More than half of adolescents participating in baseball experience shoulder or elbow pain during a competitive season (Lyman, Fleisig, Andrews, & Osinski, 2002;​ Lyman et al., 2001), increasing future risk of overuse injury by 7.5x (Yang et al., 2014). Because of these unique demands, baseball has the greatest percentage of injuries in high school athletics resulting in surgery (32.3%), with 60.3% of these injuries due to overuse. Injuries to the shoulder and elbow comprise 53-63% of all injuries in baseball (Collins & Comstock, 2008;​ Shanley, Rauh, Michener, & Ellenbecker, 2011), believed to occur as a result of repeated microtrauma to soft-tissues caused by the repetitive mechanical strain throwing (Andrews & Fleisig, 1998). Research and practice has suggested that pitchers who generate and transfer greater energy from the lower extremity will exhibit better performance and reduced stress on the arm, thereby decreasing the risk of injury. The purpose of this dissertation was to investigate the generation, dissipation, and transfer of mechanical energy during the pitching motion as it relates to performance and upper extremity joint kinetics, with the goal of maximizing performance while minimizing shoulder and elbow joint kinetics.

Aim 1 investigated the role of ground reaction forces in baseball pitching performance. The results of this study showed that contrary to commonly held belief, drive leg ground reaction forces demonstrated no association to hand velocity in adult baseball pitchers. Stride leg ground reaction forces, however, were significantly correlated to hand velocity, particularly posteriorly directed ground reaction force. This relationship suggests that the production of force and stiffening of the stride leg upon landing helps a pitcher to halt the linear energy of the body towards home plate, allowing maximum transfer of energy to the pelvis and trunk at the initiation of the kinematic sequence (McNally, Borstad, Oñate, & Chaudhari, 2015).

Aim 2 described the energetics of the legs, trunk and arm during baseball pitching, and evaluated the relationship between increased energy generation and transfer from the legs, trunk, and arm to performance in baseball pitchers. Results showed that the stride leg transferred the least amount of energy up the kinetic chain, relative to the total energy input into the hand during the pitch. However, despite transferring the least energy, stride leg energy was also the most variable between pitchers, and explained an additional 6.9% of variance in hand velocity, compared to height and mass alone, in a group of high school baseball pitchers.

Aim 3 extended the results of the first two aims, evaluating the relationship between mechanical energy transfer from the legs, trunk, and arm, and the relationship to shoulder and elbow joint kinetics after accounting for the effect of increased performance. Overall, hand velocity and mass were the largest predictors of shoulder and elbow joint torques, explaining between 65.1% and 80.7% of variance in joint kinetics. Stride leg energy transfer was a significant predictor of elbow varus torque at maximum shoulder rotation, and trunk energy transfer to the arm was associated with changes in peak shoulder internal rotation torque and peak shoulder distraction force. These effects, however, only explained an additional 1.8% to 4.0% of variance in respective joint kinetics.

Findings from this study indicate that in support of previous theory, increasing energy output from the lower extremities can increase the total amount of energy available to be transferred to the hand, and may lead to an increase in throwing velocity. In particular, the stride leg seems to play a particularly important role in its ability to transfer energy generated by the drive leg up to the trunk. However, the hypothesis that generating a greater amount of energy with the legs leads to reduced joint kinetics on the arm was refuted, as energy generated by the legs must still be transferred across the arm to the ball via greater joint torques and forces.