Upper arm leverage positions have humeral longitudinal axes in front of or in line with acromial lines. During upper arm accelerations, shoulder joint muscles mioanglosly extend, inwardly rotate, horizontally flex and abduct humeruses. Because transitions move humeruses to upper arm leverage positions, humeruses move with shoulder girdles and thoraxes. Shoulder joint muscles lock these structures together. When pitchers abduct humeruses overhead beyond vertical posteriorly, shoulder joint extension muscles drive humerus toward home plates.
Pitchers reduce pitching arm centrifugal stress with near vertical upper arm accelerations.
During forearm accelerations, shoulder joint muscles mioanglosly inwardly rotate humeruses. Until elbow and forearm joints accelerate forearms, pitchers cannot powerfully inwardly rotate shoulder joints. During decelerations, shoulder joint muscles plioanglosly flex, horizontally extend and abduct humeruses. Flexions stop humeral extensions. Horizontal extensions stop horizontal flexions.