MECHANICS OF BODYROLL IN FRONT-CRAWL SWIMMING

INTRODUCTION: 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

BUOYANCY: THE PRIMARY SOURCE OF BODYROLL

The purpose of the study was to determine the rotational effect of buoyant force (buoyant torque) and its contribution to the bodyroll exhibited during front crawl swimming performed at a distance pace. Three-dimensional videography was used to measure the position and orientation of the body segments of eleven competitive swimmers performing at a distance pace. The buoyant torque was computed with the method described by Yanai (1999). The bodyroll generated by the buoyancy torque was determined from the double time-integral of the buoyancy torque and the principal moment of inertia of the body. The buoyancy torque changed in the mean range from –7.7 Nm to 7.7 Nm with a systematic pattern at stroke frequency. The buoyant torque generated the bodyroll of the peak-to-peak amplitude 1.26 radians, accounting for 88 % of the peak-to-peak amplitude (mean = 1.43 radians) of the bodyroll exhibited by the swimmers

FORCES ON THE LOWER BACK DURING ROWING PERFORMANCE IN A SINGLE SCULL

The purpose of this study was to determine the compressive force on the lower back during rowing performance during trials in which Hatchet sculling blades were used and trials in which Macon sculling blades were used. Compressive force was determined using an inverse dynamics approach using the lower back model developed by Chaffin (Chaffin and Andersson, 1990). The results indicate that: (1) the peak compressive force on the lower back was found to be 5344N and 4876N for Macon and Hatchet blades respectively, (2) there was no difference in peak compressive force between the trials with Hatchet blades and Macon blades; (3) with the Hatchet blades the compressive force increased immediately after the entry of the blade

3D SCAPULAR KINEMATICS AND SCAPULOHUMERAL RHYTHM IN SWIMMERS AND BASEBALL PITCHERS

The present study was conducted to describe characteristics of scapular kinematics and scapulohumeral rhythm (SHR) in baseball pitchers and swimmers. The participants were 16 swimmers, 19 baseball pitchers and 8 non-athletes. Each participant was asked to perform three tasks, arm abduction, shoulder horizontal abduction (HA) and shoulder internal/ external rotation (IR/ER). An electromagnetic tracking device was used to record the 3D data of shoulder complex. The SHR during the arm abduction and the range of motion for shoulder complex were determined for each task. The results showed that swimmers had significantly greater ranges of shoulder HA and scapular upward rotation while pitchers had a significantly greater range of shoulder ER and a significantly smaller range of shoulder IR. There was no obvious difference in SHR between the groups