Which muscle is primarily responsible for shoulder abduction?

The deltoid muscle is primarily responsible for shoulder abduction due to its anatomical position and muscle fibers' orientation. The deltoid is a large, triangular muscle that covers the shoulder joint and consists of three distinct parts: the anterior, lateral, and posterior fibers. Among these, the lateral fibers play a crucial role in raising the arm away from the body, effectively facilitating abduction.

During the initial phase of shoulder abduction, the supraspinatus (another shoulder muscle) assists in initiating the movement, but its contribution is limited to the first 15 degrees of abduction. Beyond this range, the deltoid takes over to perform the action more effectively.

The other muscles mentioned, such as infraspinatus and latissimus dorsi, have different primary functions. The infraspinatus is primarily involved in external rotation of the shoulder, while the latissimus dorsi is responsible for actions such as shoulder adduction and extension, not abduction. This specialization of muscle function helps to understand why the deltoid is regarded as the main muscle for shoulder abduction.