Most of us take it for granted, but walking isn't as simple as it looks. With the natural human gait the ankle exerts force to push off the ground. A typical prosthesis doesn’t reproduce the force exerted by a living ankle, resulting in amputees spending much more energy in comparison to walking naturally. A new prototype artificial foot recycles energy that is otherwise wasted in between steps to significantly cut the energy spent per step, making it easier for amputees to walk.

The prosthetic foot created by Art Kuo, professor in the University of Michigan (U-M) departments of Biomedical Engineering and Mechanical Engineering, and U-M graduate, Steve Collins, was designed to capture the energy that is wasted when the ankle hits the ground. To enhance the power of the ankle push-off a microcontroller tells the foot to return the energy at precisely the right time.

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. They found that test subjects spent 23 percent more energy walking with a conventional prosthetic foot, compared with walking naturally.

"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," said Kuo.

However, based on metabolic rate measurements, the test subjects spent just 14 percent more energy walking in the energy-recycling foot than they did walking naturally.

“We know there's an energy penalty in using an artificial foot," Kuo said. "We're almost cutting that penalty in half."

Although all prosthetic feet store and return energy, they don’t provide a choice about when and how this is done.

“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."

Also, 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 provided through a small, portable battery.

The engineers are now testing the foot on amputees at the Seattle Veterans Affairs Medical Center and commercial devices based on the technology are under development.

A paper about the device, "Recycling Energy to Restore Impaired Ankle Function during Human Walking," is published in the Feb. 17 edition of in the journal PLoS ONE.

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