Influences of Bat Inertia Properties on the Generating Mechanism of Bat Motion with the Upper Extremities

Abstract

This study aimed to investigate the influences of bat inertia properties on the generating mechanism of bat swing motion through the conduction of a sensitivity analysis based on forward dynamics contribution analysis. The human body was modeled as an articulated body consisting of six segments (left and right upper extremities) and two additional segments (upper torso and head), and the bat was added to constitute a multi-body system model with a total of nine segments. To consider the anatomical degrees of freedom of each joint in the body, the connections between the right and left hands and the bat were modeled as constrained with a zero degree of freedom virtual joint. One experienced baseball player participated in the experiment and hit a ball placed on a tee stand. An instrumented bat was used to solve the closed-loop problem of the upper limbs. From the results obtained in an induced speed analysis, sensitivity analysis was conducted by varying the mass and the moment of inertia around the center of gravity of the bat. The results obtained in this study showed that both the motion-dependent term and joint torque term were more influenced by mass than by the moment of inertia around the center of gravity of the bat. This result indicates that changes in bat mass have more influence on the swing mechanism than those in the moment of inertia around the bat’s center of gravity. This suggests that altering bat mass would be useful if the aim was to change the swing speed or the joint load on each joint.