So you want to work on your speed.
The fastest man can run at 27 mph.
Usain Bolt is "the
world's fastest man" because he has the record for the 100 metre sprint at
9.58 seconds.
But could runners go
faster?
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 and Tim
Dorn 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.
WIRED's Robbie Gonzalez
explores the science of extreme sprinting speed.
http://www.bbc.com/future/story/20120712-will-we-ever-run-100m-in-9-secs
Video: https://www.youtube.com/watch?v=SdMo9hbt2nI
