Final answer:
The subscapularis muscle is the shoulder muscle in question; it is one of the four rotator cuff muscles that provide primary support for the glenohumeral joint.
Step-by-step explanation:
The muscle of the shoulder in question is the subscapularis muscle. It originates on the anterior scapula and performs the function of medially rotating the arm. Moreover, shoulder muscles that originate on the scapula also include the supraspinatus, infraspinatus, teres major, teres minor, and coracobrachialis, each contributing to various movements of the arm. Critically important to the shoulder's function and stability are the four rotator cuff muscles: supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles provide primary support for the glenohumeral joint by attaching to the humerus and encapsulating it with their tendons, which then fuse to the joint capsule forming the rotator cuff.