Muscles connecting the upper extremity to the vertebral column (posterior view).
Infraspinatus muscle (shown in red) seen from behind.
|infraspinous fossa of the scapula|
|middle facet of greater tubercle of the humerus|
|suprascapular and circumflex scapular arteries|
|Actions||Lateral rotation of arm and stabilizes humerus|
|Anatomical terms of 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.
The muscle arises by fleshy fibers from the medial two-thirds of the infraspinatous fossa, 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 teres 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 trapezoidal insertion of the infraspinatus onto the humerus is much larger than the equivalent insertion of the supraspinatus, the reason why the infraspinatus is involved in rotarcuff tears about as frequently as the supraspinatus. 
The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. These muscles function to abduct and laterally rotate the arm, respectively.
The infraspinatus is frequently fused with the teres minor.
The infraspinatus is the main external rotator of the shoulder. When the arm is fixed, it abducts the inferior angle of the scapula. Its synergists are teres minor and the deltoid.
Additionally, the infraspinatus reinforces the capsule of the shoulder joint.
From an evolutionary prospective, the pectoral muscles — the pectoralis major and pectoralis minor — is 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. 
- Gray's Anatomy, see infobox.
- Mochizuki, T.; Sugaya, H.; Uomizu, M.; Maeda, K.; Matsuki, K.; Sekiya, I.; Muneta, T.; Akita, K. (2008). "Humeral Insertion of the Supraspinatus and Infraspinatus. New Anatomical Findings Regarding the Footprint of the Rotator Cuff". The Journal of Bone and Joint Surgery 90 (5): 962–9. doi:10.2106/JBJS.G.00427. PMID 18451386.
- Platzer, Werner (2004). Color Atlas of Human Anatomy, Vol. 1: Locomotor System (5th ed.). Thieme. p. 138. ISBN 3-13-533305-1.
- "Infraspinatus". Wheeless' Textbook of Orthopaedics. Retrieved January 2011.
- Brand, R. A. (2008). "Origin and Comparative Anatomy of the Pectoral Limb". Clinical Orthopaedics and Related Research 466 (3): 531–42. doi:10.1007/s11999-007-0102-6. PMC 2505211. PMID 18264841. , p 541
|Wikimedia Commons has media related to Infraspinatus muscles.|
- Origin, insertion and nerve supply of the muscle at Loyola University Chicago Stritch School of Medicine
- Anatomy figure: 03:03-04 at Human Anatomy Online, SUNY Downstate Medical Center
- infraspinatus+muscle at eMedicine Dictionary