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Endurance Fatigue: Perception Is Everything

A fascinating study proves that fatigue in endurance is nothing more and nothing less than quitting.

A fascinating study proves that fatigue in endurance is nothing more and nothing less than quitting.

The design was simple, but the results were profound. Samuele Marcora, an Italian-born exercise physiologist at Wales’ Bangor University, and his colleague Walter Staiano brought 10 male athletes into their lab and had them perform a simple exercise protocol. Each pedaled on a cycle ergometer as hard as he could for 5 seconds (a test of maximal voluntary cycling power, or MVCP), and his power output was recorded. Then the subjects rode the same bikes as long as they could at a fixed power output level that corresponded to 90 percent of their individual VO2 max. Immediately after completing this ride to exhaustion, which ended when the required wattage simply could not be sustained any longer (or approximately 12 minutes, on average), each subject then repeated the 5-second maximum power test.

Marcora and Staiano found that, on average, power output in the second 5-second MVCP, performed in a state of exhaustion, was roughly 30 percent lower than the power produced in the first MVCP, performed in a fresh state. Yet the power output in the second MVCP was still roughly three times greater than the power that each cyclist was required to maintain in the ride to exhaustion.

Wait a minute: If the subjects cycled at roughly 242 watts until they physically could not complete another pedal stroke at that level, how were they able to pedal at 731 watts for 5 seconds immediately afterward?

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Marcora and Staiano’s answer to this question could not be simpler, yet it completely shatters the concept of endurance fatigue that most of us believe in. In a paper on their study published in the European Journal of Applied Physiology, they wrote:

It is traditionally assumed that exhaustion during high-intensity aerobic exercise occurs because fatigued subjects are no longer able to generate the power output required by the task despite their maximal voluntary effort…We have demonstrated for the first time that this is not the case… [I]f our subjects were able to voluntarily produce 731W for 5s immediately after exhaustion, they must have been physiologically able to produce 242W for much longer. The most likely explanation for the very high MVCP produced immediately after exhaustion is psychological. Subjects knew that the final MVCP test was going to last only 5s, and such knowledge motivated them to exert further effort after the time to exhaustion test which had a longer and unknown duration.

As intuitively sensible as this explanation is, it is scientifically revolutionary, as it supposes that the true cause of endurance fatigue is perception of effort (i.e. psychological suffering), whereas perceptions are traditionally seen as having no causal force in exercise physiology.

In 2010, I spoke to Dr. Samuele Marcora about the broader implications of his provocative new study. Here is the transcript of that interview:

You propose to replace the conventional model of endurance fatigue, which centers on the muscles and the cardiovascular system, with what you call a psychobiological model of endurance fatigue. Please explain that.

My proposal is actually based on general motivation theory. What we call exhaustion is not the inability to continue; it’s basically giving up. The reality is that the neuromuscular system is actually able to continue. My idea is that it’s basically a safety mechanism like many other sensations. So you have sensations motivating you to take a certain course of action for survival. Think about thirst or hunger or pain. All these sensations are there to make us do something. That is actually beneficial for our survival, and I think perception of effort does the same.

There’s this idea that perceptions are mere perceptions and can be overridden through conscious will in a way that purely physiological limitations cannot. Have you encountered resistance to your model on this point?

My physiology colleagues think that because something is a perception, in some way it is less real and can be overcome. Obviously, it can be overcome  to a certain extent, but that doesn’t make it any less powerful. That’s why people believed this assumption [that fatigue is caused by hard physiological limits] for so long, because it feels like that. The perception of effort makes you feel like you can’t continue. You feel, “I am giving my maximal effort. I feel I cannot continue. Therefore I’ll stop.”

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If you think about pain, pain is created in your brain based on a certain signal such as a broken foot. If you didn’t feel that pain, of course you can keep going. There are people who genetically don’t feel pain and they usually die quite young. Just because something is a perception doesn’t make it any less powerful in controlling your behavior. If you are very thirsty you might kill somebody for a glass of water. You wouldn’t keep running if you ran over a piece of glass and cut your foot. If you didn’t have perception of effort, you could run your marathon much faster, definitely!

