Lower-Limb Muscle Synergy Characteristics during Sprinting in Front-Side Swing-Dominant and Back-Side Propulsion-Dominant Sprinters

Purpose: This study aimed to examine differences in lower-limb muscle synergy characteristics during sprinting between front-side swing-dominant and back-side propulsion-dominant sprinters, and to elucidate the neuromuscular control features underlying distinct lower-limb technical strategies. Methods: Eight male sprinters with systematic training backgrounds were recruited and classified as front-side swing-dominant or back-side propulsion-dominant according to the front-side/back-side action ratio during the sprinting phase. High-definition video and surface electromyography were synchronously collected during flying sprint trials to obtain kinematic and lower-limb muscle ac-tivation data from the gluteus maximus, biceps femoris, semitendinosus, lat-eral gastrocnemius, medial gastrocnemius, soleus, rectus femoris, and tibialis anterior. Non-negative matrix factorization was used to extract lower-limb muscle synergy modules. Between-group statistical comparisons focused on motor-module muscle weights, whereas motor-primitive activation patterns were examined descriptively. Results: The two groups showed distinct mus-cle-weight distributions across multiple synergy modules. In SYN_2, front-side swing-dominant sprinters exhibited greater weights for the biceps femoris and semitendinosus, whereas back-side propulsion-dominant sprinters exhibited greater weights for the gluteus maximus and soleus, indicating a clear divergence in posterior-chain recruitment patterns. In SYN_3, back-side propulsion-dominant sprinters showed greater weights for the lateral gastrocnemius, medial gastrocnemius, and soleus, whereas front-side swing-dominant sprinters showed greater tibialis anterior weight, suggesting group-specific differences in ankle-muscle coordination. Additional differences in muscle weights were ob-served in SYN_4 and SYN_5, involving the rectus femoris, medial gas-trocnemius, lateral gastrocnemius, and biceps femoris. Conclusion: Front-side swing-dominant and back-side propulsion-dominant sprinters exhibit distinct patterns of lower-limb muscle synergy organization during sprinting. Front-side swing-dominant sprinters are characterized by greater involvement of the hamstrings, tibialis anterior, and selected triceps surae muscles in swing-to-stance transition control, whereas back-side propulsion-dominant sprinters rely more strongly on the gluteus maximus, soleus, and triceps surae, reflecting a posterior-chain propulsion-oriented coordination strategy. These findings indicate that differences in sprinting technique are not limited to lower-limb kinematic patterns, but are also reflected in muscle synergy recruitment and neuromuscular control strategies, providing a mechanistic basis for sprint technique diagnosis and individualized training optimization.