What is an agonist muscle?

• A. The muscle that contracts and causes movement
• B. The muscle that relaxes to allow movement
• C. A muscle that stabilizes joints
• D. A muscle that lengthens

Answer: (A)

An agonist muscle is the muscle that contracts to produce the main movement—the prime mover for that action. When you bend your elbow, for instance, the biceps brachii contracts to bring the forearm up toward the shoulder, so it serves as the agonist for that flexion. The muscle on the opposite side, the triceps, lengthens and controls the movement, acting as the antagonist to allow smooth motion. Other muscles in the region may stabilize the joint or refine the movement, but the defining idea is that the agonist is the contracting muscle driving the action.