Infraspinatus muscle

Origin and insertion

Infraspinatus muscle lies on the dorsal surface of the scapula, deep to trapezius, and parts of the deltoid and latissimus dorsi muscles. It originates with strong muscle fibers directly from the medial two-thirds of the infraspinous fossa, whereas its tendinous fibers arise from the ridges of the fossa. Some fibers also originate from the deep surface of the thick infraspinous fascia covering the infraspinatus muscle, that separates it from the teres major and minor muscles.

From these origin points, the fibers converge into a narrow tendon at the superolateral edge of the scapula. The tendon crosses the posterior side of the shoulder and inserts onto the middle impression of greater tubercle of humerus, as well as the posterior part of the shoulder joint capsule.

Relations

Infraspinatus muscle is found on the dorsal surface of the scapula, arising lateral and inferior from the supraspinatus muscle and medial and superior to the teres major and minor. Due to their close anatomical relation and being separated only by the spine of scapula, infraspinatus and supraspinatus are considered as one functional unit.

The infraspinatus muscle fibers course towards the shoulder joint almost parallel to the teres minor and major, separated from them by a thick fascia. As it approaches the shoulder joint, the infraspinatus is sometimes separated from the neck of the scapula by the infraspinatus bursa, which may communicate with the shoulder joint cavity.

Innervation

Like the supraspinatus, infraspinatus is also innervated by the suprascapular nerve that arises from the upper trunk of the brachial plexus derived from roots C5 and C6.

Blood supply

Infraspinatus receives arterial blood supply from suprascapular and circumflex scapular arteries.

Function

Rotator cuff muscles form a functional musculotendinous unit that both stabilizes and produces movements on the shoulder joint. The function of this muscular apparatus is to provide a wide range of arm motion while keeping the head of humerus centralized within the glenoid cavity.

The tendons of rotator cuff muscles blend with the fibrous capsule of the glenohumeral joint and form a tendinous sleeve called the rotator capsule. Each muscle has its own pulling axis that results in a certain movement (prime mover), while altogether, via rotator capsule, they create a concavity compression. This is a stabilizing mechanism in which the rotator cuff compresses the humerus into the concavity of glenoid fossa, preventing its dislocation by translating forces. When the arm is abducted, infraspinatus works together with teres minor to oppose the upward pull of the deltoid muscle and to stabilize the humeral head against the glenoid fossa. This prevents the upward displacement of the humeral head as well as impingement on the coracoid process. Supraspinatus complements the this action of infraspinatus, however its precise role in this movement is controversial. 

Acting as a prime mover, infraspinatus is a powerful lateral rotator of the humerus. This action is important in preloading the upper extremity in backward extension and external rotation on the shoulder joint, seen in actions such as hitting overhead. The muscle is contracted eccentrically to smoothen the upper extremity during deceleration of powerful overhead movements.

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