MECHANICS OF BODYROLL IN FRONT-CRAWL SWIMMING
Keywords: kinetics, angular momentum, external torque, buoyant force
AbstractINTRODUCTION: This study investigated the mechanical factors that would explain the rolling action of the body (bodyroll) in front-crawl swimming. METHODS: Eleven competitive swimmers performed front-crawl swimming at a self-determined sprinting speed. The performances were recorded using two panning periscopes and the three-dimensional movement of the subjects was reconstructed from digitezed video recordings (Yanai et al., 1996). The subject’s body was modelled as fourteen simply-linked cylindrical rigid segments, and angular momentum of the body was determined (Dapena, 1978). The first timederivative of the angular momentum of the body was computed and the external torque acting on the body about the long-axis of the trunk was determined. RESULTS AND DISCUSSION: The angular momentum of the whole body about the long-axis of the trunk was found to change in a systematic manner, oscillating from 4.15 (left) to - 4.39 (right) kgm2/s, indicating that an external torque was acting on the body. Of the total angular momentum, 40 % was due to the rolling action of the trunk and head, and 60 % due to the actions of the upper and lower limbs. The angular momenta of the upper and lower limbs were directed in opposing directions for 56 % of the stroke time. Such a counter-action of the limbs was observed while the trunk was rolling without twist of the trunk. It would indicate that the trunk acts as a rigid link to transfer the reactions of the torques that generated the angular momenta in upper and lower limbs against the fluid resistance. The external torque was found to attain peak values (± 64 Nm) shortly before the arm exited the water and also in the middle of the recovery phase of the stroke. Kicking action seemed to be a major drive in generating the torque in the former period, which helped the body to attain a large bodyroll angle. The torque in the latter period stopped the bodyroll and initiated the roll toward the other side. Force due to inward pull, and the torque due to the buoyant force seemed to generate the torque in the latter period. CONCLUSIONS: External torque was necessary to maintain the rolling action of the body: The external torque was hypothesized to be generated by the kick at the arm exit, the inward pull of the stroke, and the torque due to buoyant force: and While rolling, the trunk acted as a rigid link to transfer the twisting torques. REFERENCES: Dapena, J. (1978). A Method to Determine the Angular Momentum of a Human Body About Three Orthogonal Axes Passing Through its Center of Gravity. Journal of Biomechanics 11, 251-256. Yanai, T., Hay, J. G., Gerot, J. T. (1996). Three-Dimensional Videography with Panning Periscopes. Journal of Biomechanics 29, 673-678.
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