This paper presents a human-driven robot leg that works as an energetically conservative variable stiffness spring. Using a pedaling mechanism, the leg stores energy supplied by the person and allows the stiffness to be increased before the stored energy is released. A prototype confirms force amplification, power amplification through stiffness modulation, and stiffness adjustment at low energy cost.
Why it matters: The gears of a bicycle amplify the force of human legs, but not the power. Conventional variable stiffness springs can amplify both, but increasing the stiffness costs energy in proportion to the energy already stored. This leg amplifies force and power at low cost, much like shifting gears while pedaling a bicycle, and points toward human-driven robot legs that extend human performance.