The subscapularis is the rotator cuff muscle responsible for internal rotation of the shoulder. It is the only rotator cuff muscle located on the front, or costal surface, of the scapula.
The subscapularis fills the subscapular fossa and attaches to the lesser tubercle of the humerus. When it contracts, it turns the upper arm inward and also helps adduct the arm and stabilize the humeral head in the shallow glenoid socket.
A useful memory aid for the four rotator cuff muscles is SITS: supraspinatus, infraspinatus, teres minor, and subscapularis. The infraspinatus and teres minor primarily produce external rotation, which makes them common distractors in this question. The word “small” is slightly misleading because the subscapularis is a broad, powerful muscle rather than the smallest cuff muscle.
Clinicians assess its tendon with tests such as the lift-off, belly-press, and bear-hug tests. Its upper and lower subscapular nerves arise from spinal nerve roots C5 and C6.