Which muscle is responsible for medial rotation of the arm at the shoulder?

The subscapularis muscle is primarily responsible for the medial rotation of the arm at the shoulder. This muscle is one of the four rotator cuff muscles and is situated on the anterior surface of the scapula. When it contracts, the subscapularis pulls the humeral head inward and downward, facilitating medial rotation. This motion is crucial during various activities that involve bringing the arm closer to the body, such as reaching across the chest or performing certain types of lifting movements.

In contrast, while other muscles listed may have roles in shoulder movement, they do not specialize in medial rotation the way the subscapularis does. The teres minor, for example, primarily assists in lateral rotation of the arm, and the pectoralis major contributes to various shoulder movements, including flexion and adduction, but not specifically medial rotation. The serratus anterior plays a pivotal role in the upward rotation of the scapula and stabilization in dynamic movements but does not have a direct influence on the medial rotation of the arm.