Our brain initiates an electrical signal, it passes through the motor neuron found in the somatic nervous system that actually innervates our bicep muscle. And that creates a contraction that initiates an action potential which creates the muscle contraction in the bicep. So as I'm pulling these two movable bones, my radius and my owner, towards my body, the bicep is increasing in thickness, it's contracting. And at the same time that the radius is decreasing between this bone and our hinge, our joint, our muscle in the back, our tricep muscle is actually elongating. It's stretching out, it's extending. And the muscle that contracts is known as our agonist, while the muscle that is elongating is known as our antagonist.