Which muscle is primarily involved in shoulder adduction?

The latissimus dorsi is primarily involved in shoulder adduction because it is one of the main muscles responsible for bringing the arm closer to the body from an abducted position. This muscle originates from the lower back and inserts on the humerus, allowing it to pull the arm down and back, effectively executing adduction. Its role in movements such as rowing, pulling, and other exercises targeting the upper body further highlights its importance in shoulder adduction.

While the deltoid, supraspinatus, and infraspinatus muscles play significant roles in shoulder movement, they are not primarily responsible for adduction. The deltoid is more involved in shoulder abduction, especially its lateral fibers, while the supraspinatus assists in initiating abduction. The infraspinatus mainly focuses on external rotation of the shoulder and stabilizing the shoulder joint rather than adduction. Thus, the predominant action of the latissimus dorsi in shoulder adduction makes it the correct answer.