Which muscle forms the rounded contour of the shoulder and abducts the arm?

• A. Supraspinatus
• B. Deltoid
• C. Biceps Brachii
• D. Infraspinatus

Answer: (B)

The deltoid muscle forms the rounded contour of the shoulder and is the main muscle that lifts the arm away from the body. Its middle fibers are especially responsible for abduction, moving the arm outward from about 15 degrees up to the side. The deltoid sits over the shoulder joint, creating that smooth, rounded silhouette therapists often notice. While another muscle (the supraspinatus) helps start abduction in the first 0–15 degrees, the bulk of the lifting and the recognizable shoulder contour come from the deltoid. The other muscles listed have different roles—one assists with the initial start of abduction but not with shaping the shoulder, another rotates the arm, and the biceps brachii mainly flexes the elbow and contributes less to shoulder abduction.