Pectoralis minor
A small muscle under the pec that pulls on the shoulder blade, not the arm — and carries a large amount of posture folklore.
- Type
- Muscle
- Region
- Shoulder
- Standard name
- m. pectoralis minor
- Origin
- Third, fourth and fifth ribs, near their cartilages
- Insertion
- Coracoid process of the scapula
- Actions
- scapular depressionscapular protractionscapular downward rotation
- Verify
- TA2 2305 TA98 A04.4.01.006
What it is
A thin triangular muscle lying underneath pectoralis major, running from three ribs up to the coracoid process — a hook of bone projecting forward from the scapula.
The crucial structural point: it attaches to the shoulder blade, not to the arm. Everything it does is done to the scapula.
The axillary artery and the nerves of the brachial plexus pass behind it on their way into the arm.
What it does
Pulls the scapula forward and downward around the ribcage, and tips it so the socket rotates downward. It also assists in lifting the ribs when the shoulder blade is fixed, which is why it is sometimes listed as an accessory muscle of breathing.
What loads it
Movements pulling the shoulder blade forward and down against resistance, and forceful breathing. Nothing loads it in isolation.
What is commonly got wrong about it
This muscle carries more posture folklore per gram than almost any other, so it is worth being precise.
"Tight pec minor causes rounded shoulders." The mechanism is plausible on paper — a short muscle would tip the scapula forward. The evidence connecting measured pec minor length to measured shoulder posture, and connecting either to pain, is much weaker than the confidence of the claim. Studies have found relationships that are small, inconsistent, or absent.
"Rounded shoulders" as a condition needing correction. Posture varies enormously between healthy people and the link between resting posture and pain is far weaker than it is usually presented. This is one of the areas where the fitness industry's confidence most exceeds the research.
Thoracic outlet syndrome as something to self-treat. Compression of the nerves and vessels passing behind this muscle is a real, uncommon and difficult diagnosis. It is not a stretching project — arm symptoms involving numbness, weakness or colour change need proper assessment.
That you can stretch it in a doorway. A doorway stretch moves the shoulder joint and reaches several structures. Nothing about it isolates this muscle.