"Blade Runner" Has No Clear Edge
Double amputee Oscar Pistorius, who runs on artificial legs made of carbon fiber, definitely sprints differently than intact-legged elite runners, according to newly released research. While he appears to have no clear advantage over runners with two legs, it remains uncertain how individual components of his unique stride affect his speed. Even the scientists who carried out the research disagree about how to interpret the findings, highlighting just how difficult it is to identify the variables that are most important for sprint speed.
“We found that the amount of metabolic energy and his rate of fatigue were not distinct from runners with intact limbs,” says Hugh Herr, director of the Biomechatronics Group at the MIT Media Lab and one of the scientists involved in the study. “However, his biomechanics were distinct from intact-legged runners.”
Pistorius, a South African sprinter, has been at the center of a high-profile battle over whether athletes with prostheses should be allowed to compete against intact-limbed athletes in events including the Olympics. Pistorius, nicknamed “the Blade Runner” because of his J-shaped Cheetah Flex-Foot carbon fiber artificial limbs, has set a number of Paralympic world records, and in 2007 he placed second in the South African National Championships–an able-bodied event. His impressive performances have made some wonder whether prostheses specially designed for running might give amputees a special advantage.
In the spring of 2007, the International Association of Athletics Federations (IAAF), the international governing body for athletics, instituted a rule prohibiting the use of technical devices that might give athletes a competitive advantage, and later prohibited Pistorius from participating in competitions under its rule. Pistorius appealed the decision, enlisting the aid of several U.S. scientists, and the Court of Arbitration for Sport (CAS), an international arbitration body, ruled that the IAAF had not provided sufficient evidence to show that the blades gave him an advantage. It reversed the ban in May 2008, enabling Pistorius to try out for the 2008 Olympics. (He narrowly failed to qualify for the South African team in the 400-meter dash.) The U.S. researchers have now published the results of their study in the Journal of Applied Physiology.
According to Peter Weyand, a physiologist and biomechanist at Southern Methodist University, in Dallas, and lead author of the study, much of Pistorius’s hearing focused on the wrong issue. “There was a lot of attention given to the question of whether his blades allowed him to run with less energy than other runners, which is pretty much irrelevant in sprinting,” says Weyand. “It’s sort of like arguing that a Volkswagen will beat a Porsche in a drag race because it gets better gas mileage.” Fuel economy is not the determining factor in sprint races, he explains: “When sprinting, animals are not energy limited; the mechanics are the limiting factor.”
Weyand’s study does, however, suggest that Pistorius has different running mechanics than intact-legged runners do: he hits the ground with much less force and stays in contact with the ground longer, a pattern that previous research suggests would put him at a disadvantage. “We know from past experiments that what separates really fast runners is how hard they can hit the ground in relation to their body weight,” says Weyand.
Previous research also shows that both elite and ordinary runners with intact legs tend to move their limbs at a similar speed. Pistorius, on the other hand, “can reposition his limbs a lot faster than anyone we’ve ever measured,” says Weyand. But the scientists don’t yet know how to interpret this finding: does it represent an advantage of his comparatively light carbon limbs, or is it merely compensation for the fact that he can’t hit the ground with as much force as intact-limbed runners? “There is no real evidence he has an advantage over others, and there is some evidence the prostheses are a hindrance,” says Daniel Ferris, a biomechanist at the University of Michigan, in Ann Arbor, who was not involved in the study.
“The science is still immature, and we don’t know for certain why he’s mechanically distinct–whether it’s because of his prostheses or because of his biology,” says Herr. One way to answer that question would be to study a runner with one intact and one prosthetic leg and directly compare the biological side to the artificial side–an experiment that Herr says is in the works.
One of the major areas of controversy surrounding Pistorius’s performance is the fact that he can run the second half of the 400-meter race faster than the first half–an unusual pattern for sprinters. Some believe that this is direct evidence that his prostheses give him an advantage: they argue that because he does not have muscle below the knee, he would not suffer the same fatigue. But Weyand and his collaborators found that all runners, including Pistorius, appear to follow the same fatigue curve.
One possible explanation for Pistorius’s unusual pattern, says Herr, is that because he does not have calf muscles, the amputee runner is actually at a disadvantage during the first 200 meters–the acceleration phase of the race. It may be in the second half of the race that Pistorius’s inherent talent becomes clear. “Oscar is an outlier,” says Herr, who is a double amputee himself. “The Cheetah has been available to athletes for 15 years, but no one has been able to run as fast as Oscar.” However, Herr says that scientists haven’t yet studied Pistorius and others as they accelerate.
The research is also helping scientists better understand the basics of running. “The Oscar Pistorius case has injected a great deal of interest in the area of bipedal sprinting,” says Herr. “By looking at the differences between amputee and intact-legged runners, we can more fundamentally understand the running mechanism and what is most important for speed.” Relatively little research has been done on the mechanics of sprinting, even in intact-legged runners, partly because it’s difficult to study people moving at such fast speeds. The new research was done using a special treadmill–one of only two or three such machines in the country.
Ferris says that the findings also point to ways that running prostheses could be improved. “One thing to try would be a prosthesis with adjustable stiffness,” he says. “That way, runners may be able to generate higher forces at certain points in the race.”
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