useful information regarding muscle balance around the knee.

PMID: 18174936 [PubMed – indexed for MEDLINE]

Ankle sprain injuries: a 2-year prospective cohort study in female Greek professional basketball players.

J Athl Train. 2007 Jul-Sep;42(3):388-94

Authors: Kofotolis N, Kellis E

Abstract
CONTEXT: Ankle sprains are a common basketball injury. Therefore, examination of risk factors for injury in female professional basketball players is worthwhile.
OBJECTIVE: To examine rates of ankle sprains, associated time missed from participation, and risk factors for injury during 2 consecutive seasons.
DESIGN: Prospective cohort study.
SETTING: Eighteen professional basketball facilities.
PATIENTS OR OTHER PARTICIPANTS: We observed 204 players from 18 female professional basketball teams for 2 consecutive seasons during a 2-year period.
MAIN OUTCOME MEASURE(S): Using questionnaires, we recorded the incidence of ankle sprains, participation time missed, and mechanisms of injury in games and practice sessions. Potential risk factors, such as age, body mass, height, training experience, and history of ankle sprain, were examined using multivariate logistic regression.
RESULTS: Fifty of the 204 participants sustained ankle injuries; injuries included 32 ankle sprains, which translated to an ankle sprain rate of 1.12 per 1000 hours of exposure to injury. The 32 players missed 224.4 training and game sessions and an average of 7.01 sessions per injury. Most injuries occurred in the key area of the basketball court and were the result of contact. Injury rates during games were higher than injury rates during practice sessions. Centers, followed by guards and forwards, had the highest rate of injury. Players who did not wear an external ankle support had an odds ratio of 2.481 for sustaining an ankle sprain.
CONCLUSIONS: Female professional basketball athletes who did not wear an external ankle support, who played in the key area, or who functioned as centers had a higher risk for ankle sprain than did other players.

PMID: 18059995 [PubMed – indexed for MEDLINE]

Effect of strength and aerobic training in children with cerebral palsy.

Med Sci Sports Exerc. 2007 Nov;39(11):1902-9

Authors: Unnithan VB, Katsimanis G, Evangelinou C, Kosmas C, Kandrali I, Kellis E

Abstract
PURPOSE: The aim of this study was to evaluate the effectiveness of a strength and aerobic interval training program on arm-cranking economy, aerobic capacity, and gross motor function in children and adolescents with spastic diplegic cerebral palsy (CP).
METHODS: Seven subjects (five females, two males; mean age 15.9, range 14-18 yr) participated in a 12-wk aerobic interval and strength training program, three times per week, 70 min per session. The aerobic interval training required the subjects to perform 60-m uphill walking repetitions. Six individuals (four females, two males; mean age 15.7, range 14-17 yr) who did not receive the training program served as controls. Pre- and postexercise tests were performed to assess changes in arm-cranking economy, aerobic capacity, and gross motor function.
RESULTS: The results demonstrated significant (P < 0.05) reductions in the oxygen cost of arm cranking (mean +/- SD: pre: 13.2 +/- 3.22 mL x kg(-1) x min(-1), post: 12.7 +/- 3.38 mL x kg(-1) x min(-1)) and %VO2peak (pre: 75.8 +/- 3.55%, post: 61.8 +/- 7.47%) in the training group. Significant (P < 0.05) increases in aerobic capacity (pre: 17.5 +/- 4.45 mL x kg(-1) x min(-1), post: 20.8 +/- 5.93 mL x kg(-1) x min(-1)) were also noted for the training group. Significant (P < 0.05) improvements in gross motor function measures were also noted in the training group (pre: 30.35%, post: 33.85%).
CONCLUSION: These findings demonstrate that the application of a 12-wk strength and aerobic interval training program reduced the oxygen cost of arm cranking, improved aerobic capacity, and improved gross motor function in children and adolescents with CP.

PMID: 17986896 [PubMed – indexed for MEDLINE]

Agonist versus antagonist muscle fatigue effects on thigh muscle activity and vertical ground reaction during drop landing.

