What Limits Endurance Performance? The Science of Fatigue

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science of fatigue
In endurance running, what are the factors that help us to keep going for longer, or that place a limit on how long and how fast we can keep going? 

Professor Samuele Marcora of Kent University has conducted innovative research that challenges traditional ideas about muscular fatigue. In this interview, he tells us more about this research into how the brain acts to limit performance, and the implications for training and racing…

Professor Marcora, thanks for joining us!…

Q: To start with, please can you explain the conventional ideas about the factors that limit endurance performance?

A: There is a traditional physiological model to explain why athletes fatigue and eventually stop. The basic assumption for the last few decades has been that athletes stop exertion because of muscle fatigue. Scientists have considered exhaustion to be the result of fatigue in the muscles and the central nervous system.

Based on this traditional view, the thinking was that when an athlete stops it is due to insufficient oxygen delivery to the muscle as the limiting factor. The leads to production of lactic acid, which is supposed to “poison” the muscle and make it weaker. But over the years, research has shown lactic acid to be less important in fatigue than previously thought.

Q: So your research challenges some of these assumptions. Are there other factors that are more important in limiting performance?

A: Well, it is certainly true that the muscles do fatigue during exercise. The mechanisms are well established and well researched. We understand this very well – after running a 10k you can’t use your muscles as powerfully as you could at the start of the race.

There are a lot of other changes in the muscle that make the muscle weaker over time during exertion, and are major contributors to fatigue. One of them, for example, is the depletion of glycogen, which is the main energy substrate for muscle contraction.

We also know there are even things going on in the central nervous system. For example, when it’s very hot, the brain becomes less able to activate the muscles. This leads to a reduction in muscle force/power.

So muscular fatigue is important. But it is not the reason we STOP exercise.

To research this, there has been a variety of lab studies, using “time to exhaustion” or VO2 max tests. These have looked specifically at why athletes stop, what causes them to be convinced they cannot continue. Research has consistently shown that even after the subject decided to stop, and reported they had reached exhaustion, in fact they still had reserves of energy.

A good example was an experiment which asked subjects (athletes) to complete a VO2 max test. The researchers measured levels of ATP (the fuel needed for muscle contraction) that was left in the muscles at the point when subjects reported exhaustion and stopped exercising. The ATP left in the muscles was enough to enable the subject to keep going for several minutes after they decided they reported “exhaustion”.1

In other words, muscle fatigue was not the limiting factor. There is fatigue, but despite a reduction in muscle power, there is actually still a “reserve” of function/energy. This is particularly important when you think about endurance performance, because the muscle power needed for endurance is far below maximal power. There is actually plenty of power left for this type of requirement even in fatigued muscles at the point when an athlete experiences exhaustion.

Q: So what’s really going on? What is actually the reason that athletes are unable to continue?

A: There is mounting research evidence that ultimately athletic performance is limited by perception of effort.

In a research study, we found a strong correlation between time to exhaustion (ie when the subjects stopped in their high intensity test) and their perceived effort. Asked to indicate their perceived effort on a simple scale during the high intensity effort (cycling in this study), subjects signalled a progressive increase in their perception of effort. When they reached the point where they perceived their effort as maximal, the athletes believed themselves to be incapable of continuing. But in fact the test showed they were still able to generate cycling power after choosing to stop.2

These results challenge the assumption that muscle fatigue causes exhaustion, and instead suggest ability to continue is limited by perception of effort.

Q: Does this suggest that athletes can keep going for longer simply if they try harder?!

A: Not quite! What my research indicates is that there is a psychobiological model which is better at explaining the limits of endurance performance than the traditional physiological model.

In this psychobiological model, there are two parameters: a) potential motivation, and b) perception of effort. If we consider each of these…

a) Potential motivation

This is the maximum effort an athlete is willing to commit to succeed in the task. If we consider highly motivated runners in a competitive situation, we can assume that their potential motivation is near maximum. There may be some days when they are less motivated in training, but when it comes to a race, competitive athletes will be very highly motivated. They will put in exceptional effort.

So if you have two similarly motivated and highly-trained athletes with similar abilities, why might one succeed in a race over another? The perception of effort is probably more important than their motivation.

b) Perception of effort
Perception of effort is the experience of how it feels to produce the speed required.

