Which muscle is primarily responsible for flexion of the shoulder?

The pectoralis major is primarily responsible for flexion of the shoulder due to its anatomic positioning and functional capabilities. This muscle is located in the anterior chest and spans the shoulder joint, originating from the clavicle, sternum, and the cartilage of the first six ribs, and inserting into the humerus. When the pectoralis major contracts, it draws the arm forward and upward, effectively performing shoulder flexion.

The other muscles listed have roles in shoulder movements but are not the primary movers for shoulder flexion. The deltoid muscle, specifically its anterior fibers, does assist in shoulder flexion but is not the sole or primary muscle responsible for this action. The latissimus dorsi primarily functions in shoulder extension, adduction, and internal rotation, whereas the trapezius is more involved in scapular movement rather than directly facilitating shoulder flexion. Thus, the pectoralis major is recognized for its significant contribution to this specific range of motion.