A motor unit is defined as a motor neuron (or nerve cell) and all the muscle fibers it innervates or "recruits."[1][2] All the muscle fibers in a motor unit are the same type of muscle fiber.[3] 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.[3]

Once the demand for power reaches approximately 20% of maximal, the CNS stimulation is strong enough to recruit some Type IIa fibers. As the demand for force increases, then larger waves of excitation from the CNS eventually results in the recruitment of Type IIb fibers, whic have the highest threshold of excitation. To produce maximal force, the CNS produces the largest possible stimulation and all available motor units serving the muscle are recruited (all three types). It should be noted that only trained athletes are actually able to recruit all of the available motor units. This is one of the reasons for very fast gains in strength with novices to strength training, as they learn to recruit more existing fibers.


  1. University of Oregon (n.d.). Muscle Physiology. Retrieved on 2008-05-13.
  2. Staley, Charles (n.d.). Quality Strength for Human Athletic Performance: A Guide to Speed Strength Training. Retrieved on 2008-08-03.
  3. 3.0 3.1 Leyland, Tony (July 2008). "Human Power Output and Crossfit Metcon Workouts". CrossFit Journal (71).

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