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. This cause the fibers in the bicep to actually stretch out and lengthen. So we have contraction elongation, elongation contraction. Now the muscle that contracts is known as the agonist.