Stiffness adjustment of a series elastic actuator in a knee prosthesis for walking and running: The trade-off between energy and peak power optimization

In ankle-foot prostheses a serial spring can assist the motor to reduce peak power (PP) and energy requirements (ER) during locomotion. Similar benefits can be expected for an active knee prosthesis. We compare the situation of a direct drive with a series elastic actuator optimized for minimal ER or for minimal PP. The simulations indicate that at the knee joint a serial spring can highly reduce ER and PP in running and fast walking. Around preferred walking speed (1.3m/s) only a reduction in ER is found. The optimal stiffness changes with speed and gait. Still it is possible to use one constant stiffness for a huge range of walking and running speeds with only moderate increases of ER and PP in comparison to speed and gait optimized values.