One thing that exercise physiologists are baffled by is that when very high-level endurance athletes do a physiological test, they aren’t very different from each other. They all have very high VO2 max, they all have good economy. You can’t really differentiate between them based on physiological parameters. But there is something extra that makes some of them champions. For example, I’m doing some tests on perceived ability, or what psychologists call self-efficacy, which show that beliefs about personal limits tend to be self-fulfilling. People who wish they can push harder and do more usually can. This phenomenon makes perfect sense in my model. So it certainly gives you a range of flexibility that the traditional model, where you stop regardless of your will, doesn’t give you.

Every perception is associated with a distinct set of physiological events in the brain. So is it really the conscious perception of effort that causes fatigue or is it the physiological events underlying that perception?

So the next question is, “What are the neurophysiological mechanisms underlying perception of effort?” and that’s a very big question. It’s like trying to understand the neurophysiological basis of any other phenomenon. That’s why I’m doing studies in collaboration with cognitive neuroscientists using techniques such as brain imaging and EMG, trying to tease out the neurophysiology underlying perception of effort.

I really want to tease out the mechanisms by actually studying the brain. It’s no different from studying any other perception. Hopefully, after learning more about it we can also modify it and help athletes to improve their performance, although you have to be careful, because messing around with a sensation that is there to protect you may also have detrimental effects.

Think about caffeine. Everybody now agrees that the ergogenic effects of caffeine are 90 percent mediated by the effect on the brain and therefore the perception of effort and not by the metabolic effects. So, if you like, this is the first application of the model that perception of effort is important and I think in the future we will see more and more of these things.

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Your study did find evidence of fatigue within the muscles themselves—not enough to explain the fatigue that occurred in the time to exhaustion tests, but muscle fatigue nevertheless. So, what is the role of actual muscle fatigue in your model?

In a previous study, I looked experimentally at the effect of muscle fatigue on endurance performance. So, if I pre-fatigue your muscles before I put you on a bike and ask you to do an endurance test, your endurance performance will be reduced. But the effect was relatively small. It was a 16 percent reduction in time to exhaustion. In a time to exhaustion test, a 16 percent reduction in performance is a small result.  In a time trial (it’s unpublished but we did the same thing) we had a 3 percent reduction in performance, which is also a small reduction. That study made me think about whether fatigue is the limiting factor. I reduced muscle function to the same level that people have after an endurance test. So, if muscle fatigue was the limiting factor, they shouldn’t have been able to even start the test. Instead, they latest only a couple of minutes less. So that started to put doubts in my mind.

But if muscle fatigue doesn’t cause exhaustion, why does muscle fatigue reduce performance? The reason is simple. If you cycle with muscle fatigue, your perception of effort will increase simply because, if you have fatigued muscles, in order to produce the same power output [as when your muscles are not fatigued], you will have to recruit the muscles more. The main stimulus for perception of effort is how much we are recruiting our muscles—leg muscles or inspiratory muscles. So if I am forced by having fatigued muscles, or even damaged muscles, to increase my muscle recruitment, I will perceive that as an increase in effort, and that increase in effort will make me reach my maximal level of effort and stop earlier than when I don’t have muscle fatigue. So muscle fatigue does have an effect on performance, but it is indirect. It is not a direct effect as traditionally assumed.

I did another study where, instead of using muscle fatigue, I used mental fatigue. The effect of mental fatigue on performance was the same as muscle fatigue. You wouldn’t think so. Why does playing a video game for 90 minutes reduce your endurance performance as much as muscle fatigue? It doesn’t make any sense according to the traditional model, but it actually makes perfect sense in terms of my model. I don’t know exactly what the mechanisms are, but during the cognitive task, I induced some changes in the brain that made my subjects perceive the effort required to cycle as being higher than in the normal condition. You see, it doesn’t matter why perception of effort is increased or decreased. Everything that has an effect on perception of effort will have an effect on performance.