With the advance of computing capability, many studies have attempted to understand the mechanics of sports through numerical simulations. This study focuses on baseball pitching, in which a player can throw various types of breaking balls. Because every pitcher's hand has different mechanical characteristics, the trajectory of the thrown ball varies among pitchers even if baseball pitching grips are identical. Therefore, a pitching simulation system that reflects the mechanical characteristics of individual pitchers should include not only the fluid dynamics around the ball but also the hand model by taking into account ball grip and shape, elasticity, and friction of the pitcher's hand. This study explains the modeling of the flexion motion of the finger by the flexor digitorum profundus muscle. In the hand model, the flexor digitorum profundus muscle contracts and transmits a contraction force to the finger, generating a joint torque through the flexor tendon pulleys that results in the movement of the finger. Based on this model, the acting force from the finger to the ball is calculated. Held within the hand, the ball moves toward the fingertip and is perpendicularly directed to the finger pulp. After the ball is released, its trajectory is calculated by analyzing the airflow around the ball. In the airflow analysis, the equations of the finite element method are solved by streamline upwind/pressure stabilizing Petrov-Galerkin (SUPG/PSPG) methods, which enable the stability analysis of the aerodynamic coefficients corresponding to the ball's seam position. This experiment showed that the ball's angular velocity and position at the moment of its release increased with the increasing force generated by the flexor digitorum profundus muscle and altered the subsequent trajectory of the ball.
