Which muscle performs adduction at the shoulder joint?

The subscapularis is the correct choice for performing adduction at the shoulder joint. This muscle is one of the four rotator cuff muscles and is located on the anterior surface of the scapula. It plays a vital role in internal rotation of the shoulder, but it also contributes to the adduction of the arm when the shoulder is in a certain position. The adduction movement at the shoulder joint brings the arm closer to the body's midline, which is a function the subscapularis facilitates during various movements.

In contrast, the supraspinatus primarily initiates shoulder abduction, meaning it assists in moving the arm away from the body rather than bringing it closer. The biceps brachii, while it has some roles at the shoulder, mainly functions in elbow flexion and forearm supination. The infraspinatus is more involved in external rotation of the shoulder. Therefore, the subscapularis is specifically suited for the adduction motion at the shoulder joint, distinguishing it from the other options listed.