The subscapularis is the largest and strongest of the four rotator cuff muscles, located on the anterior surface of the scapula between the bone and the rib cage, and is the only cuff muscle that internally rotates the arm rather than externally rotating it. Because it faces anteriorly, it acts as the primary dynamic stabiliser against anterior shoulder instability, forming a critical component of the restraint system that prevents the humeral head from dislocating forward. Subscapularis tears, while less common than supraspinatus tears, represent a significant surgical challenge because of the tendon's anterior position and broad attachment.
| Origin | Subscapular fossa, the anterior surface of the scapula that faces the rib cage |
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| Insertion | Lesser tubercle of the humerus; Medial lip of the intertubercular groove via the subscapular tendon |
| Nerve Supply | Upper and lower subscapular nerves (C5, C6, C7) |
| Blood Supply | Subscapular artery |
| Actions | Internal rotation of the arm, the most powerful internal rotator of the rotator cuff; Adduction of the arm; Resists anterior glenohumeral subluxation, the primary anterior stabiliser of the joint |
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In overhead athletes, the subscapularis must eccentrically control external rotation during the cocking phase and then concentrically drive internal rotation during acceleration, making it one of the most heavily loaded muscles in throwing sports, fatigue or tearing directly impairs pitch velocity and accuracy.
Subscapularis tears are often missed clinically because they are not easily visualised with standard shoulder views and require high-quality ultrasound or MRI for diagnosis. The lift-off test, belly press test, and Napoleon sign are the primary clinical assessments for subscapularis integrity. Complete subscapularis tears allow the long head biceps tendon to dislocate medially out of its groove, so medial biceps instability on ultrasound is a reliable indirect sign of subscapularis failure.
The subscapularis is not directly palpable through normal tissue, but the lesser tubercle attachment site is accessible on the anterior humerus with the arm in external rotation, tenderness here during resisted internal rotation suggests subscapularis involvement.
A partial or complete tear of the subscapularis tendon, most commonly at the superior portion at the lesser tubercle, producing anterosuperior shoulder pain, weakness in resisted internal rotation, and often medial displacement of the long head biceps tendon.
Degenerative overuse of the subscapularis from repetitive internal rotation loading in throwing athletes, producing anterior shoulder pain and reduced internal rotation strength that mimics anterior impingement but is characteristically worse with resisted internal rotation tests.
