The purpose of this study was to investigate kinematic factors to pitch fast ball by comparing motions of two groups of baseball pitchers whose ball velocities were significantly different. Twenty-two baseball pitchers participated in this study as subjects. Their pitching motion was videotaped with two high-speed video cameras to compare three dimensional kinematics. Subjects were divided into two groups, i.e., high and low ball velocity groups (HG and LG). Average ball velocity of the HG was significantly larger than that of the LG (HG, 35.7±1.0 m/s, n=10; LG, 33.2±1.1 m/s, n=12; p<0.001). From the instant of the maximal knee height of the stride knee to the middle of the second phase (from the instant of the minimum height of the throwing hand to the instant of the stride foot contact), the HG flexed the knee and hip of the pivot leg and inclined the thigh backward more than the LG. At the stride foot contact (SFC), the HG flexed the knee of the stride leg and rotated the lower trunk more than the LG. In the first half of the third phase (from SFC to the ball release), the elbow joint angle of the throwing arm of the HG was kept about 80 degrees while it was about 60 degrees for LG. The timing of the peak angular velocity of the horizontal adduction of the shoulder of the HG was significantly later than that of LG, but the angular velocity was larger in the magnitude because of longer range of motion of the horizontal adduction in HG.

The purpose of this study was to investigate three-dimensional kinetic changes in the support leg during the curve phase in long distance speed skating. Eight male long distance speed skaters performed 4000 m skating at the maximal effort, using an instrumented sensor klapskate. The skaters were videotaped with two synchronized cameras (60 fields/s) in the mid portion of the curve track by using a panning DLT technique. Push-off forces and three-dimensional coordinates of the segment endpoints and blades during the left stroke were collected over 4000 m distance to calculate kinetics of the left support leg in the first and second halves, the points of 650 m and 3450 m, by an inverse dynamics. The contribution of the knee joint to total mechanical work done by the leg joints decreased in the second half, and the positive work done by knee joint torque in the second half was smaller than that of the first half. In the first half, the hip and knee joints exerted large extension torques during the gliding phase, and the knee joint exerted large extension torque and power during the push-off phase. The knee extension torque after approximately 30% stroke and extension power of the knee joint during the push-off phase in the first half were larger than those of the second half. These results indicate that minimizing the decrease in the knee extension torque of the left support leg under fatigue condition is an important factor to maintain the skating speed in the curve phase of the long distance speed skating.

The effect of precise adjustment of muscle coordination on jump achievements and kinematics in human vertical squat jump (SQJ) was evaluated quantitatively using a musculoskeletal model and forward dynamics approach. In order to evaluate the effect of altered muscle coordination, an optimal solution muscle stimulation time was first found through an optimization method. The optimal stimulation time of muscles was systematically altered by the interval of 0.1 ms in the range±50 ms. It was found that jump performance critically depends on precise muscle control, especially in terms of inter-muscular coordination. The sensitivity of fine tuning the control of individual muscles was found to be correlated with maximum isometric muscle strength. The minimal time interval that significantly disrupted a SQJ performance amounted to less than 1 ms. It was found that intermuscular coordination is crucial in maximizing jump performance. Mutual coordination of mm. vasti and hamstrings ; m. gastrocnemius and m. rectus femoris ; m. adductor magnus and hamstrings ; m. gluteus maximus, m. adductor magnus and m. rectus femoris was found to be of great importance for jump achievement. The role of coordination of specific muscles in SQJ and possible implications of our findings to physical training have been discussed.

The kip maneuver on the horizontal bar is one of the basic skills of apparatus exercise, but it is said to be difficult for beginners to learn and execute. Although knowledge of kinetics is necessary to effectively teach the kip maneuver to beginners, there has been little research on kinetics, especially joint torques, during the kip maneuver. The purpose of this study was to investigate and compare the shoulder and hip joint torques of skilled and unskilled subjects performing the kip maneuver on the horizontal bar. The skilled subjects were made up of ten male varsity club gymnasts, while five male subjects who had no experience performing the kip maneuver comprised the unskilled group. All kip maneuvers performed by the subjects were videotaped and analyzed with two-dimensional videography to calculate trajectories of the center of mass, joint angles, angular velocities, torques, and work at the shoulder and hip joints.

Compared to the skilled group, the center of mass of the unskilled group was raised higher and closer to the bar at the instant of maximal forward swing. In addition, peak shoulder extension velocity of the unskilled group was significantly smaller than that of the skilled group (p < 0.05). Although shoulder extension torque of the skilled group increased during the backward swing phase, that of the unskilled group increased and was larger during the forward and upward swing phases than it was for the skilled subjects. The work done by the shoulder joint torque of the unskilled group was significantly smaller than that of the skilled group (p < 0.05), while there were no differences in work done by the hip joint and total work between the two groups. These results indicate that the difference between the skilled and the unskilled groups was not in the magnitude, but rather in the timing of joint torque exertion.

To improve the kip maneuver for unskilled performers, it is suggested that the shoulder extension and hip flexion torques should be exerted later and that exertion of shoulder extension torque should be emphasized.