Which muscle initiates movement and is the primary mover?

• A. Antagonist
• B. Synergist
• C. Agonist (prime mover)
• D. Fixator

Answer: (C)

The muscle that initiates movement and acts as the primary mover is the agonist, or prime mover. It is the muscle whose contraction provides the main force for a specific joint action. For example, during elbow flexion, the biceps brachii contracts to produce the primary bending of the elbow, making it the agonist in that action. The antagonist is the muscle that would produce the opposite movement, and it must lengthen or relax to allow the action to occur. Synergists assist by adding extra force or guiding the movement, while fixators stabilize the nearby joints or the area of origin to provide a solid base for the agonist’s work.