Samuele Marcora


Hello ladies and gentlemen, my name is Samuele Marcora and I am Professor or Exercise Physiology at the University of Kent in the School of Sport and Exercise Science and also a member of the Endurance Research Group.

Today I'm going to talk about the limits of endurance performance in humans and the basic question is, are we limited by our own muscles or by our own mind? First of all, let's define endurance performance: it covers a variety of sports and human activities, we define it as any whole body physical tasks such as running, cycling, rowing, lasting more than 75 seconds up to several hours or even days in the case of the Tour de France and of course the most iconic events are the Tour de France or the marathon but there are many more. And there are differences in terms of tactical and technical aspects of the sport,s however, there is a very basic feature of endurance performance that is trying to run or cycle at a speed over the course of the competition that your competitors cannot keep and this is shown here in this graph. 

This is a graph of the speed maintained by different athletes during the 2008 Beijing Olympic 10k men's race. In the solid line is the speed of the winner, and here you can see that about half way through the race a couple of athletes started to drop the pace, they couldn't keep it going anymore, this one here, in the end this guy lost because he couldn't keep the pace set by the winner. And the very basic question in exercise physiology is why he couldn't keep that pace, and the way we measure it in the lab is by measuring what we call, time to exhaustion, which is our basic test of endurance performance. Basically, it's very simple, we ask people to cycle or run on a treadmill or a cycle ergometer at a certain speed that we fix for as long as they can. We can also take a variety of physiological measures while they're doing this activity. But the basic question is, why do they stop, why can't they keep the pace that we set for them? And, just to give you an idea, at top this is a top professional cyclist he could maintain a workload of 410 watts, next time you go to the gym try that power output, most of you may not even be able to push it once, but he can keep it for 25 minutes. But the question is why couldn't he keep it for 30 or 40 or an hour. And the traditional answer is because of muscle fatigue and traditionally I mean since the early 20th century when exercise physiology started to develop, we believed and it's still important, that the oxygen delivery to the muscle, insufficient delivery of oxygen to the muscle is a limiting factor because it leads to the production of lactic acid, which is supposed to kind of poison the muscle and make it weaker.

However, over the years of course we have done much more research and now we know that lactic acid actually is not that important, there are even people that believe it is not fatiguing at all, but there are a lot of other changes within the muscle that make the muscle weaker over time during exertion. And one of them for example is the depletion of glycogen which is the main energy substrate for muscle contraction. However, now we know that there are things going on even in your central nervous system that may cause muscle fatigue. For example, when it's very hot our brain becomes less able to activate the muscles, so the combination of these muscular and brain changes reduces the ability of the athlete to produce power and this will lead to a reduction in endurance performance. This long-standing physiological model has led to a lot of innovations. For example, now top elite athletes like Chris Froome are routinely tested in the lab to measure, for example, the maximal oxygen consumption. We also tell our endurance athletes to consume a lot of carbohydrates before the competition, such as pasta and rice, in order to make sure that their muscles are full of glycogen so they can last longer. And also unfortunately these models also inspired some of the doping techniques used by many of these athletes. For example erythropoietin which is a substance that stimulates the production of red blood cells, which are the cells that carry oxygen in the blood to the muscles. 

There is however an issue and the issue is in the very basic assumption of this model. So in this graph we see this is the power output fixed in the lab at which we ask the subject to cycle or run and here is the progressive reduction in maximal power output caused by exercise, that's what we call muscle fatigue. And the assumption is that when the maximum power output coincides with the power gap we required by the exercise task the subject stops, because despite the maximal voluntary effort is not able, their neuromuscular system is not able, to produce the required power output. However, this is an assumption and actually only in 2010 we tested that and we found something very surprising. The surprising thing was that yes of course there is muscle fatigue as you can see here in the upper line there is a progressive reduction in maximal power but even immediately after exhaustion when the subject decided to stop, to cycle in this case at 242 watts, there was a significant reserve in power output so the subject could exert almost three times the power output required only a few seconds earlier, albeit for a few seconds. 

So, clearly muscle fatigue was not the limiting factor, there's been now more studies including one where immediately after exhaustion researchers took a sample of muscle from the leg muscles and measured how much ATP was in the muscle, and ATP is basically the fuel, the ultimate fuel, for muscle contraction and they calculated that with what was left in the muscles after exhaustion they could have kept going at the very high intensity for another seven to eight minutes. So there is now really mounting evidence that the muscle is not a limiting factor. So then obviously the next question is what is the limiting factor? And it seems to be the mind, specifically perception of effort. 

