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'Green' foot could make walking with a prosthetic foot easier and more efficient

In this energy-recycling foot, the University of Michigan engineers put wasted walking energy to work enhancing the power of ankle push-off. Photographs by Steve Collins.

Solar panels convert energy from the sun into electricity that can be stored and used when needed. Researchers have applied this same concept to the development of a new prosthetic foot that stores and releases energy in a way that could make it easier for amputees to walk.

The project began as a research endeavor to better understand the mechanics of human walking. "We'd been doing research on this for a number of years, and we discovered the importance of the collision of the leg with the ground," says Arthur Kuo, PhD, professor of mechanical and biomedical engineering at the University of Michigan, Ann Arbor, and co-developer of the prototype foot. "We…did many experiments to show how important this is, and we thought, ‘Well, now that we've done the science, there must be something useful we can do with it.'"

The development of a prosthetic foot turned out to be the "perfect way to both test the idea further" and also develop a biomedical application for the research, Kuo says.

The Energy Recycling prosthetic foot is the brainchild of Steve Collins, PhD, who developed the design as part of his doctoral thesis. Kuo helped develop the foot, and Peter Adamczyk, PhD, a researcher and principal officer at Intelligent Prosthetic Systems, Ann Arbor, Michigan, was involved in testing the device.

The Ankle Is the Key

One of the keys to energy economy, according to Kuo, is pushing off with the ankle. "The ankle normally produces a larger burst of work than any other joint during walking," wrote Collins and Kuo in the article "Recycling Energy to Restore Impaired Ankle Function during Human Walking," (PLoS ONE, February 17, 2010). "Ankle impairments following amputation, joint fusion, or stroke typically reduce ankle work and increase metabolic energy expenditure by at least 20 percent…. We propose an alternative, which is to restore ankle work simply by recycling energy that is normally dissipated as negative work."

Kuo says that though many prosthetic feet have some sort of elastic energy return and are quite effective for reducing impact forces and improving comfort, there is no data that shows they have an effect on energy expenditure.

"We thought one of the reasons why [prosthetic feet] might not improve energy expenditure is because they might not return the energy from the spring in the proper way," Kuo says. "The spring doesn't have any choice—it just returns energy once compressed. So rather than having the spring release spontaneously, our idea was to capture the energy and keep it locked in this spring for a period of time and release it later."

The result of the team's research effort is "a microprocessor-controlled foot that captures some of the energy that is normally dissipated by the leg and ‘recycles' it as positive ankle work," Collins and Kuo wrote in the PLoS ONE article.

The foot has separate rear- and forefoot components "that rotate about a medio-lateral axis at mid-foot," the author's continue. An onboard computer records data about where the foot is in the gait cycle, and based on that data, the computer calculates when it's appropriate to release the spring.

"The actual release of the spring is determined both by the computer and also assisted mechanically by how much weight the person has on the foot," Kuo told The O&P EDGE. "So, as the person is starting to unload from the front of the foot, the spring finally releases."

To test the effectiveness of the device, the research team compared it with a conventional prosthetic foot. Both devices were tested on able-bodied human subjects walking with an artificially immobilized ankle and a prosthesis simulator boot. Subjects were asked to walk at a controlled speed of 1.25 mph on a treadmill that had been outfitted with motion-capture and force­plate instruments.

Greater Push-off, Reduced Energy Expenditure

"The conventional prosthesis reduced ankle push-off and increased metabolic expenditure for all subjects," the authors wrote. "The Energy Recycling artificial foot captured collision energy and returned it as positive ankle work later in stance, resulting in greater push-off and lower metabolic expenditure than with the conventional prosthesis.

"The Energy Recycling foot provided ankle push-off work at more than twice the rate of the conventional prosthetic foot, restoring ankle push-off to that of normal walking," the authors continued. "The device also reduced the net rate of metabolic energy expenditure for walking with an immobilized ankle from 23 percent above normal to 14 percent."

Kuo says that his team has been testing the device in a separate study in collaboration with the VA Puget Sound Medical Center, Seattle, Washington. He says there is probably "still a good year of testing to go before we really know much about how the foot functions on actual patients."

While the Energy Recycling foot holds promise for improving walking economy for persons with amputations, there are several limitations to the foot's current design. "The prototype is still a bit heavier than what we'd like for a useful prosthetic foot," Kuo says, "so we're doing quite a bit of redesign to try and reduce that weight." Kuo and his team are also working to make the foot smaller and to reshape the components so that the foot will fit inside a standard prosthetic foot shell. Karl Zelik, another of Kuo's graduate students, will be addressing the foot's current limitations, as well as what they learn from the VA study, to design the next generation of the prototype.

The end goal, Kuo says, is to develop "a device that we can demonstrate is a significant improvement over other prosthetic feet—one that is reliable, small, and cosmetically attractive. We'll then be talking to some of the manufacturers to see if they are interested in licensing the technology." Kuo adds that the research team has already had informal discussions with two major prosthetic-foot manufacturers.

Karen Henry can be reached at