A motor unit is defined as a motor neuron (or nerve cell) and all the muscle fibers it innervates or "recruits." All the muscle fibers in a motor unit are the same type of muscle fiber. The cell bodies of Type I neurons have a lower threshold of excitation, which means that if an activity has a low demand for power, only slow-twitch fibers will be stimulated. If the need for force and power development increases, higher numbers of Type I motor units will be recruited by increasingly larger waves of excitation by the central nervous system (CNS). Eventually, all of the Type I fibers will become involved. This would be at the point where the athlete is reaching maximal aerobic capacity.