What is the action of the posterior deltoid?

• A. Extension and external rotation; assists in horizontal abduction.
• B. Flexion and internal rotation.
• C. Abduction.
• D. Extension and internal rotation.

Answer: (A)

The posterior deltoid, the rear portion of the deltoid muscle, mainly moves the arm backward and outward. Its actions are shoulder extension (pulling the arm back), external rotation (rotating the arm away from the body), and it assists with horizontal abduction when the arm is raised in front of you. This combination fits how the muscle fibers line up on the back of the shoulder to pull the humerus Posteriorly and rotate it outward, while also helping move the arm away from the midline in the horizontal plane. Flexion and internal rotation are driven more by the front part of the deltoid and other internal rotators, and pure abduction is mainly the job of the middle deltoid, so the description that matches extension with external rotation and a helper role in horizontal abduction is the best fit.