This muscle is the muscle that contracts. In this case, this muscle is our antagonist, while this muscle, our contracting muscle, is our agonist. So basically the bicep can act as the agonist but it can also act as our antagonist. So in diagram A the somatic nervous system carries that electrical signal that was created by the brain and it brings it to our bicep muscle which causes the contraction of that muscle. So as this muscle contracts, it causes this muscle to basically stretch out and it elongates as seen in the following diagram. In diagram B, the tricep muscle fibers contract and move the radius and the ulna bone, the movable bones away from the body.