Which muscle is primarily responsible for shoulder internal rotation?

The subscapularis muscle is primarily responsible for shoulder internal rotation because it is located on the anterior surface of the scapula and has a direct line of action to rotate the humerus towards the body's midline. Its positioning allows it to effectively pull the greater tubercle of the humerus backward and medially, resulting in internal rotation of the shoulder joint.

The other muscles listed—such as the infraspinatus and teres minor—are actually more involved in external rotation of the shoulder. The supraspinatus primarily assists with shoulder abduction rather than directly influencing rotation. Therefore, when considering the specific action of internal rotation, the subscapularis stands out as the key muscle due to its anatomical and functional characteristics.