The perception of effort is the conscious sensation of how hard and difficult exercise is and we use a very simple scales such as this one where we ask the subject to tell us which is the number that corresponds to how hard exercise feels, very simple measurement. And if you use these measures doing exercise this is exactly the same study as before, so cycling at 242 watts for as long as they could and on average it lasted 10 minutes. Over the course of these 10 minutes even if the power gap remains constant, there is a progressive increase in perception of effort, to the point the effort is perceived as maximal and therefore this the athletes believe that it's impossible to continue as exercise, because they've given it all and they are not capable of continuing, even if physiologically they are. The progressive increase in perception of effort is also depending on muscle fatigue because we never have to compensate the muscle fatigue by exerting more effort because the muscle gets weaker. However, we are now demonstrating that there are a variety of other factors that can affect perception of effort and therefore performance. One of them, quite surprising, is mental fatigue, that I'm sure you guys, students here and fellow academics are very familiar with, if you exert mentally for a prolonged period of time, you will feel tired after a few hours. However, this kind of fatigue according to the physiological model shouldn't really affect your endurance performance because it doesn't affect your muscle, it doesn't affect your heart, doesn't affect your blood. However, we did an experiment where we experimentally fatigue a group of subjects with 90 minutes with a very demanding cognitive task and immediately after that they were asked to do our endurance performance task which was a time to exhaustion test and we compare the performance in this mentally fatigued condition with the performance of the same group of subjects when they were not mentally fatigued, when they just watched documentaries for 90 minutes. 

What we found was a significant reduction in endurance performance measured by time to exhaustion in a mentally fatigued state. However, when we look at the physiological responses, such as oxygen delivery, heart rate, blood lactate, we couldn't find any effect of mental fatigue whatsoever. So, if I look at this data, for me it's impossible to explain why their performance was decreased from a purely physiological point of view. However, when I look at the negatives of perception of effort, the reason for the impaired performance is pretty evident. When they were mentally fatigued, their perception of effort was increased and as a result they reach their maximal effort earlier than in the control condition when they were not mentally fatigued. 

Another experiment that we did was using so-called motivational self-talk, this is a well-established psychological technique that consists of statements, positive statements, such as I'm going in there, go for it, dig deep, push it, you can do it. So try to avoid negative thinking and give yourself motivation to keep going and this very simple psychological technique has been taught to a group of subjects and another group served as control and was not taught any psychological skill. And again, we measure performance with a time to exhaustion test. Of course, in the control group I suspected there was no change in performance over a period of two weeks but in the subject who practised self-talk during their training and they used it in the endurance performance test, there was a significant improvement. And again, this was associated with a significant change in perception of effort, of course this time there was a reduction in perception of effort which means that they reach their maximum effort and decided to stop exercise later than in the control condition. So clearly those previous studies suggest, demonstrate experimentally that perception of effort is the limiting factor. So, what can we do about it? 

In terms of practical suggestions, the first one is to avoid mentally fatiguing conditions before a competition, so, for example avoid sleep deprivation, so try to sleep well before a competition, which is not always easy because you're anxious about the competition and avoid things like video games or other mentally fatiguing tasks but also avoid, for example, controlling too much your own emotions. Controlling emotions is very mentally fatiguing, mentally draining, so my advice to English athletes may be before competition do not keep a stiff upper lip because that maybe has a negative effect on your competition. The other one is to use in a systematic way these psychological skills, such as motivation, self-talk or goal setting or imagery which are being found to be effective in improving and endurance performance. In terms of testing, the suggestion is to include not only measure of the cardiovascular system such as heart rate monitors during training but also measure that such as questionnaires or the ratings or procedures ratio scale that measure basically how the athletes feel and track the other time to understand when an athlete is fatigued and needs to rest and recover. We are also developing in collaboration with the School of Computing here in Kent some objective measure of mental fatigue such as electroencephalography which is a technique to measure the electrical activity of the brain - so maybe in the future we'll be able to use these techniques to see whether an athlete needs to recover from a brain point of view. Also nutrition, caffeine is widely used by endurance athletes originally because they thought that it was going to improve the utilization of fat and improve performance that way. In reality the performance is improved because there is a direct effect on the brain which reduces perception of effort and we are now investigating other nutrients such as tyrosine which is a precursor of a neurotransmitter dopamine to see whether we can improve performance using this nutritional supplement. 

In terms of training we are developing collaboration with the Minister of Defence a new kind of training which combines classical exercise training with the mentally fatiguing task on a computer, so this additional, if you like brain workload, should make, there is actually evidence now that makes people more resistant to mental fatigue and as a result improve their endurance performance. But, of course, as for the previous physiological model there is also scope unfortunately for doping. Amphetamines and more recently this stimulant called Modafinil are banned substances and they improve performance because they reduce perception of effort. And ourselves and also other groups are researching brain stimulation which at the moment is not considered a doping method but maybe in the future especially because there is evidence that may be effective in improving endurance performance may be banned. 

The research we are doing can not only benefit endurance athletes, hopefully a better understanding of fatigue and how to reduce it will help other populations for example breast cancer patients receiving chemotherapy which is a condition associated with very profound and debilitating fatigue as well as in other conditions and of course this is not just about athletes it's about any human performance that requires endurance. For example, soldiers are required to work physically but also mentally for prolonged periods of time and obviously there, a reduction in performance may mean the difference between life and death or having a very serious injury. So, hopefully this research will lead to improvement in many areas which are affected by fatigue. Thank you very much.