Fatigue is an Emotion

I have been meaning to write part three in my series on central governors, which is supposed to cover fatigue. Well now I don’t have to, because I can just point my readers to an excellent article written by Tim Noakes, which describes in detail his central governor model for fatigue. The tile provides a good summary: Fatigue is a Brain-Derived Emotion that Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis. The basic idea is that human exercise capacity is not limited by a failure of the body, but is instead regulated by the brain to ensure that such a failure does not occur. Here is a brief summary of some of the highlights of the article.

Looking for fatigue in all the wrong places

Noakes starts with a review of the history of the study of fatigue, which mostly focused on efforts to find the ultimate limitations of human exercise capacity in the body, such as the muscles or the heart. The problem with this approach is that it does not explain why athletes almost invariably simply choose to stop exercising before such bodily limits are reached (some of which would result in catastrophic injury or death.) For Noakes “the presence of the noxious symptoms of fatigue must indicate that exercise cannot be regulated solely by an inevitable and unavoidable failure of skeletal and or cardiac muscle functions.”

Here are some common observations that are not explained by the theory that exercise is limited by some single factor in the body:

• athletes begin exercise at an intensity which is appropriate for the expected duration;
• athletes run harder in competition than in training;
• athletes speed up at the end of exercise (the end spurt).
• skeletal muscle is never fully recruited during any form of exercise - 35-50% in prolonged exercise and 60% during maximal efforts.

To explain these and other observations, Noakes and his colleagues helped develop the Central Governor Model for fatigue.

The central governor - it's all in your head

The essence of the central governor model is that fatigue is not a physical event but rather an emotion that is used by the brain to regulate exercise stress. An important implication is that all forms of exercise are submaximal since there is always a reserve of motor units that are never fully utilized.

Research shows that motor recruitment and fatigue during exercise will be affected by a huge variety of factors, including emotional state, mental fatigue, recovery from previous exercise, motivation, self belief, prior knowledge of the duration of exercise, cerebral and arterial oxygenation, muscle glycogen storage, fluid loss, thirst, heat, and more. In fact, "the prediction of this model is that potentially everything ... can potentially affect athletic performance. But that the most important of these effects begin and end in the brain."

Because fatigue is produced by the brain based on its opinions about what is going on in the body, it is subject to error, as noted by Bainbridge in 1919:

the sense of fatigue is often a very fallacious index of the working capacity of the body there is not necessarily any correspondence between the subjective feelings of fatigue and the capacity of the muscles to perform work. It is a protective feeling which tends to restrain the man from continuing to perform muscular work when this would cause injury.

In support of this idea, Noakes cites to evidence that athletes can be “tricked’ into working harder in numerous ways, such as deceiving them about the time or distance they have exercised, cooling hands and palms to make core temperature appear less elevated, or using a carbohydrate mouth rinse to fool the brain about the availability of energy reserves.

Although Noakes does not mention it in the article, the idea that the fatigue can be a "fallacious index" of actual body state has interesting implications for chronic fatigue syndrome. And there are some obvious connections to what we are learning about pain science.

The difference between winning and losing?

Noakes ends the article with some very interesting speculations about the differences between coming in first or second in an endurance event. Here are some excerpts:

In the case of a close finish the CGM was clearly successful – neither athlete died. But if the second runner did not die, why did he not run just a little faster and so approach death a little closer? For surely he could have sped up by just a fraction without dying? Yet he did not. Why not?

My unproven hypothesis is it is that in the case of a close finish, physiology does not determine who wins. Rather somewhere in the final section of the race, the brains of the second, and lower placed finishers accept their respective finishing positions and no longer choose to challenge for a higher finish.

...

According to this model, the winning athlete is the one whose illusionary symptoms interfere the least with the actual performance

…

the winner is the athlete for whom defeat is the least acceptable rationalization.

...

“The fight,” wrote Muhammad Ali “is won or lost far away from witnesses, behind the lines, in the gym, out there on the road, long before I dance under the lights.”

A little romantic maybe, but it rings true. If it's all about physiology, then why does this race feel meaningful?:

Or this one (complete with Rocky music):

I love it.