PECTORALIS MAJOR MYOLOGY
OVERVIEW
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.
LOCATION
Superior, anterior, superficial aspect of the thoracic cage
ORIGIN
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.
INSERTION
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.
FUNCTION
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)
SHORT AND LONG POSITION
Because of the fan-shaped structure, the short and long positions of Pectoralis Major will vary depending on which fibers one is attempting to target. The short position of this muscle can be found when the Pec's insertion site is closest to one of its origination sites. The long position of this muscle can be found when the insertion site is furthest away from one of its origination sites.
CLAVICULAR DIVISION
The short position of the Clavicular Division of the Pectoralis Major can be found when the arm is in a degree of shoulder flexion that does not surpass the collar bone. A degree of transverse adduction is also required for this division to become completely shortened. The combination of transverse adduction and shoulder flexion should create a diagonal path for the upper arm to travel. This diagonal path is angled toward the center of the Clavicular origin as one lifts the arm. Unilateral work can further shorten the Clavicular fibers by allowing one to adduct the arm across the body more. The long position of the Clavicular Division of the Pectoralis Major can be found when the arm is in a degree of shoulder extension and depression. The lengthened position for this division is the short position for the Iliac Division of the Latissimus Dorsi.
STERNAL DIVISION
The short position of the Sternal Division of the Pectoralis Major can be found when the arm is in a slight degree of shoulder flexion (from anatomical position) that does not surpass the sternal origin of the Pectoralis Major. Transverse adduction is also required to bring the insertion of the Pec to the center of the body; toward the Sternal origin. Unilateral work can further shorten the Sternal fibers by allowing one to adduct the arm across the body more.
The long position of the Sternal Division of the Pectoralis Major can be found when the upper arm is in extension and transverse abduction. This division will also benefit by allowing the shoulder to retract and depress slightly for more length. The lengthened position for this division is the short position of the Lumbar Division of the Latissimus Dorsi.
COSTAL DIVISION
The short position of the Costal Division of the Pectoralis Major can be found when the shoulder is depressed and the arm is transversely adducted, brining the insertion of the Pectoralis Major closest to the center of the Costal origin. Unilateral work can further lengthen the Costal fibers by allowing one to adduct the arm across the body more.
The long position of the Costal Division of the Pectoralis Major can be found when the arm is in a degree of shoulder extension and retraction that aligns with the short position of the Thoracic Division of the Latissimus Dorsi.
ANTAGONIST
Latissimus Dorsi: The three divisions of the Latissimus Dorsi correspond antagonistically to the three divisions of the Pectoralis Major.
Clavicular Pec Division = Iliac Lat Division
Sternal Pec Division = Lumbar Lat Division
Costal Pec Division = Thoracic Lat Division