The modern prosthetic running blade is now one of the most recognisable symbols of Para sport.
Its curved carbon-fibre profile has become familiar through the Paralympic Games, amputee athletics and increasingly recreational sport. But the technology did not emerge from an attempt to make an artificial foot look more realistic.
It came from one amputee asking a much more fundamental question: what does a prosthetic foot actually need to do?
A recent feature published by Hackaday revisits the story of Van Phillips, the inventor whose work on the Flex-Foot helped change the direction of lower-limb prosthetics and ultimately led to the energy-storing-and-returning feet used throughout the profession today.
For CPOs, it remains one of the clearest examples of how questioning an established design assumption can transform an entire category of prosthetic technology.
A water-skiing accident changed Phillips’ career
Phillips was a 21-year-old university student studying business when he lost his left leg below the knee following a water-skiing accident in 1976.
The prosthetic options available to him at the time were functional, but limited.
Traditional feet were largely passive structures intended to provide stability and allow the user to progress over the foot during walking. They could imitate the appearance of a biological foot, but they did relatively little to reproduce the dynamic function of the muscles and tendons of the lower limb.
Phillips wanted considerably more from his prosthesis.
Rather than accept those limitations, he became interested in prosthetic design and changed the direction of his education.
He eventually studied prosthetics at Northwestern University’s Prosthetic-Orthotic Center and also worked within the University of Utah’s prosthetics laboratory, gaining access to the expertise and equipment needed to experiment with new concepts.
The problem with making a prosthetic foot look like a foot
One of the key ideas highlighted in the history is deceptively simple.
Traditional prosthetic design had understandably attempted to recreate the shape of the human foot.
But a biological foot is not simply a foot-shaped structure.
The lower leg contains muscles, tendons and ligaments that deform during loading, storing energy and then returning part of it during movement. The Achilles tendon in particular behaves somewhat like a spring during walking and running.
A conventional rigid prosthetic foot could reproduce the shape without reproducing that function.
Phillips therefore began moving away from the question of how to imitate the anatomy and towards the question of how to reproduce the mechanical behaviour.
That shift would become the foundation of the Flex-Foot.
Carbon fibre provided the spring
Phillips’ solution was to use a flexible structural element as the prosthetic foot itself.
Rather than relying on a rigid artificial ankle and foot, the design incorporated a curved composite member capable of deforming under load.
As the prosthetic user applied weight, the structure flexed and stored mechanical energy. As the limb unloaded during forward progression, some of that energy was returned.
Today this concept is familiar to virtually every lower-limb prosthetist.
At the time, however, it represented a significant departure from conventional prosthetic design.
The material that made the approach possible was carbon-fibre composite.
In the late 1970s and early 1980s, carbon fibre was still comparatively exotic. Developing a prosthetic structure capable of repeatedly supporting body weight while surviving millions of loading cycles required careful control of fibre orientation, resin, laminate thickness and geometry.
Phillips reportedly tested prototypes on himself, with some early designs failing before the construction and geometry were refined sufficiently for reliable use.
Flex-Foot is born
Phillips founded Flex-Foot Inc. in 1984, and the concept developed into a wider family of prosthetic feet.
The breakthrough was not merely the use of carbon fibre.
It was the realisation that once a prosthetic foot no longer had to resemble an anatomical foot, engineers and prosthetists had much greater freedom over its shape.
That principle ultimately contributed to the development of the long curved structures now associated with running prostheses.
Instead of attempting to hide the technology inside a cosmetic foot shell, the shape could be determined by mechanical performance.
The running blade was effectively form following function.
From Flex-Foot to the Cheetah
Flex-Foot was acquired by Icelandic prosthetics company Össur in 2000.
The company’s later Cheetah running feet carried the energy-storage concept further, using long J-shaped or similarly curved carbon-fibre structures engineered specifically for running.
Unlike conventional cosmetic prostheses, there is little attempt to make such devices resemble human anatomy.
The carbon structure itself is visible.
Load it and the blade flexes. Remove the load and it returns towards its original shape, releasing part of the mechanical energy stored during loading.
The design has since become synonymous with amputee sprinting and Para athletics.
When prostheses became ‘too good’
The success of running blades eventually created an unusual problem.
Prosthetic technology had progressed from attempting to restore basic walking ability to enabling amputees to compete at the highest levels of athletics.
That led to a controversial question: could a prosthesis provide a competitive advantage?
The debate became particularly visible around South African double amputee sprinter Oscar Pistorius, who competed using Össur Cheetah Flex-Foot prostheses.
In 2007–2008, the International Association of Athletics Federations — now World Athletics — sought to prevent Pistorius from competing against non-disabled athletes on the basis that the prostheses might provide an unfair mechanical advantage.
The case ultimately reached the Court of Arbitration for Sport, which overturned the ban in 2008 after concluding that sufficient evidence had not been presented to establish the claimed overall metabolic advantage.
The debate did not end there.
Running prostheses are lighter than biological lower limbs and can return substantial amounts of elastic energy, but they do not produce their own power and cannot reproduce all the functions of biological muscle.
Acceleration, ground-contact time, limb mass, force production and curve running can each lead to different conclusions about whether a particular characteristic creates an advantage or disadvantage.
The fact that such a debate became necessary is itself evidence of how far the technology had advanced.
From extraordinary invention to everyday P&O
Phillips received the Lemelson-MIT Prize in 1997 and was inducted into the National Inventors Hall of Fame in 2008.
But perhaps the most significant measure of the Flex-Foot’s influence is that its underlying principle no longer appears unusual.
Energy-storing-and-returning feet are now a routine part of modern prosthetics.
They are used not only by elite athletes but by everyday prosthetic users seeking smoother rollover, greater activity and more dynamic mobility.
That is often how genuinely transformative P&O innovation looks in retrospect.
A technology that once appeared radical eventually becomes part of standard clinical practice.
An important lesson for today’s CPO
The Flex-Foot story remains highly relevant as the profession enters another period of rapid technological change.
3D printing, powered prostheses, microprocessor components, digital sockets, wearable sensors and AI-assisted design are all creating new possibilities.
Yet the lesson from Phillips’ work is not simply that new materials produce better prostheses.
It is about defining the clinical and functional problem correctly.
Phillips did not begin by asking how to manufacture a more realistic-looking foot.
He asked what the biological system was achieving mechanically, and whether another structure could achieve a similar useful function.
That distinction matters.
Innovation does not always require reproducing anatomy more accurately. Sometimes it requires abandoning anatomical appearance altogether.
Relevance across IMEA
That thinking is particularly important for the IMEA region.
Across India, the Middle East and Africa, CPOs work in environments ranging from advanced private prosthetic centres to high-volume public hospitals and humanitarian programmes.
The best solution for a patient will not always be the technology that most closely imitates the appearance of the missing anatomy.
It may instead be the solution that provides the appropriate combination of function, durability, repairability, cost and access.
The Flex-Foot succeeded because it focused on what the user wanted to do, rather than what the device was expected to look like.
Fifty years after Phillips lost his leg, that principle remains one of the most useful questions a prosthetist can ask:
What does this patient actually need the prosthesis to accomplish?
The answer to that question helped create the modern running blade.
It may also continue to shape the next generation of prosthetic innovation.
- Hackaday – Bladerunners and the Mother of Invention
- Össur
- Össur – Cheetah running prostheses
- Northwestern University Prosthetics-Orthotics Center
- National Inventors Hall of Fame
- International Paralympic Committee
- IMEA CPO