Muscle synergy characteristics during hands-and-knees crawling in children aged 4–6 years

Hands-and-knees crawling requires complex multi-joint coordination, managed by the central nervous system (CNS) via muscle synergies. While infant crawling is well-studied, neuromotor control in 4–6-year-olds remains under-investigated. This study examined the spatiotemporal consistency and developmental trajectories of muscle synergies during crawling in this age group. Kinematics and surface electromyography (sEMG) from 16 muscles were recorded from 49 healthy children performing self-paced crawling, segmenting ten cycles per participant. Muscle synergies were extracted using non-negative matrix factorization (NMF). Spatiotemporal characteristics were evaluated using cosine similarity (CS) and temporal phase dispersion (Circular SD). An average of 7.82 ± 1.05 muscle synergies were extracted from the 16 recorded muscles, with synergy number increasing significantly with age ( p < 0.05). Cluster analysis identified 12 group-level synergies. Proximal-dominated core shared synergies (M4–M7, M11) exhibited high prevalence (73.47%–87.76%) and stable spatial organization (mean CS = 0.68–0.86), without significant age-related differences. Conversely, specific synergies for distal adjustments (M1–M3, M8–M10, M12) showed substantial inter-individual variability in activation timing (Circular SD: 20.68%–44.32%). While their internal structures remained stable, their population prevalence exhibited substantial fluctuations across age groups. These fluctuations, characterized by wide inter-individual variability, suggest that rather than undergoing deterministic neural reorganization, the CNS adopts a “proximal freezing” strategy for basic stability and a “distal freeing” strategy via active sensorimotor exploration to manage redundant degrees of freedom in 4- to 6-year-olds.