Which of the following muscles is primarily responsible for adduction of the shoulder?

The primary muscle responsible for adduction of the shoulder is the latissimus dorsi. This broad, flat muscle spans from the lower back to the upper arm and plays a key role in various movements of the shoulder joint. During adduction, it pulls the arm closer to the body's midline and is especially engaged during activities like pulling and lifting. Its positioning and fiber orientation make it effective for this action.

While the other muscles listed also have roles in shoulder movements, they do not primarily focus on adduction. The deltoid mainly assists in arm abduction, the pectoralis major can assist with adduction but is more known for flexion and medial rotation, and the trapezius primarily functions in scapular elevation and rotation rather than directly contributing to adduction of the arm. Understanding the specific roles of these muscles helps clarify why the latissimus dorsi is recognized as the primary muscle for shoulder adduction.