Runners who've faced off against Oscar Pistorius say they know when the South African is closing in on them from behind. They hear a distinctive clicking noise growing louder, like a pair of scissors slicing through the air—the sound of Pistorius's Flex-Foot Cheetah prosthetic legs.

It's those long, J-shaped, carbon-fiber lower legs—and the world-class race times that come with them—that have some people asking an unpopular question: Does Pistorius, the man who has overcome so much to be the first double amputee to run at an Olympic level, have an unfair advantage? Scientists are becoming entwined in a debate over whether Pistorius should be allowed to compete in the 2012 London Games.

Pistorius was born without fibulas, one of the two long bones in the lower leg. He was unable to walk as a baby, and at 11 months old both of his legs were amputated below the knee. But the growing child didn't let his disability slow him down. At age 12 he was playing rugby with the other boys, and in 2005, at age 18, he ran the 400-meter race in 47.34 seconds at the South African Championships, sixth best. Now 25, the man nicknamed the “Blade Runner” has qualified for the 2012 Summer Olympics in London, just three weeks before the games were to begin. But should he be allowed to compete?

The question seems preposterous. How could someone without lower legs possibly have an advantage over athletes with natural legs? The debate took a scientific turn in 2007 when a German team reported that Pistorius used 25 percent less energy than natural runners. The conclusion was tied to the unusual prosthetic made by an Icelandic company called Össur. The Flex-Foot Cheetah has become the go-to running prosthetic for Paralympic (and, potentially Olympic) athletes. "When the user is running, the prosthesis's J curve is compressed at impact, storing energy and absorbing high levels of stress that would otherwise be absorbed by a runner's ankle, knee, hip and lower back," explains Hilmar Janusson, executive vice president of research and development at Össur. The Cheetah's carbon-fiber layers then rebound off the ground in response to the runner's strides.

After the German report was released, the International Association of Athletics Federations (IAAF) banned Pistorius from competing. Pistorius hired Jeffrey Kessler, a high-powered lawyer who's represented athletes from the National Basketball Association and National Football League. It soon became clear that the IAAF's study was very poorly designed, so when Pistorius's team asked for a new study they got it. Soon scientists gathered at Rice University to figure out just what was going on with Pistorius's body.

The scientific team included Peter Weyand, a physiologist at Southern Methodist University who had the treadmills needed to measure the forces involved in sprinting. Rodger Kram, at the University of Colorado at Boulder, was a track and field fan who studied biomechanics. Hugh Herr, a double amputee himself, was a renowned biophysicist. The trio, and other experts, measured Pistorius's oxygen consumption, his leg movements, the forces he exerted on the ground and his endurance. They also looked at leg-repositioning time—the amount of time it takes Pistorius to swing his leg from the back to the front.

After several months the team concluded in a paper for The Journal of Applied Physiology that Pistorius was "physiologically similar but mechanically dissimilar" to someone running with intact legs. He uses oxygen the same way natural-legged sprinters do, but he moves his body differently.

The results of the Rice University study—physiologically similar, mechanically different—were presented to the Court of Arbitration for Sport (CAS) in Switzerland in 2008, which decided that Pistorius should be allowed to run, revoking the IAAF's decision. He missed qualifying for the 2008 Beijing Olympics by 0.7 second.

But then scientific controversy arose. Members of the team that had published the paper began to express very different ideas about what, exactly, "mechanically different" meant. One group said that Pistorius's differences leave him on a level running field with all the other athletes. The other said that Pistorius is mechanically different in a way that confers a serious competitive advantage.

Weyand, the scientist with the treadmills, believes that Pistorius's prosthetics allow him to move in a way that no non-prosthetics wearer could, giving him an advantage. Kram, the biomechanics expert, believes that the Blade Runner's blades hinder him just as much as they help.

One of the biggest points of contention is limb-repositioning time. The average elite male sprinter moves his leg from back to front in 0.37 second. The five most recent world record holders in the 100-meter dash averaged 0.34 second. Pistorius swings his leg in 0.28 second, largely because his Cheetah's are lighter than a regular human leg. Pistorius's rivals are swinging a lower leg that weighs about 5.7 kilograms, whereas his lower leg only weighs 2.4 kilograms.

Kram and his researchers countered with a paper claiming to have measured Walter Dix, a 100-meter sprinter, swinging his leg faster than Pistorius. But they used television footage of Dix rather than the standard, high-speed research video generally used to make such measurements. "The differences here are relatively small, so doing it with TV video isn't going to cut it," says Jesus Dapena, a biomechanics researcher at Indiana University Bloomington who was not involved in the Rice study. High-speed footage for Dix from that same season does exist, Weyand says, and it shows the runner clearly repositioning his limbs at around the same rate as the average Olympic sprinter.

Swing time is important because it affects some central factors that determine how fast a person can run. Repositioning his legs faster means Pistorius can keep his foot on the ground longer than everyone else. It's a bit counterintuitive, but Weyand argues that a runner's speed is largely determined by how long he can keep his feet on the ground, rather than in the air. The longer a foot remains on the ground, the more time the person has to generate force that will propel him forward. More force generally means more speed.

Kram argues, however, that because the Cheetahs are made of carbon fiber, and are lighter, they can't transmit nearly as much force to the ground as a human leg can, creating less forward propulsion. So Pistorius has to push down harder than most people to get the same amount of force against the ground. Weyand counters that Pistorius simply doesn't need to push as hard to run just as fast.

Of course, other researchers have other theories about a possible advantage. Because Pistorius's Cheetah's don't tire, his lower leg stays springy throughout the entire race. For most 400-meter runners the second half of the race is where the real battle happens. Jim Matin, a researcher at the University of Utah, says that the lower leg is what weakens and slows runners. Martin thinks that if Pistorius ran in a competitive 600-meter race, Pistorius could set the world record.

Some of the arguing may be moot. The fact that Pistorius runs differently does not necessarily indicate an advantage, because even the most elite sprinters have their own running styles, says Jill McNitt-Gray, a researcher at the University of Southern California who wasn't involved in the Rice study. One sprinter might use his hips more than the next. Another may rely more on his arm thrust. Amputees develop ways to interact with their prosthetic that makes sense for them. "Your body is going to figure out how best to use [the prosthetic]," she says.

In many ways, studying Pistorius is difficult. There's only one of him, and only one good study that uses his specific physiology. There are no other Olympic-level double amputees, and single-leg amputees run totally differently. Imagine your right leg could swing 10 percent faster than your left; your left leg simply could not keep up. A person with one prosthetic and one intact leg can only go as fast as his slowest leg—generally the biological one.

To complicate matters further, science doesn't totally understand how running works. "We really don't know exactly the mechanics of running," Dapena says. They have a working idea, he says, but it's possible that the forces Weyand and Kram are debating aren't important. "It's a good logic," he says, "but it's not necessarily down pat that way."

Weyland will not say outright whether or not Pistorius should be allowed to run in the Olympics. Perhaps, he says, the sprinter represents something more important than the dispute over his light, springy legs. "I admire the heck out of him," he adds. "He's an excellent athlete who's worked like crazy and persevered and overcome."

For Kram, whether Pistorius should run comes down to power. "Oscar derives all of his power from what he had for breakfast." Athletes should be in a different race only when motors or alternate power sources are introduced, he says. "When you're tired you can't just twist the throttle. You have to find that desire or have that physiological ability to push. That's what makes the Olympics special." It's what makes Pistorius special, too, Kram says. He's pushed his whole life.

Now Pistorius will represent South Africa in the 400-meter race and the 4 x 400-meter relay. And if there’s one thing everyone agrees on, it’s that the races will be intriguing to watch.