Infraspinatus

Infraspinatus is one of the four muscles that make up the rotator cuff, the others being: supraspinatus, teres minor and subscapularis.

• origin: infraspinatus fossa of the scapula
• insertion: greater tuberosity of the humerus
• innervation: suprascapular nerve (C4-6)
• action: external rotation

Origin

It attaches medially to the infraspinous fossa of the scapula and laterally to the greater tubercle of the humerus.

It arises by fleshy fibers from its medial two-thirds, and by tendinous fibers from the ridges on its surface; it also arises from the infraspinatous fascia which covers it, and separates it from the teres major and minor.

Insertion

The fibers converge to a tendon, which glides over the lateral border of the spine of the scapula, and, passing across the posterior part of the capsule of the shoulder-joint, is inserted into the middle impression on the greater tubercle of the humerus.

Innervation

The infraspinatus muscle is supplied by the suprascapular nerve (C5 and C6), which arises from the superior trunk of the brachial plexus and passes laterally through the posterior triangle of the neck and through the scapular notch on the superior border of the scapula. After supplying fibers to the supraspinatus muscle, it supplies articular branches to the capsule of the shoulder joint.

Relations

The tendon of this muscle is sometimes separated from the capsule of the shoulder-joint by a bursa, which may communicate with the joint cavity.

Action

It acts with teres minor to externally rotate the glenohumeral joint , and with other rotator cuff muscles to stabilise the shoulder.