The muscle that serves as the prime mover during a movement is called the

Agonist

Explanation:

The muscle that serves as a prime mover during a movement is called "agonist". Agonist refer to the leader muscles that contract and cause the movement.  

For example, the deltoid muscle is the prime mover or agonist. Contraction of deltoid muscle causes abduction of the arm. An agonist for one type of movement may serve as antagonist or fixator for some other movement.

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