Born to Run

Could an 18-year-old double amputee perched on a pair of carbon-fiber springs have an edge over able-bodied athletes? Spectrum Online asked leading experts, and the answers are as different as they are surprising.

Photo: John Super/AP Photo

The head of R&D on foot products at Iceland-based Ossur is audibly shaken in a phone interview as she ponders the implications of carbon-fiber sprinting prostheses that might one day help an amputee exceed normal human performance on the track.

As the person in charge of designing sports prosthetics for the world's leading manufacturer of "running feet," as disabled athletes call them, Heidrun Gigja Ragnarsdottir well knows just how far things could go, though the implications give her pause. A revolution in new materials, the ever-shrinking microprocessor, and the power of CAD design tools have all pushed the technology of prostheses, in the words of Massachusetts Institute of Technology engineer Hugh Herr, to the "threshold of a new age" [see sidebar, "Building a Better Leg"]. The bionic man--or at least a microprocessor-controlled bionic leg--is already a reality. But even in the realm of passive prostheses, which by definition do not produce energy but only store and release it, recent changes have made it possible for a lower-limb amputee to run faster than ever seemed imaginable.

Maybe too fast. Is it conceivable that an amputee sprinter fitted with one, or even two, prostheses might, disability notwithstanding, gain an advantage over able-bodied competitors by virtue of using high-tech carbon-fiber leaf springs instead of feet? And if that athlete ran fast enough to qualify, for example, for the Olympics, should he or she be allowed to run?

"You don't want to disqualify someone because he is disabled--that is unfair," says Ragnarsdottir, thinking aloud. "I don't envy the people who have to decide on this question."

That would be General Secretary Istvan Gyulail and his colleagues at the International Association of Athletics Federations (IAAF), the Monaco-based body that sets the qualifying criteria for world-class competition, including the Olympics. "It would seem inhumane and against the sport to say 'go away and compete in events for the disabled,'" said Gyulail. Indeed, if a disabled sprinter can muster enough grit and gumption to give the world's fastest human being a run for their money, how could anyone possible deny that athlete the chance?

But good will is not necessarily good science, and from a strictly biomechanical point of view, it remains an open question as to whether running with a prosthesis could ever enable an athlete to surpass what would have been his able-bodied performance were he not missing one or both of his natural feet. What series of tests or experiments could possibly tease out the impact of a prosthesis from among the dozens of other variables that determine performance levels?

The IAAF has not yet taken a position on this issue, for the simple reason that an amputee running in the Olympics was "never foreseen as a realistic possibility," to quote Gyulail. But what would have seemed less than a year ago like a safely abstract and hypothetical question straight out of a graduate course in biomechanical engineering is today a real live case study.

Photo: Otto Bock

Meet Oscar Pistorius , a boyish 18-year-old high school student from Pretoria, South Africa, who was born with a congenital condition that left him with lower legs but no feet. In January 2004, nevertheless, Oscar decided to try running track, after an injury forced him to give up his spot as a second-string player on the school rugby team. An athlete by nature, he wasn't about to let the fact that he was a double amputee stop him from breaking a sweat on the field.

What nobody anticipated is that he would also start to break records.

At first Pistorius ran on a pair of improvised prostheses built by an engineer from a South African aircraft company. But his new carbon-fiber running feet, had an unnerving tendency to shatter, so Oscar's prosthetist put him in touch with an American colleague, Brian Frasure. Not only was 34-year-old Frasure a highly experienced prosthetist--a profession he chose after losing a leg trying to hop a moving train while in college--he also held the world record in the 200 meter dash for amputees, a blistering 22.69 seconds, barely 3 seconds shy of the able-bodied mark.

