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.
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.
The Infraspinatus and Teres minor rotate the head of the humerus outward (external rotation); they also assist in carrying the arm backward. Studies by Lastayo, w., etc. have shown the infraspinatus to be the major external rotator of the shoulder in comparison with the teres minor.