Quadriceps Strength Correlations and Biomechanical Differences During the Single-Leg Squat Test With Posterior Versus Neutral Standardization After ACL Reconstruction

Background: After anterior cruciate ligament reconstruction (ACLR), the single-leg squat test (SLST) may be included in an initial/baseline performance testing battery. During the SLST, the position of the nonstance limb is known to affect the biomechanics/kinematics of the test, making SLST standardization important for comparing outcomes after ACLR. Hypothesis: The position of the nonstance limb during the SLST will influence the correlation between peak knee flexion limb-symmetry index (LSI) and quadriceps strength LSI, and affect lower extremity biomechanics during the SLST. Study Design: Biomechanical cohort study. Level of Evidence: Level 3. Methods: A total of 66 athletes completed quadriceps strength and single-leg squat testing 5 to 7 months after ACLR. Based on their nonstance limb position during the SLST, athletes were placed in either a posterior standardization (PS) or neutral standardization (NS) group for biomechanical analyses. Correlations between peak knee flexion LSI during the SLST and quadriceps strength LSI were investigated, and between- and within-group analyses of lower extremity joint biomechanics were completed. Results: Performing the SLST with PS produced a medium correlation between peak knee flexion LSI and quadriceps strength LSI ( R = 0.37, R 2 = 0.14), whereas NS produced a small correlation ( R = 0.27, R 2 = 0.07). In both groups, the surgical limb showed significantly less peak knee flexion (PS; P = 0.02) (NS; P = 0.03) and knee moment-contribution (PS; P = 0.04) (NS; P = 0.0001), as well as significantly more hip moment-contribution (PS; P = 0.03) (NS; P = 0.0001). Surgical limb change in anterior center of pressure was significantly greater with NS relative to PS (P = 0.03), and nonsurgical limb ankle moment-contribution was significantly greater with PS relative to NS (P = 0.02). Conclusion: Although surgical limb knee moment contribution is reduced significantly in both groups after ACLR, peak knee flexion LSI may be a better indicator of quadriceps strength LSI during the SLST with PS than with NS. Clinical Relevance: After ACLR, single-leg squatting with PS may be preferred for knee-specific performance testing and as an exercise regression for horizontal hopping, whereas NS may be preferred as an exercise regression for vertical hopping. Strength-of-Recommendation Taxonomy (SORT): B-level recommendation (biomechanical, Level 3 cohort study).