An energy storing prosthetic knee actuator and method of using such an energy storing prosthetic knee actuator comprising a hydraulic or pneumatic cylinder with a piston and a piston rod, wherein the piston and the hydraulic or pneumatic cylinder are movable with respect to each other, an energy storage accumulator and a control valve provided in piping connecting the hydraulic or pneumatic cylinder with the energy storage accumulator for controlling an exchange of fluid or gas between the accumulator and the cylinder, wherein the cylinder and the piston are connected to a user's upper leg and lower leg respectively or vice versa, and wherein the actuator is arranged to close the control valve when the user is sitting down and to open the control valve when the user is going to stand or is standing up.
