Artificial Foot Recycles Energy for Easier Walking

This recycling results in a reduction of the increased metabolic energy expenditure caused by using a conventional prosthesis from 23% to only 14%.

The proof-of-concept device approximates the size and form of a conventional prosthetic foot, but has separate rear foot and fore foot components that rotate about a mediolateral axis at mid-foot. When the heel contacts the ground at the beginning of a stride, the rear foot component rotates and compresses a coil spring. At maximum compression, the rear foot is latched by a continuous one-way clutch. But rather than releasing the energy spontaneously as in conventional elastic prostheses, the device stores it until sufficient load is detected on the fore foot; it then releases the fore foot, and the spring provides push-off as the person begins to unload the trailing leg, with timing similar to normal ankle push-off. A small return spring resets the device during the ensuing swing phase, so that the rear foot is in position for the next step. All of the energy capture is performed passively, so that the only active elements are a microcontroller and two micromotors that release the energy-storing spring and reset the mechanism, both powered by a small battery generating about 0.8 W of electricity. The study presenting the energy-recycling prosthesis was published in the February 2010 issue of the journal PLoS ONE.

"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 codeveloper Art Kuo, Ph.D., a professor in the departments of biomedical engineering and mechanical engineering at the University of Michigan. "All prosthetic feet store and return energy, but they don't give you a choice about when and how. 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.”

Human beings normally dissipate significant energy during walking, largely at the transitions between steps. The ankle then acts to restore energy during push-off, which may be the reason that ankle impairment nearly always leads to poorer walking economy. The replacement of lost energy is necessary for steady gait, in which mechanical energy is constant (on average) and the external dissipation of energy is negligible, resulting in no net work performed over a stride. Ankle impairments following amputation, joint fusion, or stroke typically reduce ankle work and increase metabolic energy expenditure by at least 20%.

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