Just found this in last weeks new scientist. Should be
useful for all you competitive types out there. Got a fair
bit of relevence to cyclists. If you want to see it in its
proper home try: archive.newscientist.comartOpen ↗
icle/article.jsp?rp=1 &id=mg18124395.400 but it requires a
free registration.
Running on empty
New Scientist vol 181 issue 2439 - 20 March 2004, page 42
Your lungs are bursting and your muscles are screaming at
you to stop. Can it really be possible that fatique is all
in the mind? Rick Lovett reports
TIMOTHY NOAKES will never forget the day he encountered the
hill from hell. It was 1976 and he was running the gruelling
Comrades Marathon, an annual 90-kilometre road race between
Durban and Pietermaritzburg in South Africa. About 20
kilometres from home he rounded a bend and saw a steep
incline he hadn't known was there. Even before he started
climbing, he suddenly began to feel overwhelmingly tired.
At the time it was just a case of gritting his teeth. But
Noakes, a professor of exercise physiology at the University
of Cape Town, South Africa, soon came to see that hill as an
intellectual mountain, too. Why had the very thought of it
made him feel so tired?
Conventional wisdom on muscle fatigue can't explain what
happened that day. For the better part of a century,
scientists and athletes have presumed, not unreasonably,
that fatigue originates in the muscles themselves. Precise
explanations have varied, but all have been based on the
"limitations theory". In other words, muscles tire because
they hit a physical limit: they either run out of fuel or
oxygen or they drown in toxic by-products.
In the past few years, however, Noakes and his colleague
Alan St Clair Gibson have taken a hard look at the standard
theory. The deeper they dig, the more convinced they have
become that physical fatigue simply isn't the same as a car
running out of petrol. Fatigue, they argue, is caused not by
distress signals springing from overtaxed muscles, but is an
emotional response which begins in the brain.
The essence of their new theory is that the brain, using a
mix of physiological, subconscious and conscious cues, paces
the muscles to keep them well back from the brink of
exhaustion. When the brain decides it's time to quit, it
creates the distressing sensations we interpret as
unbearable muscle fatigue. This "central governor" theory
remains controversial, but it does explain many puzzling
aspects of athletic performance, as well as suggesting some
revolutionary approaches to training and offering
tantalising hints as to the cause and maybe even the cure of
chronic fatigue syndrome.
The hill from hell might have set Noakes thinking about
fatigue, but it was a more recent discovery that made him
start researching it in earnest. He calls this the
"lactic acid paradox". Lactic acid is a by-product of
exercise, and its build-up is often cited as a cause of
fatigue. But when research subjects exercise in a
decompression chamber designed to simulate high altitude,
they become fatigued even though lactic acid levels
remain low. Nor has the oxygen content of their blood
fallen too low for them to keep going. Obviously, Noakes
deduced, something else was making them tire well before
they hit either physiological limit.
Noakes and St Clair Gibson decided to probe further. For
their first study, published in 2001 (American Journal of
Physiology - Regulatory Integrative and Comparative
Physiology, vol 281, p R187), they recruited seven
experienced cyclists and asked them to pedal 100-kilometre
time trials on stationary exercise bikes. On several
occasions during the time trial, they asked the cyclists to
sprint for 1000 or 4000 metres. Throughout the experiment,
the cyclists wore electrical sensors taped to their legs to
measure the nerve impulses travelling to their muscles.
It has long been known that during exercise, the body never
uses 100 per cent of the available muscle fibres in a single
contraction. The amount used varies with the length of the
endeavour, but in endurance tasks such as the cycling test
the body calls on about 30 per cent, spreading the load by
rotating in fresh ones as needed. And because separate nerve
filaments send signals to each fibre, sports scientists can
determine what fraction of the muscle is being used by
measuring the electrical impulse travelling to it.
Noakes reasoned that if the limitations theory was correct
and fatigue was due to muscle fibres hitting some limit, the
number of fibres used for each pedal stroke should increase
as the fibres tired and the cyclist's body attempted to
compensate by recruiting an ever-larger fraction of the
total. But his team found exactly the opposite. As fatigue
set in, the electrical activity in the cyclists' legs
declined - even during the sprints, when they were striving
to cycle as fast as they could.
