Altered Central Motor Control After ACL Reconstruction: A Muscle Synergy Analysis During High-Risk Landing

Purpose: Anterior cruciate ligament reconstruction (ACLR) patients face elevated secondary injury risk due to residual neuromuscular deficits, yet the neuromuscular coordination strategies underlying altered muscle coordination remain unclear. Muscle synergy (MS) analysis offers a valuable framework for elucidating these mechanisms. This study characterized MS patterns in ACLR patients during high-risk landing and their correlation with lower-extremity biomechanics. Methods: Twenty-two patients who had undergone unilateral ACLR performed a single-leg drop–land–lateral jump task using both affected and nonaffected limbs, whereas sixteen healthy controls performed the task using their dominant limb. with the limb-level data were subsequently categorized into three groups: affected, nonaffected, and control. Surface electromyography (14 muscles), kinematics, and ground reaction forces were recorded. Non-negative matrix factorization was used to extract MS spatial (muscle weighting vectors) and temporal (activation profiles) components across pre-landing, impact, and loading phases. Joint angles/moments were computed via OpenSim musculoskeletal models. Mixed-effects models and Pearson correlations assessed group differences and synergy-biomechanics associations. Results: Compared to controls, ACLR limbs showed lower hip abduction and ankle plantarflexion angle but greater knee flexion angle at landing, accompanied by excessive pre-landing quadriceps weighting, reduced gluteal/semimembranosus contributions, and earlier but lower-amplitude synergy recruitment. Post-landing, quadriceps weighting decreased and knee-dominant synergy recruitment delayed, with compensatory redistribution toward gluteal, gastrocnemius, and tibialis anterior muscles, corresponding to reduced knee extension and increased hip extension moments. Critically, during impact, ACLR limbs lacked a dedicated frontal-plane stabilization synergy, MS4 (gluteus medius/peroneus longus), relative to the control group. A greater number of synergies in the affected limb was associated with more normative joint biomechanics. Conclusions: ACLR limbs exhibit persistent, limb-specific MS abnormalities across all landing phases, including altered sagittal-plane quadriceps weighting, impaired frontal-plane modular control, and dysregulated synergy timing. These central neuromuscular coordination alterations correlate with maladaptive knee biomechanics, potentially increasing secondary ACL injury risk. From a sports training perspective, targeting these specific synergy alterations, particularly the missing frontal-plane stabilization module, may inform more effective neuromuscular training protocols for safe return to sport.