Blade runners: Do high-tech prostheses give runners an unfair advantage?
This article was originally published by Scientific American.
Paralympic long jump champ Markus Rehm’s bid to compete in the 2016 Rio de Janeiro Olympics fell short in July when he could not prove that his carbon-fiber “blade” prosthesis didn’t give him an advantage. His baffling case serves as a reminder that four years after South African sprinter Oscar Pistorius propelled himself into history as the first amputee Olympic athlete to compete using blade prostheses, the technology’s impact on performance remains unclear despite ongoing research.
Blade prostheses, like Rehm uses on his right leg and Pistorius used on both, share some characteristics with biological limbs. The blades store energy as they bear the runner’s weight and then release it as the runner pushes off the ground, much the way a leg’s calf muscles and Achilles’ tendons spring and recoil. But an important difference is the foot, which on a blade prosthetic does not pivot or generate its own energy. A biological foot has muscle fibers that help it push off the ground in a way that creates “metabolic efficiency so your muscles don’t have to put all of the work back in with every step as you’re running,” says David Morgenroth, an assistant professor in the University of Washington’s Department of Rehabilitation Medicine.
A runner using biological limbs can also adjust the stiffness of leg muscles and the angle at which a foot strikes the ground on the fly to accommodate any changes in a running surface. But the stiffness and shape of a blade cannot be changed once it has been fitted to the runner, as it is custom-optimized for a particular athlete to run under very specific conditions. “That could be a disadvantage when you’re trying to get up to speed as a runner, such as when you’re coming out of the starting blocks. It’s an entirely passive system,” Morgenroth says.
The blades do have competitive benefits, however. Once a runner on blades accelerates to top speed, one potential advantage lies in the ability to move the prostheses faster and with less effort — because the blades weigh less than a competitor’s lower legs and feet. Researchers who have studied blade prostheses disagree fiercely over the net impact of these pros and cons on overall performance.
The Pistorius factor
Shortly after track and field’s governing body, the International Association of Athletics Federations (IAAF), banned Pistorius in 2008 from competing against so-called “able-bodied” competitors, he underwent a series of tests at Rice University’s Locomotion Laboratory in an attempt to be reinstated. The researchers concluded that Pistorius used 17 percent less energy than that of elite sprinters on intact limbs. The tests also revealed that it took the South African 21 percent less time to reposition, or swing, his legs between strides. Big disagreements arose over how to interpret the research.
Southern Methodist University’s Peter Weyand and Matt Bundle from the University of Montana saw a clear overall advantage in Pistorius’s faster leg swings and more energy-efficient stride, which they said could create up to a seven-second advantage in the 400-meter race. “The more mass you have closer to the axis — in this case, your hips — the easier it is to stop the rotation and then turn it around,” Bundle says. “Whereas if you had that same amount of mass located a long way away from the axis — in your lower legs and feet — it becomes much more difficult to stop it and get it going in the opposite direction.”
The other researchers — including head of Massachusetts Institute of Technology Media Lab’s Biomechatronics research group Hugh Herr, former Media Lab Biomechatronics postdoctoral fellow Alena Grabowski and Rodger Kram, an associate professor in the University of Colorado Boulder’s Integrative Physiology Department — determined there was “insufficient evidence” to prove Pistorius’ carbon-fiber Flex-Foot Cheetah prostheses gave him an advantage. Their work helped persuade the Court of Arbitration for Sport to overturn the IAAF ban. The sprinter would go on to compete in the 400-meter and 400-meter relay races at the 2012 Summer Olympics in London.
Leap of faith in science
Rehm, whose lower right leg was severely damaged by a boat propeller when he was a teenager, won the German national long jump title in 2014. Officials from Germany’s track and field governing body later barred him from competing in the 2014 European Championships in Zurich due to concerns that his blade was creating an unfair advantage. In 2015, as Rehm sought a way to continue competing in mainstream events, the IAAF changed its rules, requiring amputee athletes prove a prosthesis does not give them an edge. In a bid to compete in the 2016 Rio Olympics, Rehm turned to Grabowski, now director of the University of Colorado Boulder’s Applied Biomechanics Laboratory, and her colleagues at the German Sport University in Cologne and National Institute of Advanced Industrial Science and Technology in Japan.
Long jumpers such as Rehm rely on a fast run-up sprint followed by an efficient takeoff technique to propel them both vertically and horizontally over a sand pit. During takeoff a jumper lowers his center of mass and pushes off of one leg to quickly get as high in the air as possible without sacrificing forward velocity. In general, the faster the run-up speed the farther a competitor will jump. Grabowski and her colleagues found that Rehm and other world-class long jumpers with a below-the-knee amputation use a fundamentally different technique than competitors who do not need a prosthesis. The blade’s passive-elastic nature may limit a jumper’s top sprinting speed (a disadvantage) but it enables better takeoff technique (an advantage), Grabowski says. Ultimately the researchers could not say conclusively whether or not Rehm’s prosthetic gave him an overall advantage, effectively ending his hopes of competing in Rio. Rehm is now part of an IAAF working group studying prosthesis use in athletic competition, in hopes of competing in the 2017 IAAF World Championships.
Although Grabowski’s long jump study was inconclusive, her research is far from the finish line. In March Grabowski, Kram and research associate Paolo Taboga reported in that athletes with a left leg prosthesis are at a disadvantage in track events of 200 meters or more. Having their blade leg on the inside of the counter-clockwise curve made them generally 4 percent slower than those wearing right leg prostheses. The disparity was less pronounced in the outer lanes where the curve radius is not as great. “One of our objectives was to understand how [a] prosthesis affects performance and to — down the line — design better prostheses that could allow someone to negotiate the curves better,” Grabowski says.
Grabowski and her colleagues continue to research the effects of blade height and stiffness on performance. After Brazilian sprinter Alan Oliveira beat Pistorius in the 2012 Paralympic Games 200-meter race, the South African complained that Oliviera’s longer blades made him faster. The International Paralympic Committee (IPC), which governs the Paralympic Games, regulates prosthetic length for double-leg amputees based on a number of factors including wingspan (from the tips of one’s fingers on one hand to those on the other hand while the arms are held perpendicular to the body) and height. The researchers tested five amputee athletes running at different heights on blade prosthetics made by three companies. “We’re actually finding that within a range of four centimeters it’s not really having an effect on top speed,” Grabowski says. The researchers hope to publish their findings within the next six months.
The question of whether a carbon-fiber prosthetic offers athletes an unfair advantage may never be fully answered, given how much research is still being done to understand what makes a runner — any runner — faster and more efficient, the University of Washington’s Morgenroth says. Meticulous lab testing is important but it can never replicate what actually happens on the track in the heat of competition.