Which muscle is primarily responsible for shoulder abduction?

The deltoid muscle is primarily responsible for shoulder abduction due to its anatomical structure and placement around the shoulder joint. This muscle is shaped like an upside-down triangle and makes up the bulk of the shoulder. It has three distinct parts: the anterior (front), lateral (middle), and posterior (back) fibers. While the anterior and posterior fibers have roles in shoulder flexion and extension respectively, it is the lateral fibers that are most actively engaged in lifting the arm away from the body, which characterizes abduction.

The deltoid's effectiveness in facilitating abduction occurs primarily beyond the first 15 degrees of movement, as the supraspinatus muscle initiates this movement up to that point. Therefore, when considering all aspects of shoulder abduction, the deltoid is recognized as the primary mover during this motion.

In the context of the other muscles listed, the serratus anterior assists with the movement of the shoulder blade, which is important for full shoulder mobility but does not primarily accomplish shoulder abduction. The latissimus dorsi is involved in actions like shoulder extension, adduction, and internal rotation, while the trapezius supports shoulder elevation and scapular movement but is not the main muscle for abduction. Thus,