What is the primary action of the anterior deltoid at the shoulder?

• A. Flexion and medial rotation of the humerus; assists in shoulder abduction after initial 15 degrees.
• B. Abduction of the shoulder.
• C. Extension and lateral rotation.
• D. Flexion of the elbow.

Answer: (A)

The main action of the anterior deltoid at the shoulder is to flex the arm and medially rotate the humerus, with a secondary role in aiding abduction once the arm has passed the initial lift. This matches the described action: flexion with medial rotation as the primary movements, plus a contribution to shoulder abduction after the first 15 degrees. Abduction is mainly performed by the middle deltoid and the supraspinatus in the early phase, and the anterior fibers only assist in lifting as the arm rises. The other choices describe actions outside this muscle’s primary role: extension and lateral rotation come from the posterior deltoid, pure abduction is primarily middle deltoid with supraspinatus, and elbow flexion is not a deltoid action.