Answer to Question 12
Let me try a different way of explaining my basic argument:
There are no particular or obvious limits on the flexion, extension or rotation of an intervertebral joint. Facet joints will limit rotation to a different extent in different parts of the spine. Similarly facet joints will tend to suggest a limit to flexion when they cease to overlap or extension when contact occurs beyond the articular surface. There will be an average range of motion for most joints of the spine if we measure a large number of people but we all know that variation will be large. If for a particular joint we select a normal range of motion based on say the 90th percentile then there still doesn't seem to be any reason why an individual with a different range of motion should suffer any particular pain or problem. The actual limit in a particular joint is usually defined by ligaments local to that joint but these ligament lengths will depend on use. If regular use stretches them they will be longer without damage or pain.
Obviously there will be postural implications but opposite compensations in different points of the spine can often counteract these.(or not-)
On top of this there is the muscle system. This is extremely complex, coordinating movement of the body, locally and generally . This is not something I need to (or can!) understand. I don't need to. It just works -- incredibly well in most cases.
Then there is the case where muscles local to a joint become permanently tighter than they should be. They limit the mobility of that joint or area the spine and there will be pain or ache because muscles that are permanently tight do ache and can cause spasm type pain. To me this is what chiropractic is all about. This is the chiropractic subluxation. (I call it a sublesation for reasons explained at (1) below) I suspect that we won't agree on this but this is not important here. This is not the subject that I am trying to explain.
If we go back to my first paragraph and look at the flexion extension shapes of disks throughout the spine we find a lot of variation and nothing very significant except at the lowest two disks L4 5 and L 5 S. If we have chosen people without back problems we find the surprising result that the extended shape is so steeply wedge shaped that even the fully flexed shape maintains the wedge shape in the same direction. Wide end anterior. (Answer to question 1) Very surprising but when you think about it, it's a fairly obvious result of evolving to bipedalism while maintaining a birth canal. (Answer to question 3)
Certainly a surprising, and almost unrecognised, detail of the human spine. But on the principal defined in my first paragraph there is no immediate reason why someone whose (sedentary?) lifestyle happens to distort the shape of their spine such that either of these lowest two joints does flex by say 10° either side of the parallel sided shape should therefore suffer from pain. The whole of the intervertebral joint L4-5 could work perfectly satisfactorily even if the range of flexion and extension differs from the average.
Assume for moment that back pain doesn't exist. For a particular individual, lifestyle or some incident has resulted in the lumbar spinal shape being different from the average with the lowest joints flexing beyond parallel sided and the upper lumbars, and lower thoracics, being more extended than usual to compensate posturally. Everything will work fine for this individual. But one day he may try to lift something heavy with a fully flexed lower spine. The layout of the spinal musculature is such that he will be setting up the conditions for a sudden disc prolapse as shown by Adams and Hutton. (Question 5)
He will get no warning that this is going to happen because nothing is damaged or stretched beyond its normal shape or length. The failure will be instantaneous, as it was in the laboratory but for that individual, two or three million years ago on the plains of Africa, it will almost certainly be fatal.
There isn't really a mechanical solution to this. The whole layout of lower spine and pelvis makes this sudden and catastrophic failure unavoidable. The solution that has evolved is a pain which simply depends on the shape of the lowest two spinal joints. It is initially a very severe pain designed to stop further activity and also very persistent to encourage care in all subsequent activities.
The actual mechanism of the pain I suggest in the answer to question 4. This seems to fit with a lot of recent "evolution" of thought on discogenic and nonspecific low back pain. (See for instance my letter to Nicolai Bogduk. Ref (2) below)
(1) Sublesation http://www.naturaljointmobility.info/Sublesation.htm
(2) letter to Author of “The Clinical Anatomy of the Lumbar Spine” Nikolai Bogduk. Link to letter