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Isokinetic muscular strength and performance in youth football : relationships with age, seasonal variation and injury

Forbes, Hollie Samantha


Hollie Samantha Forbes


Matthew Paul Greig

Lars McNaughton

Remco Polman

Jason Siegler

Monika Lohkamp

Sam L. Nabb


The primary aim of the current project was to investigate the isokinetic muscular strength and performance of elite male youth footballers, and the relationships with age, seasonal variation and injury. A secondary aim was to use the information gathered to target muscle strain injury prevention strategies to particular age groups and times, and evaluate the effect.

The primary aim was achieved by establishing normative patterns for muscular strength and performance of elite male youth footballers (grouped according to chronological and biological age) across a competitive season of youth football in Chapters Four and Five. Isokinetic muscular strength (characterised by peak torque (PT) and peak torque relative to body weight (PTBW)) of the hamstrings (H) and quadriceps (Q) using both concentric (CQ, CH) and eccentric muscle actions (EH) was evaluated. Muscular performance of the same muscle groups (characterised by H:Q ratios (conventional (CHQ) functional (FHQ)), asymmetry (dominant (dom):non dominant (ndom) leg ratios (e.g. CQ:CQ)), and angle of peak torque (AoPT)) was also investigated which necessitated an isokinetic speed of 60 °/s. Isokinetic evaluation was completed three times over the course of a regular playing season (start of season (SS) mid season (MS) and end of season (ES)).

Participants were grouped according to chronological age (n=152, under 12 (U12) - under 18 (U18)) and biological maturation (according to Pubertal Development Scale (PDS 1 - 5) n=134). Forty seven participants completed SS, MS and ES isokinetic evaluation. Bilateral isokinetic evaluation consisted of five maximal repetitions of CQ and CH, followed by five repetitions of EH, leg dominance was counter-balanced. Repetitions two-four were used to calculate PT, PTBW, dom:ndom and AoPT for CQ, CH and EH, CHQ and FHQ; these measures were compared across chronological and biological age groups using a mixed model ANOVA. Dom:ndom CQ, CH and EH were compared across chronological and biological age groups using a one way ANOVA, while the relationship between AoPT and PT/PTBW was considered using a Pearson’s correlation. Additionally, the relationship between chronological and biological age, and PT/PTBW was investigated using a mixed model ANOVA within PDS group three. For analysis of seasonal variation a mixed model ANOVA was applied for all isokinetic measurements which considered time (SS, MS, ES), leg dominance (dom, ndom) and age group (U12 -U15) with a further mixed model ANOVA performed on CQ:CQ, CH:CH and EH:EH. Where appropriate SIDAK corrections were applied and the level of significance was accepted at p≤0.05.

The main findings were that youth footballers did not increase their PT and PTBW EH in-line with CQ and CH as chronological and biological ageing progressed, this lead to a significant FHQ imbalance at U18. Dom:ndom CH comparisons identified that the chronologically younger and biologically less developed groups displayed a significantly stronger dom leg which may be explained through the concepts of skill acquisition and trainability. Biological age was not found to exert any additional effect over and above that of chronological ageing as significant differences in muscle strength still existed according to chronological age group within PDS group three. Additionally, AoPT EH and PT EH were found to be significantly negatively correlated on both legs which supported a potential mechanism for non contact hamstring muscle strain injury during running. Analysis of seasonal variation revealed that all PTBW measures showed a MS decrease. This may be related to breaks in normal training activity and links appropriately to times of peak injury incidence highlighted in youth football.

In order to achieve the secondary aim of the current project Chapters Four, Five and Six investigated the relationship between isokinetic muscular strength and performance, muscle strain injury of the thigh, and injury risk attenuation.

A retrospective and prospective injury audit was undertaken for the elite male youth football participants. For the retrospective approach participants were grouped according to chronological age (n=147) or biological age (n=128) and indicated using a self-report injury form their history (ever, (Hx)) or recent history (12 months, (Hx12)) of hamstring, quadriceps and adductor injuries. Approximately each player had an Hx of muscle strain injury and 0.56-0.59 of players had an Hx12. The hamstrings were the most commonly injured muscle group and the prevalence of muscle strain injury Hx and Hx12 increased with chronological and biological age. The prospective audit (n=50) identified that 0.16 of players sustained a muscle strain injury during the season, 0.08 of these being to the hamstrings.

Between group comparisons (one way ANOVA with SIDAK correction) were also performed to investigate the difference in isokinetic measures between those participants who had an Hx12 of muscle strain injury and those who did not. It was discovered that for Hx12 of an injury to the dom hamstrings the injured group had less PTBW CH and EH on the dom leg. The injured group also had more inner range AoPT CH. These findings linked appropriately to the reported mechanisms and risk factors for hamstring injury but the exact direction of cause and effect could not be established. To this end a logistic regression analysis was undertaken in an attempt to predict which group (injured vs. non injured the 50 participants would belong to, using evidenced based risk factors in the experimental model. No predictive relationship between risk factors (including altered isokinetic muscular strength and performance) could be established. The information regarding the relationship between injury and muscular strength and performance may highlight a role for isokinetic screening to ensure adequate rehabilitation from injury.

Injury risk attenuation strategies were investigated through an exercise intervention using the U18 age group following a break from football activity. The participants were split based on their FHQ at initial isokinetic evaluation (via odd and even placing) to form control (n=8) and intervention groups (n=8). Isokinetic evaluation was conducted as previously outlined and the exercise intervention targeted the hamstrings. Only six of the control group and seven of the intervention group completed the study and were compared using a mixed model ANOVA. Results showed that the intervention group were not significantly different to the control group post intervention for any of the isokinetic muscular strength and performance measures, though both groups significantly improved over time for the ndom leg CHQ and PTBW EH, and FHQ improved for both legs. Contamination of the control group may explain the lack of significant difference between groups. However, the exercise intervention was not targeted to individuals who displayed prior alterations to isokinetic muscular strength and performance, and this approach was discussed using the results of one member of the intervention group.

In summary, the current project achieved the stated aims by discovering normative patterns of isokinetic muscular strength and performance according to age and seasonal variation. Injury risk attenuation strategies were targeted appropriately to the U18 age group following a break from football activity. However, the applied evidence based exercise may have been more effective if targeted to ‘risk’ after isokinetic screening.


Forbes, H. S. (2012). Isokinetic muscular strength and performance in youth football : relationships with age, seasonal variation and injury. (Thesis). University of Hull. Retrieved from

Thesis Type Thesis
Deposit Date May 7, 2013
Publicly Available Date Feb 22, 2023
Keywords Sport science
Public URL
Additional Information Department of Sport, Health and Exercise Science, The University of Hull
Award Date Jun 1, 2012


Thesis (2.2 Mb)

Copyright Statement
© 2012 Forbes, Hollie Samantha. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

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