Which muscle is primarily responsible for shoulder abduction?

The deltoid muscle is primarily responsible for shoulder abduction, making it the correct choice for this question. The deltoid is a large, triangular muscle covering the shoulder joint and comprises three distinct parts: anterior, lateral, and posterior fibers. Among these, the lateral fibers are primarily engaged during the abduction of the arm, which is the movement of raising the arm laterally away from the body.

While other muscles, such as the supraspinatus, do assist in this motion, the deltoid is the main muscle that provides the strength required for raising the arm to the side, especially beyond the initial 15 degrees of abduction where the supraspinatus is more contributory. The pectoralis major and latissimus dorsi are involved in different shoulder movements; the pectoralis major aids in flexing and rotating the shoulder, and the latissimus dorsi primarily contributes to shoulder extension and adduction. Thus, while these muscles play significant roles in shoulder movement, they are not the primary muscles for abduction.