J Electromyogr Kinesiol. 2009 Feb;19(1):55-64

Authors: Kellis E, Kouvelioti V

Abstract
BACKGROUND: Agonist and antagonist co-activation plays an important role for stabilizing the knee joint, especially after fatigue. However, whether selective fatigue of agonists or antagonist muscles would cause different changes in muscle activation patterns is unknown.
HYPOTHESIS: Knee extension fatigue would have a higher influence on landing biomechanics compared with a knee flexion protocol.
STUDY DESIGN: Repeated-measures design.
METHODS: Twenty healthy subjects (10 males and 10 females) performed two sets of repeated maximal isokinetic concentric efforts of the knee extensors (KE) at 120 degrees s(-1) until they could no longer consistently produce 30% of maximum torque. On a separate day, a similar knee flexion (KF) fatigue protocol was also performed. Single leg landings from 30cm drop height were performed before, in the middle and after the end of the fatigue test. The mean normalized electromyographic (EMG) signal of the vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF) and gastrocnemius (GAS) at selected landing phases were determined before, during and after fatigue. Quadriceps:hamstrings (Q:H) EMG ratio as well as sagittal hip and knee angles and vertical ground reaction force (GRF) were also recorded.
RESULTS: Two-way analysis of variance designs showed that KE fatigue resulted in significantly lower GRF and higher knee flexion angles at initial contact while maximum hip and knee flexion also increased (p<0.05). This was accompanied by a significant decline of BF EMG, unaltered EMG of vastii and GAS muscles and increased Q:H ratio. In contrast, KF fatigue had no effects on vGRFs but it was accompanied by increased activation of VM, BF and GAS while the Q:H increased during before landing and decreased after impact.
CONCLUSION: Fatigue responses during landing are highly dependent on the muscle which is fatigued.

PMID: 17888681 [PubMed – indexed for MEDLINE]

Reliability of EMG power-spectrum and amplitude of the semitendinosus and biceps femoris muscles during ramp isometric contractions.

J Electromyogr Kinesiol. 2008 Jun;18(3):351-8

Authors: Kellis E, Katis A

Abstract
Appropriate reliability is a necessary condition for the use of surface EMG for evaluation of hamstring muscle function in cases of knee joint pathologies or ligament injuries. The aim of the study was to investigate the test-retest reliability of power spectrum and amplitude of surface electromyographic (EMG) measurements of semitendinosus (ST) and biceps femoris (BF) during ramp isometric contractions. Eleven males performed maximum isometric contractions (MVC) of the knee flexors in two sessions, a week apart with simultaneous recording of surface EMG of the BF and ST. Intra class correlation (ICC) and standard error measurements (SEM) were applied to assess test-retest reliability of the averaged EMG (aEMG) and the median frequency (MF) over 10 levels of force, from 0% to 100% of the maximum. The ICC values ranged from 0.38 to 0.96 for the aEMG with SEM values reaching 11.37% of MVC. For the MF, the ICCs ranged from 0.44 to 0.98 (SEM range 4.49-18.19Hz). In our set up, ramp contractions can be used to examine hamstring EMG patterns with acceptable reliability.

PMID: 17254801 [PubMed – indexed for MEDLINE]

Ankle sprain injuries and risk factors in amateur soccer players during a 2-year period.

Am J Sports Med. 2007 Mar;35(3):458-66

Authors: Kofotolis ND, Kellis E, Vlachopoulos SP

Abstract
BACKGROUND: Although ankle sprains are common in soccer, the role of various risk factors in amateur soccer players is unclear.
PURPOSE: To identify the incidence of ankle sprain injuries, associated time loss of participation, and risk factors during two consecutive seasons in amateur players.
STUDY DESIGN: Descriptive epidemiology study.
METHODS: Of 336 athletes enrolled in the study, 312 male amateur soccer players were observed during a 2-year period. Ankle sprain injury incidents, participation time loss, injury mechanisms, ankle region injured, and other risk factors were recorded in games and practice sessions using questionnaires.
RESULTS: During the study 208 ankle injuries were recorded, of which 139 were ankle sprains. These led to 975 sessions lost (on average, 7 lost sessions per injury). Most incidents (80.6%) were contact injuries, occurring mostly in defenders. Injury rates were equal between games and practice, while 61.1% of injuries were observed toward the end of each half of the game (P < .05). The injury incidence rate was higher during the first 2 months of the season as opposed to the last month (P < .05). Multinomial logistic regression showed that previous ankle sprain (P < .05) is a significant predictor of ankle sprain injury.
CONCLUSION: Ankle sprain injuries in amateur soccer players are primarily contact injuries, occurring mainly in defenders and during both games and practice. It appears that more injuries occur in players with previous ankle injury. Injury rates are higher toward the end of a game and chiefly occur during the first 2 months of the season.

