Quadriceps Strength, Muscle Activation Failure, and Patient-Reported Function at the Time of Return to Activity in Patients Following Anterior Cruciate Ligament Reconstruction: A Cross-sectional Study

STUDY DESIGN: Cross-sectional. OBJECTIVES: To determine if quadriceps activation failure (QAF) moderates the relationship between quadriceps strength and physical function in individuals following anterior cruciate ligament (ACL) reconstruction. BACKGROUND: Quadriceps activation failure may impair the recovery of physical function after ACL reconstruction, given that QAF reduces strength and strength is related to physical function. Evidence of this relationship has been found in individuals with knee osteoarthritis, wherein patients with lower strength and greater QAF had lower levels of physical function. METHODS: Participants consisted of 52 individuals who were cleared for return to activity at an average. SD of 7.4. 1.2 months post-ACL reconstruction. Quadriceps activation failure was assessed using the superimposed burst technique, and quadriceps strength was assessed using concentric isokinetic contractions (Newton meters per kilogram). Physical function was quantified using a combined variable of a physical measure (single-leg hop for distance) and a self-reported function measure (International Knee Documentation Committee [IKDC] Subjective Knee Evaluation Form), calculated using a principal-component analysis. Simple correlations were then performed to determine the order in which variables were entered into the regression model to evaluate if QAF moderates the relationship between quadriceps strength and physical function. RESULTS: The combination of quadriceps strength and the interaction of strength by QAF predicted 30% of the variance in physical function (R-2 = 0.30, P<. 001; principal-component analysis, -0.61 strength + 0.02 interaction -1.896); however, the interaction of strength by QAF only accounted for 7% of the capabilities of the model (P =.023). CONCLUSION: Physical function is largely influenced by the recovery of quadriceps strength and minimally attenuated by QAF. These data suggest that QAF may affect individuals post-ACL reconstruction differently, and to a lesser extent, than individuals with knee osteoarthritis. This trial was prospectively registered in a public registry (NCT01555567).