Plenty in the tank
To Noakes, this was strong evidence that the old theory was
wrong. The cyclists may have felt completely done in, he
says, but their bodies actually had considerable reserves
that they could theoretically tap by using a greater
fraction of the resting fibres. This, he believes, is proof
that the brain is regulating the pace of the workout to hold
the cyclists well back from the point of catastrophic
exhaustion.
More evidence comes from the fact that fatigued muscles
don't actually run out of anything critical. Muscle biopsies
have shown that levels of glycogen, which is the muscles'
primary fuel, and ATP, the chemical they use for temporary
energy storage, decline with exercise but never bottom out.
Even at the end of a marathon, ATP levels are 80 to 90 per
cent of the resting norm. And while glycogen levels approach
zero, they never get there. Post-marathon muscles also still
have substantial reserves of other fuels, notably fat.
Still more evidence in favour of the central regulator comes
from observations of the closing stages of distance races.
Top athletes almost always manage to go their fastest during
the last kilometre of a race, even though, theoretically,
that's when their muscles should be closest to exhaustion.
In particular, Noakes says, the end spurt makes no sense if
fatigue is caused by muscles poisoning themselves with
lactic acid. If lactic acid build-up is the limiting factor,
racers would progressively slow down and would find it
impossible to sprint for the finish line.
But with the central governor theory, the explanation is
obvious. Knowing the end is near, the brain slightly relaxes
its vigil and allows the athlete to tap a bit of the body's
carefully hoarded reserves.
But the central governor theory does not mean that what's
happening in the muscles is irrelevant. The governor
constantly monitors physiological signals from the muscles,
along with other information, to set the level of fatigue. A
large number of signals are probably involved, but the ones
Noakes is most sure about include the body's remaining
stores of carbohydrates, the levels of glucose and oxygen in
the blood, the rates of heat generation and heat loss, and
the rate at which muscles are working. Where the central
governor theory differs from the limitations theory is that
these physiological factors are not the direct determinants
of fatigue - they are just information to take into account.
Conscious factors can also intervene. Noakes believes that
the central regulator evaluates the planned workout, and
sets a pacing strategy accordingly. Experienced runners
know, for example, that if they set out on a 10-kilometre
training run, the first kilometre feels mysteriously easier
than the first kilometre of a 5-kilometre run, even though
there should be no difference. That, Noakes says, is because
the central governor knows you have farther to go in the
longer run and has programmed itself to dole out fatigue
symptoms accordingly.
This can be verified by putting people on treadmills and
telling them they're going to run one distance when in fact
you have another planned. When the subjects are given the
real story midway through the test, their reported levels of
fatigue suddenly adjust to account for the new information.
It also explains Noakes's experience on the hill from hell.
"The central governor had been pacing me for another 20
kilometres," he says, "but it had presumed it was going to
be flat. Now, it suddenly had to take the hill into account,
and it forced me to slow down."
St Clair Gibson believes there is a good reason why our
bodies are designed to keep something back. That way,
there's always something left in the tank for an emergency.
In ancient times, an emergency might take the form of a lion
or pack of wolves at the end of a long, gruelling hunt.
Today, the "wolf" might be a mugger hiding in an alley, or a
lightning storm near the end of a long hike. But the same
concept applies: life would be too dangerous if our bodies
allowed us to become so tired that we couldn't move quickly
when faced with an unexpected need.
Drugs and hypnosis
The team also believes the central governor theory helps to
explain why hypnosis helps block sensations of fatigue,
allowing athletes to work harder. If fatigue were merely the
result of hitting the muscles' physiological limits, this
shouldn't be possible. But it is. Amphetamines have a
similar effect, and again it could be down to the central
governor. Blocking the sensation of fatigue with drugs,
however, makes it much easier to work yourself to death.
Normally, fatigue will force even the most iron-willed
competitor to quit before they succumb to heatstroke, but
this didn't happen for the British cyclist Tom Simpson, who
died after taking amphetamine during the Tour de France in
1967, the year before drug tests started. Ecstasy, Noakes
adds, is an amphetamine-like substance that could have the
same effect on clubbers.