So in June last year Pistorius went to Charlotte, N. C., where Frasure fitted him with a new pair of cutting-edge, sickle-shaped feet made by Ossur specifically adapted to the young athlete's weight and preferred distance, also the 200. The results were staggering: in a couple of months Pistorius lopped nearly 3 seconds off his best time, an improvement that Frasure described as "unheard of" in a sport in which sprinters are ecstatic if they can trim a tenth of a second off their race. On 22 September 2004 at the Athens Paralympics, Pistorius became the first amputee to break the 22-second barrier in the 200, clocking a phenomenal 21.97.

Frasure-the-runner saw the fruits of his labors as Frasure-the-prosthetist from up close that day: he was on the track next to Pistorius when his patient-turned-protégé thundered past him on his way to shattering the American's world record. "I should have waited until next year to make his legs," Frasure said wryly, with a wan smile. To put Pistorius's achievement and potential in perspective, consider this: the able-bodied world record for 200 meters is 19.32 seconds, and the qualifying time for a South African sprinter going to the Olympics would be 20.75 seconds. Pistorius shaved another tenth of a second off his Athens record earlier this year, which puts him barely over 1 second shy of a the time he needs to have a shot at participating in the world's premier sporting event. Taking into account the fact that, in his own words, "sprinters usually peak somewhere between 26 and 29 years old," his chances of lining up one day in the starting blocks shoulder-to-shoulder with the fastest males on the planet seem fair to excellent. "I have no doubt that Oscar will eventually run fast enough to compete in an able World Championship," Frasure said in April. "He could be ready to qualify for South Africa by the 2008 Olympics," he added, agreeing that Pistorius is probably a decade away from his physical peak.

There's more. Everyone close to Pistorius agrees that his greatest potential lies in an event that he has only begun to run in competition: the 400. The fact that he is a bilateral amputee slows Pistorius down as he comes out of the starting blocks. Once he builds up speed, however, that same liability becomes an asset: his balanced and extra-long stride--of which more below--give him "a top speed that is ridiculous," comments American sprinter Marlon Shirley, the only amputee to have ever run the 100 meter dash in under 11 seconds.

To what extent can Pistorius's jaw-dropping performance be attributed to his innate talent and determination, and to what extent is it a by-product of technology? Even on technical grounds, there is no clear answer. Factor in the fundamental issue of fairness, and the waters get even muddier. "The ethical question that has to be resolved--but which so far hasn't even been profoundly addressed--is how to distinguish fair advantages that emerge from technology from unfair ones," comments Anita Silvers, an ethicist in San Francisco State University's philosophy department who has written extensively on disability.

To date, none of the relevant able-bodied sports federations, national or international, have an opinion on the matter, official or otherwise. "This is a big surprise to me," the IAAF's Gyulail said earlier this year, though he acknowledged that Pistorius's performance suggests that an amputee running fast enough to qualify for the Olympics is now "clearly a possibility." Disabled sports federations, prompted by a complaint from the U.S. Paralympics committee, are currently rewriting the rules on prostheses, which should be in force for the 2008 Games in Beijing.

Photo: Otto Bock

As streamlined in form as they are sophisticated in engineering, 21st-century sprinting prosthetics are a radical departure from the past. Unlike an earlier generation of artificial devices, which sought with well-intentioned literal-mindedness to recreate the highly complex articulation of the human foot, the heel-less carbon-fiber leaf spring used by nearly all Paralympics sprinters today was designed with a different purpose: quite simply, to allow the athlete to move as quickly as possible. Of what use, after all, is a heel to a sprinter who runs entirely on the balls of his feet? At the Athens Paralympics last year this journalist saw only one lone sprinter--in the 400--wearing an old-fashioned prosthesis with an artificial foot inside a track shoe, and watching him limp seventy meters behind the field was a painful reminder of just how revolutionary was the changeover.

 

But were it not for a freak water-skiing accident in 1976, current technology might not be all that different from what it used to be. When 21-year-old Van Philips, a college student in Arizona, sought to resume an active sports life shortly after losing his foot, he was appalled to find the best running prostheses on the market clumsy and painful. "Some of the feet available in those days were made of balsa wood," Philips recalled. "They were light but they had no flexibility. Some had ankle movement, but no method of storing energy. Many were a sloppy fit." There must be a better way, he thought.