PMID: 17218660 [PubMed – indexed for MEDLINE]

Biomechanical characteristics and determinants of instep soccer kick.

J Sports Sci Med. 2007;6(2):154-65

Authors: Kellis E, Katis A

Abstract
Good kicking technique is an important aspect of a soccer player. Therefore, understanding the biomechanics of soccer kicking is particularly important for guiding and monitoring the training process. The purpose of this review was to examine latest research findings on biomechanics of soccer kick performance and identify weaknesses of present research which deserve further attention in the future. Being a multiarticular movement, soccer kick is characterised by a proximal-to-distal motion of the lower limb segments of the kicking leg. Angular velocity is maximized first by the thigh, then by the shank and finally by the foot. This is accomplished by segmental and joint movements in multiple planes. During backswing, the thigh decelerates mainly due to a motion-dependent moment from the shank and, to a lesser extent, by activation of hip muscles. In turn, forward acceleration of the shank is accomplished through knee extensor moment as well as a motion-dependent moment from the thigh. The final speed, path and spin of the ball largely depend on the quality of foot-ball contact. Powerful kicks are achieved through a high foot velocity and coefficient of restitution. Preliminary data indicate that accurate kicks are achieved through slower kicking motion and ball speed values. Key pointsSoccer kick is achieved through segmental and joint rotations in multiple planes and via the proximal-to-distal sequence of segmental angular velocities until ball impact. The quality of ball – foot impact and the mechanical behavior of the foot are also important determinants of the final speed, path and spin of the ball.Ball speed values during the maximum instep kick range from 18 to 35 msec(-1) depending on various factors, such as skill level, age, approach angle and limb dominance.The main bulk of biomechanics research examined the biomechanics of powerful kicks, mostly under laboratory conditions. A powerful kick is characterized by the achievement of maximal ball speed. However, maximal ball speed does not guarantee a successful kick: in each case, the ball must reach the target. As already explained, when the player is instructed to hit the ball accurately, joint and segment velocities are lower as opposed to a fast and powerful kick performance. It is therefore apparent that future research should focus on biomechanics of fast but accurate kicking.

PMID: 24149324 [PubMed]

Hamstring antagonist moment estimation using clinically applicable models: Muscle dependency and synergy effects.

J Electromyogr Kinesiol. 2008 Feb;18(1):144-53

Authors: Kellis E, Katis A

Abstract
The purpose of this study was to examine whether prediction of antagonist moment (M(flx)) of the hamstrings using clinically applicable models depends on the muscles examined. Nine healthy males performed maximal isometric knee extension and flexion contractions at 0 degrees , 45 degrees and 90 degrees angles. Calibration knee flexion efforts were also performed at different levels of intensity. The resulting electromyographic (EMG) – moment curves were fitted using polynomial equations which were then used to estimate M(flx) from the antagonist EMG. Analysis of variance designs showed that the M(flx) predicted using the biceps femoris EMG was not significantly different compared with those predicted using the semitendinosus EMG data (p>0.05). Further, prediction of M(flx) using the EMG of both muscles or a combination of EMGs and architectural properties reduced estimation error but did not provide significantly different predicted values compared with the simpler single-muscle EMG – moment models (p<0.05). It appears that M(flx) prediction using the present EMG – moment model is not muscle specific. Prediction using models which combine EMG data and anatomic parameters of the hamstring muscle components yielded more accurate estimates and therefore their use to examine co-contraction levels is recommended.

PMID: 17055745 [PubMed – indexed for MEDLINE]

Strength and speed characteristics of elite, subelite, and recreational young soccer players.

