Lower Limb Muscle Synergies During Table Tennis Forehand Topspin Stroke: A Muscle Synergy Theory-Based Analysis

Based on the muscle synergy theory, this study aimed to investigate the lower limb coordination strategies and their individual variations during table tennis players' forehand topspin strokes. Surface electromyography (sEMG) signals were recorded from eight ipsilateral lower limb muscles in ten players. Non-negative matrix factorization (NMF) was applied to extract motor module composition and temporal activation patterns. Inter-individual similarity was evaluated using K-means clustering and cosine similarity. The results showed that: (1) Lower limb muscle synergy modules could be classified into three clusters: Cluster 1 (rectus femoris/vastus medialis), Cluster 2 (gluteus maximus/ gluteus medius/ biceps femoris/ tibialis anterior), and Cluster 3 (lateral gastrocnemius/ soleus). The composition of motor modules exhibited high inter-individual similarity across clusters, with Cluster 2 demonstrating significantly greater consistency than other clusters (p < 0.01); (2) Cluster 2 and Cluster 1 reached peak activation during the early and mid-late forward phases, respectively, while Cluster 3 showed double peak activation during the backswing and backward phases. Considerable inter-individual variability was observed in temporal activation patterns, with Cluster 2 demonstrating significantly lower similarity than Cluster 3 (p < 0.01); (3) Activation areas differed significantly between stroke phases, with Cluster 2 greater than Cluster 3 in forward phase, while Cluster 3 higher than Cluster 2 in backward phase. The findings indicated that: The lower limb utilized three fundamental muscle synergy patterns during table tennis forehand topspin strokes. These synergies demonstrated phase-specific functional roles while maintaining temporal coordination. Athletes can optimize their performance by precisely adjusting temporal parameters while maintaining a standardized lower-limb movement structure, a regulatory capability particularly evident during the forward phase.