Which muscle is primarily responsible for shoulder abduction?

• A. Deltoid
• B. Latissimus dorsi
• C. Triceps brachii
• D. Pectoralis major

Answer: (A)

Shoulder abduction is carried out mainly by the deltoid muscle, especially its middle fibers, because they are best positioned to lift the humerus away from the body through a wide range of motion. The supraspinatus helps at the very start, initiating the first 15 degrees of lift, but once that initial phase is over, the deltoid does the bulk of the work to raise the arm.

Other muscles don’t serve as the primary abductors. Latissimus dorsi mainly pulls the arm downward, inward, and backward (adduction/extension), so it doesn’t drive abduction. Triceps brachii is primarily an elbow extensor, with the long head capable of assisting shoulder actions like extension, but not abduction. Pectoralis major adducts and medially rotates the arm and can flex it, but it does not abduce the shoulder.