Res Sports Med. 2006 Jul-Sep;14(3):205-14

Authors: Gissis I, Papadopoulos C, Kalapotharakos VI, Sotiropoulos A, Komsis G, Manolopoulos E

Abstract
The purpose of the present study was to compare maximal isometric force, force-time curve characteristics, pedaling rate, vertical jump, and sprint performance among young soccer players from different competition levels. Fifty-four (54) young soccer players were divided into three groups according to competition level: the elite group (n=18) consisted of soccer players from the national youth soccer team of Greece, the subelite group (n=18) consisted of youth soccer players who participated in the local championship, and the recreational group (n=18) consisted of recreational soccer players. All groups were evaluated for maximal isometric force, explosive force at 100 msec, peak force relative to body mass, rate of force development, squat and drop jump heights, 10 m sprint time, and pedaling rate. The elite group presented significantly (p < 0.05) higher maximal isometric force, vertical jump height, and pedaling rate, and lower 10 m sprint time in comparison with the subelite and recreational groups. No significant differences were observed in strength and speed characteristics between the subelite and recreational young soccer players. The findings of the present study suggest that the elite young soccer players can be distinguished from subelite and recreational young soccer players in strength and speed characteristics. These strength and speed measures can be used for strength and speed diagnosis, and for designing and evaluating training programs.

PMID: 16967772 [PubMed – indexed for MEDLINE]

Effects of two 4-week proprioceptive neuromuscular facilitation programs on muscle endurance, flexibility, and functional performance in women with chronic low back pain.

Phys Ther. 2006 Jul;86(7):1001-12

Authors: Kofotolis N, Kellis E

Abstract
BACKGROUND AND PURPOSE: Improving functional performance in patients with chronic low back pain is of primary importance. The purpose of this study was to examine the effects of 2 proprioceptive neuromuscular facilitation (PNF) programs on trunk muscle endurance, flexibility, and functional performance in subjects with chronic low back pain (CLBP).
SUBJECTS: Eighty-six women (40.2+/-11.9 [mean+/-SD] years of age) who had complaints of CLBP were randomly assigned to 3 groups: rhythmic stabilization training, combination of isotonic exercises, and control.
METHODS: Subjects trained with each program for 4 weeks with the aim of improving trunk stability and strength. Static and dynamic trunk muscle endurance and lumbar mobility were measured before, at the end of, and 4 and 8 weeks after training. Disability and back pain intensity also were measured with the Oswestry Index.
RESULTS: Multivariate analysis of variance indicated that both training groups demonstrated significant improvements in lumbar mobility (8.6%-24.1%), static and dynamic muscle endurance (23.6%-81%), and Oswestry Index (29.3%-31.8%) measurements.
DISCUSSION AND CONCLUSION: Static and dynamic PNF programs may be appropriate for improving short-term trunk muscle endurance and trunk mobility in people with CLBP.

PMID: 16813479 [PubMed – indexed for MEDLINE]

Laboratory strength measures of talented and less talented Greek children ages 9 to 10 years.

Percept Mot Skills. 2006 Feb;102(1):239-46

Authors: Papadopoulos C, Kalapotharakos VI, Gantiraga E, Noussios G, Gissis I

Abstract
The purpose was twofold: (1) to assess whether laboratory tests can distinguish talented groups and less talented groups of children identified by their total scores in a battery of field tests and (2) to evaluate whether scores in field and laboratory tests are correlated. 36 children (ages 9 to 10 years) were divided into groups of higher and lower total scores on standing long jump, medicine ball throw, and sprint. Both groups were tested for maximum isometric force, force produced at the initial 100 msec., reactive force index, drop jump height, squat jump height, and maximum pedaling rate. Analysis showed the talented group had significantly (p < or = .05) higher scores on maximum isometric force, force produced in the initial 100 msec., and drop jump height than less talented children. Moderate to high correlations were found among scores for field performance tests with maximum pedaling rate. In conclusion, the laboratory tests distinguished the talented and less talented children in maximal isometric force, the force produced at the initial 100 msec., the reactive force index, and the drop jump height.

PMID: 16671624 [PubMed – indexed for MEDLINE]

Muscle-contraction properties in overarm throwing movements.

J Strength Cond Res. 2006 Feb;20(1):117-23

Authors: Grezios AK, Gissis IT, Sotiropoulos AA, Nikolaidis DV, Souglis AG

Abstract
On the basis of dynamic and kinematic data, this study identifies the type of muscle contraction in unloaded overarm throwing movements. An unloaded throw or nearly unloaded throw is defined as the throw in which the external resistance is too small (e.g., the team handball, baseball, and water polo throws as well as the tennis and badminton smashes). A special arm-force-measuring apparatus was constructed to imitate an overarm throw. Forty-two subjects were placed into 3 groups: untrained subjects, weight-trained athletes, and team handball players. The measured parameters included the velocity of the initial movement, the release velocity, the velocity of the first 50 milliseconds of the concentric phase, the force value at the moment of deceleration of the initial movement, and the impulse values during the eccentric and concentric phases of the test movement. Statistically significant higher values of the above parameters (p < 0.05) were determined in that test at which the initial speed of movement was higher. Also, the correlation coefficients of the parameters of the initial phase of the throw movement were very high (p < 0.001), especially the parameters related with the movement’s first 50 milliseconds. The results support the thesis that the stretch-shortening cycle is the type of muscle contraction in unloaded overarm throws. Furthermore, it is possible to increase the throw velocity by increasing the velocity of the initial movement (i.e., by provoking higher inertia forces).

