Quadriceps Strength Asymmetry After Anterior Cruciate Ligament Reconstruction Alters Knee Joint Biomechanics and Functional Performance at Time of Return to Activity.

Background: Quadriceps strength deficits are observed clinically after anterior cruciate ligament (ACL) injury and reconstruction and are often not overcome despite rehabilitation. Given that quadriceps strength may be important for achieving symmetrical joint biomechanics and promoting long-term joint health, determining the magnitude of strength deficits that lead to altered mechanics is critical. Purpose: To determine if the magnitude of quadriceps strength asymmetry alters knee and hip biomechanical symmetry as well as functional performance and self-reported function. Study Design: Cross-sectional study; Level of evidence, 3. Methods: A total of 73 patients were tested at the time they were cleared for return to activity after ACL reconstruction. Quadriceps strength and activation, scores on the International Knee Documentation Committee form, the hop for distance test, and sagittal plane lower extremity biomechanics were recorded while patients completed a single-legged hop. Results: Patients with high and moderate quadriceps strength symmetry had larger central activation ratios as well as greater limb symmetry indices on the hop for distance compared with patients with low quadriceps strength symmetry (P < .05). Similarly, knee flexion angle and external moment symmetry were higher in the patients with high and moderate quadriceps symmetry compared with those with low symmetry (P < .05). Quadriceps strength was found to be associated with sagittal plane knee angle and moment symmetry (P < .05). Conclusion: Patients with low quadriceps strength displayed greater movement asymmetries at the knee in the sagittal plane. Quadriceps strength was related to movement asymmetries and functional performance. Rehabilitation after ACL reconstruction needs to focus on maximizing quadriceps strength, which likely will lead to more symmetrical knee biomechanics.