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There are plenty of stories about inventors who see a problem and decide they can do better. But Van Phillips had a little more motivation than most. The problem was his own leg. In 1976, Phillips was a 21-year-old college student when a water-skiing accident cost him his left leg below the knee. If that wasn’t bad enough, the prosthetic leg he received afterward wasn’t exactly a technological marvel. Prosthetic limbs of the era were generally designed to look and act something like a biological leg and foot, but “act” might be giving them too much credit. They were passive structures that provided something to stand on and roll over while walking.

Phillips wanted to do more than walk. There was just one problem: he wasn’t an engineer. Before the accident, he had been studying business. So, if he was going to build a better leg, first he was going to have to learn how.

Back To School

Phillips became fascinated with prosthetics and eventually studied prosthetic design at Northwestern University’s Prosthetic-Orthotic Center. He also worked at the University of Utah’s prosthetics laboratory, where he had access to both the people and equipment he needed to experiment.

The conventional wisdom was that a prosthetic foot should imitate a human foot. That seems perfectly reasonable — evolution has had quite a long time to work on the design. But there’s a problem with simply copying the shape. A real foot isn’t just a foot-shaped object attached to the bottom of your leg. Muscles, tendons, and ligaments store and release energy as you walk or run. Your Achilles tendon, in particular, acts very much like a spring. A conventional prosthetic foot might look right, but it didn’t have anything corresponding to that spring.

Phillips eventually stopped worrying so much about making something that looked like a foot. Instead, he decided to make something that worked like one.

Spring In Your Step

The basic idea behind what became the Flex-Foot is simple, but unobvious: use a spring as the foot. As is often the case, the concept is the easy part. Making a spring that will repeatedly support the weight of a human being, flex the right amount, survive millions of cycles, fit into a practical prosthesis, and not occasionally snap and dump its owner onto the pavement is considerably harder. The answer was carbon-fiber composite.

Today, carbon fiber turns up everywhere from airplanes to bicycles to suspiciously expensive automotive trim. In the late 1970s and early 1980s, though, it was still an exotic material, and the design problem was genuinely nasty. A prosthetic foot spends every waking hour of its owner’s life under cyclic load — tens of thousands of flex cycles a week, millions over a product’s lifetime — with a full body weight snapping through it on every step and several times that on every stride of a run. Get the fiber layup or resin choice wrong, and you don’t get a gentle failure; you get delamination or a sudden snap, with the wearer still attached. Tuning it added another layer: the same blade has to feel right for wildly different body weights and gait styles, which meant iterating on the curvature, the laminate thickness, and where along the blade the flex was concentrated, rather than just picking “carbon fiber” off a shelf and calling it done. Phillips wasn’t merely drawing this stuff on paper, either — he tested prototypes on himself, and by his own accounts the early ones failed more than once before the layup and geometry held up to real use.

Instead of a rigid ankle and artificial foot, the Flex-Foot used a curved composite member extending down the leg and underneath it. When the wearer put weight on the prosthesis, the carbon-fiber structure flexed and stored energy. As the wearer moved forward and unloaded it, the spring returned some of that energy — much closer to what the biological machinery in your lower leg actually does.

Form Follows Function

Phillips founded Flex-Foot, Inc. in 1984, and the design evolved into a family of prosthetic feet. One particularly interesting development was that once you abandon the requirement that a prosthetic foot has to look like a foot, you have quite a bit of design freedom. That’s where the familiar running blade comes from.

The later Cheetah running foot, made by the Icelandic company Össur after it acquired Flex-Foot, takes the idea to its logical conclusion. There’s hardly any pretense that it is an artificial human foot. It is a long, curved carbon-fiber spring, generally shaped something like a J. Load it and it bends. Release the load, and it springs back. If you’ve watched the Paralympics, you’ve almost certainly seen them.

There is something particularly satisfying about the design from an engineering standpoint. Your first instinct is to mimic the original device — in this case, a human foot and leg. Phillips, instead, identified what the system actually needed to do. That’s a lesson that applies well outside prosthetics. Nature’s solution isn’t necessarily the only solution, and copying the appearance of a biological system doesn’t necessarily reproduce its behavior. Airplanes don’t flap their wings, either.

Too Good?

Of course, once prosthetic legs became good enough to let amputees run competitively, an odd question arose: could they become too good? That became a formal legal dispute in 2007–2008, when the IAAF (now World Athletics) moved to bar South African sprinter Oscar Pistorius, a double amputee running on Össur’s Cheetah Flex-Foot blades, from competing against able-bodied athletes.

The IAAF’s case leaned heavily on research from German sports scientist Gert-Peter Brüggemann, who argued the blades were mechanically efficient enough to constitute an unfair aid under the federation’s own rule against springs, wheels, or other advantage-granting devices. Pistorius’s side countered with independent testing led by MIT biomechanics researcher Hugh Herr, arguing there was no measurable edge. When the case reached the Court of Arbitration for Sport in 2008, the panel sided with Pistorius: it found the IAAF had not shown sufficient evidence of a metabolic advantage from using the Cheetah Flex-Foot, and overturned the ban.

That ruling didn’t settle the underlying biomechanics so much as expose how messy the question really is. Running blades are light and return a meaningful share of stored elastic energy, but they aren’t motors — they can’t give back energy that wasn’t put into them, and they lack the active muscle contribution a biological leg supplies, particularly out of the blocks and around a curve. Depending on which variable you isolate — limb mass, ground-contact time, force production, or acceleration mechanics — the prosthesis can look like an advantage or a disadvantage. It’s a genuinely open biomechanics problem dressed up as a sports-eligibility ruling.

The controversy eventually became as much a question about what constitutes a “normal” human body as it was an engineering problem. That’s a pretty remarkable problem for an invention to have. In a few decades, the question had gone from “can we make an amputee walk better?” to “is this prosthetic leg unfair to people who still have legs?”

Better Than A Fake Foot

Flex-Foot was acquired by Össur in 2000. Phillips received the Lemelson-MIT Prize in 1997 and was inducted into the National Inventors Hall of Fame in 2008 — the same year the CAS ruling put his design at the center of a sports-eligibility debate. Descendants of his original design are still in use today: energy-storing-and-returning feet are now an ordinary part of modern prosthetics, and specialized running blades have become almost synonymous with amputee athletics.

Even if you never design a prosthetic, the lesson is a valuable one. When solving a problem, it is natural to mimic something that already exists, either natural or an earlier design. But real innovation often comes when you stand back and consider what you actually need. Not every hole has to be made with a drill.

If you aren’t big on springs, you could 3D print a foot. We think it is cool to be able to create your own prosthetics.