INTRODUCTION In crawl swimming, rotational movement around the long axis of the trunk is performed in conjunction with the stroke movement of the upper limbs, and this movement is defined as trunk rotation movement. In crawl swimming, since the upper and lower limbs are moved alternately, contraction and control of the trunk muscles are important [1]. It has been reported that the timing of switching the torsion motion of the knee shifts to an early stage [2]. The purpose of this study was to clarify the trunk muscle activation with changes in swimming speed during crawl swimming and to investigate the relationship with trunk twisting movements. METHODS The subjects were 7 male swimmers. After a sufficient warm-up, the first trial was a 100m swimming speed trial (V_100m), and the second trial was a 50m swimming speed trial (V_50m). The trial was performed in an experimental circulating water tank. The target muscles were the muscles of the trunk, and a total of 6 muscles were attached (Latissimus dorsi, Erector spinae, abdominal oblique). We attached LED markers to 11 locations on the body, divided the movement into phases, and calculated the maximum rotation angle (MRA) for each "Shoulder roll, Hip roll, Trunk twist". In the propulsion phase, the phase up to MRA was called Roll Phase (RP), and the phase after MRA was called Roll Back Phase (RBP). The EMG was converted to an RMS waveform and normalized by the peak value (mvc) obtained by manual muscle strength measurement, which was defined as "%mvcRMS". From these values, Active Phase (%) in each phase was compared between trials. RESULTS Higher swimming speed decreased stroke time (V_50m: 0.99 s, V_100m: 1.09 s, p<0.01) and increased trunk twist rotation angle (V_50m: 27.92°, V_100m: 30.97°, p<0.05). A significant increase in the active phase of the trunk muscle group was observed in the right oblique muscle of the trunk twist MRA RP between trials. Similarly, a significant increase in trunk twist MRA RBP was also observed in the left erector spinae muscle. DISCUSSION At higher swimming speeds, the active phases of the erector spinae and abdominal oblique increased significantly with increasing trunk torsion angles. This suggests that these two muscles may contribute to the torsional movements during crawl swimming.
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