PMID: 16503670 [PubMed – indexed for MEDLINE]

Aging effects on postural responses to self-imposed balance perturbations.

Gait Posture. 2005 Nov;22(3):250-7

Authors: Hatzitaki V, Amiridis IG, Arabatzi F

Abstract
The present study investigated how young and older individuals organize their posture in response to self-induced balance perturbations evoked by oscillatory single limb movements. Eleven old (70.1+/-4.3 years) and nine young (20.1+/-2.4 years) participants performed repeatedly for 5s hip flexion/extension movements using full range of motion and maximum velocity. Two-dimensional joint kinematics (sampling rate: 60Hz), center of pressure (CoP) and EMG activity of tibialis anterior (TA), medial gastrocnemius (MGAS) rectus femoris (RF) and, semitendinosus (ST) in the stance limb were recorded and analysed. Cross-correlation function (CCF) analysis was used to identify the degree of coupling between the swinging limb (SL), center of gravity (CoG) and CoP motions. Old adults significantly limited SL, CoG and CoP range of anterior/posterior (A/P) motion in response to the forceful leg swinging. In the stance limb, significantly lower levels of ankle muscle activity resulted in reduced hip and knee joint excursions and increased ankle instability. By contrast, young performers produced sufficient ankle muscle activity to stabilize the foot to the ground while progressively increasing joint range of motion from the ankle to the hip. Center of pressure and SL movements were strongly correlated in an anti-phase relationship in both age groups. In older adults, however, the relationship between CoG-SL and CoG-CoP movements was neither strong nor synchronous, reflecting a weaker coupling and lack of coordination between component movements. It is concluded that insufficient ankle muscle activity, central integration deficits and increased anxiety to postural threat are important factors implicated for the weaker postural synergies and freezing of degrees of freedom seen in the elderly during performance of single limb oscillations.

PMID: 16214664 [PubMed – indexed for MEDLINE]

Effects of load on ground reaction force and lower limb kinematics during concentric squats.

J Sports Sci. 2005 Oct;23(10):1045-55

Authors: Kellis E, Arambatzi F, Papadopoulos C

Abstract
The purpose of this study was to examine the effects of external load on vertical ground reaction force, and linear and angular kinematics, during squats. Eight males aged 22.1 +/- 0.8 years performed maximal concentric squats using loads ranging from 7 to 70% of one-repetition maximum on a force plate while linear barbell velocity and the angular kinematics of the hip, knee and ankle were recorded. Maximum, average and angle-specific values were recorded. The ground reaction force ranged from 1.67 +/- 0.20 to 3.21 +/- 0.29 times body weight and increased significantly as external load increased (P < 0.05). Bar linear velocity ranged from 0.54 +/- 0.11 to 2.50 +/- 0.50 m x s(-1) and decreased significantly with increasing external load (P < 0.05). Hip, knee and ankle angles at maximum ground reaction force were affected by external load (P < 0.05). The force-barbell velocity curves were fitted using linear models with coefficients (r2) ranging from 0.59 to 0.96. The results suggest that maximal force exertion during squat exercises is not achieved at the same position of the lower body as external load is increased. In contrast, joint velocity coordination does not change as load is increased. The force-velocity relationship was linear and independent from the set of data used for its determination.

PMID: 16194981 [PubMed – indexed for MEDLINE]

Reliability of a practicable EMG-moment model for antagonist moment prediction.

