The Science and Controversy of Running Blade Prosthetics

In the 1970s television show The Six-Million Dollar Man, a severely-injured test pilot is rebuilt with nuclear-powered bionic limbs. These cybernetic parts allowed the pilot to suddenly become superhuman—the new-and-improved Colonel Steve Austin could not only run faster than a car, he could lift said car with just one bionic arm (which, as viewers discovered in episode six, also contained a Geiger counter to save the world from doomsday). Every week, the opening credits would intone with great seriousness:

Gentlemen, we can rebuild him. We have the technology. We have the capability to make the world’s first bionic man. Steve Austin will be that man. Better than he was before.

Better... stronger... faster.

The Six-Million Dollar Man could have been an icon for inclusivity—people with disabilities can do cool things! Instead, the takeaway was that artificial body parts are bionic wonders, increasing ordinary human powers beyond the realm of possibility. From the action-adventure series Bionic Woman, which featured Steve Austin’s cybernetic counterpart, Jamie Sommers, to the whatzits and whirlygigs within the arms of the animated Inspector Gadget, pop culture has created a fantastical story of phenomenal feats contained within ordinary prosthetic limbs. Even today, prosthetics are largely misunderstood, which impacts the way users are treated in a variety of personal and professional settings.

Perhaps no better example of this effect is Blake Leeper’s recent lawsuit against World Athletics, the governing body of track and field. The American sprinter, who was born with both legs missing below the knee, sued for the right to compete against runners with biological legs at the 2021 Tokyo Olympics. The eight-time Paralympic Track and Field medalist, world record holder, and three-time American record holder was told his prosthetic legs were an “artificial competitive advantage” under rule 6.3.4, which intends to prevent disabled athletes from competing against able-bodied athletes with mechanical aids that overcompensate for the loss created by a disability.

Leeper isn’t the first athlete with a limb difference to sue for the right to compete against athletes with biologically intact limbs. South African sprinter Oscar Pistorius filed a similar lawsuit after being banned from able-bodied competition in 2008. The prevailing belief was that Pistorius’s prosthetics, known as “Flex-Foot Cheetahs,” enabled him to use less energy than non-amputee athletes while covering the same distance, and therefore run faster. Pistorius appealed the ban and won, making history as the first amputee runner to compete at the Olympics in 2012.

Still, no clear precedent was set for future athletes, and in 2019 Leeper found himself in the same fight as Pistorius. In his appeal to compete against all athletes, Leeper was tasked with the burden of proving he was not, in fact, a bionic man.

Leeper, like many runners with limb differences, uses a type of prosthetic limb known as “running blades,” a carbon-fiber leg that attaches to the residual limb via a socket. Running blades were invented in the late 1970s by Van Phillips, who lost his lower leg in a water skiing accident. Depressed by the limited athletic function of prosthetics at the time, Phillips enrolled as a student at Northwestern University Medical School, where he studied prosthetics. His unique perspective as a user of prosthetics allowed him to identify what others had missed: most were designed with structure in mind, so replacing the bone was central to keeping a leg upright. Aesthetics were also a key element—something that could be concealed with a pant leg and a shoe.

But Phillips wanted function, and he wanted it in the form of ligaments and tendons. These human structures are a key element of absorbing, storing, and releasing kinetic energy when running and jumping—when sprinting, a biological leg returns 249% of stored energy, propelling the body forward. By observing artificial means of kinetic energy in the form of diving boards and pole vaulting, Phillips developed a J-shaped piece of carbon fiber named the Flex-Foot, which returned 90% of energy while sprinting. Since the initial release of the Flex-Foot, engineers have been working to improve on Phillips’s design to build a better running blade.

Aron Albertsson is one of those engineers. As the lead product designer for sports feet at prosthetic design firm Össur, Albertsson spends his days trying to make fake ligaments that return as much energy as the real thing. Like many engineers, Albertsson speaks bluntly, without embellishment. His office at the Össur research headquarters in Reykjavik, Iceland, is white and uncluttered. Throughout the video call, Albertsson looks away from the camera as he talks, not wanting to be distracted as he explains the complexities of his research.

