Infraspinatus muscle

Human anatomy

Structure

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

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 rotator cuff tears about as frequently as the supraspinatus.

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.

Innervation

The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. These muscles function to abduct and laterally rotate the arm, respectively.

Variation

The infraspinatus is frequently fused with the teres minor.

Function

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. The infraspinatus and teres minor rotate the head of the humerus outward (external, or lateral, rotation); they also assist in carrying the arm backward. Additionally, the infraspinatus reinforces the capsule of the shoulder joint.

In other mammals

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.

Meat

In butchery, the infraspinatus of cattle forms the top blade subprimal, part of the shoulder clod. The large, tough, and chewy sheet or seam of gristle that runs down the middle is the shoulder tendon.

Used whole, the infraspinatus is a top blade roast; cut perpendicularly, it gives top blade steaks; filleted into two long, flat cuts by seam cutting along the tendon then portioned, it gives flat iron steak.

Additional images

|File:Infraspinatus muscle 1.JPG|The infraspinatus shown on a cadaver, left shoulder, posterior view. |File:Thorax section 8.jpg|Imaged transverse section of the body through the thorax.

References

Saladin, Kenneth. Anatomy and Physiology: the Unity of Form and Function. 7th ed., McGraw Hill Education, 2014. pp. 343, 346, 491, 543. Funk, Lennard. Rotator Cuff Biomechanics. Shoulderdoc.co.uk. TheFresh Healthcare Marketing, 11 Feb 2016. Web.

References

1. (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.
2. "Infraspinatus". Wheeless' Textbook of Orthopaedics. link
3. Platzer. Werner. "Color Atlas of Human Anatomy, Vol. 1: Locomotor System". [[Thieme Medical Publishers. Thieme]]. (2004)
4. (2008). "Origin and Comparative Anatomy of the Pectoral Limb". Clinical Orthopaedics and Related Research.
5. Underly, Kari. (2011-08-16). "The Art of Beef Cutting: A Meat Professional's Guide to Butchering and Merchandising". John Wiley & Sons.