Throwing effort-dependent disruption of muscle synergies in baseball players with the yips

The yips is a psychoneuromuscular disorder characterized by context-dependent disruption of fine motor control, with baseball players often struggling more during low-effort than maximal-effort throws. This study investigated effort-dependent alterations in the spatiotemporal organization of muscle synergies in baseball players with the throwing yips. Twenty-two collegiate players (12 with yips, 10 controls) performed throws at low, moderate, and maximum-effort levels. Using nonnegative matrix factorization, electromyographic activity from 13 upper-limb muscles was decomposed into spatial and temporal synergy components, which were classified using k-means clustering. Within-player consistency across effort levels and between-group similarity were evaluated using cosine similarity for spatial components and zero-lag cross-correlation for temporal components. Self-reported symptom severity in the yips group was highest at the low-effort level. The number of muscle synergies did not differ between groups or effort levels (3.5-3.7 synergies), and no yips-specific synergies were identified. The variance accounted for by the first muscle synergy was higher in the yips group than in the control group under low-effort (0.66 vs. 0.62, p = 0.012) and moderate-effort (0.67 vs. 0.63, p = 0.001) conditions. Reduced consistency and similarity of muscle synergy patterns were observed specifically at the low-effort level in clusters associated with the arm cocking motion. These findings indicate that effort-dependent yips symptoms reflect subtle spatiotemporal disruptions of shared muscle synergies rather than the loss or emergence of specific synergies.