It’s such detailed work, and Albertsson speaks so expertly on it, that I assume he went into the field of prosthetic design with intent. “No, in college I actually never thought it was interesting,” Albertsson says frankly. “I studied mechanical engineering in university, and I assumed I would be working in a more traditional job here in Iceland, like geothermal power plants. But my girlfriend at the time, now my wife today, was actually studying to become a prosthetist. Because of her, I ended up doing an internship at Össur, and I stayed.”

As it turned out, designing a prosthetic running blade was a technical challenge on par with (or even exceeding) geothermal energy. The human body is incredibly complex, and replicating the tangle of bones, muscles, ligaments, and tendons that work in tandem to make movement happen requires both creativity and precision. There are also variations in running styles and even individual runners. In short, it’s an engineer’s dream.

To the untrained eye, running blades don’t look that much different from the original Flex-Foot of the 1970s. But a side-by-side progression of running blades over the years reveals subtle-yet-powerful differences. The original Flex-Foot was shaped like the letter L, with a heel to mimic the natural shape of the foot. But runners don’t use their heels in motion, so the design was rounded out to a J shape to mimic the natural movement of running on toes. J-shaped blades, though effective for sprinters, proved cumbersome for long-distance runners, who run with an entirely different form. A C-shaped curve was created to perform in line with the biomechanics of endurance running. The latest development in blade design is the split toe for better control when rounding the curve of a track or the final corner of a 5K.

“People think running blades are all the same, but there are different blades for different activities,” says Albertsson. “It’s not just the shape of the blade, but the stiffness of it. If you’re a sprinter, you want a plate that is quick and can keep up with you, so you select a stiffer plate. If you’re running a marathon, you get a softer blade that has less impact on your body.”

Stiffness is adjusted through the manipulation of the carbon fiber used to create the blades. At least thirty and as many as ninety sheets of carbon fiber, each sheet thinner than the width of a human hair, are layered in a specific way to achieve the desired stiffness or softness.

The weight of the user is a factor in blade design, as is the type of amputation (an above-the-knee amputation is different, biomechanically, from one where the amputation is located on the lower leg). Unilateral limb differences, where one leg is intact, requires a different setup from bilateral, or prosthetics worn on both legs.

Each new design is run through a battery of tests to confirm the safety and efficacy of the structure. From there, it’s placed on the athlete for testing: “I’m looking at the numbers on the computer screen while they’re running,” says Albertsson. “We see the rollover pattern, the force with each step, the ground contact times. We’ve got a 3D measurement system where we look at gait.”

The ultimate goal is to create a comfortable running stride for the user. It is not, as many believe, to make an exact replication of the gait of someone with intact, biological legs. “Whether you’re a unilateral or bilateral [limb difference], you’re missing your bones and muscles. All those joints and things that help you adapt to the ground are missing,” Albertsson explains. “If you’re a unilateral, you’re pushing off more from your sound side and keeping momentum with the blade. If you’re amputated above the knee, you’re missing that important knee joint. You’re using different muscles in different ways.”

Athletes who use running blades also have to contend with a different set of challenges that athletes with biological legs don’t have to face. Because a blade is not as dynamic as a human ankle or knee, the energy absorbed in one direction is returned in the opposite direction. If a sprinter is rounding a curve, they spring back on the same plane they press down, instead of the way they’re aiming to go. The same is true for a marathoner going downhill or a trail runner on uneven terrain. Though the blade does not generate energy on its own, someone using running blades must learn how to create and respond to the forces of absorption and propulsion from the running blades. One cannot flex a prosthesis like a calf muscle, nor dexterously maneuver through steep or technical terrain; it is not much different from a biological ankle trying to run in a ski boot.

