The chest is composed mainly of one large muscle, the pectoralis major, located on each side of the sternum (breast bone), This muscle spreads out in a fan-like manner from the upper arm to the clavicle (collarbone) entire sternum and the sixth and seventh ribs. The uppermost fibers run almost parallel to the anterior deltoid and are known as the clavicular or upper portion of the pectoralis major. The muscle fibers below the uppermost portion and which attach to the sternum run horizontally and constitute the sternal or lower portion of the pectoralis major. The fibers in the lowermost portion of the muscle (those which are attached to the sixth and seventh ribs) run in line with the external oblique abdominal muscle. (This abdominal portion is usually linked together with the sternal section.)
All the muscle fibers of the pectoralis major converge into a 3-inch wide tendon which undergoes a 180° twist before insertion on the upper arm directly in front of the uppermost portion of the humerus. Because of the 180° twist, the lowermost fibers of the pectoralis major insert highest on the bone and the upper fibers lowest. This unique arrangement allows for six muscle actions which include extension, in which you pull the arm down in front of the body from an overhead position, medial rotation in which the arm rotates inward, adduction in which the arm is pulled down sideways to the body from an overhead position, abduction when the arms are raised sideways above the horizontal position, horizontal adduction in which you pull the arm in toward the middle of the body from a position out to the sides and flexion in which raise the arm directly in front of the body.
The upper or clavicular portion of the pectoralis major is a prime mover for shoulder joint flexion and in arm abduction, but only after the arm passes the level position. The clavicular head reaches it’s strongest contraction at about 115° of flexion (25° above level). The middle portion of the pectoralis major which includes part of the clavicular and part of the sternal divisions work together in horizontal adduction, in which the arms are brought from a position of out to the sides to in front of the body. Because the upper portion of the sternal and the lower fibers of the clavicular division fibers run almost parallel, they are involved in any movement in which you bring the arm directly across the body (as in the bench press).
The lower fibers of the sternal division, having a downward and inward angle of pull require exercises in which you pull the arm down and inward( cable crossovers). The abdominal division of the pectoralis major works together with the abdominals. For example, when you do straight arm pull-downs or rope pull-downs and pull the arms down there is also a crunching action to involve the abdominals. Another muscle of the chest, the pectoralis minor, lies underneath the pectoralis major and contributes some muscle mass to the chest. It is a prime mover for abduction and downward rotation of the scapula and has no effect on arm movements as does the major (although they work in synchronization).
In essence, the pectoralis minor pulls the scapula away from the midline (as when you raise the arm in front) and rotates it downward whenever the arm is lowered from an overhead position. The pectoralis major is very efficient in many movements because it uses only certain portions of the muscle. As a result, you do not waste any energy and maximize the use of this large muscle. To get full development, however, you must do exercises to target each segment.