What muscle is primarily involved in shoulder adduction?

The pectoralis major is primarily responsible for shoulder adduction because it is a large muscle that spans across the chest and attaches to the humerus (the upper arm bone). When this muscle contracts, it pulls the arm toward the body's midline, effectively performing the movement known as adduction. Additionally, the pectoralis major is involved in various arm movements, including flexion and internal rotation, particularly when the arm is raised in front of the body. This role is crucial in many activities, such as pushing, pulling, or lifting objects.

The other muscles mentioned, while they do have functions related to the shoulder and arm, do not primarily facilitate adduction. For instance, the latissimus dorsi primarily acts in shoulder extension and internal rotation; the deltoid muscle is more associated with shoulder abduction and flexion rather than adduction; and while the trapezius plays a role in moving and stabilizing the shoulder girdle, it does not primarily contribute to the adduction of the arm. Therefore, the pectoralis major stands out as the key muscle involved in shoulder adduction.