An new prosthetic foot has been developed and explained in a PLoS-One paper by Steven H. Collins (Department of Bio-mechanical Engineering, Delft University of Technology) and Arthur D. Kuo (Departments of Mechanical and Biomedical Engineering, University of Michigan).
The innovative new artificial foot uses a micro controller to operate a device in the foot which stores energy from the down step and then releases it on the up step and mimicking the natural movement of an unmodified human ankle.
Research has indicated that subjects walking with a conventional artificial ankle increased net metabolic energy expenditure by 23% compared to normal walking. The new energy recycling ankle reduces this amount to 14%.
For more information you can read the PLOS One document here