Final answer:
An antagonist is a muscle that opposes or reverses a particular movement. They work in opposition to the prime mover, or agonist, to control rapid movement and maintain body or limb position.
Step-by-step explanation:
A muscle that opposes, or reverses, a particular movement is called an antagonist. Antagonists play an important role in muscle function by maintaining body or limb position and controlling rapid movement. They work in opposition to the prime mover, or agonist, which is the principal muscle responsible for a movement.