Final answer:
The subscapularis muscle is responsible for medial (internal) rotation of the humerus, as part of the rotator cuff which stabilizes and mobilizes the shoulder.
Step-by-step explanation:
The subscapularis muscle can create medial (internal) rotation of the humerus. This muscle is an integral part of the rotator cuff, a group comprising the tendons of four muscles, including the subscapularis, which are vital for shoulder stability and movement. The subscapularis muscle originates on the anterior scapula and its action results in the rotation of the humerus, bringing the anterior surface of the limb towards the midline of the body. This motion is distinct from the superior and inferior rotations of the scapula itself, which involve the scapula rotating around a point inferior to the scapular spine with associated movements of the glenoid cavity up or down respectively.