Neurosci Lett. 2005 Aug 05;383(3):266-71

Authors: Kellis E, Kouvelioti V, Ioakimidis P

Abstract
Although the use of practicable EMG-moment models for knee joint moment prediction appears promising, the repeatability of the estimated forces remains unclear. The purpose of this study was to apply an EMG-moment model to predict the antagonist moment of the knee flexors (Mflx) during maximal isometric knee extension efforts. Nine healthy males performed maximal isometric knee extension and flexion contractions at 0 degrees , 45 degrees and 90 degrees angles with recordings of the net moment and EMG of thigh muscles. Calibration knee flexion efforts were performed at different levels of intensity and the resulting EMG-moment curves were fitted using second-order polynomials. The polynomials were then used to predict Mflx. This procedure was repeated a week after. The results indicated non-significant differences in test-retest Mflx. Intraclass correlation coefficients ranged from 0.852 to 0.912 indicating high test-retest reliability of the estimated Mflx. For isometric contractions, the present model is suitable as a method to estimate antagonist muscle moments.

PMID: 15955419 [PubMed – indexed for MEDLINE]

Static balance improvement in elderly after dorsiflexors electrostimulation training.

Eur J Appl Physiol. 2005 Jul;94(4):424-33

Authors: Amiridis I, Arabatzi F, Violaris P, Stavropoulos E, Hatzitaki V

Abstract
The purpose of the present study was to determine the effect of dorsiflexors’ ElectroStimulation (ES) training, on postural tasks of increasing difficulty in the elderly. Twenty-one elderly adults were randomly assigned into one of two groups: a Training (TG) and a Control Group (CG). The TG (n = 10) performed (4 weeks, 4 s/week, 40 min/session) superimposed (electrically evoked and voluntary activation) isometric dorsiflexions (ankle 100 degrees ) while seated. Biphasic, rectangular symmetrical pulses (300 ms, 70 Hz, 20-60 mA) were used to provoke maximal muscle activation. Participants performed three static balance tasks (Normal Quiet Stance, Sharpened Romberg, and One-Legged Stance) during which postural sway was quantified using maximum range and standard deviation of Centre of Pressure displacement (Kistler 9281C, 1,000 Hz). Bipolar surface electrodes were used to record the Electromyographic activity (EMG) of Tibialis Anterior, Medial Gastrocnemius, Rectus Femoris and Semi-Tendineous. Two-dimensional kinematic data were collected (60 Hz) and analyzed using the APAS Motion Analysis software. The body was modeled as a five-segment rigid link system. Isometric dorsiflexion moment/angular position relationship was also established using a Cybex dynamometer. ES training resulted in decreased postural sway (P < 0.05), greater ankle muscles EMG activity (P < 0.001), greater stability of the ankle joint (P < 0.05) and significant changes in mean position of all three joints of the lower limb. In addition, dorsiflexion moment significantly (P < 0.001) increased as a result of ES training. It is concluded that dorsiflexors’ ES training, could reduce postural sway and the use of ankle muscles, more characteristic of young adults, might appear in the elderly as well.

PMID: 15843959 [PubMed – indexed for MEDLINE]

Strength training effects on physical conditioning and instep kick kinematics in young amateur soccer players during preseason.

Percept Mot Skills. 2004 Oct;99(2):701-10

Authors: Manolopoulos E, Papadopoulos C, Salonikidis K, Katartzi E, Poluha S

Abstract
The purpose of this study was to examine effects of lower limb strength training on physical conditioning and kinematic characteristics of instep kicking in 16 young amateur soccer players who participated in initial and final laboratory tests. In addition to their standard preseason soccer program, 8 players comprised the experimental group, who performed an 8-wk. strength-training program. Maximal and relative isometric force of the lower limbs were significantly improved. Moreover, toe and ankle linear velocity during ball contact, ball velocity, as well as ankle, knee and hip angular velocities of the kicking leg were significantly increased. It is concluded that conditioning and kinematic indices of the kicking performance could be improved after strength training of the lower limbs.

PMID: 15560363 [PubMed – indexed for MEDLINE]

Knee biomechanics of the support leg in soccer kicks from three angles of approach.

