How do antagonist pairs relate to each other?

• A. They perform the same movement.
• B. They lengthen at different rates.
• C. They are opposite of one another.
• D. They both are prime movers.

Answer: (C)

Antagonist pairs are muscles that oppose each other across a joint. Their defining relationship is that they produce opposite actions: when one muscle shortens to create a movement, its partner lengthens and helps control or slow the motion, keeping the movement smooth and preventing overshoot. For example, when you bend the elbow, the biceps acts as the mover, while the triceps opposes that action; as the biceps contracts, the triceps lengthens to allow the bend and to help stop the motion at the end range.

The idea that fits best is that antagonists are opposite of one another. The other options miss this core relationship: muscles that produce the same movement are working together as agonists or synergists, not antagonists; the rate at which muscles lengthen isn’t what defines an antagonist; and prime movers are the muscles that initiate the action, not the opposing partner.