So we also have synergist muscles that are involved in aiding in aiding the agonist antagonist muscles. Now we can also refer to these muscles in a different way. We also have flexor muscles and extensor muscles. So the flexor muscle is the muscle that contracts and decreases the angle at the joint with respect to our moveable bone while the extenter is the muscle that lengthens and increases the angle at the joint. So in this particular case, our bicep is acting as our flexor because it is decreasing that angle with respect to the joint and our arm that is moving. In this case, the bicep is acting as the extensor because as we are extending our arm the angle basically increases.