Final answer:
The nerve supply for the subscapularis muscle, the only rotator cuff muscle that medially rotates the arm, comes from the upper and lower subscapular nerves.
Step-by-step explanation:
The nerve supply for the only rotator cuff muscle that medially rotates the arm is the upper and lower subscapular nerves. These nerves innervate the subscapularis muscle, which is responsible for medially rotating the arm. The subscapularis is one of the four muscles comprising the rotator cuff, also known as the musculotendinous cuff, which plays a vital role in maintaining the stability and movement of the shoulder joint. Baseball pitchers often experience injuries in this area due to repetitive motion and the stress of throwing fast pitches.