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origin: infraspinous fossa of the scapula
insertion: middle facet of the greater tubercle of the humerus
innervation: suprascapular nerve (C5-6)
action: external rotation of the humerus
It arises medially from the infraspinous fossa of the scapula, specifically from 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 muscles.
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 facet of the greater tubercle of the humerus.
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.
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.
It acts with teres minor to externally rotate the glenohumeral joint, and with other rotator cuff muscles to stabilize the shoulder.
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