And so necessity once again gave birth to an invention, this one designed, patented and built by Philips, who changed to an engineering major in school and later founded a company, called Flex-Foot, that supplied several generations of elite disabled runners with performance-enhancing feet. One measure of the significance of Philips innovation is the addition last year of his first "C-Sprint" prosthetic--so named due to its curved shape--to the permanent collection of the National Museum of American History, along with drawings, sketches and prototypes tracing its conception and development. Philips sold his company in 1999 to Ossur, which has continued to improve and refine the basic design.

The kind of device worn by Pistorius, Frasure, and Shirley actually consists of three parts. The first is a hard socket made of polypropylene or woven carbon fiber composite materials that fits over the stump of the lower leg. A custom-fitted silicon rubber liner provides an interface between the two, protecting sensitive tissue while creating a snug fit. Finally, to replace the two bones that provide the support structure of the lower leg (the tibia and fibula) along with the ligaments and muscles that capture and generate energy, an ultra-stiff and curved length of carbon fiber about the width and thickness of a ski--a mix of woven and unidirectional fibers, on the one hand, and filament wound fibers, on the other--is bolted onto the outside of the socket. Compared to any other prosthesis, this inverted J-shaped spring is "the most effective at storing and releasing energy during walking and, in particular, recreational and competitive sports activities," concluded Linda J. Marks and John W. Michael in a clinical review published in the British Medical Journal.

"The key is that the polymer matrix has a combination of elastic and viscous behavior when subjected to cyclical load" as the prosthetic springs are compressed and "fired" with each step taken by the athlete, explains Mike Jenkins, a mechanical engineer and materials expert at Birmingham University in England who specializes in prosthetics. But even if advances in material design ensure that energy storage is maximized (up to 95 percent, in some cases) a carbon fiber foot "can never release more energy than it stores, and there will always be some dissipation or damping" caused by the imperfect interface between stump and socket, he adds.

Conclusion? "If you considered the case of identical twins, subjected to the same conditioning but one twin having two Cheetah limbs [made by Ossur], then I think the 'able-bodied' competitor would win," Jenkins said in an e-mail exchange. Ajit Chaudhari, a biomechanical engineer at Stanford University's Biomotion Laboratory, points out that a below-the-knee amputee is missing quite a few important muscles. Their absence, he explains, would outweigh any gains from increased efficiency or storing energy in the carbon springs.

Even new developments in osseointegration--the insertion of a titanium implant directly into the remaining femur bone, thus eliminating energy loss stemming from the need for a socket and silicon liner--would never yield more than a 100 percent return on the load, which is two to three times the weight of a sprinter at full throttle. Given that the muscles in an anatomical limb

Photo: Otto Bock

generate, in addition, at least as much energy as is stored, it would seem obvious that Pistorius is running, so to speak, with one hand tied behind his back.

Case closed? Hardly. Biomechanical engineers and physicians point to several variables suggesting that Pistorius might be able to compensate for his disability, and perhaps even gain a slight edge. To start with, the technology will inevitably improve, allowing disabled athletes to run faster, jump higher, throw further, and to continue closing the gap with their able-bodied peers. "The movement we are trying to get out of the prosthesis is merely replacing the energy that was lost with the limb," says Ragnarsdottir of Ossur. "But we are starting to borderline on a gray area of designing something that could exceed normal performance."

"We simply don't know what the perfect [artificial] foot is," says Robert Gailey, a Miami-based prosthetist with a doctorate in prosthetics design and a leading expert on below-the-knee amputee sprinting. "The one we have was drawn on a napkin." When the first Flex-Foot model hit the market, Gailey recalls, it was designed to be mounted on the front of the socket that fits over the amputee's residual limb. But when an Australian sprinter lacking instructions put it on backwards by accident, the designers realized it worked better that way.

