Which muscle is primarily responsible for extending the shoulder?

The latissimus dorsi is primarily responsible for extending the shoulder due to its anatomical positioning and function. This large, flat muscle spans from the lower back to the upper arm and facilitates several movements at the shoulder joint. When the latissimus dorsi contracts, it pulls the upper arm backward, which is the action of shoulder extension.

In contrast to the primary role of the latissimus dorsi, the other muscles listed have different primary functions. The deltoid, for example, mostly facilitates shoulder abduction, flexion, and extension, but it is not solely responsible for shoulder extension. The rhomboid muscles primarily function to retract the scapula, and the trapezius is involved in a variety of movements including elevation, retraction, and rotation of the scapula but is not specifically a primary mover for shoulder extension. Therefore, the latissimus dorsi is the most significant muscle associated with this specific movement of extending the shoulder.