The effects of hip function on plyometric performance during a battery of single-leg hop tests following anterior cruciate ligament reconstruction
Background: Hip weakness results in abnormal biomechanics that increase anterior cruciate ligament (ACL) injury risk. Muscle strength and function qualities are related to single-leg hop performance outcomes in ACL injured individuals; however, the extent to which hip abductor (HAB) strength and activation impacts the plyometric performance of reactive strength, rate of force development, and ground reaction force impulse after ACL injury has not been explored. Objectives: To examine the effect of anterior cruciate ligament reconstruction (ACLR) on HAB rate of torque development (RTD) and electromyography (EMG) activity as well as phase-specific plyometric performance asymmetry during a battery of single-leg hop tests that included a drop vertical jump, hop for distance, and rebound side hop. A second objective was to examine the relationship between HAB RTD and muscle EMG activation asymmetry and plyometric performance asymmetry in individuals with ACLR and healthy control individuals. Design: A case-control comparative and exploratory analysis Setting: University Biomechanics Laboratory Methods: The study evaluated collegiate and recreational males and females (N =19) with ACLR who were cleared to return-to-sport and matched healthy controls (N=19). Bilateral isometric HAB RTD from 0-50 ms was tested to assess explosive strength. Plyometric performance asymmetry was assessed for each hop test. Between- and within-group comparisons were analyzed for HAB RTD (strength) and muscle activation (function) using dependent and independent t-tests. Between-group and within-phase comparisons of eccentric and concentric RTD and ground reaction force impulse asymmetry during three single-leg multidirectional hop tasks were evaluated using 2x2 mixed ANOVAs. Independent t-tests compared between-group asymmetry differences for concentric RTD and ground reaction force impulse, reactive strength, jump height, and ground contact time variables during three single-leg multidirectional hop tasks. Finally, Pearson correlations were performed between HAB RTD and EMG activation, eccentric and concentric RTD development and ground reaction force impulse and reactive strength asymmetry. Multivariate all-subsets linear regressions that included HAB RTD and muscle activation were conducted to predict plyometric ability asymmetry during the three-hop test battery. Results: No significant differences were found in HAB strength and function between the ACLR limbs and groups. Correlations were weak-to-moderate and none were statistically significant (r < .60, p > .05). The adjusted R squared value for each hop was less than 0.1. Significant between-group differences were identified in ACLR for RSI (p = 0.005, d = -1.00), jump height (p = 0.005, d = -1.00), and ground reaction force impulse asymmetry (F = 6.05, p = .016, ȠP2 = 0.08) during a single-leg drop vertical jump. Significant between-phase differences in ground reaction force impulse asymmetry were also identified (F = 6.95, p = .01, ȠP2 = 0.09) during a single-leg drop vertical jump. Conclusion: Hip abductor strength and function are not impaired after returning to sports in ACLR individuals. These variables have minimal influence on plyometric performance, suggesting that these variables do not play a role in the increased reinjury risk observed following ACLR. Additionally, deficits in GRF impulse asymmetry (greater in the concentric phase than eccentric phase), reactive strength, and jump height remained in the ACLR group after returning to sport. This suggests that for clinical testing, jump height is a sufficient surrogate for reactive strength, and force production should be a key focus of rehabilitation.