In September's Paralympics, a pair of carbon-fiber legs carried 17-year-old Oscar Pistorius to stardom. The unheralded South African won bronze in the 100 meters and set the world record —21.97 seconds—in taking the 200 gold. In the latter event, the teenager walloped Marlon Shirley and Brian Frasure, veteran American sprinters who were wearing the same model of racing leg as Pistorius, the Flex Foot Cheetah. But even as the Americans congratulated their foe, they were looking his legs up and down. Pistorius, they said, had unfairly rigged his Cheetahs—he was "racing tall."
Unlike Frasure and Shirley, both single amputees, Pistorius was racing with two Cheetahs. Unilateral and bilateral amputees often race separately in the Paralympics, but this year the two classes were combined because of a lack of competitors. If you're a single amputee, the vertical length of your prosthetic depends on the length of your good leg. But a runner like Pistorius, who had both his lower legs amputated shortly after birth, must have his leg length determined by a prosthetist.
Pistorius' first prosthetist was a South African named Francois Vanderwatt. Unable to find a suitable pair of racing legs in Pretoria, Vanderwatt eventually convinced an engineer from a local airplane company to design some carbon-fiber limbs. But according to Vanderwatt, these airplane prostheses kept breaking. So, a few months before the Paralympics, Pistorius and Vanderwatt went to see the American sprinter Frasure, who's both a Paralympics veteran and a well-connected prosthetist. Frasure offered his young rival some sprinting tips and, along with Vanderwatt, fitted Pistorius with some top-shelf Cheetahs. According to Vanderwatt, they used wingspan and femur measurements to come up with a conservative estimate of Pistorius' anatomical height. Pistorius left the United States racing short, not tall.
But, according to Frasure, Pistorius had a growth spurt before he showed up in Athens—his Cheetahs appeared to be roughly 2 inches longer. Perhaps the South African's blazing speed owed less to natural talent than to his new, unnatural stride. (Reached on his cell phone in South Africa, the young sprinter's father, Henke Pistorius, called the allegations a "bunch of bull." Henke said that Oscar was unavailable for comment because he was studying for his high school's final exams.)
The International Sports Organization for the Disabled does have a formula to determine the maximum allowable leg length for bilateral amputees: (length of thigh bone minus 13 centimeters) divided by .4. But according to Chris Cohen, the chairman of athletics for the International Paralympic Committee, Paralympic officials have never whipped out a ruler to measure athletes' thigh bones or Cheetahs.
If the IPC were to start checking up on leg height, that would violate the spirit of the Paralympics *. Recent crackdowns have halted breakthroughs in designer steroids and blood doping, but the world's track-and-field scientists remain unfettered in their quest to build the perfect prosthetic leg. While the IPC has adopted rules to stop, for example, runaway wheelchair technology, they have a hands-off policy toward legs. According to the IPC rule book, "hopping is not allowed"—in other words, no prosthetic pogo sticks. Otherwise, anything goes. If a racer wants to show up wearing 30-foot stilts made of Kevlar and bamboo, that's OK.
The market for state-of-the-art sprinting limbs is minuscule, but prosthesis companies spend heavily on research and development for the same reason car companies build fuel-injected racers—racing-leg technology might one day help create a commercially viable leg that's perfect for pickup basketball. The first leap forward in racing-leg tech came in the mid-1980s. After growing frustrated with wooden legs, Van Phillips, an outdoorsy American engineer who had lost a leg in a water-skiing accident, teamed up with an aerospace engineer to create the first carbon-fiber prosthetic leg. When the new limbs debuted in the 1988 Paralympic Games, the world record in the 100-meter dash fell nearly 1.5 seconds to 11.73.
In the past 15 years, carbon-fiber sprinting legs have evolved to look less and less like the real thing. Practical concerns outweigh cosmetic ones: Since a high-end prosthesis has to act like a spring, it makes sense that it would look like a spring. The Flex Foot Cheetah, made by the Icelandic company Ossur, looks less like the spotted, fuzzy leg that propels the world's fastest animal across the Serengeti than a short cross-country ski bent in the shape of a sickle. The synthetic black blade terminates in a small, flat foot that looks like a table coaster.
A bad racing leg feels stiff like a baseball bat. A good one feels bouncy like a diving board. That's because leg designers have to maximize the limb's "energy return," making sure that when the foot hits the ground, as much of that energy as possible gets pushed back toward the sprinter. An efficient racing leg coils on impact and then promptly recoils, propelling the racer toward the finish line. Because of its total lack of shock absorption, walking long distances on a Cheetah is precarious and uncomfortable—this is not an everyday leg. After a race, sprinters change into something that looks a bit more like a human leg for the walk home.
Soon enough, legs won't just look strange on the outside. European scientists have started experimenting with osseo-integration, in which the prosthetic is anchored directly to an amputee's remaining bone. Researchers have also dabbled with myoelectric gams that use the natural electricity given off by muscles to trigger electromechanical devices that control the body's basic motor functions. And why stop at electricity when you can use a whole circuit board? In 1997 *, a German company introduced a computer-controlled device called the C-Leg. A microprocessor collects information from the shin and foot 50 times per second, then uses the data to regulate the amputee's next step. So far, the C-Leg has defied the trickle-down theory of leg technology. While its improved stability on stairs has already proved useful to everyday consumers (at least, those with a lot of cash), the C-Leg has yet to find a foothold with sprinters who run on flat surfaces.