Final answer:
The rotator cuff muscle responsible for internal rotation at the shoulder joint is the Subscapularis.
Step-by-step explanation:
The rotator cuff muscle that performs internal rotation at the shoulder joint is the Subscapularis. This muscle originates on the anterior surface of the scapula and inserts onto the lesser tubercle of the humerus. The Subscapularis is responsible for medially rotating the arm, distinguishing it from the other rotator cuff muscles, which include the Supraspinatus, Infraspinatus, and Teres Minor. The Supraspinatus mainly assists with abduction of the arm, whereas the Infraspinatus and Teres Minor primarily contribute to the lateral rotation of the arm.
As a group, the rotator cuff muscles are responsible for stabilizing the shoulder joint, by providing the "fine tuning" movements of the head of the humerus within the glenoid fossa. They are deeper muscles and are very active in the neuromuscular control of the shoulder complex during upper extremity movements.