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, said actuator further comprising 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 energy storage accumulator and the hydraulic or pneumatic cylinder, wherein: - the hydraulic or pneumatic cylinder and the piston are connected to a user's upper leg and lower leg respectively or vice versa, and wherein the activator is: provided with a single control valve； and is ar- ranged to - close the single control valve when the user is sitting down； and - open the single control valve when the user is going to stand or standing up.
