The subscapularis muscle is the largest of the four rotator cuff muscles. It is responsible for internal rotation and adduction of the your arm at the shoulder. Most importantly, in conjunction with the other rotator cuff muscles, it is responsible for dynamic stabilization of the shoulder joint. The shoulder has less connective tissue structure than other major joints. The rotator cuff muscles act as a sort of dynamic muscle/ligament hybrid to stabilize the shoulder joint as you move. Of the important function of subscapularis is to provide dynamic restraint to the  back side of the shoulder joint.

Trigger points in your subscapularis cause pain in the back of the shoulder, and sometimes extending down the back of your arm. Referred pain can also develop on the back side of your hand, wrist, forearm.

Subscapularis trigger points are often involved with Frozen Shoulder Syndrome. They are frequently the foundation of an entire range of shoulder dysfunction.