The theory could also help to unravel the mystery of chronic
fatigue syndrome. Perhaps something has interfered with the
brain's regulation of fatigue so that you always feel
exhausted even though you are not. Successfully puzzling out
the workings of the central governor might open the door to
a long-awaited cure, Noakes suggests.
St Clair Gibson and Noakes are presently trying to find
where the central governor is located in the brain by
studying the electroencephalograms (EEGs) of tiring
cyclists. "We're finding that a lot of areas of the brain
are involved," St Clair Gibson says, "but we haven't yet
found the stop switch." However, the mix of such areas is
interesting, and includes the frontal lobe (which is
involved in decision making), the parietal lobe (which is
involved in sensation), and, for some reason, the visual and
speech centres.
The central governor theory has found favour with other
exercise physiologists. George Brooks at the University of
California, Berkeley, for example, recently amended his
textbook to include it. But for some it remains
controversial.
One critic is Jere Mitchell, a cardiologist at the
University of Texas Southwestern Medical Center, Dallas.
He points to treadmill tests in which people run up ever-
steeper slopes while having their oxygen consumption
measured. Shortly before the subjects collapse in
exhaustion, their oxygen consumption reaches a plateau
beyond which it won't increase, no matter how hard they
try to work.
This maximum rate of oxygen consumption, called VO2 max, can
be boosted by increasing the number of red blood cells in
circulation - for example, by re-injecting blood that was
taken several weeks earlier. This proves that fatigue has
nothing to do with any central governor, Mitchell argues.
Instead, it kicks in at the point at which the body has
bumped into a very real physiological limit - the amount of
oxygen the blood can transport.
Peter Wagner of the University of California, San Diego,
concurs. He has conducted treadmill tests in which athletes
are tested under two different conditions: on normal air,
and on pure oxygen. That is enough to produce an 8 to 10 per
cent increase in the amount of oxygen going to the muscles,
he says, producing a measurable increase the VO2 max in well-
trained athletes.
Noakes and St Clair Gibson, however, argue that the central
governor theory can explain both studies. The brain, they
say, senses the elevated amount of oxygen in the blood and
then "resets" the pace to allow the athlete to work harder,
while still maintaining a reserve. "So there is a ceiling of
oxygen use," says St Clair Gibson, "but at a level decided
by the brain, with a wide margin of reserve for error."
If the central governor theory does prove to be correct, can
coaches use it to improve athletes' performance? Noakes's
experience on the Comrades Marathon underscores the
importance of knowing the course beforehand - particularly
its later stages. Top athletes and coaches figured that one
out many years ago. In fact, says Brooks, trainers are often
ahead of the science. "Coaches, by experience, have
discovered things which scientists take longer to
understand," he says. But Noakes argues that the central
governor theory helps make sense of interval training, a
"sharpening" technique in which athletes do repetitive bouts
of high-intensity exercise interspersed with recovery breaks
(see Graphic).
In a recent experiment, Noakes took a group of cyclists who
had never done intervals before and asked them to add them
to their normal training, once or twice a week for six
weeks. At the end of this programme the cyclists, who were
fast recreational riders but not professional racers, had
shaved a startling 15 minutes, or approximately 10 per cent,
off their previous times on a 100-kilometre time trial.
Similarly dramatic improvements are often observed when
runners are introduced to interval training. Traditional
theory says that the improvement is due to physiological
changes in the muscle cells that make them better able to
use oxygen or tolerate the build-up of metabolic waste
products. But Noakes doesn't see how major physiological
improvements can occur so quickly. And in any case, he says,
interval training seems to induce very little, if any,
biochemical change in the muscle. He believes that interval
training works largely by teaching the central governor that
going faster won't do you any harm.
Perhaps, then, the central governor idea can be used to give
athletes an important mental edge. Simply telling them that
even when they are feeling completely exhausted their bodies
actually have a lot in reserve should provide an incredible
psychological boost, says St Clair Gibson. "When athletes
know that," he says, "it's going to be exciting."