It’s also difficult for the runner to identify the exact position of the prosthetic foot at initial contact. That’s one element many people don’t consider in the discussion of running blades. In replicating the structures of the human leg, running prosthetics are missing a small-but-critical element of running performance: the ability to feel the terrain. Bones, muscles, and ligaments are missing, but so are the channels of the nervous system that allow humans to feel texture, pressure, and pain.

When a biological foot strikes the ground, it sets off a series of tiny electrical impulses that transmit information to the central nervous system—how hard or soft the ground is, whether the surface is level or sloped, if that brown spot is sharp like a rock or slippery like mud. Almost instantly, the brain and spinal cord neural circuits send back a series of impulses: Lean forward. Don’t land so hard. Now move your foot ever-so-slightly to the left. It’s easy to say we do this without thinking, but a more accurate way might be to say that we don’t think about our thinking—the nervous system is constantly running in the background.

Humans are not born with such advanced coordination. If they were, it’d be possible to walk right out of the womb and into the bassinet. Instead, it takes almost two years, on average, to learn how to stand without falling over and walk without tumbling. Watch a toddler learn to toddle, and you will see an intense amount of concentration as she tries to recall the specific activation patterns required to pick up a leg, move it forward, and repeat on the other leg.

“‘Touch’ isn’t just a single sense,” explains Dr. Gregory Arthur Clark of the University of Utah. “It’s twenty senses of touch and movement: temperature, pain, vibration, proprioception. We have to learn how to make sense of all of these different bits of information before we can put them to use.”

Each one of those senses is transmitted through information-specific channels (composed of specific classes of nerve cells called “neurons”). Electrical signals travel from the receptors in the skin or muscle to the processing center in the brain or spinal cord, which tells the body how to move in response. With time and practice, this process becomes so seamless, a toddler soon becomes a runner who can step on a sharp rock or begin to slip on a muddy patch and instantly recruit the muscles needed to stay upright without consciously thinking about it.

But when a running blade is placed on a residual limb, these electrical channels are not artificially extended. Historically, such a feat was thought to be impossible. There are billions of neurons in the central nervous system, each firing off electrical signals in an effort to talk to the brain and each other. Picture a game of telephone in a packed concert arena, transmitting multiple messages from person to person from the highest seats in the balcony to the roadies backstage. The potential for error is simply too high.

But recent developments in neuroscience have made it possible to track the electrical impulses in the body to determine which messages travel through which channels; more importantly, it can replicate those messages, even when key components of the pathways are missing. Clark and his team use a device called the Utah Slanted Electrode Array, a matrix no bigger than a pencil eraser that serves as an interface between a prosthetic and the neuromuscular system. Through this interface, the user can receive touch and movement information from the prosthesis as well as control the prosthesis dexterously and intuitively simply by thinking about it.

“It’s Luke Skywalker style,” Clark says grandly. “If we put in two or three of these devices, we can talk to 300 different nerve fibers or listen to up to 300.”

Currently, Clark’s work focuses on hands. In his lab, a robotic hand known as the DEKA LUKE arm is outfitted with the Utah Electrode Array, then mapped for the user to determine which channels in the nervous system respond to various elements of touch or movement. The interface then helps the user not only control the hand’s opening and closing mechanisms, but to feel specific elements of what he or she is touching. The user can tell whether an object is hard or soft, pick up a heavy, fragile object with just the right amount of grip force, and generate a consistent grip force to hold an item. They also get an accurate sense of where their prosthetic hand is in space, even if the hand is covered by a box or table—something that is not possible with a standard prosthesis. They can even incorporate the hand into their subjective body image so that it becomes integrated into their sense of self—something that is not always possible with a standard prosthesis.

This technology, known as sensory feedback, has exciting implications for the future of running prosthetics, as proprioception is a key element of movement. Lower back pain is the chief complaint of people who use running blades, and experts hypothesize that being able to make micro-adjustments to one’s gait in response to the terrain could reduce this complication as well as make running a safer and more enjoyable experience.

