Perceived exertion, not muscle failure, determines “exhaustion”


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There’s an interesting preprint available online from the European Journal of Applied Physiology, by Samuele Marcora and a colleague from Bangor University in Wales. Its title is “The limit to exercise tolerance in humans: mind over muscle?,” so you might think it’s another paper supporting Tim Noakes’s “central governor” theory. It is, and it isn’t — but either way, it’s interesting.

Marcora took 10 elite rugby players and (after various preliminary testing and so on) had them do a five-second “maximum voluntary cycling power” (MVCP) test. Then they did a very intense cycling trial to exhaustion which took about 10 minutes (they offered cash prizes to the top performers and circulated the results publicly to stimulate competition and make sure the subjects went all-out), followed immediately (within one second) by another MVCP test.

Now, if you subscribe to traditional exercise physiology, you’d say that the subjects stopped the test-to-exhaustion when they were no longer physically able to generate enough power to continue. Possible reasons for their failure would include “limited oxygen delivery, metabolic and ionic changes within the active muscles, supraspinal reflex inhibition from muscle afferents sensitive to these changes, and altered cerebral blood flow and metabolism.” But that’s not what Marcora saw. The subjects had to maintain an output of (on average) 242 watts in the test to exhaustion. But as soon as they stopped, one second later, they were able to output (on average) 731 watts in a five-second burst — nearly triple the required power! Clearly the subjects didn’t stop the test because their couldn’t physically produce the needed power:

These results challenge the long-standing assumption that muscle fatigue causes exhaustion during high-intensity aerobic exercise, and suggest that exercise tolerance in highly motivated subjects is ultimately limited by perception of effort.

The interpretation of these results gets a little tangled. Marcora is an advocate of something he calls the “psychobiological model of exercise tolerance,” which seems to basically mean that we stop exercising when it gets hard. He says this is different from — and much simpler than — Noakes’s central governor theory. I’m not sure I really a see a difference that extends beyond semantics, but perhaps that’s because I haven’t given it enough thought. I downloaded a couple of Marcora’s other papers where he explains the theory in more detail, so I’ll be interested to see what he has to say. Either way, these results are certainly interesting in that they once again support the notion that, when we collapse from exhaustion, we’re generally running up against barriers imposed by our brain rather than absolute physical limits imposed by our body.

17 Replies to “Perceived exertion, not muscle failure, determines “exhaustion””

  1. If I was being chased by a lion, I would not be able to outrun it and it certainly would not be because of a lack of motivation.

  2. @Richard, good point, but we can’t test ‘escaping from a lion’ as a true experiment. There’s no pre-test for being chased by a lion, but I would imagine a subject would perform better in this life-threatening situation. That doesn’t necessarily reject the idea that collapsing from exhaustion in a lab is limited by mental barriers. You’re just adding the idea that there is an absolute limit beyond that, when motivation is in sufficient supply.

  3. No, of course not, there are physical limits. But the point is that when we stop, it is not because we have reached those physical limits, but because we have reached a mental limit. If you were being chased by a lion, you would probably have a higher output of power than if you were doing intervals around a track. The session might not last as long though…

    I wonder if there’s a bit of a bug in this in that maybe the two tests were on different systems. Is 242 watts using the aerobic system (which has more endurance and less power) while the 731 watts is anaerobic or ATP/CP(which produces more power). Maybe they stopped because they aerobic system was tapped, but since the anaerobic or ATP system recovers more quickly, it was then ready to go.

  4. Interesting discussion — I think you guys have pretty much answered Richard’s point. As John says, there ARE physical limits; this (and other) research just suggests that we reach the point where we believe we can’t continue before we reach those limits. And that may happen even if you’re being chased by a lion. The point isn’t that we’re all mentally weak and decide to stop because it’s easier. We’re responding to powerful signals from our brain telling us to stop before serious harm occurs, in the same way that you instinctively pull your finger out of a fire before the flesh is actually charred.

  5. Here’s how the authors respond to your second point, John:

    “It may also be argued that physiological differences between the two tasks (e.g., motor unit recruitment and energy substrates) could explain the large difference between MVCP immediately after exhaustion and the power output required by the time to exhaustion test. However, this is irrelevant to the aims of our study. In fact, even in fatigued subjects, there is a hyperbolic relationship between power output and tolerable exercise duration. Therefore, if our subjects were able to voluntarily produce 731 W for 5 s immediately after exhaustion, they must have been physiologically able to produce 242 W for much longer.”

    So I guess they’re saying that, yeah, maybe the energy systems are different — but there WAS energy available that the subjects could have used to extend their time in the exhaustion test. And not just a little bit of energy, a dramatic increase.

  6. @Johnny Hall I used a lion because it expresses the motivator of survival well but yes the lion can run faster but that misses the point.

    We can change it up a little by having a 10,000m race. The winner gets $100,000,000.00 and the loser tortured and crucified. Kenenisa Bekele stands at the start line. Not as much fun but perhaps matches the exhaustion criteria better.

    I think the flaw in this experiment is the motivator. In order to determine the absolute physical limit, we must use an absolute motivator. Anything else is not going to work. The subjects would need to truly believe without a doubt that their lives are at risk. Anything else would skew the results.

