Which muscle is described as the chief flexor and chief extensor at the shoulder joint?

• A. Deltoid
• B. Supraspinatus
• C. Infraspinatus
• D. Teres minor

Answer: (A)

The deltoid is a large, multi‑part muscle that crosses the shoulder and has different actions in its fibers. The anterior (front) part of the deltoid flexes the arm at the shoulder, while the posterior (back) part extends the arm. Because these actions are produced by distinct portions of the same muscle, the deltoid can powerfully bring the arm forward and also pull it back, making it the primary muscle associated with both flexion and extension at the shoulder.

Other muscles mainly contribute to different actions. Supraspinatus initiates abduction, not flexion or extension. Infraspinatus and teres minor primarily rotate the arm laterally. Latissimus dorsi and teres major are strong extensors and adductors, but the deltoid provides the primary, broad capability for flexion and extension across the shoulder range when needed.