Hello ladies and gentlemen, my name is Samuele Marcora
and I am Professor of Exercise Physiology at the University of Kent in the School
of Sport and Exercise Science and also 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
sports 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 of the of the
competition that your competitors cannot keep nd 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 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 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 fatigue muscle fatigue. And the assumption is that when
the maximum power output coincides with the power gap were 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 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 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 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
get 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 the 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 this 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 experiments 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
practiced 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 condition 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 have 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 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 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 Defense 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 they 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.