The important aspect for distance running is that perception of effort increases over time. Think of a 5k race. During the first kilometre, your perception of effort will be relatively low, maybe 5 out of 10. But by the fourth kilometre you would probably say your effort is 8 or 9 out of 10. You may be going at the same pace, but your perception of effort has significantly increased.

Why does this happen? There are physiological reasons why you feel things have become harder:

  • One reason is simply fitness. Perception of effort will increase over time more slowly if you are fitter. If you are less fit, you will reach your perceived maximal effort sooner, and feel that you cannot keep going or will have to slow down. So two runners could have equally high motivation, but if one is slightly fitter they will have a slower increase in perceived effort, and feel more comfortable and able to keep going for longer.
  • Another reason is indeed muscle fatigue – as muscles tire, they are less able to produce required power. Therefore more effort needs to be exerted in order to maintain the same pace.

However, these physiological factors of fitness and muscle fatigue are not the only determinants of perception of effort.

Another crucial factor is mental fatigue.

science of fatigue

Perceived effort increases due to mental fatigue as well as physical fatigue.

Q: What is mental fatigue, and how does it impact on endurance performance?

A: This is where things get really interesting! If you exert effort mentally over a period of time, you feel tired. But mental fatigue, according to the traditional physiological model, shouldn’t really affect your endurance performance because it doesn’t affect your heart or your muscles or blood.

But we have tested the impact of mental fatigue on physical performance. We conducted a research experiment 3 where all subjects were asked to complete a time to exhaustion test in two different conditions. In the first condition (mental fatigue), subjects were given a prolonged (90 min) and demanding cognitive task, then immediately afterwards perform the time to exhaustion test. In the second condition (control), subjects did not do the mentally fatiguing task before the time to exhaustion test. We found a significant difference in performance: in the mental fatigue condition, the subject could not keep going for as long as they did in the control condition.

When mentally fatigued, the test subjects showed their perception of effort was increased, and as a result they reached maximal effort earlier than when not mentally fatigued. But when we looked at the physiological data, couldn’t see any effects of the mental fatigue on physiological measures of heart rate, cardiac output, oxygen consumption or blood lactate.

Mental fatigue was acting even more strongly than the physiological factors as a limit to performance.

Q: Where does perception of effort come from?

A: It is commonly thought that the body sends signals to the brain to give us a perception of our effort. The brain is passed information from the muscle receptors about lactic acid and other signals in the body (such as the heart and lungs), so the brain can interpret how stressed the body is.

But actually, this isn’t the same as perceived effort. While there are signals that do get sent to the brain, that’s not what defines perceived effort.

Instead to understand perceived effort, there’s a very helpful analogy by running author Matt Fitzgerald. He describes a race horse and jockey. The horse is your body and the jockey is your brain. Perceived effort is not how stressed the horse is, but how hard the jockey is whipping the horse [ok it’s just a useful analogy so don’t worry about animal cruelty for a moment!… ]

science of fatigue

The analogy of a jockey whipping a race horse helps explain how the brain has to work hard to activate tired muscles.

The brain activates the muscles to make you run. As the muscles fatigue, the brain has to work harder to activate them. So when running at the same pace over 5k, part of the reason why you feel more tired at 4k than at 1k is that the “jockey” needs to work harder to “whip” the “horse”. The horse is tired, therefore jockey has to whip harder, and the jockey feels the effort of doing so.

The jockey can also become fatigued as in the case of mental fatigue, and therefore activating the muscles (whipping the horse) also feels harder.

Q: Has this been demonstrated in research experiments?

A: Yes, research has been done to measure how hard the brain has to work when you lift a submaximal weight. Tests have used electrodes placed on the scalp to measure brain activity, and we can see the brain activity which relates to muscular movement. These tests have shown that the brain is working harder when the muscles are fatigued, compared with when the same task is done with muscles that aren’t fatigued.

Q: But how are these tests relevant for endurance performance? And for ordinary runners? After all, we’re not running to exhaustion or close to VO2 max in endurance races.

A: If you want to beat your PB, it is helpful to understand why you can (or can’t) keep up a certain pace. Looking at what ultimately limits your performance matters, because if you can improve your time to exhaustion you can also improve your endurance time in a race.