Med Sci Sports Exerc. 2004 Jun;36(6):1017-28

Authors: Kellis E, Katis A, Gissis I

Abstract
PURPOSE: To examine knee joint kinematics, electromyographic (EMG) activity patterns and ground reaction forces (GRF) during an instep soccer kick from three different approaches relative to the ball.
METHODS: Ten male soccer players performed maximum kicks from 0 rad (K0), 0.81 rad (K45), and 1.62 rad (K90) angle between the players’ starting position and the position of the ball. GRF data and 3-D kinematics and EMG activity of the vastus medialis (VM), vastus lateralis (VL), and biceps femoris (BF) muscles of the lower leg were recorded.
RESULTS: colon; Compared with K0, K90, and K45 demonstrated higher medial and posterior GRF and lower anterior GRF. K90 and K45 also demonstrated higher external rotation displacement, maximum flexion, internal rotation, abduction, and adduction velocity of the tibia relative to the femur of the support leg compared with K0 (P < 0.01). The BF EMG before and immediately after ground contact was also higher in K90 and K45 compared with K0 (P < 0.01).
CONCLUSION: Soccer kicks using a high angle of approach increase the medial and posterior GRF, which is indicative of an altered stance during the kick, resulting in an altered balance. Such kicks are accompanied by significant alterations in knee joint kinematics and an increased BF activation around ground contact. Soccer kicks from an angled approach may induce significant loads to knee joint structures of the support leg.

PMID: 15179172 [PubMed – indexed for MEDLINE]

Age-induced modifications of static postural control in humans.

Neurosci Lett. 2003 Oct 30;350(3):137-40

Authors: Amiridis IG, Hatzitaki V, Arabatzi F

Abstract
We examined how young and older adults adapt their posture to static balance tasks of increasing difficulty. Participants stood barefoot on a force platform in normal quiet, Romberg-sharpened and one-legged stance. Center of pressure (CoP) variations, electromyographic (EMG) activity of ankle and hip muscles and kinematic data were recorded. Both groups increased postural sway as a result of narrowing the base of support. Greater CoP excursions, EMG activity and joint displacements were noted in old compared to younger adults. Older adults displayed increased hip movement accompanied by higher hip EMG activity, whereas no similar increase was noted in the younger group. It is concluded that older adults rely more on their hip muscles when responding to self induced perturbations introduced by increased task constraints during quiet standing.

PMID: 14550913 [PubMed – indexed for MEDLINE]

The effects of a calisthenics and a light strength training program on lower limb muscle strength and body composition in mature women.

J Strength Cond Res. 2003 Aug;17(3):590-8

Authors: Tsourlou T, Gerodimos V, Kellis E, Stavropoulos N, Kellis S

Abstract
This study evaluated and compared the effectiveness of an aerobics-calisthenics (A-CAL) and an aerobics/weight training (A-WT) programs on lower limb strength and body fat (%). Thirty-five adult women (age 42.1 +/- 5.2 years) were randomly assigned to A-CAL (n = 14), A-WT (n = 14), or a control group (n = 7). The A-CAL and A-WT trained 3 days per week for 10 weeks. Maximal bilateral isometric and isokinetic knee extension (KEXT) and flexion (KFLEX) torque, squat jump (SJ), and body fat (%) were measured before and immediately after training. The results revealed nonsignificant differences between A-CAL and A-WT (p > 0.05). Both A-CAL and A-WT improved SJ (p < 0.001). A-WT increased isometric torque of KEXT and KFLEX (p < 0.05), isokinetic torque of KFLEX (p < 0.05), and decreased body fat (%) (p < 0.05) when compared with controls. In summary, the application of a 10-week light-weight training program improved selected strength parameters of healthy women, compared with controls, but the effectiveness of the calisthenics exercises as an independent form of strength training is dubious.

PMID: 12930192 [PubMed – indexed for MEDLINE]

Antagonist moment of force during maximal knee extension in pubertal boys: effects of quadriceps fatigue.

Eur J Appl Physiol. 2003 May;89(3-4):271-80

Authors: Kellis E

Abstract
The examination of the moment exerted by the hamstrings during maximum isokinetic knee extensor tests is useful when comparing isokinetic strength and muscle activity patterns between children and adults. The purpose of this study was to examine the effect of antagonist moment of the hamstrings on the isokinetic moment of the knee extensors in pubertal children and to determine whether this effect is altered following a fatigue task. Eighteen healthy pubertal males [age 14.3 (0.5) years] performed 34 maximal isokinetic concentric efforts of the knee extensors at 60 degrees.s(-1). The average moment of force and electromyographic (aEMG) signal of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) at 11-30 degrees, 31-50 degrees, 51-70 degrees and 71-90 degrees of knee flexion were calculated for each repetition. The hamstrings antagonist moment was determined before and after the fatigue task by fitting the aEMG-moment relationship at different levels of muscle effort using second-degree polynomials. The percentage contribution of the antagonist moment to the resultant joint moment ranged from 7.1 % to 60.4 % throughout the range of motion, with the highest percentage observed close to full knee extension (11-30 degrees). The antagonist effect was significantly greater during concentric tests of the knee extensors compared to the corresponding eccentric tests ( p<0.05). Following the fatigue test, there was an overall decline of the resultant joint moment, but no changes in the predicted hamstrings moment were observed. These results indicate that when testing maximal knee extensor isokinetic strength in pubertal boys, activity of the hamstrings implies a reduction of the net extensor moment as compared to the isolated capacity of the knee extensors. However, this antagonist effect is not altered following the performance of an isokinetic fatigue knee extension task.

