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 contribution to 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 mean radial error (the average Euclidean distance from individual pitch locations to the target across trials)—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 provide evidence consistent with the notion that the SMA contributes to 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.