Which muscle is primarily responsible for shoulder abduction?

The medial deltoid muscle is primarily responsible for shoulder abduction. This muscle is located on the outer aspect of the shoulder and plays a crucial role in lifting the arm away from the body. It is anatomically designed for this motion, as the fibers of the medial deltoid are positioned to effectively produce this movement.

When the arm is raised to the side, the medial deltoid contracts to facilitate the movement, usually initiating around 15 to 30 degrees of shoulder abduction when the supraspinatus, a rotator cuff muscle, assists in the process. As the arm continues to elevate, the deltoid maintains its role in controlling the movement throughout its range.

The pectoralis minor contributes more to movements such as scapular protraction and depression, rather than shoulder abduction. The superior trapezius is primarily involved in shoulder elevation and neck extension, while the latissimus dorsi mainly functions in shoulder adduction, extension, and internal rotation. Thus, the function of these muscles is distinct and does not include the primary action of shoulder abduction provided by the medial deltoid.