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

This paper demonstrates that the engineers' idea works. They 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.

The paper is called "Recycling Energy to Restore Impaired Ankle Function during Human Walking."