Running on empty

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Running on empty

 

New Scientist vol 181 issue 2439 - 20 March 2004, page 42

http://www.newscientist.com/news/news.jsp?id=ns99996208

 

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. "