Final answer:
The subscapularis muscle is the only rotator cuff muscle that medially rotates the arm at the shoulder joint. It is one of the four rotator cuff muscles that stabilizes the shoulder joint and allows for various movements.
Step-by-step explanation:
The only rotator cuff muscle that is a medial rotator of the arm is the subscapularis. The rotator cuff, also known as the musculotendinous cuff, includes four muscles: the supraspinatus, infraspinatus, teres minor, and the subscapularis. These muscles arise from the scapula and attach to the greater or lesser tubercles of the humerus.
The subscapularis is the only muscle among them that performs the role of medial (internal) rotation of the arm at the shoulder joint, drawing the limb towards the midline of the body. This contrasts with the primary actions of the other rotator cuff muscles: the supraspinatus assists with abduction, the infraspinatus and teres minor with lateral rotation of the arm. Overall, these dynamic ligaments provide the primary structural support for the shoulder joint.