Answer to Question 5
The severity and persistence of low back pain is difficult to explain when there seems to be no tissue damage. Undoubtedly much pain is muscular but this is not a full explanation and rarely provides a solution.
The mechanical structure of the spine, pelvis and associated muscles provides a reason for such severity. To an engineer the layout is obviously an “over-centre mechanism” (like the clips often used on a suitcase to provide extra force for the last bit of closing –and on a multitude of other clamping mechanisms)
This is an “over centre mechanism” because the point of attachment of the muscles (mainly via the erector spinae aponeurosis) is
below the hinge point in the spine.
The seriousness of such an extrusion of the nucleus is obvious and fully explains the evolution of a severe and persistent pain
if there is a source for such a pain before tissue damage occurs.
The answer to Question 4 provides just such a source and it so happens, probably by chance, that the angle of spinal flexion that begins to become dangerous, corresponds to the flexion through and beyond the parallel sided shape (for the lowest two disks as shown below.)
This is, I suggest, the reason that low back pain has evolved to be so severe and persistent. The Evolution of Low Back Pain
(1) Adams MA, Hutton WC Prolapsed Intervertebral Disc. A Hyperflexion Injury Spine 7: 184-191, 1982