Science and techno world topic: Future
To understand how fast a human can ultimately run, we
need to go beyond the record books and understand how Usain Bolt's legs work.
To understand how fast a human can ultimately run, we
need to go beyond the record books and understand how Usain Bolt's legs work.
In 2008, at the Beijing Olympic Games, Jamaican sprinter
Usain Bolt ran the 100m in just 9.69 seconds, setting a new world record. A
year later, Bolt surpassed his own feat with an astonishing 9.58-second run at
the 2009 Berlin World Championships. With the 2012 Olympic Games set to begin
in London, the sporting world hopes Bolt will overcome his recent hamstring
problems and lead yet another victorious attack on the sprinting record. He is
arguably the fastest man in history, but just how fast could be possibly go?
That’s a surprisingly difficult question to answer, and
ploughing through the record books is of little help. “People have played with
the statistical data so much and made so many predictions. I don’t think people
who work on mechanics take them very seriously,” says John Hutchinson, who
studies how animals move at the Royal Veterinary College in London, UK.
The problem is that the progression of sprinting records
is characterised by tortoise-like lulls and hare-like… well… sprints. People
are getting faster, but in an unpredictable way. From 1991 to 2007, eight
athletes chipped 0.16 seconds off the record. Bolt did the same in just over
one year. Before 2008, mathematician Reza Noubary calculated that “the ultimate
time for [the] 100 meter dash is 9.44 seconds.” Following Bolt’s Beijing
performance, he told Wired that the prediction “would probably go down a little
bit”.
John Barrow from the University of Cambridge – another
mathematician – has identified three ways in which Bolt could improve his
speed: being quicker off the mark; running with a stronger tailwind; and
running at higher altitudes where thinner air would exert less drag upon him.
These tricks may work, but they’re also somewhat unsatisfying. We really want
to know whether flexing muscles and bending joints could send a sprinter over
the finish line in 9 seconds, without relying on environmental providence.
To answer that, we have to look at the physics of a
sprinting leg. And that means running headfirst into a wall of ignorance. “It’s
tougher to get a handle on sprinting mechanics than on feats of strength or
endurance,” says Peter Weyand from Southern Methodist University, who has been
studying the science of running for decades. By comparison, Weyand says that we
can tweak a cyclist’s weight, position and aerodynamic shape, and predict how
that will affect their performance in the Tour de France. “We know down to 1%,
or maybe even smaller, what sort of performance bumps you’ll get,” he says. “In
sprinting, it’s a black hole. You don’t have those sorts of predictive
relationships.”
Our ignorance is understandable. By their nature, sprints
are very short, so scientists can only make measurements in a limited window of
time. On top of that, the factors that govern running speed are anything but
intuitive.
Sole power
Weyand divides each cycle of a runner’s leg into what
happens when their foot is in the air, and what happens when it’s on the
ground. The former is surprisingly irrelevant. Back in 2000, Weyand showed
that, at top speed, every runner takes around a third of a second to pick their
foot up and put it down again. “It’s the same from Usain Bolt to Grandma,” he
says. “She can’t run as fast as him but at her top speed, she’s repositioning
her foot at the same speed.”
That third of a second in the air – the swing time – is
probably close to a biological limit. Weyand thinks that there is very little
that people can do to improve on it, with a notable exception. Oscar Pistorius,
the South African double-amputee, runs on artificial carbon-fibre legs that
each weigh less than half of what a normal fleshy limb would do. With this
lighter load, he can swing his legs around 20% faster than a runner with intact
limbs, moving at the same speed.
For most runners though, speed is largely determined by
how much force they can apply when their foot is on the ground. They have two
simple options for running faster: hit the ground harder, or exert the same
force over a longer period.
The second option partly explains why greyhounds and
cheetahs are so fast. They maximise their time on the ground using their bendy
backbones. As their front feet land, their spines bend and collapse, so their
back halves spend more time in the air before they have to come down. Then,
their spines decompress, giving their front halves more time in the air and
their back legs more time on the ground.
Such tricks aren’t available to us two-legged humans, but
technology provides alternatives. In the 1990s, speed skaters started using a
new breed of “clap skates” where the blade is hinged to the front of the boot,
rather than firmly fixed. As the skaters pushed back, the new design kept their
blades in longer contact with the ice, allowing them to exert the same force
over more time. Speed records suddenly fell.
People have tried to duplicate the same effect with
running shoes, but with little success. That’s because a running leg behaves a
bit like a pogo stick. As it hits the ground, it compresses. As it steps off,
it gets a bit of elastic rebound. Technologies that try to alter a runner’s
gait tend to interfere with this rebound, and diminish the leg’s overall performance.
“It’s hard to intervene in a similar manner to the clap-skates without
buggering up the other mechanics of the limb,” says Weyand. (Again, Pistorius
bucks the trend because his artificial legs are springier than natural ones,
and give him around 10% longer on the ground than other runners.)
Ground force
For those with intact limbs, one option remains: exert
more force on the ground. Put simply, fast people hit the ground more
forcefully than slow people, relative to their body weight. But we know very
little about what contributes to that force, and we are terrible at predicting
it based on a runner’s physique or movements.
We know that champion male sprinters can hit the ground
with a force that’s around 2.5 times their body weight (most people manage
around two times). When Usain Bolt’s foot lands, it applies around 900 pounds
(400kg) of force for a few milliseconds, and continues pushing for around 90
more.
Weyand likes to imagine a weightlifter trying to apply
the same force in a one-legged squat – they would come nowhere close. “What we know about force under static
conditions under-predicts how hard sprinters hit by a factor of two,” he says.
“We just don’t have the ability to go from the movements of the body to the
force on the ground.” Even if a sprinter’s muscles were eventually boosted by
gene doping techniques, we have no way of calculating how much faster their
owners would run.
Studies are underway to fill in those gaps, and Weyand is
hoping that we’ll be able to make better predictions in five or 10 years. Just
a few months ago, Marcus Pandy from the University of Melbourne used computer
simulations of sprinters to show that the calf muscles, more than any others,
determine the amount of force that runners apply to the ground. At top speeds,
the hip muscles become increasingly important too. “Maybe if you train a
sprinter, you could potentially train them to have really strong calves,” says
Hutchinson.
For the moment, however, any predictions about the
ceilings of human speed are still ill-informed ones. The only way to work out
if Bolt or some other sprinter will smash the existing record is to watch them.
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