Prime mover muscles are the muscles that initiate the source of movement during physical action. These muscles do not act alone. They act simultaneously with other muscles recruited to perform the specific action.
Muscles are defined within four groups: agonist, antagonist, synergist and fixator. Prime movers are the agonist muscles, and they are assisted by the synergistic muscles. The antagonistic muscles are the muscles that oppose the primer mover by slowing it down. The fixator muscles assist the movement of the other three groups by holding the bones associated with the muscle groups.
There are many muscles that serve as prime movers depending on the location and type of movement. It is also common for muscles to assume multiple roles, including the prime mover. Take the example of the arm movement that occurs on the stroke during cross-country skiing. In this motion, the bicep acts as the prime mover by lifting the ski pole. The tricep is relaxed in this movement and plays the role of the antagonist. Once the pole has been planted in the snow, the roles of the muscles are reversed. It is common for muscles to have different roles depending on whether the motion is static, concentric or eccentric.