スピードスケートにおける加速理論の再検討

Abstract

Many researchers have attempted to measure the change in velocity of the center of gravity (CG) for speed skaters in stroking. However, there have been some difficulties in measuring the velocity change in speed skating; they include the very long stride length (about 10m) and the three-dimensional behavior of the skater's CG. The purposes of this study were to investigate the push-off technique for top-level Japanese speed skaters and the change in velocity during the push-off phase using 3D cinematography, and then to examine acceleration theory during the push-off phase in speed skating. Skaters participating in the 500m race of the All Japan student championship (1989) were videotaped (60 fields/s) by 10 VTR cameras over 20m at the crossing zone of the back straightaway. Twenty-two male skaters were selected as subjects and classified into two groups on the basis of the performance of the competition. 3D coordinates of the segment endpoints were obtained on five sub-areas (each 4m in length) using a DLT method. Displacement and velocity of the CG and the angles of the hip, knee and ankle joint were calculated. The results obtained are summarized as follows: 1) Push-off movement for the top group skaters placed the CG further forward than that of the second group. 2) The vector derived from the push-off movement for the top group skaters was directed forward, and accelerated the CG of the skaters effectively. 3) Increase in the velocity in skating direction for all subjects seemed to contribute more than expected to the acceleration of the CG. It has been proposed that acceleration in speed skating occurs by push-off of the leg in a direction perpendicular to the gliding direction of the skate, since the force applied to the opposite direction of gliding cannot contribute to acceleration of the CG due to very small frictional force. However, this theory cannot thoroughly explain the findings obtained for the top skaters in this investigation. Therefore, the acceleration theory should be modified to reflect the fact that the CG of the skater during speed skating is accelerated not only by the push-off perpendicular to the gliding direction but also by an increase in velocity vector in a gliding direction.