The pectoralis major is the dominant anterior chest muscle with a unique twisted insertion that reverses the cranio-caudal arrangement of its origins. The clavicular and sternocostal heads have distinct actions and are separately innervated.
| Origin | Clavicular head: anterior surface of medial clavicle half; Sternocostal head: anterior sternum and costal cartilages 1-6; Abdominal head: anterior rectus sheath |
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| Insertion | Lateral lip of the bicipital groove — the clavicular fibres insert most inferiorly, the sternocostal fibres most superiorly (twisted insertion) |
| Nerve Supply | Medial and lateral pectoral nerves (C5-T1) |
| Blood Supply | Pectoral branches of the thoracoacromial artery |
| Actions | Adducts and medially rotates the humerus; Clavicular head: flexes and horizontally adducts the humerus; Sternocostal head: extends and depresses the humerus from elevation |
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Pectoralis major tendon rupture (typically at the bicipital groove insertion) produces asymmetric anterior axillary fold with a positive anterior fold asymmetry sign. Ruptures occur in weight-lifters during eccentric bench press loading. MRI identifies the retracted tendon. Surgical repair to the bicipital groove restores full strength. The pectoralis major pedicled flap is the workhorse of head and neck reconstruction.
The anterior chest wall dominant muscle — visible as the anterior axillary fold during resisted adduction.
Avulsion of the pectoralis major from the bicipital groove in a bench-press athlete producing anterior axillary fold asymmetry, repaired surgically to restore full adduction and rotation strength.
