Final answer:
A muscle that assists the agonist is called a d. synergist. The synergist aids the main muscle involved in a movement, either by adding force or by acting as a fixator to stabilize the movement's origin.
Step-by-step explanation:
A muscle that assists the agonist is known as a d. synergist. The agonist, or prime mover, is the main muscle responsible for a specific movement. For example, the biceps brachii acts as the prime mover when lifting a cup, while the brachialis assists the biceps brachii, thus functioning as a synergist.
Synergists play a critical role in supporting the prime mover by adding extra force or by reducing unnecessary movement. Additionally, a synergist can act as a fixator, stabilizing the origin of the prime mover to provide a solid base from which it can act.
An example of synergists at work is during the flexion of the lower arm, where the brachialis and brachoradialis act as synergists to the biceps brachii. In contrast to synergists, an antagonist muscle has the opposite action to the prime mover, helping to maintain limb position and controlling rapid movement.