The pectoralis major muscle originates from multiple locations, including the costal cartilage of ribs two through six, the sternum, the medial half of the clavicle. The pectoralis major muscle divides into two muscular heads: the sternal and clavicular heads. These two muscular heads converge into a singular attachment at the humerus.
The pectoralis major muscle is responsible for adduction, internal rotation and forward flexion of the shoulder. All body builders know the bench press and flies stress the pectoralis major. With extreme force, such as bench press with heavy weight and eccentric load, the force on the pectoralis exceeds its strength and tears.