Which muscle initiates shoulder abduction?

• A. Supraspinatus
• B. Latissimus Dorsi
• C. Serratus Anterior
• D. Frontalis

Answer: (A)

The initial lift of the arm away from the body is driven by the supraspinatus. This small rotator cuff muscle sits above the shoulder joint and its fibers attach to the humeral head to pull it up and away from the torso at the very start of abduction, roughly the first 10–15 degrees. This action also helps stabilize the humeral head in the glenoid as the arm begins to elevate, setting the stage for the deltoid to take over as abduction continues.

Latissimus dorsi, by contrast, is more associated with pulling the arm downward and backward (adduction and extension), not initiating abduction. Serratus anterior primarily protracts and upwardly rotates the scapula, aiding overhead motion but not the start of abduction itself. Frontalis has no role in shoulder movement.