The sternocostal head is the larger and more powerful component of the pectoralis major, providing the primary force for arm adduction and internal rotation during exercises such as the bench press, flyes, and push-ups. Sternocostal head tears are the most common pectoralis major injury, typically occurring at the distal tendon-bone junction during heavy resisted shoulder extension (eccentric loading during the lowering phase of a bench press). The medial muscle roll visible at the anterior axillary fold is primarily the sternocostal head.
| Origin | Anterior surface of the sternum and costal cartilages of ribs 1 through 6 |
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| Insertion | Lateral lip of the bicipital groove of the humerus through the posterior (deep) lamina of the pectoralis major tendon, the lower fibres twisting to insert most superiorly |
| Nerve Supply | Medial pectoral nerve (C8, T1) and lateral pectoral nerve (C5, C6, C7) |
| Blood Supply | Pectoral branches of the thoracoacromial artery and lateral thoracic artery |
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The lower sternocostal fibres fold upon themselves to insert above the clavicular fibres at the humerus, creating the unique twisted insertion that allows the lower fibres to adduct the arm more powerfully when it is above horizontal. This arrangement is why the lower chest exercises with the arms above shoulder height selectively target the lower pectoralis.
Pectoralis major tendon tears usually involve the sternocostal head at its humeral insertion, often during the lowering phase of a heavy bench press. They produce bruising, a medial roll of muscle as the torn part recoils, loss of the anterior axillary fold definition (visible as a web when the arm is abducted) and weakness of internal rotation and horizontal adduction. MRI confirms the tear site and retraction. Surgical tendon-to-bone repair, ideally within about 6 weeks, gives better strength than delayed repair or non-operative care. Because the lower sternocostal fibres receive much of their supply from the medial pectoral nerve, EMG can help tell medial from lateral pectoral nerve injury.
The sternocostal head forms the bulk of the pectoral muscle mass from the sternum to the anterior axillary fold, becoming maximally contracted during resisted horizontal adduction and adduction of the arm from the abducted position.
Distal tendon avulsion from the humerus during heavy resisted shoulder extension producing medial muscle roll, anterior axillary fold loss, and internal rotation weakness requiring surgical repair within 6 weeks.