PMID: 12736835 [PubMed – indexed for MEDLINE]

The effects of electromyostimulation training and basketball practice on muscle strength and jumping ability.

Int J Sports Med. 2000 Aug;21(6):437-43

Authors: Maffiuletti NA, Cometti G, Amiridis IG, Martin A, Pousson M, Chatard JC

Abstract
The aim of this study was to investigate the influence of a 4-week electromyostimulation training program on the strength of the knee extensors and the vertical jump performance of 10 basketball players. Electromyostimulation sessions were carried out 3 times weekly; each session consisted of 48 contractions. Testing was carried out before and after the electromyostimulation training program (week 4) and once more after 4 weeks of normal basketball training (week 8). At week 4, isokinetic strength increased significantly (p < 0.05) at eccentric and high concentric velocities (between 180 and 360 x s(-1)); this was not the case for low concentric velocities (60 and 120 degrees x s(-1)). Electromyostimulation training increased also isometric strength at the two angles adjacent to the training angle (p < 0.01). Squat jump increased significantly by 14% at week 4 (p < 0.01), whereas counter movement-jump showed no change. At week 8, gains in isokinetic, isometric strength and squat-jump performance were maintained and the counter movement jump performance increased significantly by 17% (p<0.01). Electromyostimulation as part of a short strength-training program enhanced knee extensor strength and squat jump performance of basketball players.

PMID: 10961520 [PubMed – indexed for MEDLINE]

Velocity-specific training in elbow flexors.

Eur J Appl Physiol Occup Physiol. 1999 Sep;80(4):367-72

Authors: Pousson M, Amiridis IG, Cometti G, Van Hoecke J

Abstract
The purpose of this study was to show that velocity-specific training may be implicated in modifications in the level of coactivation of agonist and antagonist muscles. Healthy males (n = 20) were randomly placed in to two groups: one group trained using concentric contractions (n = 12), the other was an untrained control group (n = 8). The training group underwent unilateral resistance training at a level of 35 (5)% of a one-repetition maximal contraction of the elbow flexors, executed at maximal angular velocity. Training sessions consisted of six sets of eight consecutive elbow flexions, three times per weak for a total of seven weeks. The velocity of the ballistic movements executed during training were measured using an optoelectronic measuring device (Elite), both at the beginning and at the end of the training period. Subjects were tested pre- and post-training during isokinetic maximal elbow flexions with constant angular torque (CAT) at 90 degrees (0 degrees = full extension), and at different velocities (60, 120, 180, 240 and 300 degrees x s(-1)) for concentric actions, and -60 and -30 degrees x s(-1) for eccentric and isometric contractions at 90 degrees. In order to verify the levels of activation of the agonist biceps brachii (BB) muscles and antagonist triceps brachii (TB) muscles during maximal voluntary activation, their myoelectrical activities were recorded and quantified as root mean square (RMS) amplitudes, between angles of 75 and 105 degrees . The results show that mean angular velocities between elbow angles of 75 and 105 degrees were similar before [302 (32) degrees x s(-1)] and after [312 (27) degrees x s(-1)] the training period. CAT significantly increased measures at angular velocities of 240 and 300 degrees x s(-1) by 18.7% and 23.5%, respectively. The RMS activity of BB agonist muscles was not significantly modified by training. Post-training normalized RMS amplitudes of TB antagonist muscles were inferior to those observed at pre-training, but values were only significantly different at 300 x s(-1). In conclusion, in this study we attempted to show that an increase of CAT to 240 and 300 degrees x s(-1), though velocity-specific training, may be due, in part, to a lowering of the level of coactivation.

PMID: 10483808 [PubMed – indexed for MEDLINE]