Which muscle is primarily involved in horizontal adduction?

The pectoralis major is primarily responsible for horizontal adduction of the arm. This muscle originates from the clavicle and sternum and inserts into the humerus. When the pectoralis major contracts, it pulls the arm across the body, facilitating the movement known as horizontal adduction. This action is commonly engaged in various exercises and functional movements such as bringing your arms together in front of your chest or performing a bench press.

The deltoid muscle, while also playing a role in shoulder movements, is more associated with shoulder abduction and flexion rather than horizontal adduction specifically. The latissimus dorsi primarily aids in shoulder extension, adduction, and internal rotation, and does not significantly contribute to horizontal adduction. The trapezius, which is located in the upper back, is primarily involved in movements of the scapula and does not directly affect horizontal adduction of the arm. Thus, the pectoralis major is the clear choice as the muscle that most directly performs horizontal adduction.