Some promising studies also show sensory feedback could be a way to address a phenomenon known as Phantom Limb Pain, or ongoing painful sensations that seem to be coming from the part of the limb that is no longer there. “The limb is gone, but the pain is real,” says Clark. “It’s a weird thing—they have phantom pain because they have a phantom limb. Their physical part is not there, but their brain fills in the empty space, and it hurts. But when someone has sensation in that limb [through sensory feedback] and can embody the prosthesis into their own brain, the phantom doesn’t have a place to live anymore.”

Will sensory feedback make future users of running prosthetics faster? Possibly. Clark says speed could come from an increase in confidence—surefooted runners are faster than cautious ones. “It’s like running on trails or running barefoot,” he says. “You start out slow and dodging rocks and mud, but soon you just plow through. You learn how to make those tiny adjustments.”

Sensory feedback is a technological marvel that even Clark talks about with a kind of stunned reverence, despite his years of work within the field. It’s the same awe people expressed when they first saw The Six-Million Dollar Man. Which begs the question: Where is the line between optimizing movement patterns and creating a superhuman? Have prosthetic limbs entered the realm of “artificial competitive advantage”?

In legal terms, “precedent” refers to a court decision that is considered as authority for deciding subsequent cases involving identical or similar facts. The case of Oscar Pistorius, who won the right to compete in the Olympics using his running blades, could have set a precedent for athletes like Leeper, who petitioned for the same opportunity. Instead, World Athletics instituted a rule after the Pistorius case that athletes using such “mechanical aids” must take it upon themselves to prove their blades do not give them a competitive edge. If Leeper wanted to race against all athletes, he had to prove he would be just as fast with intact limbs as he is with running blades—a race against himself.

“I suppose the perfect scientific study would be to measure the same person with and without legs,” says Dr. Alena Grabowski of the University of Colorado–Boulder. “But you’re never going to get anyone to sign up for a study where we amputate your legs. And even if we had someone who was a sprinter before and after amputation, you could argue slower times could be due to age, or trauma, or time off from training, or secondary injuries.”

Grabowski, who studies the physiology of runners with limb differences, was tasked with creating a protocol to determine if Leeper’s running blades were, in fact, performance-enhancing. For weeks, Leeper sprinted around the indoor track and on treadmills at the CU Boulder lab while Grabowski collected performance and physiological data. Because there was no way to collect data on Leeper with biologically-intact legs, Grabowski compared his performance metrics to an extensive database of amputee and non-amputee athletes.

“We collect data from treadmills and overground experiments, where we measure the forces that athletes exert on the ground,” says Grabowski. “We also use motion capture to determine how a person moves. We can measure things like torque at the ankle, power at the knee, or work at the hip, things like that. And then we measure metabolic cost, and that’s a really important measure of effort.”

Grabowski admits it’s not perfect. The comparison is “apples to oranges,” and some things, like the exact loss of power from a detachable prosthesis to the residual limb, are difficult to quantify. But by building a large database of performance metrics in amputee and non-amputee athletes, patterns begin to emerge. These patterns give a better view of how a prosthetic leg functions versus a biological leg.

“We’ve shown that across the board, [running blades] aren’t an unfair advantage. If anything, they are a disadvantage,” explains Grabowski. “Because prostheses are passive, they can’t generate power. They can only store and return energy. That’s one of the fundamental differences between prostheses and biological limbs.”

Through controlled studies, Grabowski has found that prosthetic limb users do not accelerate faster at the start, cannot achieve faster maximum sprint velocities, do not run around a curve as fast, do not run faster at maximum aerobic capacity or otherwise exhibit superior sprinting endurance when compared to runners with biological limbs. In fact, prostheses render the user at a significant disadvantage in certain racing elements, such as block starts and rounding curves. These patterns held true for Leeper in testing, and the evidence was presented to the Court of Arbitration for Sport, where the case was being heard.

