Electric-Energetic Consequences of Springs in Lower-Extremity Powered Prostheses on Varied Terrain

We present electric-energy consequences from the primary mechanical design trade-offs in lower-extremity, knee and ankle powered prostheses walking on varied terrain. Previous work has studied level-ground walking and running, but has not yet examined the influence of design parameters in large range of motion tasks such as stair ascent and descent. There are four main hardware components commonly implemented in bionic ankles and knees that can be tuned to achieve desired performance: motor, reduction ratio N, series spring stiffness Ks, and parallel spring stiffness Kp. The allowed joint range of motion is a fifth parameter that can strongly affect electric energy consumption. Using a kinematically clamped analysis we evaluate the electric cost of transport (eCOT) for knee and ankle stair ascent and descent in addition to level-ground walking. Results show parallel springs (PS) improve level-ground energetics but can be costly on stairs. Variable transmissions combined with PS can improve energetics but not more than simply limiting range of motion, while the knee does not benefit greatly from this complexity.