Infraspinatus
The main external rotator of the shoulder, and the largest of the four cuff muscles.
- Type
- Muscle
- Region
- Shoulder
- Standard name
- m. infraspinatus
- Origin
- Infraspinous fossa of the scapula
- Insertion
- Middle facet of the greater tubercle of the humerus
- Actions
- shoulder external rotationshoulder stabilisation
- Verify
- TA2 2458 TA98 A04.6.02.008
What it is
A thick triangular muscle filling the large hollow below the spine of the scapula — the biggest of the four rotator cuff muscles, and the one covering most of the back of the shoulder blade.
Its tendon blends with the joint capsule on its way to the greater tubercle, as all the cuff tendons do. That blending is why the cuff is described as a cuff: the four tendons form a continuous sleeve around the head of the humerus rather than four separate straps.
What it does
Externally rotates the shoulder — turns the arm outward — and it is the principal muscle for that, contributing the majority of the force alongside teres minor.
Like the rest of the cuff, it also holds the head of the humerus centred in the socket. During overhead movement it pulls the head downward and backward, keeping it seated as the arm rises.
External rotation strength is typically much lower than internal rotation strength, because the internal rotators include several large muscles — pectoralis major, latissimus dorsi, subscapularis and teres major — against two small ones.
What loads it
External rotation against resistance, and any overhead or throwing motion where the arm has to be decelerated.
The inverted row and pull-up load it as a stabiliser throughout, though neither trains external rotation directly.
What is commonly got wrong about it
That the rotator cuff is one muscle. It is four, with different actions — one abducts, two externally rotate, one internally rotates. "A rotator cuff injury" describes a region, not a structure.
That the cuff's job is rotation. The name says rotator, and the more important job is stabilising. The rotation is what you can measure; the compression into the socket is what keeps the joint working.
That external rotation exercises are rehab-only. Nothing on this site prescribes for injury, but it is worth saying plainly that the external rotators are genuinely weak relative to their opponents in most people, and that is an ordinary training observation rather than a clinical one.