The hydraulics of the lower leg are strong enough to keep the pseudo spine relaxed and straight while standing, this keeps permanent "kinks" out of the leg. While running the hydraulic tissues contract and release to give spring and shock absorption to the runner as seen in the figure below.
Red arrows = Retraction and contraction of hydraulic tissues
Blue arrows = Extension of leg by extending hydraulic tissues
Purple arrows = Shock absorption
The arm operates in the same way but does not have the ability to retract as much as the leg.