In terms of horizontal adduction of the shoulder, which muscle acts as the prime mover?

The pectoralis major is the prime mover for horizontal adduction of the shoulder due to its anatomical positioning and muscle fiber arrangement. This muscle has a broad origin on the clavicle and sternum, allowing it to effectively pull the arm across the body when it contracts. It is particularly active in movements that bring the arm closer to the midline, characteristic of horizontal adduction, such as during certain phases of lifting or reaching movements.

Additionally, the pectoralis major is primarily responsible for actions that involve bringing the arm forward and across the body, reinforcing its role in horizontal adduction. It works efficiently in conjunction with other shoulder muscles but stands out as the primary mover for this specific motion.

While the latissimus dorsi, deltoid, and teres major contribute to shoulder movements, they do not primarily specialize in horizontal adduction. The latissimus dorsi primarily extends, adducts, and medially rotates the shoulder, the deltoid has multiple roles depending on its fibers but is more involved in abduction and flexion, and teres major also aids in shoulder adduction and medial rotation but is not the primary player for horizontal adduction.