No Sprinting Advantage With Prosthetic Limbs

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No Sprinting Advantage With Prosthetic Limbs

By Michael Torrice
ScienceNOW Daily News
4 November 2009

In 2007, South African double-amputee sprinter Oscar Pistorius became the first disabled athlete to compete against able-bodied runners, placing seventh in the British Grand Prix. But his J-shaped carbon-fiber prostheses, called the Össur Flex-Foot Cheetah, sparked a debate within the athletic world: Do the devices give him an unfair advantage over able-bodied competitors? The answer, according to a new study of six amputee sprinters, is no.

Scientists debate whether the Össur Cheetah boosts performance. Some experts believed that Pistorius's setup would allow him and other amputee sprinters to move their legs faster than able-bodied runners and reach high speeds more easily. But last summer, a team of researchers at the Massachusetts Institute of Technology (MIT) in Cambridge showed that Pistorius's prosthetic limbs didn't generate as much force against the ground as biological legs.

Biomechanicist Alena Grabowski, a member of the MIT lab, and colleagues wanted to find out if these results were true for amputee sprinters in general. The scientists decided to study single amputees. That allowed them to analyze prosthetic and biological limbs on the same person and thus provide an "on-board comparison," Grabowski says. The researchers recruited six elite amputee sprinters who used devices similar to Pistorius's and had them run on a treadmill. The runners slowly lowered themselves onto the moving belt from handrails and tried to keep up with it for 10 strides. "They can reach really high speeds," Grabowski says. "It's amazing to watch."

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Great strides. Scientists studied the running mechanics of six single-amputee athletes as they sprinted on a treadmill.

Credit: Orthopedic Specialty Hospital

The treadmill measured the force, called ground reaction force (GRF), each limb generated as it struck the belt. More force means faster speeds. At all speeds, the sprinters' prosthetic limbs produced 9% less GRF than their biological limb, the scientists report online today in Biology Letters. A similar reduction in able-bodied sprinters--with no other change to their running mechanics--would drop their top speed by 9%. Moreover, the researchers found no difference in leg swing times between the two limbs, suggesting that--even though the prostheses are lighter than biological legs--amputee sprinters don't move their legs faster.

To back up their laboratory data, Grabowski and colleagues also studied videos of the 2008 Paralympic and Olympic 100-meter men's finals. They observed no significant difference between the two groups' leg swing times. "From the data we've collected so far, there doesn't seem to be any advantage in using a running-specific prosthesis," Grabowski says.

"The results are hard to refute," says physiologist Daniel Ferris of the University of Michigan, Ann Arbor. Young-Hui Chang, a comparative physiologist at the Georgia Institute of Technology in Atlanta, agrees, but he says the debate may be missing how much amputee athletes must overcome mechanically to compete alongside able-bodied runners. These devices can't generate power on their own, like biological legs can, he says. "To think that [prostheses] give you an unfair advantage may be overlooking the obvious."

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