The efficiency produces all the torque needed to let a 165-pound person walk, but with just a 30W to 60W motor versus SPARKy's 150W -- a big help to battery life that also reduces the AMP-Foot 2.0's weight to that of the fleshy kind.
ENGADGET: AMP-Foot 2.0 prosthesis gives the power of real feet, keeps a light step (video)