What is an antagonist?

• A. Muscle that does most of the work
• B. Muscle that helps the prime mover in a movement
• C. Muscle that opposes or reverses a prime mover
• D. Muscle that stabilizes joints

Answer: (C)

Antagonists are muscles that oppose or reverse the action of the muscle that’s primarily producing the movement. This opposite action helps control, slow, and stabilize motion for smooth, coordinated movement. For example, when the elbow is flexed by the biceps (the prime mover), the triceps serves as the antagonist to extend or slow that motion. Antagonists can also contract eccentrically to decelerate a limb and provide braking. The other roles described—doing most of the work, assisting the prime mover, or stabilizing joints—fit other functional categories, so the term that best fits is the opposing muscle.