In human anatomy, the infraspinatus muscle is a thick triangular
muscle, 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 ...
.
['' Gray's Anatomy'', see infobox.] As one of the four muscles of the
rotator cuff
The rotator cuff is a group of muscles and their tendons that act to stabilize the human shoulder and allow for its extensive range of motion. Of the seven scapulohumeral muscles, four make up the rotator cuff. The four muscles are the supraspi ...
, 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
The infraspinous fossa (infraspinatus fossa or infraspinatous fossa) of the scapula is much larger than the supraspinatous fossa; toward its vertebral margin a shallow concavity is seen at its upper part; its center presents a prominent convexi ...
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
The greater tubercle of the humerus is the outward part the upper end of that bone, adjacent to the large rounded prominence of the humerus head. It provides attachment points for the supraspinatus, infraspinatus, and teres minor muscles, thr ...
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
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 attach ...
which covers it, and separates it from the
teres major
The teres major muscle is a muscle of the upper limb. It attaches to the scapula and the humerus and is one of the seven scapulohumeral muscles. It is a thick but somewhat flattened muscle.
The teres major muscle (from Latin ''teres'', meaning ...
and
teres minor
The teres minor (Latin ''teres'' meaning 'rounded') is a narrow, elongated muscle of the rotator cuff. The muscle originates from the lateral border and adjacent posterior surface of the corresponding right or left scapula and inserts at both the ...
.
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
The greater tubercle of the humerus is the outward part the upper end of that bone, adjacent to the large rounded prominence of the humerus head. It provides attachment points for the supraspinatus, infraspinatus, and teres minor muscles, thr ...
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
The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial join ...
by a
bursa, which may communicate with the joint cavity.
Innervation
The
suprascapular nerve
The suprascapular nerve is a nerve that branches from the upper trunk of the brachial plexus. It is responsible for the innervation of two of the muscles that originate from the scapula, namely the supraspinatus and infraspinatus muscles.
Structur ...
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
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 ...
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 and
pectoralis minor
Pectoralis minor muscle () is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major in the human body.
Structure
Attachments
Pectoralis minor muscle arises from the upper margins and outer surfaces ...
– are thought to have evolved from a primitive muscle sheet that connected the
coracoid
A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
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.
[, 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