Subscapularis

The subscapularis is a large, triangular muscle on the anterior surface of the scapula, occupying the subscapular fossa. It forms the anterior portion of the rotator cuff and contributes substantially to the shoulder’s dynamic stability. The muscle fibers converge to a tendon that inserts primarily on the lesser tubercle of the humerus, with some fibers blending into the anterior capsule of the glenohumeral joint.

The subscapularis is innervated by the upper and lower subscapular nerves, which arise from the posterior cord of the brachial plexus (typically C5–C6). Its blood supply is chiefly from the subscapular branch of the axillary artery, with contributions from nearby arterial sources.

Functionally, the subscapularis medially rotates the humerus and assists in adduction. It also plays a key role in stabilizing the glenohumeral joint by compressing the humeral head into the glenoid, helping to prevent anterior dislocation and to centralize the head during arm movements. The muscle works together with other rotator cuff muscles to control shoulder rotation and transfer loads during overhead activities.

Clinical relevance includes subscapularis tears, which can be isolated or occur with other rotator cuff injuries. Tears cause weakness in internal rotation and anterior shoulder pain, and may be associated with biceps tendon pathology due to shared anatomic proximity. Diagnosis relies on clinical examination and imaging, with MRI commonly used to assess tendon integrity. Treatment ranges from conservative management to surgical repair, depending on severity and functional impact.