Final answer:
Synergists are the muscles that assist the primary movers, or agonists, in an action. The brachialis is an example of a synergist when it aids the biceps brachii in lifting a cup.
Step-by-step explanation:
The muscles that assist the primary movers (prime movers or agonists) during an action are known as synergists. An example of this is the action of lifting a cup, where the biceps brachii serves as the prime mover, while the brachialis, which is located deep to the biceps, assists as a synergist muscle. Synergists can also act as fixators; for instance, when the brachialis stabilizes the bones to which the biceps are attached, ensuring a smooth movement.
It's important to note that a muscle performing the opposite action of a prime mover is termed an antagonist. Antagonists are essential for maintaining body positions and controlling rapid movements, such as in the act of balancing the arm or during quick reflex actions.