## 4 Discussion
So, although the shoulder is a key joint for overarm throwing that has a substantial effect on ball release speed, the mechanisms behind the production of ball release speed start in the trunk. Of the three joint angle changes at the shoulder, upper arm external/internal rotation had the largest effect on ball release speed. The external rotation of the upper arm is produced by the inertial lag of the forearm and hand as the more proximal segments rotate forward [8]. On the other hand, the extreme external rotation of the upper arm is due mainly to the sequential actions of the flexion and abduction muscles at the shoulder [30]. The subsequent stopping of the external rotation and production of internal rotation of the upper arm is due mainly to the actions of the internal rotation proximal joint torque at the shoulder [30]. The third key joint in the kinematic/kinetic chain was the elbow, where forearm extension/flexion had the most effect on ball release speed if it was kept at a constant value (Table 1). In the last 50 ms prior to ball release, the throwing arm extended rapidly through a range of about 60 deg (Figure 4). The acceleration of the forearm in the direction of elbow extension is caused primarily by the interactive moments resulting from the linear acceleration of the shoulder and the angular velocity of the upper arm [8,31]. As the trunk is rotated counter-clockwise and the upper arm undergoes abduction and extension rotations relative to the trunk, forces applied to the forearm at the elbow and directed along the longitudinal axis of the upper arm are essential to maintain the centripetal acceleration of the elbow relative to the shoulder [8,18,31,32]. Mechanically, when a force acts on the trunk, a force with equal magnitude but in the opposite direction will act on the upper arm. These forces will propagate to the elbow joint, forearm, and further outward to produce the angular movements of the adjacent segments. The final joint in the kinematic/kinetic chain that would appear to have some effect on ball release speed was the wrist, where if the wrist extension/flexion angle was held constant the ball speed changed by 1.6 m/s (Table 1). However, observation of the wrist extension/flexion angle from the 15 trials shows that it was almost constant prior to ball release (Figure 4). The wrist joint is not able to generate large flexion velocities which contribute to the ball speed [33] because the hand is mainly extended in either the arm-cocking phase or the arm acceleration phase, and slightly flexed at ball release [34]. The strategy used by the central nervous system (CNS) is that the elbow and shoulder contribute to the adjustments of ball speed, but the wrist does not [35,36]. As the length of the hand is shorter than that of the forearm and upper arm, the angular velocity of the wrist is less effective for increasing ball speed than the angular velocity of the elbow and shoulder, which is one of the reasons why the wrist does not contribute to the adjustment of ball speed [35]. In addition, the role of the wrist in fastball pitching is to simplify the control of the finger grip force for an accurate ball release. One element in the kinematic/kinetic chain that surprisingly did not appear to have substantial effect on ball release speed was the motion of the scapula. The effect of scapula external/internal rotation on ball release speed was quite small when the scapula external/internal joint angle was fixed at the value at ball release (Table 1). However, examining the joint angle time histories from the 15 trials shows that prior to ball release the slope of the time history was quite high (Figure 4); this warrants further investigation. This study focused on the role of the trunk and throwing arm on ball release speed and did not directly include a physical representation of the lower limbs beyond constraining the pelvis to move in the same way as the recorded performances. Consequently, a possible limitation of the simulation model is that the legs were not included explicitly.