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.