Final answer:
When the lesser tubercle of the humerus is visible in profile for the AP shoulder joint, it indicates the humerus is internally rotated. This medial rotation brings the anterior surface of the arm closer to the body's midline.
Step-by-step explanation:
If the lesser tubercle is in profile for the AP (anterior-posterior) shoulder joint, the humerus is in a position of internal rotation. This internal or medial rotation of the humerus occurs when the anterior surface of the arm moves toward the midline of the body. It is distinguished from external or lateral rotation, where the anterior surface moves away from the midline. The positioning of the tubercles is crucial for understanding shoulder anatomy as well as identifying potential dislocations or injuries that predominantly occur in an inferior direction due to the strong support around the humerus provided by muscles and ligaments.