The first objective of this study was to clarify the impact point that produced the greatest ball velocity immediately after impact in instep soccer kicking. The second objective was to calculate the impact force during the ball impact phase.

Five experienced male university soccer players performed maximal instep kicks using a one-step approach in varying the impact point. The kicking motions were captured two-dimensionally by a high-speed video camera at 2,500 fps. The impact point was calculated as the distance from the center of mass of the foot (projected onto the dorsal aspect of the foot) to the contact point. The influence of the impact point on the ball-foot velocity ratio (ball velocity immediately after impact/foot velocity immediately before impact) was examined. The impact force was calculated from ball deformation based on the Hertz contact theory.

Impact on the surrounding area of the center of mass of the foot produced the greatest ball velocity and little angular displacement of the ankle. As an example, in a trial with a ball velocity of approximately 24 m/s, the peak ball deformation was approximately 4.5 cm and the peak impact force was over 2,000 N. Time-series of the impact force calculated in this study can be used in kinetic analysis during the ball impact phase.

The purpose of this study was to investigate kinetic factors affected the take-off angle of the grab start. Twelve male college swimmers participated in this study. They were instructed to perform the grab start with three different take-off angles ; usual take-off angle (Normal Trial : NT), higher take-off angle than Normal (High Trial : HT), lower take-off angle than Normal (Low Trial : LT). The starting movements were recorded by a high speed camera (250 fps) and the 2-DLT method was used for calculating the kinematical data. Ground reaction force was measured with force plate (Kistler, 9253B11) under the starting block. Joint torque at shoulder, hip, knee and ankle were calculated by the inverse dynamics. Joint torque was analyzed after subject's hands released from starting block, because there was the closed loop of hands and feet on the force platform.

Mean horizontal take-off velocity of HT was significantly slower than NT and LT. Changes in each joint torque did not differ among HT, NT and LT. However, segment absolute angles of foot, shank, thigh and trunk at the peak knee joint torque (t_pk) decreased along with take-off angle decrement. In HT, mean negative peak value of knee joint torque power (P_k) significantly decreased, and then, mean maximum knee joint torque significantly increased than NT and LT. These results suggested that effective use of Stretch-Shortening-Cycle (SSC) contributed to the increment of knee extension torque.

In conclusion, take-off angle was changed by inclination of whole body when joint extension torque generated.