Which muscle serves as the agonist for horizontal abduction of the shoulder?

• A. Pectoralis Major, Deltoid (Anterior)
• B. Latissimus Dorsi, Teres Major
• C. Trapezius, Levator Scapulae
• D. Deltoid (Posterior)

Answer: (D)

Horizontal abduction of the shoulder is produced primarily by the posterior portion of the deltoid. When the arm is raised to about 90 degrees in front of you, the fibers of the posterior deltoid line up to pull the upper arm backward and outward, moving it away from the midline in the horizontal plane. This makes it the prime mover for this action.

Other choices don’t fit as the main mover. The pectoralis major and the anterior deltoid are more involved in flexion and horizontal adduction of the shoulder. Latissimus dorsi and teres major are strong internal rotators and adductors, pulling the arm toward the body. The trapezius and levator scapulae mainly move the scapula and don’t directly drive horizontal abduction at the shoulder joint.