A second factor is the human body's extraordinary capacity to adapt. "The amputee athlete uses the muscles crossing the hip and knee much more than they are used normally," observes Chaudhari, "and it is possible in theory that they could more than compensate for the lack of muscles crossing the ankle." But even as they marvel at the body's ability to develop new muscle groups to compensate for a missing limb, Chaudhari and Gailey remain skeptical that such changes could result in a net gain of energy and speed--in a single amputee, that is. In the case of double amputees such as Pistorius, the potential to leverage the use of two prostheses into an advantage becomes greater, not less. This is not how the International Paralympics Committee (IPC) has seen the situation. Its medically based classification of disability ranks a bilateral amputation as a more debilitating handicap than the loss of a single leg, which seems intuitively plausible. How could an athlete missing both feet--and all the muscles and ligaments attached to them--possibly gain an edge over a single-leg amputee? Just ask Shirley, the U.S. sprinter who holds the world record in the 100-meter dash for single-leg amputees and who, in a desperate bid to beat Pistorius in the 200 meters in Athens, injured himself by deliberately using an ultra-stiff prosthesis designed for the 100.

"It's simple: the length of his legs gives him an extreme advantage mechanically over the other [single-amputee] athletes in the field," Shirley said after his loss in the 200. "You could just see the difference it makes coming down the back stretch. He took almost four steps less than I did.

When asked if the advantage he was describing was technical, Shirley shook his head. "No, it's biomechanical. I have the same technology--carbon fiber sokets, carbon fiber feet--but I can't manipulate my anatomical structure like he can." By augmenting his height, in other words, Pistorius is increasing the length of his stride and thus his speed. Shirley is quick to point out his beef is with the IPC and not with Pistorius, whom he admires. And in some ways his complaint is a red herring: even if the two athletes run in the same Paralympics races, technically they are competing in different categories. The only reason they line up on the same track is that there are not enough elite bilateral amputee sprinters in the world to create a separate event.

But on the more fundamental question of whether Pistorius has a biomechanical edge over Shirley, the engineers and prosthetists Spectrum Online consulted suggest that Shirley is right and the IPC is wrong. "In running, stride length times stride frequency equals speed. If you can find that magic height--the maximum height without decreasing frequency--you can build a better runner," said Gailey. Ossur's Ragnarsdottir agrees: "Pistorius is covering more ground as he runs."

How much of an advantage could a double-amputee like Pistorius get from use of artificial feet? Engineer Kevin Harney, business development manager of the lower extremities division of international prosthetics giant Otto Bock HealthCare GmbH, points out that a unilateral amputee is faster out of the blocks in the acceleration phase of a sprint but starts to fatigue on his sound side during the stabilization and terminal phases, making it more and more difficult to keep loading the prosthesis efficiently.

For double amputees, however, the situation is reversed. "Once bilateral amputees reach their natural frequency response"--the optimal balance of stride length and frequency "all the athlete need to do is ensure that he applies the necessary forces to maintain that frequency. This means they are faster in the terminal phase, and "this is typically where they win the race," says Harney. This point was obviously not lost on Pistorius, who had a sudden "growth spurt"--he lengthened his prostheses-- between May 2004, when Frasure fitted him with his first pair of top-notch sprinting feet, and September, when he showed up for the Paralympics. "Oscar was at least two inches taller in Athens than the height I set him up at in North Carolina," Frasure confirmed by e-mail.

All three of these experts hasten to point out that there is a limit. "You cannot run on stilts. You may be taller, but you are not more efficient," in Gailey's words. Miriam Wilkens, the IPC's communications director, agrees that too much height creates instability. Indeed, when Pistorius won the 200 in Athens, he was running so tall he nearly fell over at the end of his race.

True. But he also broke the world record.

