Which muscle primarily abducts the shoulder after the initial 15 degrees?

• A. Supraspinatus
• B. Middle fibers of the deltoid
• C. Infraspinatus
• D. Teres minor

Answer: (B)

Abduction of the shoulder starts with a quick lift by the supraspinatus for the first roughly 15 degrees, acting to initiate the motion and stabilize the humeral head. After that initial portion, the middle fibers of the deltoid become the primary abductors. Their lateral pull on the humerus provides the strongest, most consistent lifting force as the arm continues to move away from the body, making them the main driver of abduction beyond that initial 15 degrees. The other muscles listed don’t take on the principal role in this range: infraspinatus and teres minor mainly rotate the arm laterally and stabilize the joint, and while supraspinatus assists early on, the dominant ongoing abductor is the middle deltoid.