Final answer:
The relationship between the pronator quadratus and supinator is described as antagonists, as they perform opposite actions. The pronator quadratus pronates the forearm, while the supinator supinates it. In contrast, synergists assist prime movers in muscle actions.
Step-by-step explanation:
The term that best describes the relationship between the pronator quadratus and supinator is antagonists. These two muscles perform opposite actions in the forearm. The pronator quadratus pronates the forearm, turning it posteriorly, whereas the supinator supinates the forearm, turning it anteriorly. This opposed action is typical of muscle antagonists, which work against each other to control motion and position.
A muscle that helps an agonist work is called a synergist. Synergists assist prime movers by performing, or helping to perform, the same motion. For example, the biceps brachii is a prime mover in forearm flexion, and the brachialis and brachoradialis are synergists to this motion. Fixators can also act as synergists by stabilizing the origin of the prime mover, ensuring effective movement.