Infraspinatus

backMusculus infraspinatus

The infraspinatus covers most of the back of your shoulder blade and rotates your upper arm outward. It produces most of the shoulder's external rotation strength.

The infraspinatus is a broad triangular muscle filling the large hollow below the ridge on the back of the shoulder blade. Its fibres converge outward into a tendon that passes behind the shoulder joint and attaches to the back of the top of the arm bone. On a lean, well-trained back it is visible as a flat plane just below the rear deltoid.

Its action is external rotation: with the elbow tucked at your side and bent to ninety degrees, sweeping your forearm outward away from your stomach. Most of the strength for that movement comes from here, with the teres minor just below contributing the rest.

Along with the teres minor, it also has a constant job during pressing and overhead movements — holding the head of the arm bone down and back in the socket while the larger muscles drive the arm forward. It works hardest exactly when the arm is overhead, which is where the joint is least inherently stable.

It is a much bigger muscle than its reputation suggests. Discussion of the rotator cuff tends to treat all four as small stabilisers, but the infraspinatus is comparable in size to a rear deltoid and produces most of the shoulder's external rotation strength.

Training notes

External rotation with a band or a light cable is the direct movement, and the setup that matters is keeping the elbow at the side. Once the elbow drifts away from the ribs, the rear deltoid starts contributing and the specificity is gone. A rolled towel held between elbow and ribs is a crude but effective way to enforce it.

External rotation strength is typically a fraction of internal rotation strength, and that gap widens in people who press a great deal and pull very little. The two or three light sets a week that address it are among the lowest-cost additions available to a program.

Face pulls and rear-delt work involve the infraspinatus but do not train it through rotation, since the arm's rotation barely changes across the movement. They are worth doing on their own merits; they are not a substitute for rotation work.

This is not injury advice. The point here is that a muscle producing most of the shoulder's external rotation is worth training, in the same way any large muscle producing a movement is worth training.

Anatomy

Origin
  • Infraspinous fossa of the scapula
Insertion
  • Middle facet of the greater tubercle of the humerus
Actions
  • Externally rotates the humerus, turning the arm outward
  • Holds the head of the humerus centred in the socket
  • Assists in drawing the arm backward and outward at shoulder height
Innervation
Suprascapular nerve (C5–C6)

Exercises

Antagonists

Synergists