Heterogeneous neuromuscular control strategies in the soccer instep kick: A cross-sectional study of synergy structure mapping across skill levels

The instep kick is a fundamental soccer skill for generating high ball velocity and requires coordinated activation of trunk and lower-limb muscles. However, quantitative evidence on how sEMG-derived neuromuscular coordination patterns differ across skill levels remains limited. This cross-sectional study investigated skill-level differences in muscle synergy organization during the instep kick. Surface electromyography was recorded from 12 trunk and lower-limb muscles on the dominant side. Individual muscle synergies were extracted using non-negative matrix factorization, and group-level patterns were identified through clustering. Synergy similarity, sparseness, muscle weight distribution, temporal activation features, and their associations with ball velocity and phase timing were analyzed. Three highly similar shared synergies were identified in both groups (SP > 0.8), suggesting preserved basic sEMG-derived coordination patterns across skill levels. The professional group demonstrated a slightly longer leg-cocking duration (Δ = 0.007 s, p = 0.001, r_rb = 0.379) and a shorter leg-acceleration duration (Δ = 0.004 s, p = 0.030, r_rb = 0.325). Within professional players, ball velocity was positively associated with relative leg-cocking duration ( r = 0.59, p = 0.012) and negatively associated with relative leg-acceleration duration ( r = −0.48, p = 0.044). The activation of professional-specific synergies was also positively associated with ball velocity ( r = 0.67, p < 0.001). Professional players showed higher synergy sparseness, additional professional-specific synergy clusters, and merging-like and fractionation-like cross-group mapping patterns. These findings indicate that professional players exhibit more differentiated and concentrated sEMG-derived muscle-weighting patterns associated with higher ball velocity.