Pectoralis major tendon rupture is a partial or complete tear of the pectoralis major tendon. This structure is the tendon attachment of the pectoralis major muscle to the arm (humerus). The pectoralis major muscle becomes a tendon at the outer chest and the tendon attaches into the humerus. A rupture causes the loss of continuity between the pectoralis major muscle and the humerus resulting in the loss of this muscle function. The function of the pectoralis major is to forcefully bring the arm to the midline or across the body, as well as to rotate the arm inward. Usually this injury involves the tendon pulling off the humerus, although occasionally the muscle may tear in the mid-belly or at the junction where the muscle becomes tendon.