Neural Fatigue: A Short Investigation into Neural Fatigue During Exercise

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For anyone that has been an avid athlete or have just been involved in sports, we all know that fatigue is the killer of any great performance.  At this level, we have likely all experienced muscle fatigue (insufficient energy or high levels of muscle discomfort) and environmental fatigue (Work, school, sleep, and stress).  The next two factors of fatigue are Neuroendocrine and Neural fatigue.   Although these can potentially be felt at a lower level of activity, elite level athletes most likely have all had battles with one or the other.   Muscle fatigue is temporary and can be fixed quite quickly with proper rest and nutrition (3)  but neuroendocrine and neural fatigue tends to take much longer to recover from.  Before bodybuilding, as an elite level hockey player, I always wondered why I was never able to maintain consistency during times where I needed to be as consistent as possible. Question is, what causes neural fatigue?

The area of investigation, as commented on by Dr. Mark Davis (1), first states that there are many definitions of neural fatigue, but the best of which would be to define it as “an acute impairment of exercise performance that includes both an increase in the perceived effort necessary to exert a desired force or power output and the eventual inability to produce that force or power output” (1).

More studies have been done on muscle fatigue than neural fatigue, but in the little studies that have been done on this topic, a simple common theory is that ability to maintain central motor drive has more to do with willingness and motivation rather than afferent and efferent neurons working effectively (1).  Some disagree with this, however, and believe that the actual reduction associated with central nervous system drive is most likely associated with 2 factors; The first, being that there is a large reduction in descending impulses to the receiving motor neurons, and the second reason,  a lack of motoneuron excitability by neurally activated afferent feedback from the muscles (1).  This theory has been somewhat proven in a test done with monkeys, showing that between the 1st and 20th repetition, the motor neuron discharge depletes significantly (2).  More convincing evidence was show later in studies done with humans.

In conclusion, find these this research very interesting, because I can immediately rule out the first theory of lack of motivation.  I was trained as a goalie to be as focused as possible, and my fight to win was evident in every situation. What faltered wasn’t my mental state, but my body’s ability to continue, which would better explain the final two points stated above. This could potentially be why as a hockey player, we had immense difficulty peaking performance for playoffs, and continuing to stay at that peak condition for any extended period of time until we made it to the finals.  By then we quite possibly could have been in a stage of CNS fatigue.  I still evidently remember allowing the opposing team to score a goal, but actively thinking and trying to move my body, but to no avail,  which out of personal experience leads me to believe that the theory explaining the reduction of descending impulses the most accurate.

Denon Maximchuk

Natural Path Personal Training

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References:

  • Davis JM, Bailey SP, epartment of Exercise Science, School of Public Health, University of South Carolina, Columbia 29208, USA.

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Medicine and Science in Sports [1997, 29(1):45-57]

  • Maton, B. Central nervous changes in fatigue induced by local work. In: Muscle Fatigue: Biochemical and Physiological Aspects. G. Atlan, L. Beliveau, and P. Bouissou (Eds.). Paris: Masson, 1991, pp. 207-221.
  • Weiss, LW. (1991). The obtuse nature of muscular strength: The contribution of rest to its development and expression. Journal of Applied Sports Science Research. 5: 219-227.
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