Which muscle is primarily responsible for abducting the arm?

The deltoid muscle is the primary abductor of the arm, particularly during the first 90 degrees of arm elevation. It is a thick, triangular muscle that covers the shoulder joint and is composed of three distinct parts: the anterior (front), lateral (middle), and posterior (back) fibers. Among these, the lateral fibers are specifically responsible for arm abduction.

When the deltoid contracts, it pulls the humerus away from the body, facilitating movements such as raising the arm sideways. This action is critical in various activities, from reaching and lifting to throwing and swimming, illustrating the deltoid’s significant role in upper limb mobility and functionality.

The other muscles listed serve different primary functions. The triceps are primarily involved in extending the elbow, while the biceps are mainly responsible for flexing the elbow and supinating the forearm. The latissimus dorsi aids in movements such as extension, adduction, and internal rotation of the shoulder, but does not primarily abduct the arm.