Developmental changes in upper limb muscle synergies during throwing: A comparison between preschoolers and schoolers.

Throwing is a fundamental motor behavior that undergoes marked refinement during childhood, yet the neuromuscular mechanisms underlying this process remain unclear. We compared upper limb muscle synergies in preschool-aged (3-5 years; n = 13) and school-aged (6-9 years; n = 8) children during overarm throwing. Participants threw balls toward a fixed target while electromyographic activity from 16 upper limb muscles and ball kinematics were recorded. Muscle synergies, extracted using non-negative matrix factorization, were analyzed for structure and sparseness. School-aged children exhibited higher ball velocity, improved accuracy, and greater synergy sparseness, reflecting more selective muscle recruitment. Cluster analysis identified eight synergy clusters in schoolers versus six in preschoolers, with several preschooler synergies fractionated into functionally specialized components in schoolers. These refinements-across the upper limb-likely enhance energy transfer efficiency and release control. The findings highlight fractionation-driven specialization as a key developmental adaptation that supports improved throwing performance.