The purpose of this study was to investigate kinematic changes in the baseball batting motion for high, middle and low hitting points. Subjects were ten right-handed male skilled batters of a varsity baseball club. Nine hitting points were set according to the baseball rules : three heights (high, middle, low) times three courses (inside, center, outside) based on the subject's height and the width of a home plate. The subjects were requested to hit a ball on a batting tee randomly assigned one of nine hitting points at each trial. Angular kinematic data were collected at l20Hz with a Vicon 612 system.

In case of the high hitting point, the right shoulder flexion and left shoulder horizontal-adduction at the instant of Left upper arm perpendicular to the hitting direction (LUP), and the left shoulder flexion at the Impact (IMP) were smaller than those of the middle height. In case of the low hitting point, the right hip flexion at the Toe-on, and the right shoulder flexion, extension of both elbows, and flexion of both hips at the LUP were greater than those of the middle height. The left elbow extension, left shoulder flexion, adduction and flexion of both hips were greater at the IMP than those of the middle height.

These results indicated that to hit a ball at high and low points appropriately a batter first adjusted flexion-extension and abduction-adduction of the hip joints, and then swing the bat adjusting both arms, especially the left arm just before the impact.

During running, the lower extremity swings forward in swing phase. The greatest flex in the hip joint occurs just before heel contact, and the thigh swings to the most forward position. Then, the hip joint extends, and the thigh moves slightly backward before the knee joint extends, and the leg swings to its most forward position. In this study, the mechanism of phase difference between forward swings of the thigh and leg in swing phase was investigated in terms of interaction torques of hip and knee joints.

The typical pattern of change in the rotation angles of the hip and knee joints in running was modeled from data obtained during long-distance running. Comparative patterns involving phase difference during which forward swings of the thigh and leg were altered successively also were generated. Lower extremity movements in typical and comparative patterns were computer simulated, and net, muscle, interaction, and gravity torques were calculated.

Results indicated that as phase difference became larger than that found in the typical pattern, larger hip and knee muscle torques were required to compensate for interaction torque in the early swing phase. As phase difference decreased, larger muscle torques were required to overcome the interaction torques in the late swing phase. Consequently, hip and knee net torques could be produced through minimal change of muscle torques in the typical pattern.

Thus, it is likely that phase difference between forward swings of the thigh and leg occurs to minimize change in muscle torque during regulation of the interaction torque.

This study investigated the dynamic mechanism and technique, which particularly focused on horizontal rotation, of double leg circles (DLC) on the pommel horse. The forces exerted on the pommel during DLC performed by 17 male university gymnasts were recorded using a pommel horse model containing force plates, and the performances were videotaped with two digital video cameras. Based upon the result that three judges scored each of the performances on the video, the best 5 (S-group) and worst 5 (U-group) out of the 17 subjects were analyzed. Changes in the horizontal velocity of the center of mass (CM) accorded with the change of tangential components of horizontal reaction forces exerting on CM. The horizontal velocity of CM was minimal during both-hand support phase (BHS), and maximal during the one-hand support phase (OHS). The horizontal velocity of the ankles and the horizontal velocity of CM of the head and trunk were minimal during OHS, maximal during BHS. However, the horizontal velocity of CM of the total leg had no apparent peak. The mechanical energy of CM decreased during BHS. The body lateroflexed to the direction of the releasing hand during OHS, and the direction of lateroflexion changed during BHS. In the comparisons between S-group and U-group, the S-group showed smaller angles of body flexion, but the two groups were almost the same in the angle of body lateroflexion. From these results, it was thought that the DLC was composed of two kinds of rotations: rotation of CM and rotation of the body about CM. The rotational velocity of the leg was influenced by their rotations and the ratio of influences varied with phases. And the importance of lateroflexion of the body during the DLC was suggested.