Muscle synergy asymmetry between the affected and unaffected limbs during the stance phase in unilateral flatfoot

BACKGROUND: Musculoskeletal conditions can disrupt neuromuscular coordination during gait, leading to asymmetry and altered muscle synergy patterns. Understanding how the nervous system reorganizes synergies in individuals with unilateral flatfoot (UF) is essential for improving clinical assessment and rehabilitation strategies. METHODS: Surface electromyography (sEMG) of major lower-limb muscles during the stance phase was collected from 24 individuals with UF. Synergists were extracted by non-negative matrix factorization (NMF) and analyzed with vector coding (VC) to examine bilateral phase relationships. Independent t-tests compared muscle weights within synergies, paired t-tests compared temporal waveforms between sides, and chi-square tests examined differences in dominant quadrant distribution. RESULTS: Both affected (AFF) and unaffected (UNAFF) sides exhibited two synergies. The tibialis anterior (TA) (Syn1: AFF = 0.47 ± 0.40, UNAFF = 0.46 ± 0.38; Syn2: AFF = 0.36 ± 0.38, UNAFF = 0.46 ± 0.39) and medial gastrocnemius (MG) (Syn1: AFF = 0.41 ± 0.40, UNAFF = 0.49 ± 0.43; Syn2: AFF = 0.52 ± 0.41, UNAFF = 0.45 ± 0.41) showed significantly higher weights than the mean value within each respective synergy (p < 0.001). Both sides mainly exhibited an In-phase pattern, with Syn1 more prevalent on the UNAFF side (p < 0.05) and Syn2 on the AFF side (p < 0.05). CONCLUSION: The combined NMF-VC analysis effectively captured the structural and temporal features of muscle synergies. UF individuals exhibited MG compensation on the AFF side, suggesting that nervous system may maintaining stability through strengthening synergy patterns dominated by major muscles. This study revealed the neuromuscular reorganization in UF gait and provided a quantitative method for synergy-based rehabilitation assessment.