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Infraspinatus Muscle Anatomy

Last updated on: January 3, 2025

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The infraspinatus muscle occupies the medial three quarters of the infraspinous fossa and is partly covered by the deltoid and trapezius. In addition to helping stabilize the glenohumeral joint, the infraspinatus is a powerful lateral rotator of the humerus.

Infraspinatus Muscle Anatomy

The infraspinatus lies deep to the deltoid and trapezius muscles. It shares borders with the supraspinatus superiorly, teres minor inferiorly, and the scapular spine superiorly. The muscle’s anatomical position makes it particularly vulnerable during posterior shoulder approaches.

See Also: Rotator Cuff of the Shoulder

Origin & Insertion

The infraspinatus muscle is a triangular muscle that originates from the infraspinous fossa and converges into a tendon that inserts into the greater tubercle of the humerus.

Innervation & Blood Supply

The suprascapular nerve (C5, C6) provides motor innervation to the infraspinatus muscle. The suprascapular artery, accompanied by circumflex scapular vessels, ensures adequate blood supply. Understanding these neurovascular relationships proves crucial during surgical procedures and rehabilitation protocols.

See Also: Glenohumeral Joint Anatomy

Origin	Infraspinous fossa of scapula
Insertion	Middle facet on greater tuberosity of humerus
Innervation	Suprascapular nerve (C5 and C6)
Blood Supply	Suprascapular and circumflex scapular arteries
Action	The main function of infraspinatus muscle is external rotation of the humerus, as well as stabilization of the glenohumeral

Functional Significance

The infraspinatus functions as a primary external rotator of the glenohumeral joint, contributing approximately 60% of external rotation strength. It also acts as a dynamic stabilizer of the shoulder joint, working synergistically with other rotator cuff muscles to maintain optimal humeral head positioning during arm movement.

Clinical Implications

Infraspinatus dysfunction commonly presents in overhead athletes and manual laborers. Weakness in external rotation, particularly when tested with the arm at the side, often indicates infraspinatus pathology. Clinicians should note that isolated infraspinatus tears occur in approximately 10% of rotator cuff injuries.

To test the infraspinatus muscle, the person flexes the elbow and adducts the arm. The arm is then laterally rotated against resistance. If acting normally, the muscle can be palpated inferior to the scapular spine. To test the function of the suprascapular nerve, which supplies the supraspinatus and infraspinatus, both muscles must be tested as described.