Study the image to become familiar with the pectoralis major’s origin and insertion as well as its muscle fibers and their direction of influence.
The Pectoralis Major is a thick, fan-shaped muscle located on the superior, anterior surface of the thoracic cage. The pectoralis major originates from the sternal half of the clavicle, the anterior surface of the sternum, the cartilage of all true ribs (minus 1 and 7), and the aponeurosis (sheet of fibrous connective tissue) of the external oblique. The fibers of the pectoralis major run laterally toward their insertion site, converging into a single flat tendon that attaches on the superior, lateral region of the humerus. Because of these attachment sites, the direction of the fibers, and the joint involved, the pectoralis major is responsible for adducting the arm at the glenohumeral joint, transversely adducting the arm at the glenohumeral joint, medially rotating the humerus, producing shoulder flexion (clavicular head), and depressing the shoulder (sternocostal head). The sternocostal head can also engage in shoulder extension depending on the starting position of the arm.
Superior, anterior, superficial aspect of the thoracic cage
Clavicular head - The clavicular head originates on the anterior surface of the medial half of the clavicle.
Sternocostal head - The sternocostal head originates on the anterior surface of the sternum, the superior six costal cartilages, and the aponeurosis of the external oblique muscle.
The fibers of the pectoralis major run laterally from its sites of origin and conver into a single flat tendon that attaches on crest of greater tubercle of humerus.
Adduction the arm at the glenohumeral joint (clavicular head)
Transverse adduction the arm at the glenohumeral joint
Medial rotation of the humerus
Shoulder flexion (clavicular head)
Shoulder depression (sternocostal head)
The sternocostal head can also engage in shoulder extension depending on the starting position of the arm.
SHORT AND LONG POSITION
The fan-shaped structure of the pectoralis major means that the short and long position of this muscle will vary depending on the fibers one is attempting to target. The short position of this muscle can be found when the pectoralis major’s insertion site is closest to one of its origination sites. Arm placement for the short position will vary vertically depending on the target fibers but the humerus will be transversely adducted, placing the upper arm closest to the body. The long position of this muscle can be found when the insertion site is furthest away from one of its origination sites. Arm placement will also vary vertically for the long position depending on the target fibers but the arm will be more transversely abducted, placing the upper arm further away from the body. Target fibers to consider will be the upper, mid, or lower region of the pectoralis major.