This prosthetic knee joint disposes of a quick exhaust valve. During development of this joint, it was noticeable, that during fast walking at the beginning of the forward motion of the prosthesis leg, an air spring, which was too strong, effected the prosthesis joint; walking appeared to be unphysiological. The leg moved forward too quickly and could not be slowed down enough effectively. The solution: An iglidur® J piston ring from igus®. The axial play of the piston ring in the ring groove is simultaneously used to switch the quick exhaust valve. The axial movement of the piston is a few tenths of a millimetre only, but it is enough to get rid of an air cushion that disturbs in the swing phase at the right time.