Which muscles are primary abductors of the shoulder?

• A. Supraspinatus, Deltoid
• B. Pectoralis Major, Latissimus Dorsi, Triceps Brachii, Infraspinatus, Teres Minor
• C. Infraspinatus and Teres Minor
• D. Subscapularis

Answer: (A)

Abduction of the shoulder is driven mainly by two muscles acting together. The supraspinatus sits at the top of the shoulder blade and starts the movement, lifting the arm away from the body for the first roughly 15 degrees. After that initial phase, the deltoid, especially its middle fibers, provides the bulk of the force to lift the arm through the rest of the range. The other muscles listed don’t serve as primary abductors: pectoralis major and latissimus dorsi are strong adductors and internal rotators; teres major duplicates that action; infraspinatus and teres minor mainly external rotate and stabilize, with only minor direct contribution to abduction. Subscapularis is an internal rotator. So the combination of supraspinatus and deltoid best explains the primary abductors.