Final answer:
The triceps brachii is the antagonist to the biceps because it extends the elbow. Muscles work in pairs with one flexing (biceps) and the other extending (triceps) for smooth limb movement.
Step-by-step explanation:
If the biceps brachii causes flexion of the elbow, then the triceps brachii, which causes extension of the elbow, would be considered an antagonist to the biceps. Our muscles operate in pairs, so while the biceps contract to flex the elbow, the triceps extend the elbow when they contract. This cooperative action allows for the smooth movement of our limbs. An example that illustrates this relationship is the act of bending and straightening the elbow when performing exercises such as bicep curls or tricep extensions.
The muscles in our arms also typically exert much larger forces within the body than the limbs apply to the outside world, due to the fact that the muscles are attached to bones via tendons near joints, providing a mechanical advantage that results in an input force greater than the output force.