Which motion is associated with the subscapularis muscle?

The subscapularis muscle is primarily responsible for internal rotation of the humerus at the shoulder joint. This muscle is one of the four rotator cuff muscles, and its position and orientation allow it to effectively mediate internal rotation as it is located on the anterior surface of the scapula.

When the subscapularis contracts, it pulls the humeral head anteriorly and medially, facilitating the internal rotation movement, which is important for various activities that involve reaching across the body or rotating the arm inward.

This characteristic makes internal rotation the correct answer in the context of identifying the motion associated with the subscapularis muscle, clearly distinguishing it from the other options related to different shoulder movements.