This study analyzed upper torso rotation, which is crucial for improving pitch velocity and minimizing upper limb load during pitching. A simplified model was developed to facilitate the analysis which focused on torso torsion. In this model, the instantaneous torque transfer ratio and torso mobility were defined as key indices. We experimentally verified how these indices vary under three conditions: no torsion, static torsion, and dynamic torsion. The results demonstrate that torso mobility increases in conjunction with the instantaneous torque transfer ratio. Both static and dynamic torsion conditions resulted in higher instantaneous torque transfer ratios compared to the no-torsion condition. In static torsion, the maximum torso mobility was, on average, 26% greater than that observed under no torsion. In dynamic torsion, the maximum torso mobility was, on average, 69% greater than that observed under no torsion. These results indicate that torsion is effective in increasing both the instantaneous torque transfer ratio and torso mobility and that this effect is stronger in dynamic torsion. Therefore, torsion increased the angular velocity of the upper torso in response to torque input from the lower limb, potentially resulting in higher ball velocity and a reduction in upper limb strain. Additionally, the findings imply that the input required from the lower limbs to achieve a specified pitch velocity may be reduced. The simplified model and indices proposed in this study provide a foundation for designing exercise intervention techniques, evaluating athletic performance, and assessing injury risk related to torso rotation.

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