Which of the following muscles causes movement at a joint in the direction opposite to that of its agonist?

The correct answer is the antagonist. Antagonist muscles play a crucial role in joint movement by opposing the action of the agonist, which is the primary muscle responsible for generating a specific movement. When the agonist contracts to produce a movement, the antagonist must relax and lengthen to allow that movement to occur smoothly. For example, during a bicep curl, the biceps brachii serves as the agonist, contracting to flex the elbow, while the triceps act as the antagonist, relaxing to enable that action. This opposing function is essential for joint stability and coordination of movement, ensuring that motions are controlled and preventing injury.

The other options reflect different roles in muscle movement but do not describe this opposing action. Prime movers refer to the main muscles responsible for a specific movement, while synergists assist the prime movers by stabilizing joints and providing additional support. Flexors are a specific category of muscles that decrease the angle between body parts, but they do not inherently define the relationship between opposing muscle actions.