Final answer:
The muscle with the primary responsibility for causing a particular movement when several muscles are contracting is called the prime mover, or agonist.
Step-by-step explanation:
When several muscles are contracting at the same time, the muscle that has the major responsibility for causing a particular movement is called the prime mover, or agonist. Muscles that assist the prime mover are known as synergists, and they can also act as fixators to stabilize the bone that serves as the attachment point for the prime mover's origin. The muscle with the opposing action to the prime mover is known as the antagonist. For example, when lifting a cup, the biceps brachii acts as the prime mover with the brachialis serving as a synergist in this action.