  7. @Richard Ayotte What you’re arguing shows exactly what the experiment was trying to demonstrate: that the point at which we stop maximal exercise is determined by whether motivation exceeds discomfort, rather than by physical limits. In this case, cash prizes and competition weren’t enough to get the competitors anywhere near “total physical failure.”

    You’re suggesting that being chased by a lion or racing your hypothetical 10,000m race would constitute some form of “absolute” motivator. Maybe, maybe not — at this point, that’s nothing but a thought experiment. In real-life situations, though, it appears that human performance is limited by a brain that shuts down physical systems BEFORE they can make your heart explode.

    This doesn’t mean that you would be able to beat Kenenisa Bekele if only you tried hard enough. His 10,000m time is almost certainly beyond your absolute physical abilities. The point is simply that your own limits are imposed preemptively by your brain, as are Kenenisa Bekele’s.

  8. @alexInteresting subject. Yes, your explanation makes sense. Within someone’s physical limits, perception of effort is the ultimate limiting factor.

    I’m having problems with this quote though.

    These results challenge the long-standing assumption that muscle fatigue causes exhaustion during high-intensity aerobic exercise, and suggest that exercise tolerance in highly motivated subjects is ultimately limited by perception of effort.

    Here’s the famous Sian Welch & Wendy Ingraham video.

    After watching it, it’s hard to believe that muscle fatigue does not cause exhaustion.

  9. Ha! I’d never seen that video. Sooooo painful to watch! It’s interesting — it’s certainly possible that these two athletes ran right to the edge of their physical limits. On the other hand, it could simply be that their legs were cramping, something that happens to athletes all the time — and not just when they’re at the limits of their physical endurance. There are plenty of physical factors that can force people to stop — they pass out from heat stroke, or their legs seize up, or whatever. They’re not “choosing” to stop. But the debate (and it’s a hot one, far from settled) among exercise physiologists is whether the ultimate signal to stop is because part of the body HAS failed, or because the brain’s subconscious control is afraid that something WILL fail. Functionally, of course, the result is the same: you can’t go any further.

    There are no absolutes, of course. When someone dies of heat stroke, clearly something failed!

  10. Samuele Marcora, the lead researcher, shared a few thoughts on this post over on the Canadian Running site (where this blog is mirrored):

    “Hi Alex, thanks for your interest in my research. You summed it uo very well! I just want to clarify that the difference between my psychobiological model and the central governor model is more than semantics. I propose that performance is regulated by us, i.e. our conscious brain. Noakes proposes that performance is regulated by a subconscious intelligent system (the central governor) inside our brain. I think Noakeā€™s central governor is a typical example of a common mistake when trying to explain how our mind works: the homunculus argument. See Wikipedia for more details:
    In order to develop new interventions, we need to understand the neurobiological bases of psychological constructs (e.g., motivation and perception of effort) relevant to endurance performance. Hypothesising a central governor does not help in this respect. It just transfer the problem of how our mind works without solving it.”

  11. Picking up on Richard’s point, I was thinking that an interesting experiment which my be mildly informative in this debate may be testing subjects ability to exercise to “exhaustion” where the psychological capacity was increased without an increase in physiological capacity. Specifically I was thinking (using my basic reasoning that is not well informed by any formal qualification in the relevant sciences) that the introduction of adrenalin or some other mental stimulant may go some way to determining the impact of psychology. This may have an impact on the brain’s capacity to handle more work, but may not have as great a physiological impact. I do, however, admit that the results would still be open to interpretation about the proportion of any increase in work performed which was due to changes in psychology vs changes in physiology. I’m also not sure about the ethical implications of giving people stimulants which may result in them performing exercise which exceeds their physical capabilities and thereby risking long term physical harm.

    Just a thought anyway. Very interesting stuff and good for me to know next time I’m hanging on to the back of the pack by my fingernails. It’s all mental…it’s all mental….

  12. @Dave
    Thanks for the comments, Dave — excellent points. The study you’re looking for was published last fall in the British Journal of Sports Medicine: It’s called “Exercising with reserve: evidence that the central nervous system regulates prolonged exercise performance.” Here’s the abstract:

    OBJECTIVE: The purpose of this study was to measure the effects of an amphetamine (methylphenidate) on exercise performance at a fixed rating of perceived exertion of 16. METHODS: Eight elite cyclists ingested 10 mg methylphenidate in a randomised, placebo-controlled crossover trial. RESULTS: Compared with placebo, subjects receiving methylphenidate cycled for approximately 32% longer before power output fell to 70% of the starting value. At the equivalent time at which the placebo trial terminated, subjects receiving methylphenidate had significantly higher power outputs, oxygen consumptions, heart rates, ventilatory volumes and blood lactate concentrations although electromyographic activity remained unchanged. The ingestion of a centrally acting stimulant thus allowed subjects to exercise for longer at higher cardiorespiratory and metabolic stress indicating the presence of a muscular reserve in the natural state. CONCLUSIONS: This suggests that endurance performance is not only “limited” by mechanical failure of the exercising muscles (“peripheral fatigue”). Rather performance during prolonged endurance exercise under normal conditions is highly regulated by the central nervous system to ensure that whole-body homeostasis is protected and an emergency reserve is always present.

  13. this makes sense. an old training buddy – his name was Arto- had a motto, which i believed and still do, that ‘we can make ourselves suffer’. this is something that takes some convincing.

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