Age-related differences in whole-body muscle synergies during overarm throwing in children

Overarm throwing is a uniquely human skill that requires precise whole-body coordination. Although throwing behavior emerges early in childhood, the neuromuscular mechanisms that support its development remain poorly characterized. Here, we provide novel evidence for the age-related differences of whole-body muscle synergies during maximum-effort throwing in preschool-aged (PS) and school-aged (SA) children. Electromyography was recorded from 16 muscles, and non-negative matrix factorization was applied to extract muscle synergies. We compared ball speed, number of synergies, synergy structure, and temporal consistency between groups. Ball speed was significantly higher in SA than PS (p = 0.0059). Yet, the number of synergies did not differ (p = 0.6227), suggesting that the dimensionality of synergies is largely established by the preschool years. Instead, developmental improvements were driven by structural and temporal reorganization. Trunk and upper-limb synergies merged into a single synergy in SA, reflecting improved postural integration. Moreover, the temporal variability of synergy activation was reduced in SA (p = 0.0006), indicating that movement sequences became more precise and stable. These findings reveal that early gains in throwing arise not from expanding synergy number but from reorganizing their structure and temporal coordination, offering mechanistic insight into how complex whole-body skills are refined during childhood.