Fatigue & Performance

Why did we say that fatigue was a complex phenomenon in the introduction to the ABOUT FATIGUE section? It suffices to see all the possible causes of fatigue in athletes listed the diagram below to get a grasp on just how complex fatigue is.

First there are the so-called peripheral causes, that is to say localized in the muscle, which occur at several levels. The reasons most often put forward (lactic acid and lack of energy substrates) may play a role, but in reality, their role remains relatively modest. Other chemicals are far more detrimental to muscle function.

But fatigue is not only in the muscles! And this is much less known. What happens in the brain during strenuous exercise?

First of all, there may be the gradual onset of so-called central fatigue. In summary, even when we give our maximum, we are no longer able to recruit all our muscle fibers. This can be measured by stimulating either a muscle or the brain directly, while the athlete tries to produce the greatest possible force: we find, compared to maximum voluntary output, artificial stimulus increases force, proving that the muscle is able to do more, and that the limit is in the nervous system. Oddly, central fatigue is intense after an ultra-marathon… but also after a maximum effort of 2 minutes without a break. Even if the reasons which explain this central fatigue can be very different, the cause is to be sought either in the brain, or in the spinal cord, or in connection with sensory fibers in muscles, tendons, and joints.

When we get tired, we also note an increase in the perception of effort for a given physical task. Take the example of the chair against a wall exercise. You are not heavier after 2 minutes than at the beginning. And yet it is more difficult, why? Two mechanisms are involved:

1. Feedback: the sensory fibers, which we mentioned earlier, send painful information to the brain in response to the chemical and mechanical changes that occur during muscle exertion.

2. Feed-forward: when muscles get tired or motor neurons in the spinal cord become less excitable, the motor cortex must increase the amount of signal it sends towards the periphery if the same intensity is to be maintained effort (e.g. maintaining the same running speed). However, a copy of this signal is sent to the sensory cortex and the exercise seems more difficult.

Since this feeling of arduousness cannot increase indefinitely, we decide either to reduce the pace to a level which seems bearable to us (when it is possible, case of a marathon for example), or we abandon the matter (case of the chair against the wall). In any case, it is the brain that has the last word, the muscles are not limiting.

In fact, our perception of effort is more complicated. Other factors can influence this perception, and it is for this reason that the Flush model  was created. For example, when you perform a mental tiring task or when you are sleep deprived, you leave with a certain level of water in the tank. The flush model also has the advantage of clearly highlighting what is called the safety reserve and which explains why, unlike horses, humans do not die of exhaustion. All of this is well explained here.