Infraspinatus
Musculus infraspinatus
A thick, triangular rotator cuff muscle occupying the infraspinous fossa. It is the main lateral rotator of the humerus.
Overview
Infraspinatus arises from the medial two-thirds of the infraspinous fossa and its dense fascia. It converges on a tendon that crosses the back of the shoulder joint to the middle facet of the greater tubercle. Its lower edge is partly fused with teres minor. Part of it lies under the skin in the triangle between deltoid, trapezius and teres major, and it can be felt contracting there during resisted lateral rotation.
It is the chief lateral rotator of the arm and a key posterior stabiliser of the humeral head, resisting posterior subluxation. It is also active in the deceleration (follow-through) phase of throwing, where it works eccentrically. It is supplied by the suprascapular nerve, which reaches the fossa around the spinoglenoid notch.
Anatomy
- Origin
Medial two-thirds of the infraspinous fossa and the infraspinous fascia.
- Insertion
Middle facet of the greater tubercle of the humerus.
- Action
Lateral rotation of the arm; holds the humeral head in the glenoid.
- Innervation
Suprascapular nerve.
- Nerve roots
C5–C6
- Arterial supply
Suprascapular and circumflex scapular arteries.
- Relations
Covered by deltoid and trapezius above. Teres minor and teres major lie along its lower border.
Function
- Laterally rotates the humerus
- Stabilises the humeral head (posteriorly)
Clinical significance
Spinoglenoid notch entrapment
A paralabral ganglion cyst or traction in overhead athletes (volleyball players) compresses the suprascapular nerve at the spinoglenoid notch. This causes isolated wasting of infraspinatus and weak external rotation, while supraspinatus is spared.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.