Are field-based assessments of lower-limb muscle function associated with muscle injuries during an amateur male football season?

Introduction: Despite growing interest in injury prevention in the professional realm, there is limited research at the amateur level. Injuries in amateur sport not only affect the athlete’s ability to participate but also bear a financial cost. In the amateur environment, there is often a lack of team medical staff to diagnose and record injuries, therefore limiting the ability to follow injury recording methods. Therefore, this study utilised an electronically delivered self-reporting injury survey to record all injuries. Often, players experience symptoms of injury whilst participating but the injury is not recorded until a more severe time-loss injury eventuates. A novelty of this study is that all injuries were recorded regardless of the effect on participation. Also, in this study, tests were conducted to evaluate the efficacy of cheaper field-based physical performance measures to assess injury risk in amateur footballers, in the hope of providing a framework for injury risk assessment in the amateur environment which is more affordable that traditional laboratory-based tests. Injury risk factors assessed were lower-limb flexibility, power and strength which were measured using goniometry, counter-movement jump (CMJ) and an isometric posterior lower-limb strength test (ISO-post) tests, respectively. In addition, the utility of a portable strength test for hamstring function was assessed using a portable force platform, which can be performed in the field. This study also investigated the criterion validity of the ISO-post test compared to lab-based isometric and eccentric isokinetic strength measures. Surface EMG was used to measure muscle activity during each strength test in order to further investigate the relationship between lab-based and field-based strength tests. Methods: In study 1 (i), 21 amateur football(soccer) players were recruited from three local football clubs to investigate laboratory isometric (ISOdyno) and eccentric (ECCdyno) dynamometry strength measures relative to the field-based isometric posterior lower limb (ISO-post) test using a portable force platform. Force output (N and N.m) were measured directly for isometric and isokinetic tests. Simultaneously, surface electromyography (EMG) was used to record muscle activation in the lateral and medial hamstrings. In study 2 (ii), 38 amateur football (soccer) players from three local clubs with a sub-group of n=20 were recruited. Lower-limb flexibility, CMJ performance, and ISO-post strength were assessed during three testing phases (pre-season, mid-season and post-season). Lower limb injuries were documented weekly throughout the season using the selfreported OSTRC questionnaire employing the “any physical complaint” definition of injury recorded using, for three testing phases (pre-season, mid-season and post-season). Results: i) No statistically significant correlation was seen between the ISO-post with ISOdyno and ECCdyno strength tests. Significant correlations were observed between peak muscle activity of the medial and lateral hamstrings measured during the ISO-post and ECCdyno tests which identifies a substantial relationship between the two tests. However, significantly lower medial hamstring activation during the ISO-post compared to the ISOdyno highlights that medial hamstrings did not achieve maximal activation during the ISO-post test, unlike the lateral hamstring muscles. Despite this, the lateral hamstrings are the more commonly injured during running movement and consequently justifies the inclusion of the ISO-post for injury risk assessment. ii) Examining injuries using the self-reported OSTRC questionnaire whilst using the definition of injury “any physical complaint” increases the number of injuries reported, 351 total injury reports compared to 24 time-loss injuries and 212 injury reports compared to 15 time-loss injuries for 5 target muscle groups (calves, hamstrings, quadriceps, adductors and lower-back). Negative correlations were reported between quadriceps flexibility and hamstring injuries, hamstring flexibility and quadriceps injury, and groin injuries and adductor flexibility. Conflicting results were seen for lower-back injury risk, as flexibility for the calves, quadriceps and adductors were positively associated with lower-back injuries, whilst quadriceps and adductor were negatively associated with lower-back injuries. CMJ was negatively correlated to groin injury, ISO-post was negatively correlated with hamstring injuries. The %Δ of CMJ and ISO-post performance pre- to mid-season and pre- to post-season were also negatively correlated indicative of less/more reported injuries with improvements/decrements in performance. Conclusion: Future research may consider weekly testing to enable coaching staff to identify rapid changes in performance measures. Further research is needed to determine the test-retest reliability of amateur coaching staff performing flexibility, CMJ and/or ISO-post measures in the field. Finally, future research could consider the effects of rotation of the lower-limb during the ISO-post test to favour each medial and lateral hamstrings activation and the relationship these changes in position have with sensitivity to match-induced fatigue