The subscapularis is the only muscle in the rotator cuff group that is responsible for internal rotation. Its origin is along the medial (axillary/inside) border on the anterior surface of the scapula. It inserts into the lesser tubercle on the front of the humerus, medial to the long head of the biceps brachii.

Internal rotation is the primary function of the subscapularis, but the muscle's contraction also adducts the arm (laterally lowers the arm).

Large degrees of external rotation can add unnecessary stress to the subscapularis, particularly at increasing degrees of abduction (laterally raising the arm).

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See also: supraspinatus, infraspinatus, teres minor.