The term for the muscle's prime mover in a joint action is what?

• A. Antagonist
• B. Agonist
• C. Synergist
• D. Fixator

Answer: (B)

The muscle that directly produces the main movement at a joint is the agonist. It’s the primary driver of the action. For example, when you bend the elbow, the biceps brachii contracts to bring the forearm up—that muscle acts as the agonist for elbow flexion. The antagonist is the muscle that opposes that movement, such as the triceps during elbow flexion, which lengthens to allow the bend. Other muscles called synergists help the action by adding force or guiding the motion, like the brachialis and brachioradialis aiding elbow flexion. A fixator stabilizes a proximal joint or segment to keep the movement efficient, such as stabilizing the shoulder area during arm actions.