What best defines an agonist muscle in a movement?

• A. The muscle doing the work (contracting) to create movement
• B. The muscle relaxing to allow movement
• C. The muscle stabilizing the joint during movement
• D. The muscle that attaches to bone but does not move

Answer: (A)

The movement is produced by the muscle that actively shortens to create the action. When you bend the elbow, for instance, the biceps contracts and shortens to pull the forearm up, so it’s the muscle driving that motion. The opposing muscle lengthens or relaxes to allow the movement, which is why it’s called the antagonist. Muscles around the joint may also work as stabilizers, keeping the joint steady during the action. A muscle that just attaches to the bone but doesn’t move isn’t contributing to the movement, so it isn’t the agonist.