Final answer:
The term for a muscle that causes movement of a limb in the desired direction is an agonist, which is the principal muscle or prime mover involved in the action.
Step-by-step explanation:
A muscle (i.e., prime mover) that results in movement of a limb in the desired direction is labeled as an agonist. The prime mover or agonist is the principal muscle involved in performing a specific movement. To illustrate, when lifting a cup, the prime mover is the biceps brachii. In this action, the brachialis acts as a synergist, helping the prime mover. A synergist can also serve as a fixator to stabilize the bone to which the prime mover's origin is attached. In contrast, a muscle performing the opposite action of the prime mover is known as an antagonist, and it helps maintain body or limb position and control rapid movements.