Effects of Quadriceps Strength Recovery in the First Year After Anterior Cruciate Ligament Reconstruction on Gait Biomechanics

BACKGROUND: Aberrant walking biomechanics, a factor linked to knee osteoarthritis (KOA), are common after anterior cruciate ligament (ACL) injury. Although quadriceps weakness often persists after ACL reconstruction (ACLR), the extent to which strength recovery influences gait biomechanics over the first postoperative year, a critical window for implementing rehabilitation strategies, remains unclear. HYPOTHESIS: Distinct quadriceps strength recovery trajectories will emerge after ACLR, with the lesser strength recovery group exhibiting more aberrant gait biomechanics compared with the greater strength recovery group and controls. STUDY DESIGN: Prospective cohort study. LEVEL OF EVIDENCE: Level 2. METHODS: Of 62 enrolled patients with a primary ACL injury, 59 were eligible and matched with uninjured controls by sex, age, and body mass index (BMI). Quadriceps strength was assessed preoperatively and at 4, 6, and 12 months post-ACLR, and gait biomechanics, including vertical ground reaction force (vGRF), knee flexion angle (KFA), knee extension moment (KEM), and knee adduction moment (KAM), were assessed at 4, 6, and 12 months post-ACLR. Group-based trajectory modeling identified strength recovery profiles, and functional mixed effects modeling compared gait biomechanics between strength trajectory groups and controls. RESULTS: Two distinct strength trajectories were identified. Both the weaker (N = 31) and stronger (N = 28) ACLR groups demonstrated lesser vGRF during early stance compared with controls, with maximum differences reaching -0.15 body weight (BW). Compared with the stronger group, the weaker group exhibited lesser KFA (-1.85° and -2.18° at 6 and 12 months) and KEM (0.006 BW × height at 4 and 6 months) during early stance. Compared with healthy controls, both ACLR groups exhibited lesser KFA and KEM during early stance across postoperative timepoints, with larger and more persistent differences observed in the weaker group. Only the weaker group had significantly lower KAM relative to controls (0.006 to 0.007 BW × height at 4, 6, and 12 months). CONCLUSION: Although the stronger group exhibited more dynamic gait waveforms than the weaker group, walking biomechanics were not fully normalized to those of healthy controls. CLINICAL RELEVANCE: These findings highlight the need for complementary interventions to strength training during the first year post-ACLR to target aberrant biomechanics and potentially reduce long-term KOA risk.