Which muscle is a prime mover for shoulder extension?

The latissimus dorsi is the prime mover for shoulder extension primarily because it is a large, powerful muscle that spans a significant area of the back and attaches to the humerus, the bone of the upper arm. When the latissimus dorsi contracts, it pulls the arm downward and backward, effectively extending the shoulder joint. This action is particularly evident during movements such as pulling, reaching backward, or rowing.

While other muscles such as the triceps assist in extending the arm at the elbow joint, and the deltoid plays a role in shoulder movements, they are not the primary muscle responsible for shoulder extension itself. The deltoid muscle is generally more involved in shoulder abduction and flexion rather than pure extension. The gluteus maximus, while a powerful muscle for hip extension, does not play a significant role in shoulder extension.

Thus, the latissimus dorsi's unique positioning and anatomical structure make it the main muscle for shoulder extension, allowing it to perform this specific function effectively.