Which movement is primarily tested by the deltoid muscle?

• A. Elbow flexion
• B. Wrist extension
• C. Shoulder abduction
• D. Finger extension

Answer: (C)

The deltoid is best known for lifting the arm away from the body, i.e., shoulder abduction. Its middle fibers are the main movers in that action, especially once the arm has moved past the initial few degrees. The first portion of abduction up to about 15 degrees is primarily handled by the supraspinatus, but the deltoid takes over to raise the arm further, up to and beyond 90 degrees. While parts of the deltoid can assist with other shoulder movements depending on how the muscle fibers contract, the primary role of this muscle in movement testing is shoulder abduction.

Elbow flexion involves forearm muscles like the biceps and brachialis, not the deltoid. Wrist extension is carried out by the forearm extensors, and finger extension by the extensor muscles of the hand. So the movement most characteristic of the deltoid is lifting the arm away from the body.