Infraspinatus muscle

In human anatomy, the infraspinatus muscle is a thick triangular muscle, which occupies the chief part of the infraspinatous fossa. As one of the four muscles of the rotator cuff, the main function of the infraspinatus is to externally rotate the humerus and stabilize the shoulder joint.It attaches medially to the infraspinous fossa of the scapula and laterally to the middle facet of the greater tubercle of the humerus.The infraspinatus is the main external rotator of the shoulder. When the arm is fixed, it adducts the inferior angle of the scapula. Its synergists are teres minor and the deltoid.From an evolutionary prospective, the pectoral muscles – the pectoralis major and pectoralis minor – are thought to have evolved from a primitive muscle sheet that connected the coracoid to the humerus. In late reptilians and early mammals, this muscle structure was displaced dorsally; while most of its components evolved into the pectoralis major, some fibers eventually attached to the scapula and evolved into the supraspinatus, the infraspinatus, and parts of the subscapularis.Infraspinatus muscle (shown in red). Animation.Muscles on the dorsum of the scapula, infraspinatus is labelled 8 (posterior view).Infraspinatus muscleInfraspinatus muscleLeft scapula. Posterior surface.Left humerus. Posterior view.Infraspinatus muscleThis article incorporates text in the public domain from page 441 of the 20th edition of Gray's Anatomy (1918)