Pectoralis Major

Pectoralis major (red); pectoralis minor (purple).This is the major muscle of the chest.  It arises from the clavicle (collar bone), the sternum, and several ribs, and it inserts into the humerus on the lateral side of the anterior surface, passing over both the long head and short head of the biceps brachii.

In the relaxed position shown in the diagram, the muscle assumes a nearly 180° twist.  When the shoulder is flexed, the muscle untwists.

This muscle contributes to a number of actions in the upper arm. The most common with regard to pitching are shoulder flexion, horizontal (transverse) adduction, horizontal (transverse) flexion, and internal rotation.