Yamasaki, D. | bioRxiv Science

bioRxiv2026

Facilitating the Supplementary Motor Area Activity Reduces Variability of Ball Arrival Position in Accurate Ball Throwing Performance

Maintaining high accuracy during rapid, dynamic movements is a significant challenge for the central nervous system. The supplementary motor area (SMA) is a key cortical region for orchestrating motor commands and regulating movement variability, yet its causal role in maintaining precision during high-speed tasks remains to be fully elucidated. We investigated the impact of modulating SMA excitability on throwing performance in fourteen healthy adults with no competitive throwing experience. Participants performed maximal and submaximal (50% effort) throwing tasks before and after receiving intermittent (facilitatory) or continuous (inhibitory) theta burst stimulation (TBS) over the SMA. Outcome measures included ball speed, variability of pitch location--quantified as variable error (95% confidence ellipse area) and absolute error (Euclidean distance from the ellipse center to the target)--and introspective ratings of performance via visual analog scales (VAS). Results showed that intermittent TBS (iTBS) significantly reduced the variable error of pitch location during maximal-effort throwing--where neural noise is theoretically elevated--without compromising ball speed. In contrast, no significant changes in performance were observed following continuous TBS or during the submaximal throwing task. Notably, objective precision gains under iTBS were dissociated from subjective ratings of accuracy, which increased globally over time independent of stimulation type. These findings demonstrate that the SMA plays a critical causal role in stabilizing motor output under high motor drive. This study suggests that SMA-mediated stabilization operates as a subconscious process, offering new insights for optimizing complex motor skills. HighlightsO_LIiTBS over the SMA significantly reduces throwing variable error in novices. C_LIO_LIImproved precision occurs without compromising maximal ball throwing speed. C_LIO_LIThe SMA causally stabilizes motor output during high-speed movements. C_LIO_LIObjective precision gains are dissociated from subjective performance ratings. C_LIO_LISMA-mediated stabilization operates as a subconscious motor process. C_LI

neuroscience↗

bioRxiv2025

Developmental Reorganization of 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 developmental reorganization 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 low-dimensional coordination modules (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 (33.6 {+/-} 10.2 vs. 21.4 {+/-} 6.2 km/h, p < 0.05), reflecting improved performance. Yet, the number of synergies did not differ (PS: 6.0 {+/-} 1.1; SA: 6.4 {+/-} 1.3, p > 0.05), suggesting that the dimensionality of coordination 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 module in SA, reflecting improved postural integration, while a bilateral soleus-dominant synergy fractionated into lateralized modules, reflecting increased lower-limb specialization. Moreover, the temporal variability of synergy activation was reduced in SA (p < 0.01), indicating that movement sequences became more precise and stable with development. These findings reveal that early gains in throwing arise not from expanding synergy number but from reorganizing their structure and sharpening temporal coordination, offering mechanistic insight into how complex whole-body skills are refined during childhood. Significance StatementThrowing is a hallmark of human motor behavior, requiring precise sequencing of whole-body muscle activity. Yet how children develop this ability has remained unclear. By applying muscle synergy analysis to electromyographic recordings of preschool and school-aged children performing maximum-effort throws, we found that improvements in performance were not due to an increase in synergy number but rather to structural reorganization and greater temporal precision. Specifically, trunk and upper-limb modules merged, lower-limb modules fractionated, and activation timing became more consistent. These results identify merging and fractionation as complementary mechanisms supporting developmental refinement of motor skills. More broadly, they provide a mechanistic framework for understanding how complex whole-body actions are acquired and offer markers for pediatric training and rehabilitation strategies.

neuroscience↗