Abstract: A flexible spinal fixation rod assembly that provides relatively stiff bending compliance provided by a core rod or tube that is connected to surrounding spring and associated couplings in such a way that when the core rod is bent, the spring extends and the associated couplings move away from each other, with the core rod generating a force tending to return the core rod to its unbent configuration thereof and the spring generating a force tending to return the couplings to there unextended configuration thereof. Couplings connect each end of the rod or tube to corresponding pedicle screws for attachment to corresponding vertebrae. As the spine bends, this arrangement allows the spine to bend against limited resistance, exhibiting the necessary axial and compressive strength, and applies force to return the spine to its original configuration.