In human anatomy, the infraspinatus muscle is a thick triangular

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

, which occupies the chief part of the

infraspinatous fossa The infraspinous fossa (infraspinatus fossa or infraspinatous fossa) of the scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (coll ...

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Gray's Anatomy ''Gray's Anatomy'' is a reference book of human anatomy written by Henry Gray, illustrated by Henry Vandyke Carter, and first published in London in 1858. It has gone through multiple revised editions and the current edition, the 42nd (Octo ...

'', see infobox. 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.

Structure

It attaches medially to the infraspinous fossa of the

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...

and laterally to the middle facet of the greater tubercle of the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...

. 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 The infraspinatous fascia is a dense fibrous membrane, covering the Infraspinatous muscle and fixed to the circumference of the infraspinatous fossa; it affords attachment, by its deep surface, to some fibers of that muscle. It is intimately attac ...

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

. 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 ( grc-gre, Προῦσα, Proûsa, Latin: Prusa, ota, بورسه, Arabic:بورصة) is a city in northwestern Turkey and the administrative center of Bursa Province. The fourth-most populous city in Turkey and second-most populous in the ...

, 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 animals

From an evolutionary prospective, the pectoral muscles – the

pectoralis major The pectoralis major () is a thick, fan-shaped or triangular convergent muscle, situated at the chest of the human body. It makes up the bulk of the chest muscles and lies under the breast. Beneath the pectoralis major is the pectoralis minor, ...

and pectoralis minor – are thought to have evolved from a primitive muscle sheet that connected the coracoid to the

. 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., p 541

Additional images

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.

External links

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ExRx
{{Authority control Muscles of the upper limb Rotator cuff Shoulder lateral rotators