Final answer:
Pain during internal rotation of the shoulder is most likely caused by an issue with one of the rotator cuff muscles, specifically the subscapularis, which is responsible for this motion.
Step-by-step explanation:
The muscle affected with pain during internal rotation of the shoulder is likely one of the rotator cuff muscles. Specifically, the muscle called subscapularis is responsible for internal rotation of the arm at the shoulder. The rotator cuff is a group of four muscles, including the subscapularis, supraspinatus, infraspinatus, and teres minor, that stabilize the shoulder joint. These muscles and their tendons form a covering around the head of the humerus and are essential for various movements and maintaining shoulder stability. Pain during internal rotation could indicate an issue with the rotator cuff, such as inflammation, degeneration, or a tear, particularly in activities that involve repetitive shoulder motion like pitching in baseball.