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[Purpose] The purpose of this study was to provide information regarding the knowledge of mechanical principles related to the load applied to the shoulder and elbow during the pitching motion. [Subjects and Methods] The three-dimensional kinematics of body segments were measured in 19 amateur baseball players with a VICON motion capture system. The correlation between maximum shoulder internal torque, maximum elbow varus torque, and simultaneous kinematic parameters including joint angular acceleration and ball acceleration was investigated. [Results] Peak shoulder internal rotation torque and elbow varus torque averaged 31 Nm and 26 Nm, respectively. Ball acceleration had the strongest influence on shoulder internal rotational torque (standardized partial regression coefficient 0.69). During previous maximum external rotation (pre-MER), the main cause parameter of elbow varus torque was shoulder horizontal adduction velocity (r=0.672). On the other hand, after MER (post-MER), the main cause was ball acceleration (r=0.745). [Conclusion] Results of our study suggest that one of the main mechanical principles of joint load during the baseball pitching motion may be the ‘inertia’ yielded on the segment distal to the certain joint.