Influence of athletic training on functional lower-extremity stiffness

Abstract

Stiffness of the leg spring quantifies the relationship between the amount of leg flexion and the external load to which limbs are subjected. Lower limb stiffness is essential to facilitate athlete performance and injury risk minimisation. However, stiffness modulation is reliant upon the task requirements, the individual’s training status and the athletic training background of individuals. A systematic review highlighted a need to develop an understanding of how differing female athletic populations optimise stiffness to meet task demands and identify appropriate monitoring tools for athlete screening and subsequent longitudinal tracking of leg stiffness changes including potential associations with increased injury risk. Four studies were undertaken; 1) to investigate leg stiffness, joint stiffness and modulation strategy differences in female sub-populations from varied training backgrounds during discrete jumping tasks, 2) to evaluate the differences in leg stiffness between female sub-populations from varied training backgrounds during dynamic jumping and sports-specific tasks and to compare the observed stiffness measures between the tasks, 3) to assess differences in leg and joint stiffness in varying athletic populations during functional tasks and investigate the kinematic and kinetic mechanisms athletes utilise to modulate stiffness to meet sports-specific task demands, and 4) to evaluate longitudinal changes in stiffness across a season of training during dynamic and sports-specific tasks and evaluate potential links to injury risk in athletes. It was hypothesised that stiffness and the contributory kinetic and kinematic modulation strategies athletes utilise would differ between sub-populations. It was also theorised dynamic reactive jumping tasks may provide an adequate relationship to sports-specific tests. Additionally, it was expected that longitudinal changes in stiffness would be evident within the assessed athletic populations. Forty-seven female participants (20 nationally identified netballers, 13 high level endurance athletes and 14 age and gender matched controls) completed six unilateral tasks grouped into two categories; 1) discrete jumping tasks, traditionally utilised to assess stiffness (countermovement jump, drop jump, horizontal jump) and 2) functional sports-specific tasks (sprint, anticipated sidestep change of direction and repetitive hopping). Data was captured using a 10 camera motion analysis system (500 Hz) and force plate (1000 Hz) at three training phases; pre, post and off-season. Participants’ self-reported lower body non-contact sports related injury incidence. Statistical analysis evaluated leg stiffness, joint stiffness, contributory kinematic mechanisms and prospective injury risk. No significant differences were evident in leg stiffness measures (p=0.321-0.849) during the discrete jumping tasks despite variations in the underlying contributory mechanisms (p < 0.001-0.05). Significant differences were observed in leg stiffness, joint stiffness and the contributing mechanisms across all groups during dynamic and sports-specific tests (p < 0.000-0.017). Furthermore, results indicated the control group displayed no stiffness relationship among the sports-specific tasks, while the stiffness relationships evident between tasks within athletic populations reflected athlete training and competition demands. Pre-season results suggested repetitive hopping may serve as an intermediate monitoring tool in athletic populations. However, longitudinally it appeared differences were evident across the season in athletic populations during sports-specific tasks (p=0.005-0.042) which repetitive hopping was unable to identify. Variations in leg stiffness were also evident between the uninjured, soft tissues injury and overuse injury groups (p < 0.001-0.039). Furthermore, results of the injury risk prediction model indicated tasks relevant to an athlete’s training background predicted soft tissue and overuse injury risk within netball and endurance athletes. It would appear that leg stiffness assessment during basic maximal jumping tasks lack adequate sensitivity to identify clear modulation differences between groups with varying habitual training backgrounds. Differences in the ways groups optimise leg stiffness suggests that functional sports-specific tasks may be superior screening tools to discriminate stiffness differences between groups and as a monitoring tool to assess athletes. These findings highlight the need for practitioners to consider the appropriateness of the task utilised in leg stiffness screening. Results indicated that athletic training influences stiffness modulation strategies. Understanding the differences in the kinematic and kinetic stiffness modulation strategies athletes utilise to meet training and competition demands may provide insight into potential injury risk. Chronic athletic training appears to influence how athletes optimise stiffness across a season to meet performance demands and is related to injury risk. It appears functional sports-specific tasks are able to accurately identify and monitor longitudinal stiffness changes in athletes.