Pectoralis Major Anatomy: Origin, Insertion, Actions, and Chest Muscle Group

The pectoralis major is the large, superficial chest muscle powering pushing, hugging, and internal rotation. It has two distinct heads with different actions.

Anatomy

Feature	Clavicular Head	Sternocostal Head
Origin	Medial ½ clavicle (anterior)	Sternum, costal cartilages 1–6, external oblique aponeurosis
Insertion	Lateral lip of bicipital groove (humerus) — bilaminar tendon
Innervation	Lateral pectoral nerve (C5–C7) + medial pectoral nerve (C8–T1)
Actions	Flexion, horizontal adduction, internal rotation	Adduction, extension (from flexed), internal rotation, depression
Fiber direction	Inferolateral	Superolateral → inferolateral

Chest Muscle Group

1. Pectoralis major: Prime mover (above)
2. Pectoralis minor: Origin: ribs 3–5; Insertion: coracoid process; Action: scapular protraction, depression, downward rotation; Innervation: medial pectoral nerve
3. Serratus anterior: Origin: ribs 1–8/9; Insertion: medial scapular border; Action: protraction, upward rotation, fixation; Innervation: long thoracic nerve (C5–C7)
4. Subclavius: Clavicle → 1st rib; stabilizes SC joint

Clinical Pearls

• Rupture: Usually sternocostal head; "axillary snap"; surgical repair for active patients
• Trigger points: Refer to anterior shoulder, ulnar forearm, medial scapula
• Training: Incline emphasizes clavicular; flat/decline emphasizes sternocostal
• Stretch: Doorway pec stretch (90°/120° abduction) — avoid excessive anterior capsule stress

Functional Synergy

Pec major + lat dorsi = internal rotation/adduction power (climbing, swimming). Serratus anterior stabilizes scapula for pec force transmission. Pec minor tightness → scapular dyskinesis → impingement.