Distinct Motor Modules After Anterior Cruciate Ligament Reconstruction During Stair Descent: A One-Year Longitudinal Study

Anterior cruciate ligament (ACL) injury often leads to lasting deficits in joint stability and neuromuscular control, even after reconstruction. Understanding how these deficits manifest during functional tasks is essential for improving rehabilitation strategies. This study recruited 25 male participants who had undergone ACL reconstruction, with assessments conducted at 3, 6, 9, and 12 months postoperatively. Surface electromyography (sEMG) signals from ten lower-limb muscles were collected during a standardized stair-descent task, and non-negative matrix factorization (NNMF) was applied to extract the corresponding muscle synergy patterns. Paired-samples t-tests and Wilcoxon rank-sum tests were performed to evaluate differences in muscle synergies between the ACL-reconstructed and contralateral limbs. Motor modules analysis revealed a preserved four-synergy structure in both limbs, with VAF exceeding 90%. At 3-6 months postoperatively, the reconstructed limb exhibited significantly elevated activation of the medial gastrocnemius (MG), biceps femoris (BF), and rectus femoris (RF) (p <0.005). This was accompanied by reduced contributions from the semitendinosus (ST) and lateral gastrocnemius (LG) in Synergy 4, indicative of a compensatory, quadriceps-dominant control strategy. By <0.005). This was accompanied by reduced contributions from the semitendinosus (ST) and lateral gastrocnemius (LG) in Synergy 4, indicative of a compensatory, quadriceps-dominant control strategy. By 9 months, partial normalization of neuromuscular coordination was observed, with interlimb symmetry across all four synergies largely restored by 12 months. These findings demonstrate that recovery of physiological neuromuscular control following ACL reconstruction is a prolonged and staged process. Longitudinal motor module analysis captures the dynamic reorganization of modular control from early compensatory patterns to restored interlimb coordination and may provide a mechanism-based indicator to guide personalized neuromuscular rehabilitation.