So let's take a look at diagram A and diagram B. So let's suppose I have my arm oriented as shown and I wish to pull it, so I wish to flex my arm and pull it towards my body. So basically what's happening is the central nervous system is creating an electrical signal. 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.