Inter-subject variability in muscle synergies during squatting movements

This study investigated muscle synergies during squats, focusing on the individual variability in motor control strategies. Sixteen healthy young adults performed 20 squats at a consistent speed. Muscle synergies were extracted using non-negative matrix factorization, followed by k-means clustering and discriminant analysis to categorize similar muscle synergies. The analysis revealed an average of 3.3 ± 0.4 muscle synergies per squat. Further clustering analysis identified six distinct types of muscle synergies across subjects. Among them, one synergy involving trunk, hip, and knee extensors was universal across all subjects, representing a fundamental motor strategy for successful squat execution. In contrast, the remaining synergies varied across subjects, exhibiting mutual exclusivity, where individuals systematically select one synergy type or another during the same squatting phase. These findings demonstrate inter-subject variability in muscle synergies during the squat and underscore the coexistence of universal and individualized muscle synergies in human motor control, providing practical insights for designing personalized squat training and rehabilitation strategies guided by neuromechanical principles.