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The purpose of this study was to explore for optimum motions of the upper body in order to generate a high bat-head speed through computer simulation of baseball tee batting. A bat and upper body nine-segment computer simulation model was developed. The model has totally 17 degrees of freedom at individual shoulder, elbow, wrist and torso joints. The optimisation procedure for the performance improvement was carried out by varying values and timings of joint angle in order to increase the bat-head speed to 40 m/s at the ball impact. The changes in the joint angular velocities about the shoulder internal/external rotation and elbow flexion/extension axes of the knob-side contributed to increasing large bat-head speed. These joint angular velocities of the shoulder and elbow affect to the horizontal movement of the in the forward direction before ball impact.