Which muscle is an antagonist to the biceps brachii during shoulder flexion?

• A. Pectoralis major (lower fibers)
• B. Latissimus dorsi
• C. Deltoid
• D. Teres major

Answer: (A)

When the arm is flexed at the shoulder, the biceps brachii acts to lift the arm forward. An antagonist would be a muscle that pulls the humerus backward, opposing that forward motion. The lower fibers of the pectoralis major (the sternocostal portion) originate from the sternum and costal cartilages and insert on the humerus in a way that, when they contract, they extend the shoulder—especially from a flexed position. That backward pull counteracts the biceps’ action, making these fibers the best match as the antagonist during shoulder flexion. The other muscles listed don’t oppose shoulder flexion in the same way: some contribute to flexion themselves, while others have primary actions that don’t directly oppose this specific movement.