Upper-body muscle activity and synergies in front crawl: comparison between submaximal and maximal swimming speed under breath-holding condition

Swimming speed is a primary biomechanical constraint shaping muscle activity through modulation of stroke timing and coordination. However, how variations in speed influence both individual muscle activation and its organization into motor modules remains poorly understood. This study investigated the effect of swimming speed on upper-body muscle activation and muscle synergies during front crawl performed under breath-holding conditions. Surface electromyography was recorded bilaterally from six upper-body muscles in nine trained male swimmers while performing 25-meter trials at 80 % and 100 % of their maximal speed, after exclusion of three participants whose EMG recordings did not meet predefined signal-quality criteria. Muscle activity was analyzed using envelope-based methods and muscle synergies extracted through Non-Negative Matrix Factorization. Statistical non-Parametric Mapping (SnPM) was applied to examine amplitude- and timing-related differences across the stroke cycle. At maximal speed, muscles showed widespread increased activation amplitude and systematically earlier recruitment compared with the submaximal condition, as supported by SnPM analyses. Muscle synergy analysis identified four and six motor modules at 80 % and 100 % of maximal speed, respectively. Four synergies were preserved across conditions, but exhibited systematic anticipatory shifts. Two additional synergies were required to reconstruct muscle activity at maximal speed, primarily involving external oblique and anterior deltoid activity. These findings suggest that increasing swimming speed under breath-holding condition is associated with both earlier activation of shared muscle synergies and a reorganization of synergy modules.