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Like a hybrid car that generates electricity during braking, the device Donelan et al. developed generates electricity by assisting muscles in slowing down the leg just before it straightens out and the foot lands on the ground. That requires much less effort than conventional human power-generation -- about 1/8th the effort -- and people walking with one device on each leg produced an average of 5 watts of electricity, which is about 10 times that of shoe-mounted devices. The device churns out enough power, potentially, to serve as an "on board" charging system for power-assisted prosthetic limbs and other portable medical devices.