Anatomy Guide › Thorax
Pectoralis Major
The pectoralis major is the muscle that gives the chest its shape: a broad, fan-shaped sheet spreading from the collarbone, breastbone, and upper rib cartilages, with all its fibers converging onto a narrow tendon that grips the upper humerus. That fan architecture is the key to its versatility. The upper (clavicular) fibers pull the arm up and forward; the lower (sternocostal) fibers pull it down and across. Between them, the pec major powers pushing a door, throwing a punch, pressing a barbell, and wrapping someone in a hug.
A curious detail of its anatomy: the tendon's layers twist before inserting, so the lowest fibers of the muscle attach highest on the bone. This fold acts like a built-in strain distributor across the muscle's wide range of pulling angles. The muscle also forms the front wall of the armpit — pinch the front fold of your axilla and you are holding the pec major's edge.
In the gym era the pec major has become famous for a specific injury: tears of its tendon during heavy bench pressing, felt as a pop with sudden weakness and bruising along the front of the armpit. Complete ruptures in athletes are usually repaired surgically. The muscle is also central to chest-wall surgery — flaps of pectoralis major have long been used in reconstruction after head, neck, and sternal operations, prized for their reliable blood supply.
In Anamo, the pectoralis major covers so much of the upper chest that it is a natural heat-mapping guess: it borders the clavicle, sternum, ribs, and shoulder all at once, and its warmth reading quickly tells you which of those neighborhoods the mystery part calls home.
- Fan-shaped, arising from collarbone, sternum, and rib cartilages
- Forms the front wall of the armpit
- Its tendon layers twist so the lowest fibers insert highest
- Tendon tears during bench pressing are its signature injury
Think you could find the pectoralis major on a 3D model? Play Anamo and test yourself — every guess is color-coded by how close it lands to the mystery structure.