A dynamic fixation medical implant having at least two bone anchors includes a longitudinal connecting member assembly having at least one transition portion and cooperating outer sleeve, both the transition portion and sleeve being disposed between the two bone anchors. In a first embodiment, the transition portion includes a rigid length or rod having apertures therein for tying or otherwise attaching the rigid length to a second rigid length or to a flexible cord. Slender ties or cords extend through a plurality of apertures in the rigid lengths or are threaded, tied or plaited to the larger flexible cord or cable. In a second embodiment, a transition portion includes slender ties of a cord that are imbedded in a molded plastic of a more rigid member. The outer sleeve may include compression grooves. The sleeve surrounds the transition portion and extends between the pair of bone anchors, the sleeve being compressible in a longitudinal direction between the bone anchors.