At a September meeting in Rio de Janeiro of the International Wheelchair and Amputee Sports Federation (IWAS), the rules-making body for disability athletics, regulators addressed the question of how tall is too tall. A previous attempt a decade ago to fix a maximum allowable length of prostheses for double amputees failed, IWAS Executive Director Maura Strange explained before the meeting, because experts "were unable to define a credible anatomical formula, due to variability in arm and torso lengths in relation to leg lengths." Besides, the experts concluded, any gain in stride length would be outweighed by the loss in balance and stability.

This time, following a complaint from the United States, IWAS reversed itself and is now in favor of instituting one of two anatomical formulas to determine "natural" height for double amputees. The first, called the "demi-span" measure, said IWAS athletics director Jan Bockweg, is a variant of the rule of thumb that a person's "wingspan"--the horizontal measure of one's outstretched arms--is equal to his or her height, within five percent. The second formula is based on the distance from the midpoint of the elbow to the prominent bone of the waist.

But both these proposals are subject to challenge and have been rejected in the past. "Using just one criterion could give inaccurate results," says Frasure. "I can speak from experience--I am 6-foot-0 but my wingspan is 6-foot-4." Whatever their shortcomings, both IWAS proposals are wending their way through a complicated approval process, and one or possibly the average of both will almost certainly be put in place in time for the Beijing Games in 2008, where Pistorius hopes to make history as the first amputee to run in the Olympics.

In the end , the IAAF and International Olympic Committee will have to decide whether to let Oscar and Marlon, or other amputee athletes yet to emerge, join the club of the world's most elite runners. That decision may not be the same for each of them, and Pistorius may wind up having to trim an inch or two off his running feet. For Scott Sabolich, owner and clinical director of an eponymous prosthetics clinic and research center in Oklahoma City, that, indeed, is the solution. "Pistorius should be able to compete only if he is found to be within two percent of normal height as measured by the length of his arms at full weight bearing," he wrote in an e-mail exchange, adding that he had protested Pistorius's victory in Athens for precisely this reason. As a unilateral amputee, however, Marlon Shirley (with whom Sabolich has worked extensively) "should be able to compete because his height is not affected, even on one side," Sabolich thinks.

But ultimately the real question is not one of science but of humanity and of fairness: should Oscar be allowed to run even if he gains some measure of advantage as a sprinter from his misfortune as a human being? And how does that advantage--if there is one--compare with the good fortune of a Carl Lewis or a Michael Johnson, members of what Gailey calls "the lucky sperm club" of athletes literally built to run because of their unnaturally long legs?

A strong argument could be made that cutting a bilateral athlete such as Pistorius down to size, so to speak, is in fact a means of preventing him from ever running as fast or faster than his able-bodied colleagues. The underlying and unspoken prejudice may be that if a disabled sprinter is able to match the times of the world's best able-bodied runners, then, almost by definition, he must somehow have an "unnatural" advantage.

Indeed, "the usual complaint about prostheses in competition is that they are not 'natural,'" says Silvers, the ethicist at San Francisco State University. "But this complaint abandons consistency, because a lot of prostheses are used in competitive sports, including the Olympics, with no complaint--prosthetic skin swimwear, for example." Pistorius using a prosthetic, surely, is not the same thing as Ben Johnson taking growth hormones, or a baseball player using a corked bat. Enhancing what is already nearly perfect and repairing what is seriously damaged are qualitatively different undertakings.

Which is why, unlike Sabolich, Gailey is more inclined to let the young South African find his own perfect altitude. "If at a given moment a human being can line up and cover a given distance faster than anyone else, he should get the gold. The fact that he is using something to compensate for a missing limb should not disqualify him," Gaily says. But, he adds, "I don't think the world is ready to see an amputee win an Olympic event."

About the Author

MARLOWE HOOD (marlowe.hood@afp.com) is a journalist based in Paris. He first wrote about Oscar Pistorius in "Running Against the Wind" in the June 2005 issue of IEEE Spectrum.

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