Final answer:
A synergist muscle assists the prime mover in performing a movement by adding force or stabilizing joints and bones, enhancing the efficiency and stability of the movement.
Step-by-step explanation:
The function of a synergist muscle is to assist the prime mover (or agonist) in performing a movement. This assistance can take the form of adding extra force to the movement or by reducing undesirable or unnecessary movement.
A synergist can also act as a fixator to stabilize the joints and bones that are the attachment points for the prime mover's origin. This enables the prime mover to work more efficiently and with greater stability, effectively enhancing the movement it is designed to execute. In contrast, the antagonist muscle has the opposite role, providing a controlling and braking effect to the movement caused by the prime mover.