In sub-maximal exercise like endurance running, your brain has to compensate for muscle fatigue by increasing its effort in activating the muscles. In order to keep up the same pace, you need to increase muscle recruitment, which is taxing for the brain.

Q: So can we train our brains as a way to improve performance?

A: Well this is a very interesting area to look at. Runners absolutely need to keep doing all the usual physical training that is based on the physiological model, building physical ability of the muscles for endurance running. But what’s exciting is that the psychobiological model gives us new ways to develop, which could help to improve performance.

There are certain techniques we can use to develop our ability to delay and tolerate mental fatigue. Many of these are psychological skills, even though what’s happening in the brain is a physiological process. (In my view physiologists and psychologists could collaborate much more closely on these areas.)

Reducing the factors that cause mental fatigue before and during a race can make difference, for example…

Before a race:

  • Poor sleep increases mental fatigue. So trying to get more rest and sleep is probably helpful.
  • Trying to control your emotions too much can actually increase mental fatigue. Perhaps the “stiff upper lip” may be tiring your brain! It may be that allowing yourself to experience and express certain emotions is better, or avoiding situations which require a lot of emotional self-control.

During a race:

  • During the race, negative thoughts may increase mental fatigue. Positive self-talk can actually reduce mental fatigue (more below on this).
  • During a race, running with pacers has a positive effect. Not having to concentrate on pacing yourself reduces the mental fatigue load. You don’t need to make those decisions.

Q: Is it possible to measure the impact of techniques like these?

A: We did an experiment on the impact of self-talk on endurance performance. 4

A group of athletes were taught and practised positive self-talk skills, using motivating statements such as “hang on in there”, “go for it”, “dig deep”, “push it”, “you’re a winner”, “you can do it”, “keep going, be strong”; while a control group was not taught the skills. Both groups were asked to undertake an initial time to exhaustion test, and a second time to exhaustion test after two weeks of learning (or not learning) the skills. Over this period, there was no change in time to exhaustion for the control group, but the group taught how to use the self-talk skills and who practised the skills and used them in the test showed a significant improvement.

This demonstrated again that perception of effort is the limiting factor, and also that techniques such as self-talk can make a difference to that perception.

Q: Can we all train ourselves to tolerate mental fatigue better too?

A: Yes, all the existing physical training you do also trains your brain. There are psychobiological changes in the brain due to physical training, which also make you more resistant to mental fatigue. So all the physical training you do is also training you to cope with mental fatigue and slow down its onset.

We are also currently testing techniques to make this training for mental fatigue even more deliberate. We are conducting studies asking subjects to do low-to-moderate intensity exercise while increasing their mental fatigue by asking them to do fatiguing cognitive tasks. This research is in conjunction with the Ministry of Defence so we can look at how soldiers may be able to improve their ability to continue operating at a high cognitive level for longer. Already the results look interesting, with those trained to do tasks on a computer while exercising showing an improvement in time to exhaustion test results. This application of this should help to develop specific approaches for extending the limits of endurance performance.

Q: Finally, do you have some practical suggestions that readers could start doing in training and racing?

A: A few suggestions include:

  • Try to avoid activities that cause mental fatigue before competition/races. Avoid things like video games. Also avoid controlling your emotions too much, or being in situations that require a lot of self-control – this is very fatiguing.
  • You can make use of psychological skills including self-talk, goal setting and imagery. All have been found to be effective in improving endurance performance.
  • Think about the impact of nutrition. For example, consuming caffeine can stimulate brain performance and have direct effect on the brain, which is why some athletes use it before and during races.
  • Keep a look out for more research on brain endurance training. Athletes may be able to teach their brains to deal better with mental fatigue, with more deliberate techniques for this. It’s a growing area.

1. Morales-Alamo et al; What limits performance during whole body incremental exercise to exhaustion in humans? Journal of Physiology 2015; 593(20), 4631-4648.
2. Marcora, Staiano; The limit to exercise tolerance in humans: mind over muscle? European Journal of Applied Physiology 2010; 109 (4): 763-70
3. Marcora et al; Mental fatigue impairs physical performance in humans. Journal of applied physiology 2009; 106(3), 857-864.
4. Blanchfield et al; Talking yourself out of exhaustion: the effects of self-talk on endurance performance. Medicine & Science in Sports & Exercise; 46(5), 998-1007.

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