Which muscle is primarily responsible for extension of the shoulder?

The latissimus dorsi is the muscle primarily responsible for the extension of the shoulder joint. This broad, flat muscle originates from the lower back and extends to the humerus, which is the bone of the upper arm. When the latissimus dorsi contracts, it pulls the arm backward, effectively increasing the angle between the arm and the trunk. This motion is a fundamental action in various activities, such as rowing or pulling movements.

While the other muscles listed have roles in shoulder movement, they do not primarily facilitate extension. The pectoralis major, for instance, is more involved in flexion, adduction, and internal rotation of the shoulder rather than extension. The deltoid muscle, specifically its posterior fibers, does contribute to shoulder extension, but its overall function is more about raising the arm in various directions, including abduction and flexion. The trapezius aids in scapular movement and stabilization but does not directly extend the shoulder joint. Therefore, the latissimus dorsi stands out as the main muscle performing shoulder extension.