Pectoralis major
The large fan across the front of the chest, with two heads that pull the arm in opposite vertical directions.
- Type
- Muscle
- Region
- Shoulder
- Standard name
- m. pectoralis major
- Origin
- Medial clavicle (clavicular head) and the sternum and upper costal cartilages (sternocostal head)
- Insertion
- Lateral lip of the intertubercular groove of the humerus
- Actions
- shoulder adductionshoulder flexionshoulder internal rotation
- Verify
- TA2 2301 TA98 A04.4.01.002
What it is
The big superficial muscle of the chest, fanning from the breastbone and collarbone to a surprisingly small tendon on the front of the humerus.
It has two heads with different origins and, importantly, different lines of pull:
- Clavicular head — from the inner half of the collarbone, running downward and outward
- Sternocostal head — from the sternum and the cartilages of the upper ribs, running upward and outward
Its tendon twists as it approaches the humerus, so fibres from the bottom of the sternal head attach highest.
Because the two heads pull from above and below the joint, they oppose each other in the vertical plane while cooperating in adduction. The clavicular head flexes the shoulder; the sternocostal head extends it from a flexed position.
What it does
Adducts the arm — pulls it across and towards the body — and internally rotates it. It is the prime mover of anything hugging, pressing or throwing across the body.
The clavicular head assists shoulder flexion, raising the arm forward. The sternocostal head pulls the raised arm back down.
With the arms fixed above you, it can pull the trunk upward, which is part of climbing.
What loads it
Pressing away from the body, and adduction against resistance: push-ups, bench pressing, dips and flyes.
The push-up is the main one, with the incline push-up as the scaled version — the height of the hands sets the load.
What is commonly got wrong about it
"Upper chest" and "lower chest" as separate muscles. They are two heads of one muscle with genuinely different lines of pull, so incline and decline angles do shift emphasis — this is one of the better-supported regional claims in training. What is oversold is the precision: the notion that specific degrees of incline target specific bands is far beyond what anyone has demonstrated.
"Inner chest" exercises. There is no inner pectoral muscle. The definition down the middle of the chest is where the muscle attaches to the sternum, plus the fat over it.
That pec size prevents shoulder problems, or causes them. Claims in both directions are common and neither is well supported.