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“For amputees, what they experience when they’re trying to walk normally is what I would experience if I were carrying an extra 30 pounds,” says Art Kuo, professor of biomedical engineering and mechanical engineering at the University of Michigan.

Compared with conventional prosthetics, which don’t reproduce the force a living ankle exerts to push off the ground, the new prototype significantly cuts the energy spent per step.

Test subjects spent 23 percent more energy walking with a conventional prosthetic foot, compared with walking naturally. A paper about the device is published in the Feb. 17 edition of in the journal PLoS ONE.

To test how stepping with their device compared with normal walking, the engineers conducted their experiments with non-amputees wearing a rigid boot and prosthetic simulator.

In their energy-recycling foot, the engineers put the wasted walking energy to work enhancing the power of ankle push-off.

The foot naturally captures the dissipated energy. A microcontroller tells the foot to return the energy to the system at precisely the right time.

Based on metabolic rate measurements, the test subjects spent 14 percent more energy walking in energy-recycling artificial foot than they did walking naturally. That’s a significant decrease from the 23 percent more energy they used in the conventional prosthetic foot, Kuo says.

“We know there’s an energy penalty in using an artificial foot,” Kuo says. “We’re almost cutting that penalty in half.”

The energy-recycling foot takes advantage of power that would otherwise be lost, it uses less than 1 Watt of electricity through a small, portable battery. (Credit: Steve Collins)

Kuo explains that the new device differs from current technologies because “all prosthetic feet store and return energy, but they don’t give you a choice about when and how. They just return it whenever they want.

“This is the first device to release the energy in the right way to supplement push-off, and to do so without an external power source.”

Other devices that boost push-off power use motors and require large batteries. Because the energy-recycling foot takes advantage of power that would otherwise be lost, it uses less than 1 watt of electricity through a small, portable battery.

“Individuals with lower limb amputations, such as veterans of the conflicts in Iraq and Afghanistan or patients suffering from diabetes, often find walking a difficult task,” says Steve Collins, formerly a graduate student at University of Michigan and now an associate fellow at Delft University of Technology in the Netherlands.

“Our new design may restore function and reduce effort for these users. With further progress, robotic limbs may yet beat their biological forerunners.”

The researchers are now testing the foot on amputees at the Seattle Veterans Affairs Medical Center. Commercial devices based on the technology are under development by an Ann Arbor company.

This research was funded by the National Institutes of Health and the Department of Veterans Affairs.

University of Michigan news: www.ns.umich.edu/