Musculoskeletal kinetic models to predict injury and performance in adolescent female athletes

Background: High level adolescent female athletes in high-impact sports may be at an increased injury risk due to increased training demands coupled with pubertal growth. Participation longevity in active females requires innovative, holistic research to advance the understanding of injury prevention via longitudinal training response monitoring. Measures of musculoskeletal stiffness (MSS), which model the lower limb as a spring, may provide valuable insight on this issue. MSS has been linked to both performance and injury through cross-sectional and retrospective study designs. This thesis aimed to longitudinally investigate measures of MSS, jump performance and injury in adolescent females from high-impact, low-impact and non-sporting groups. Methods: Participants from high impact sports (gymnastics, athletics), a low impact sport (water polo), and non-sporting controls were recruited for reliability (n=16), baseline (n=113) and longitudinal (n=83) studies. A range of jump tests were performed on two portable force plates, with a focus on self-paced repeat jumps for the baseline and longitudinal (12 months) studies. MSS was calculated for short (CJb) and long effort (CJb30) continuous bent-knee repeat jumps; and short effort continuous straight-leg repeat jumps (CJs). Retrospective injury, maturational status, training hours and nutritional habits were surveyed together with injuries during the 12 month test period. Reliability statistics included intraclass correlation coefficients (ICC), coefficient of variation (CV%), percentage bias (%Bias) and Cohen’s effect size (ES). Baseline and longitudinal statistical analyses included analysis of covariance (ANCOVA) and repeated measures analysis of covariance (ANCOVA) controlling for jump frequency and body mass, linear regression models, receiver operator characteristic (ROC) curves, and logistic regressions. Results: With appropriate data reduction MSS measures used in this thesis indicated acceptable reliability (Intra-Trial: ICC: 0.98, CV%: 9.5-9.8%; Inter-Day: ICC: 0.66- 0.92, CV%: 9.1-15.2, %Bias: 1.6-16.7, ES: -0.1-0.5). Significant MSS differences were identified between high-impact athletes (e.g. gymnasts: CJs kvert = 24.16 kN/m, track and field CJs kvert = 25.52 kN/m) and controls (e.g. CJs kvert = 14.97 kN/m), with low-impact athletes demonstrating moderate MSS levels (CJs kvert = 20.26 kN/m). High-impact athletes displayed greater increases in MSS when fatigued. Longitudinally the track and field athletes exhibited increased MSS (19-37%). Whereas gymnasts and water polo players showed little change (-11.3-7.7% and 3.4- 15.9% respectively) Non-sporting participants had moderate increases (1.4-31.7%). Gymnasts and water polo athletes displayed similar longitudinal changes in MSS under fatigue. However, track and field athletes displayed reduced longitudinal MSS increases under fatigue. Prospective injury results indicated high incidence of lower limb musculoskeletal injuries in high-impact athletes. Higher MSS during the CJb and CJs jump tasks significantly predicted 74% of lower leg and 77% of overuse injuries respectively. Approximately 87% of fractures were predicted by bone strength and CJb30 stiffness change. Although not statistically significant, previous acute injury history and lower CJs stiffness suggested increased acute injury risk. Discussion: Sport-specific differences in MSS measures and jump performance may reflect more effective elastic energy storage and utilization. Stiffness increases under fatigue may enable the maintenance of jump performance, but also place high-impact athletes under increased injury risk. Longitudinal changes in MSS only partially reflected the expected pattern of stiffness change. It’s suggested that increased growth observed in the gymnasts ‘outpaced’ any training effects. Reductions in relative jump performance and stiffness measures may reflect increased growth rather than poor training adaptation. Differences between the jump tasks and participant group results suggest potential sport-specific MSS adjustments. The present results support previous retrospective literature with stress-related overuse injuries predicted by greater levels of stiffness and lower levels of stiffness suggesting acute injury risk. Although the exact mechanisms are unclear, increased forces and loading rates associated with greater stiffness levels may overload the musculoskeletal system and result in overuse injury. In contrast, less stiffness may increase acute injury risk due to increased ranges of motion and poor elastic energy transfer. Conclusions: MSS appears related to jump performance and injury risk in adolescent females which is influenced by sport-specific training. However, a complex interaction between growth, training and MSS appears evident. Coaches should be aware of the potential impact of growth on training and performance outcomes in adolescent athletes. Specific measures of MSS appear good predictors of lower limb injury in high-impact adolescent athletes. These measures have potential for both talent and injury risk identification within adolescent female highimpact athletes.