Which muscle is the prime mover for Shoulder Horizontal Abduction?

• A. Posterior Deltoid
• B. Anterior Deltoid
• C. Latissimus Dorsi
• D. Supraspinatus

Answer: (A)

Horizontal abduction involves moving the arm backward in the horizontal plane when the shoulder is flexed to about 90 degrees. The muscle best suited to pull the arm backward from that position is the posterior deltoid. Its fibers run along the back of the shoulder and attach near the humerus in a way that, when it contracts with the arm up at 90 degrees, it pulls the arm posteriorly, producing the backward, horizontal motion. It also helps externally rotate and stabilize the shoulder during this movement.

The anterior deltoid mainly facilitates flexion and horizontal adduction (moving the arm forward or across the chest), not backward. The latissimus dorsi tends to pull the arm inward and downward (adduction and internal rotation), which doesn't generate the backward pull required for horizontal abduction. The supraspinatus initiates abduction from rest but is not the primary mover for the horizontal, backward-directed motion at 90 degrees of flexion.