Which term best describes the muscle that opposes the prime mover during movement?

The term that best describes the muscle that opposes the prime mover during movement is the antagonist. In a movement, the prime mover (or agonist) is the muscle primarily responsible for executing a specific action, such as flexion or extension. While the agonist contracts to facilitate movement, the antagonist acts to resist or counterbalance this movement. This opposition is crucial for controlling the action, allowing for smoother and more coordinated movements.

For instance, during a bicep curl, the biceps brachii is the prime mover, while the triceps brachii serves as the antagonist. When the biceps contract to lift the forearm, the triceps must relax to permit this motion. If the antagonistic muscle didn't function properly, movements could become jerky and uncontrolled, leading to injury.

Synergists, on the other hand, assist the prime mover and can help stabilize joints. Flexors are a specific type of muscle that decreases the angle between two body parts but don’t necessarily describe the opposing muscle function. Thus, antagonist is indeed the most accurate term in this context.