Since the very beginnings of exercise physiology in the 1920s, it has been taught that fatigue during exercise occurs because something goes wrong in the exercising muscles.
Either the muscles run out of oxygen so that they become “anaerobic”, producing poisonous lactic acid, or else they run out of fuel, especially glycogen (the form of carbohydrate stored in the muscle), during more prolonged exercise.
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As a result, the fatigued muscles are unable to work properly, leading to what we have called a “catastrophic” failure of muscle function.
What was not immediately clear was why fatigue should be associated with such unpleasant feelings of discomfort and emotional distress. If the muscles have indeed suffered a catastrophic failure, then they should simply just stop working. Why the need to make the event so terribly uncomfortable, especially if it is during a Comrades Marathon? If the muscles truly are exhausted, then no amount of discomfort will start them working again.
Understanding the true nature of fatigue
For the past 20 years we have been working to understand the true nature of fatigue during exercise. Over the past two years it has become apparent that these traditional ideas are quite incorrect. We now believe that the brain regulates the exercise performance specifically to ensure that the "catastrophic failure" does not occur.
Thus the brain will almost always terminate the exercise before the "catastrophe" occurs. And it uses the sensations of fatigue and discomfort to ensure that the athlete does not overdo it. The nearer to the catastrophe you push your body, the more severe will be the symptoms of fatigue and discomfort that you experience.
In this way, fatigue and discomfort ensure that very few athletes ever have the mental fortitude to overcome these limiting symptoms and to damage themselves.
Lowering the symptoms of fatigue
Training and other interventions, such as carbohydrate ingestion during exercise, lower the symptoms of fatigue by increasing the physiological capacity of the body, thereby increasing the amount of exercise required to approach the point of “catastrophe.”
High intensity training (multiple repetitions) seems especially able to adapt these mechanisms quite rapidly (in days to weeks).
Self-belief also seems to influence the extent to which these symptoms are felt during exercise. Having a coach believe in your ability seems to be a particularly effective way to allow you to run faster with less discomfort. – (Prof Tim Noakes, MBChB, MD, DSC)
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