Infraspinatus

The infraspinatus is one of the four muscles of the rotator cuff. It is a thick triangular muscle that occupies the infraspinous fossa of the scapula and is the main external rotator of the shoulder².

Anatomy

The infraspinatus originates from the medial two-thirds of the infraspinous fossa of the scapula, and from the overlying infraspinous fascia that covers it. The fibers converge laterally to form a tendon that passes beneath the acromion and inserts onto the middle facet of the greater tuberosity of the humerus²³.

The infraspinatus tendon frequently fuses with the teres minor tendon near their musculotendinous junctions. This fusion is one of the most common anatomical variations of the rotator cuff².

The muscle is innervated by the suprascapular nerve, which arises from the upper trunk of the brachial plexus (C5-C6)³.

The infraspinatus has the second-largest anatomical footprint on the humerus among the rotator cuff muscles, after the subscapularis. This large insertion area allows for significant force transmission during external rotation³.

Function

The infraspinatus is the primary external (lateral) rotator of the shoulder. When it contracts, it rotates the humeral head outward, enabling movements such as throwing, reaching behind the back, and combing hair¹².

Synergists include the teres minor and the deltoid. Together with teres minor, the infraspinatus rotates the head of the humerus outward and assists in carrying the arm backward².

Additionally, the infraspinatus reinforces the capsule of the shoulder joint, providing dynamic stability to the glenohumeral joint. When the arm is fixed, the infraspinatus assists in adducting the inferior angle of the scapula (scaption)³.

Clinical Significance

Infraspinatus Tears

Like the supraspinatus, the infraspinatus is commonly involved in rotator cuff tears. The infraspinatus is the second-most frequently torn rotator cuff muscle after the supraspinatus. Tears often occur at the tendon insertion, where the tendon passes beneath the acromion³.

Nerve Entrapment

The suprascapular nerve can become entrapped as it passes through the supraspinatus and into the infraspinatus, causing suprascapular neuropathy. This condition is particularly common in overhead athletes like volleyball players and is estimated to account for 1-2% of all shoulder injuries in the general population³.

Atrophy

Infraspinatus atrophy can result from suprascapular nerve injury, chronic rotator cuff tears greater than 5 cm, or disuse. Weakness in external rotation is a key clinical finding suggesting infraspinatus pathology³.

Clinical Examination

The infraspinatus can be tested by having the patient perform external rotation against resistance with the elbow at the side and flexed to 90 degrees. Pain or weakness during this test indicates infraspinatus pathology³.

Variation

The infraspinatus is frequently fused with the teres minor, forming a combined infraspinatus-teres minor muscle. This fusion is considered a normal anatomical variation and does not typically affect function².

Evolutionary Perspective

The infraspinatus, along with the supraspinatus and parts of the subscapularis, is thought to have evolved from a primitive muscle sheet that connected the coracoid to the humerus in early vertebrates. During evolution, this muscle structure was displaced dorsally as pectoral muscles evolved into the pectoralis major while some fibers attached to the scapula became the rotator cuff muscles².