Chest

The chest, more formally called the pectoralis major, is the large fan-shaped muscle that forms the bulk of the anterior chest wall. It is one of the most prominent muscles in the upper body and a primary target in strength training, particularly in exercises like the bench press⁴.

Anatomy

The pectoralis major is a thick, fan-shaped muscle that covers the superior part of the thorax. It lies underneath the breast tissue and forms the anterior wall of the axilla. The muscle originates from multiple points: the anterior surface of the medial half of the clavicle, the anterior surface of the sternum, the first six to seven costal cartilages, and the aponeurosis of the external oblique muscle².

The fibers converge laterally to insert on the lateral lip of the bicipital groove of the humerus. The insertion is a U-shaped tendon measuring 40-60 mm wide and 3-5 mm thick. The tendon is trilaminar—formed by fibers from the clavicular head and the sternocostal head that twist as they approach the insertion¹³.

Two Heads

The pectoralis major is functionally divided into two heads. The clavicular head (also called the pars clavicularis) originates from the anterior surface of the medial half of the clavicle and is responsible for shoulder flexion. The sternocostal head (pars sternocostalis) originates from the sternum and adjacent costal cartilages and is responsible for adduction and medial rotation of the arm. The sternocostal head makes up approximately 80% of the total muscle volume⁵³.

Some anatomical texts describe a third "abdominal part" originating from the aponeurosis of the external oblique, though this is often considered part of the sternocostal head⁶.

The two heads receive different nerve supplies. The clavicular head is innervated by the lateral pectoral nerve (C5, C6), while the sternocostal head receives innervation from the medial pectoral nerve (C7, C8, T1)⁵.

Functions

The pectoralis major performs three primary actions at the shoulder joint. First, it adducts the arm, pulling the arm toward the body midline. Second, it flexes the arm when the clavicular head contracts. Third, it medially rotates the arm, turning the arm inward².

The function differs between the two heads:

• Clavicular head: Flexes the humerus (raises the arm forward when extended)
• Sternocostal head: Extends the humerus (lowers the arm when raised above horizontal) and produces horizontal adduction⁵

When the arm is raised overhead, the sternocostal head assists in extending the arm from flexed positions. The pectoralis major also plays a role in deep inspiration, helping to expand the ribcage when the body needs additional air².

Variations

Congenital absence of the pectoralis major, particularly the sternocostal head, is associated with Poland syndrome, which is characterized by unilateral absence along with ipsilateral hand anomalies. Conversely, absence of the sternocostal head with compensatory hypertrophy of the latissimus dorsi has been documented⁶.

Clinical Significance

Pectoralis Major Rupture

Pectoralis major rupture is a rare shoulder injury, most commonly seen in weightlifters during bench press. The tendon may rupture from the bone during heavy eccentric loading. Complete ruptures typically require surgical repair within 6 weeks of injury for optimal outcomes, particularly in young athletes and those involved in sports².

Testing

The two heads can be tested separately through palpation. The sternocostal head is tested by adducting the arm against resistance in the transverse plane. The clavicular head is tested by flexing the arm against resistance from a position of slight abduction⁶.

Surgical Considerations

The pectoralis major is frequently used for flap repairs in reconstructive surgery, particularly for defects arising from breast cancer treatment. The muscle's dual blood supply (primarily from the pectoral branch of the thoracoacromial artery) and reliable nerve supply make it a good candidate for tissue transfer⁸.

Training

The pectoralis major is heavily involved in all pressing movements. The bench press is the primary exercise for developing chest size and strength. Flat pressing movements emphasize the sternocostal head, while incline pressing movements emphasize the clavicular head. Decline pressing targets the lower portion of the chest⁷.

Cable flyes and dumbbell flyes provide a stretch-focused movement that targets the chest through a greater range of motion than pressing movements. Push-ups also target the pectoralis major, with elevated feet increasing the emphasis on the chest⁷.

Historical Note

The term "pectoralis" comes from Latin "pectus" meaning breast. The muscle has been studied extensively in anatomical literature, with the first detailed surgical descriptions appearing over 200 years ago⁸.