Hiking trails and other rough terrain are especially difficult for people with prosthetic legs. Now, engineers have come up with a more stable option.

The cornerstone of the new design is a kind of tripod foot that responds to rough terrain by actively shifting pressure among three different contact points. Researchers say the tool they developed for quickly emulating and improving their prototypes is just as important as the foot.

“Prosthetic emulators allow us to try lots of different designs without the overhead of new hardware,” says Steven Collins, an associate professor of mechanical engineering at Stanford University and a member of Stanford Bio-X.

“Basically, we can try any kind of crazy design ideas we might have and see how people respond to them,” he says, without having to build each idea separately, an effort that can take months or years for each different design.

Fall risk

Around half a million people in the United States have lost a lower limb, with effects that go beyond simply making it harder to move around. People with a leg amputation are five times more likely to fall in the course of a year, which may contribute to why they tend to be less socially engaged. A better prosthetic limb could improve mobility and overall quality of life as well.

Making prosthetic limbs that can better handle rough ground particularly interested researchers. Collins, graduate student Vincent Chiu, and postdoctoral researcher Alexandra Voloshina thought a tripod with a rear-facing heel and two forward-facing toes might offer a solution.

Outfitted with position sensors and motors, the foot could adjust its orientation to respond to varying terrain, much as someone with an intact foot could move their toes and flex their ankles to compensate while walking over rough ground.

But the engineers knew that perfecting the design would be tough—even with simple designs, a conventional approach can take years or more. “First you have to come up with an idea and then you prototype it and then you make a nice machined version,” Chiu says. “It could take several years, and most of the time you find out that it doesn’t actually work.”

Heel and toe

The researchers thought they could accelerate the process if they developed an emulator, which flips the design process on its head. Rather than building a prosthetic limb someone could test in the real world, the team instead built a basic tripod foot, then hooked it up to powerful off-board motors and computer systems that control how the foot responds as a user moves over all kinds of terrain.

That approach let them put their design focus on how the prosthesis should function—how hard one toe should push off while walking, how springy the heel should be, and so forth—without having to worry about how to make the device lightweight and inexpensive at the same time.

So far the team has reported results from work with one participant, a 60-year-old man who lost his leg below the knee due to diabetes and the early results are promising—making them hopeful they can take those results and turn them into more capable prosthetics.

“One of the things we’re excited to do is translate what we find in the lab into lightweight and low power and therefore inexpensive devices that can be tested outside the lab,” Collins says. “And if that goes well, we’d like to help make this a product that people can use in everyday life.”

The National Science Foundation funded the work, which appears in IEEE Transactions on Biomedical Engineering.