“If this rigorous scientific evidence is not found to be sufficient,” Grabowski wrote in evidence submitted to the court, “then we are unable to envision that any person with an amputation will ever be permitted to compete against non-amputee athletes in elite running competitions.”

The court agreed with Grabowski’s evidence, but did not rule in Leeper’s favor. Instead, they raised another issue not part of the original case: height. The court ruled that Leeper’s prostheses make him nearly six inches taller than he would be if he had biological legs, giving him an artificial performance advantage of “several seconds” over 400 meters.

Leeper, who stands at six feet, two inches, was told to shorten his legs to five feet, nine inches to comply with Maximum Allowable Standing Height, or MASH, a rule that limits stature of double-leg amputees to an estimation of what one would be with intact limbs. But these estimations are based on height data collected solely on Caucasian and Asian people, not Black athletes like Leeper.

Grabowski was shocked: “That wasn’t the rule. The rule that we were abiding by is that we needed to show whether his use of prostheses provided any sort of unfair advantage compared to biological limbs. It had nothing to do with height.”

Even if height were a factor in Leeper’s case, controlled studies by Grabowski and colleagues show it doesn’t matter. “Height doesn’t have an effect on maximum speed. You can’t get faster for free,” says Grabowski. “There’s a trade-off. To increase speed, you either need to increase stride length or stride frequency. If you increase stride length, your stride frequency goes down. So they’re making an assumption that everything else is equal somehow, and it isn’t.”

Grabowski pauses for a moment, her cheeks red.

“Sorry. I’m getting charged up now.”

Leeper has vowed to challenge the ruling in the Swiss supreme court, though he did claim one small victory already: the Court of Arbitration for Sport has stated the burden to prove competitive advantage (or lack thereof) should shift from the athlete to the governing body. For Grabowski, the fact that Leeper—or any athlete using prosthetics, for that matter—has to undergo such a process is preposterous. No one looks at a person wearing a pair of glasses and assumes they have X-ray vision. Someone with a hearing aid isn’t likely to be accused of eavesdropping through concrete walls. If you put such devices on a person with no vision or hearing impairments, they would lose their capabilities, experiencing blurred vision and loud, painful feedback. Grabowski’s research shows the reason The Six-Million Dollar Man is a fictional tale: assistive technology doesn’t create superheroes. Instead, it levels the playing field so everyone has the chance to do the thing that makes them feel heroic, whether it’s running their first 5K or qualifying for the Olympics.

Before signing off on our video call, Albertsson makes eye contact for the first time.

“Hey, before we go, can I add one more thing?”

He pauses for a second, as if trying to find the right words. “There’s so much talk about the technology of the blades and the possible advantage, but the athletes are kind of forgotten in the whole spectrum of this discussion. You can give any person a Formula One car, and it could be the best one in the world, but that doesn’t mean they’re going to win. In the end, it’s just equipment. Of course, you try to design the best equipment, but the person who uses it needs to be really skilled and put in those ten thousand hours of grit and determination.”

For Albertsson, the projects that keep him up at night are not the ones that get elite athletes across the finish lines, but the ones that get them started. His best day on the job was the day he launched a new product for Össur: running blades for children. “This one kid had been running on his everyday prosthesis, his walking foot, which wasn’t so good. But you know how kids are—they run no matter what. He put on his pediatric blade for the first time, and he was running straight away. He was like, ‘Look! Mom! Look! I’m flying!’”

The same sentiment drives Clark’s work in sensory feedback. “The human component is a huge part of human performance. Nobody’s going to outrun a cheetah, okay? We, as humans, have limits, and we push ourselves to those limits. It’s just as true for athletes that use prostheses. Everybody should get that chance. I’m not setting out to create a superhuman that’s going to outperform everybody else to take home a trophy. I’m trying to restore their life, their sense of self, their joy in existing, their ability to do things that they couldn’t otherwise do in the way that they used to be able to do. And that’s the trophy. That’s the win.”