Why do we get tired during a run?

English: Dr. Tim Noakes at the Department of Physical Education at West Point shortly before delivering a lecture on Exercise Induced Hyponatremic Encethalopathy (Photo credit: Wikipedia)

The classical theory to explain why we have to slow down during a hard run is that our leg muscles do not get enough oxygen anymore; therefore lactate builds up and further work becomes impossible. During longer, slower runs, the theory predicts that we run out of ATP (= the molecule that delivers energy), and that our cells become “poisoned” by by-products of our metabolism. Fatigue is thus seen as a “catastrophe” due to failure of our muscles.

Is that true?

As runners, we cannot help being sceptical. How can this be true if, at the end of a race, we manage to speed up when we see the finish? How is it possible that we run so much better if loved ones cheer us on? Why is motivation so important, even though there is no room for it in this theory?

If our leg muscles are too poisoned to work properly, what is happening to our hearts? Runners do not die from a heart attack at the end of a run, unless they have an underlying disease or abnormality.

Moreover, if this theory is correct, how did our prehistoric ancestors have the strength to defend themselves against wild animals after a hard day hunting?

To make matters even worse for the classical theory, scientific studies have shown that muscle cells still contain oxygen at their maximum work rate. In other words: they do not become “anaerobic” as the lactate concentration starts to increase. They have also demonstrated that athletes are not engaging all their muscle fibres at maximal effort, as you would expect according to the classical theory, but only about 30%.

Another theory

To solve all these contradictions, Prof Tim Noakes has come up with a new theory. He starts from the fact that a correct perfusion of our brains and hearts by oxygenated blood is essential for our survival. He thinks that we regulate unconsciously our running speed to make sure that our hearts and brains have enough oxygen. This means that there must be a “governor” who protects us from exercising so hard that we would damage ourselves. Tim Noakes therefore called his theory “the central governor model”.

The central governor model

At the start of the run, the unconscious brain chooses the pace by recruiting an appropriate number of fibres in the working muscles. Its choice is determined by the expected length and difficulty of the run, and by a long list of other factors, including fitness, fatigue, temperature, motivation, presence of spectators or competitors…

At each step it further adapts the pace according to information coming from the conscious brain such as the distance already covered, or the true difficulty of the terrain. It also uses information from the working muscles, such as the rate of glycogen consumption, and from other unconscious sources, e.g. amount of oxygen in the blood, loss of fluid, rise in core temperature, etc…

In that way the brain aims to prevent catastrophic changes in the body. It does so by reducing the amount of muscle fibres we are using and by making us feel tired. According to this model, fatigue is not just a physical event but an opinion of the brain. In other words: it is an emotion, which can be altered by other emotions and by physical factors.
This explains why we have some energy left for the final sprint, or to speed up when we hear friends and family shout our name.

A partnership between mind and matter

Some scientists find it difficult to accept this model, because the exact pathways and mechanisms are not well understood, but most runners find it very appealing. It indeed explains what we feel. It also makes it possible to be happy when we are tired: our brains are keeping us safe.

References

T D Noakes, A St Claire Gibson, E V Lambert. From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans: summary and conclusions. Br J Sports Med 2005; 39: 120-124

T D Noakes. Fatigue is a brain-derived emotion that regulates the exercise behaviour to ensure the protection of whole body homeostasis. Front Physiol 2012; 3: 82

R S Richardson, E A Noyszewski, J S Leigh, P D Wagner. Lactate efflux from exercising human skeletal muscle: role of intracellular PO2. J Appl Physiol 1998; 85(2): 627-634

R S Richardson, S C Newcomer, E A Noyszewski. Skeletal muscle intracellular PO2 assessed by myoglobin desaturation: response to graded exercise. J Appl Physiol 2001; 91(6): 2679-2685

A St Clair Gibson, T D Noakes. Evidence for complex system integration and dynamic neural regulation of skeletal muscle recruitment during exercise in humans. Br J Sports Med 2004; 38:797-806

J P Weir, T W Beck, J T Cramer, T J Housh. Is fatigue all in your head? A critical review of the central governor model. Br J Sports Med 2006; 40: 573-586