What muscle is responsible for internal rotation of the shoulder?

The subscapularis muscle plays a crucial role in the internal rotation of the shoulder. It is one of the four rotator cuff muscles, located on the anterior (front) surface of the scapula. Its primary function is to medially rotate the humerus (the upper arm bone) at the shoulder joint.

In contrast, the other muscles listed, such as the supraspinatus, infraspinatus, and teres minor, are primarily involved in different shoulder movements. The supraspinatus assists with arm abduction, while the infraspinatus and teres minor are responsible for external rotation of the arm. This anatomical distinction is important for understanding shoulder mechanics and the specific roles of each muscle.