Final answer:
During a biceps curl, the biceps brachii functions as the agonist, and the brachialis acts as a synergist, assisting in the movement of flexing the arm.
Step-by-step explanation:
During a biceps curl, the muscles of the biceps brachii and brachialis act as both agonists and synergists. The biceps brachii is the prime mover, or agonist, as it is the principal muscle responsible for flexing the lower arm. In the same action, the brachialis acts as a synergist to the biceps brachii, assisting in the motion of flexing the arm. Additionally, a synergist can serve as a fixator that stabilizes the origin of the prime mover's attachment, but in the case of the biceps curl, its role is more about assisting the flexing action rather than stabilization.