Subscapularis arises from the medial four-fifths of the costal surface of the scapula, from the intermuscular septa which raise bony ridges on this surface and from the concave axillary border of the scapula up to the infra-glenoid tubercle. The multipennate muscle converges over the bare area at the glenoid angle of the scapula, its musculotendinous fibers sliding over this bone and separated from it by a bursa. This subscapular bursa extends laterally and communicates with the cavity of the shoulder joint through a gap in the anterior part of its capsule. Lateral to this the tendon fuses with the capsule of the shoulder joint and is inserted into the lesser tuberosity of the humerus and the medial lip of its intertubercular groove for half an inch below. The muscle is covered by a dense fascia which is attached to the scapula at the margins of its origin.
Nerve supply of subscapularis:
It is supplied by the upper and lower subscapular nerves (C6 and 7) from the posterior cord of the brachial plexus.
Action of subscapularis:
With the other short scapular muscles the subscapularis gives stability to the shoulder joint, assisting in fixation of the upper end of the humerus during movements of elbow, wrist, and hand. Acting as a prime mover, it is a medial rotator of the humerus. There is no satisfactory test for the muscle, since its action is difficult to differentiate from other medial rotators.