Subjective effort modulates muscle synergy recruitment during the table tennis Forehand Topspin Stroke

Abstract

Human voluntary movements are produced through the coordinated activation of multiple muscles, often organized by the central nervous system into functional units known as muscle synergies. Although muscle synergies are regarded as stable and efficient control structures, it remains unclear how they adapt when performers intentionally modulate subjective effort during skilled sport-specific movements. This study examined how stepwise manipulations of subjective effort influence muscle coordination during the table tennis forehand topspin stroke, a technique requiring both high power and accurate control. Ten skilled table tennis players performed forehand topspin strokes under five effort conditions (60–100%), during which surface electromyographic signals were recorded, and muscle synergies were extracted using non-negative matrix factorization. Ball speed, ball spin, and muscle activity amplitude increased progressively with higher effort levels, confirming that participants successfully adjusted output intensity in accordance with the instructed effort. In contrast, the number of extracted muscle synergies remained constant across all effort conditions, indicating that the fundamental coordination structure was preserved despite changes in subjective effort. Clustering and scalar product analyses demonstrated that, although basic synergies were shared across effort levels, supplementary synergies were selectively recruited under high-effort conditions. These supplementary synergies became more distinct as effort increased, reflecting phase-specific adjustments in muscle contributions to meet greater output demands and support local stabilization. The high structural similarity of synergy patterns across effort levels further confirmed that skilled performers maintained stable coordination patterns regardless of effort magnitude. Overall, these findings suggest that increases in subjective effort do not reorganize muscle coordination patterns; rather, they are achieved through intensity scaling of basic synergies accompanied by the selective recruitment of supplementary synergies. This study provides new insight into effort-dependent modulation of neuromuscular coordination in a complex racket sport skill and offers a foundation for effortbased training strategies that preserve coordination stability while enhancing output control.