Final answer:
Muscles that contract concurrently with the prime mover are known as synergists. They assist the prime mover, adding force and stabilization, which supports the intended movement.
Step-by-step explanation:
The muscles that contract at the same time as the prime mover are known as synergists. These muscles assist the prime mover in performing a movement by adding extra force or by reducing undesirable movements. For instance, when lifting a cup, the biceps brachii is the prime mover for this action, while the muscle called brachialis acts as the synergist. Moreover, some synergists also play the role of fixators, which stabilize the origin bone of the prime mover, enhancing the efficiency of the prime mover's action.
It's important to understand the different roles muscles play during movement: agonists are the principal muscles responsible for movement; antagonists have the opposite action and often counteract or control the movement; synergists support the agonists; and fixators provide stability.
Therefore, the final answer to this question, in a two line explanation, is that muscles that connect at the same time as the prime mover are called synergist muscles.