Chest (pectorals)

The big fan-shaped muscle across the front of the chest. It drives everything you push away from you and pulls the arm across the body — a hug, a bench press, a shove.

This muscle in 3D

Drag to rotate · scroll to zoom — see the shape, origin and insertion of the chest (pectorals). 3D model via Sketchfab (CC-BY).

Anatomy

Muscles: Pectoralis major (clavicular & sternal heads), pectoralis minor beneath

Origin: Clavicle, sternum and upper rib cartilages.

Insertion: Intertubercular groove of the humerus (upper arm).

Actions:

Horizontal adduction (arm across the body)
Shoulder flexion (clavicular/upper head)
Shoulder extension from flexed (sternal/lower head)
Internal rotation of the humerus

How the muscle works

It converges from a wide origin onto a single point on the arm, so contracting it sweeps the arm inward and forward. The upper (clavicular) fibres press up and in; the lower (sternal) fibres press down and in — which is why incline and decline angles bias different regions.

Fibre-type bias: Mixed, with good fast-twitch capacity — responds to heavy pressing.

Functional role: All pressing and pushing; throwing; bringing the arms together.

Common problems

Pec strain or tear (bench press, sudden load)
Tight pecs pulling shoulders forward (rounded posture)

Training & stretching

Bench press (flat/incline/decline), push-ups, dips, cable flyes.

Doorway pec stretch; arm-on-wall chest opener.