While modern limb prostheses, especially of the leg and foot, have helped many injured people return to normal lives, science has still not caught up to the biological workhorse that the human ankle represents. A paper by a professor and former undergraduate at the University of Michigan's departments of Biomedical Engineering and Mechanical Engineering titled "Recycling Energy to Restore Impaired Ankle Function during Human Walking" is showing how science is catching up.

Prostheses indeed return people to their normal lives, but for many, it's still with impaired functions. There are of course purpose-built prostheses for athletics, but for the sole purpose of normal human walking, the replacements simply fall short in performance. The UofM paper cites research that concludes that 23% of walking energy is wasted by a standard foot prosthetic with every step. While all prostheses return some of the lost energy from the foot contacting the ground, they don't usually give the recipient much choice in when or how that energy is returned. Expensive battery-operated units try to mimic the ankle's push-off, but they require bulky batteries for use.

The goal of Art Kuo and Steve Collins's device is to return as much of the wasted energy as possible at the proper moment to simulate a real ankle. "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," explains Kuo, the UofM professor mentioned previously. Returning the energy properly lessens this load.

Using an interim device, studies showed that between a regular prosthetic foot's energy waste and their mechanical creation, energy waste was cut nearly in half, to 14%. This is due to the replacement's clever use of a microcontroller and energy capturing systems. The microcontroller tells the unit when to release the stored energy, better mimicking a natural foot step. The unit does still require a battery, but since it uses less than one watt of energy a small portable battery is more than enough to help power the system.

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

Testing has begun with the new artificial foot at the Seattle Veteran's Affair Medical Center while commercial development has been undertaken by an Ann Arbor-based company.

The paper has been published today in the journal PLoS ONE.

Steve Collins is now an associate research fellow at Delft University of Technology in the Netherlands.