The purpose of this study was to compare the kinematics of the lower limb and trunk motion of infielders with different skill levels while fielding grounders in baseball, and to make suggestions for coaching. Subjects were divided into a Superior Group (SG, n=10) and an Inferior Group (IG, n=10) according to fielding ability. Trials involved 2 patterns, i.e. fielding a normal grounder and fielding a bad hop. The fielding motions were videotaped with 2 high-speed video cameras (300 Hz). Three-dimensional coordinates of the segment end-points, the glove and the ball were obtained using the DLT method. The angles compared between the 2 groups were flexion-extension, abduction-adduction, and internal-external rotation for both hips, flexion-extension for both knees, dorsa-plantar flexion for both ankles, tilt backward-forward for the trunk and the center of gravity of the body (CG). The sequential data were normalized with the time from the point when the right foot made contact until catching, and then averaged.
The results can be summarized as follows: (1) the SG showed a significantly earlier the point of right foot contact in catching than the IG. There was no inter-group difference in the motion time from catching to release. (2) In catching a normal grounder, the SG showed significantly larger right hip flexion (20% time), adduction (100% time), and trunk tilt backward (0-20% time) than the IG (p<0.05). The SG showed significantly smaller displacement of the CG in a leftward direction (50-100% time) than the IG. (3) The SG showed a smaller change of catching posture between dealing with a normal grounder and a bad hop than the IG.
These result suggest that it is important for infielders to bend the right hip and reduce the change in their catching posture in order to accurately field a batted ball.
