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Unfair Disadvantage?
August 06, 2012
A double amputee sprinter sounds like the definition of an underdog, but Oscar Pistorius's prostheses allow him to do things that other humans cannot
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August 06, 2012

Unfair Disadvantage?

A double amputee sprinter sounds like the definition of an underdog, but Oscar Pistorius's prostheses allow him to do things that other humans cannot

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WHEN SOUTH African sprinter Oscar Pistorius strode into the opening ceremony last Friday night on prosthetic lower legs, the roar was commensurate with his achievements. Not only is Pistorius his country's best hope in the 400 meters and the first double amputee ever to compete in the Olympics, he is also an A-list celebrity back home. By the time the London Games end, he may be a global star. And yet this moment almost didn't occur.

In January 2008, Pistorius, whose legs were amputated below the knee when he was 11 months old because of a congenital defect, was barred from able-bodied competition by the IAAF, track's governing body, after a scientist it had commissioned to evaluate the runner's carbon-fiber Cheetah Flex-Feet claimed that the prosthetics allowed Pistorius to expend less energy than intact-limbed runners. But the ban was overturned later that year when a group of scientists proved to the Court of Arbitration for Sport (CAS) that Pistorius tires at a normal rate and that energy efficiency has no more to do with sprinting than fuel efficiency does with drag racing.

Among the scientists was Hugh Herr, an MIT professor who has more than a little in common with Pistorius. Herr was a mountain-climbing prodigy at age 17, in 1982, when he lost both lower legs to frostbite suffered on a trek. Immediately he designed climbing-specific prostheses whose lengths he could change in mid-ascent. Almost as quickly, Herr's competitors wanted him disqualified from climbing competitions. Herr has been Pistorius's most vocal supporter, insisting that if anything, Pistorius is at a disadvantage.

The CAS ruling explicitly left open the possibility that the Cheetah legs could later be shown to confer other advantages, and the year after Pistorius was reinstated, two of the sprint biomechanics experts who helped clear him on energy efficiency contended that he nonetheless has a mechanical advantage. The controversy has followed Pistorius to London, where Michael Johnson, his friend and the 400-meter world-record holder, said that he should not be in the Olympics.

IT WAS dead obvious as soon as we saw the data that Oscar has an advantage," says Peter Weyand, a biomechanist at SMU and one of the scientists who led the testing on Pistorius. To understand his reasoning, it helps to know a bit about the mechanics of sprinting.

All sprinters run essentially the same way. Sure, Usain Bolt is 6'5" and flies down the track smirking, while Tyson Gay is 5'11" and runs with his eyelids peeled back. But biomechanically they are doing the same thing. At top speed, each piston pump of a sprinter's leg slams a foot down on the ground for less than a 10th of a second. In that instant—briefer than the blink of an eye—the sprinter applies enough force to propel himself forward and lift his body back into the air for slightly more than a 10th of a second. That's how long he needs to bring the other leg forward and pound the track once again. And it isn't just top male sprinters such as Bolt and Gay who have this in common. It is also female stars such as Allyson Felix and Carmelita Jeter—and other sprinters, male and female, who have no hope of getting past first-round heats in London.

A primary difference between the best sprinters and their slower competitors lies in how much force each one applies in that fraction of a second when his foot is on the ground. (An average person running at top speed applies a force of about twice his body weight; Gay applies closer to 2½ times his body weight.) The rate at which a sprinter swings his legs through the air might also seem important in differentiating him from his rivals, but all able-bodied sprinters swing their legs at nearly the same rate: about a third of a second between strides. "All the fast guys do it the same way," Weyand says. "If you know their top speed and their leg length, without knowing anything else you can predict the time they'll spend on the ground and the time in the air and the ground forces."

In 2000, Weyand and a team of researchers at Harvard published a study showing that nearly all humans, from couch potatoes to pro sprinters, have essentially the same leg-swing times when they achieve their maximum speed. Says Weyand, "The line we use around the lab is, From Usain Bolt to Grandma, they reposition their limbs in virtually the same amount of time."

But Pistorius's leg-swing times, when measured on a specially equipped treadmill, were off the charts. At top speed the South African swings his legs between strides in 0.284 of a second, which is about 20% faster than intact-limbed sprinters with the same top speed. "His limbs are 20% lighter," Weyand says, "and he swings them 20% faster."

This is important because it allows Pistorius to circumvent a main requirement of elite sprinting: putting high forces into the ground quickly. Because Pistorius can make up time with his rapid leg swing, he can leave his foot in contact with the ground longer than other sprinters. To attain the same speed, Pistorius applies lower forces (about 20% lower) over a longer time, instead of higher forces over a briefer time. In this he's like a cross-country skier, whose boot has a hinge at the toe that allows him to leave the ski down and continue to push, prolonging the time he can apply force.

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