The movements of the vertebrae in the spine are a combination of gliding, tipping and twisting although each one individually contributes only a small degree.

Superimposed one on one, however, the net result is the grandiose wide-ranging mobility of the spine, which is so well known to us. From our towering height, we can arch backwards under a limbo bar and bend over to cut our toenails. Well, some of us can. Of the vertebral movements, gliding is the least generous. The upper vertebrae slide transversely forwards, backwards and from side to side on the vertebrae below but the actual distances travelled are minute. 

Glide is more of a background movement which better positions the vertebrae for action; it sets the stage and puts the vertebrae at its optimum starting point for the more adventurous tipping and twisting activity to follow.As we bend to touch our toes, for example, each vertebra moves forward incrementally on the one below, bringing the upper one to its perfect starting point for tipping. 

The element of glide contributes the typical following quality to the way all living creatures move (a cheetah on the run has it in spades). 

Without it, all living actions are much more clipped and jerky and not nearly as streamlined and expansive. An elderly lady tottering along the footpath has a very little glide in her joints.The right amount of vertebral glide is important; it is what healthy backs have. 

Too little or too much glide leads to trouble. If a segment has too little it will be stiff.Significantly, when the degenerative process sets in, this is the first movement to go. Although you cannot necessarily see it-first you only feel it-this lack of background movement makes your spine feel tighter; more restricted and laboured in everything it does. 

In short, it makes you feel rigid and older than you are.At the other extreme, a vertebral segment is unstable if it has too much glide. It comes to light when the spine bends over and the top vertebrae slip forward on the lower one.

This is known as segmental instability, and it actually stems from the stiffness of a segment. What distinguishes the former from the latter is the total degeneration of the motion segment.With normal segmental mobility, all the vertebrae roll around on their ball bearing discs whose fibrous walls keep everything in place. As the upper vertebrae move off-centre, tension takes up in the wall mesh and brakes the action. And as soon as the bend is incorporated into the action, another break comes on from the squirting pressure of the nucleus.With bending forward, which is our most repeated action, the nucleus is squidged towards the back of the disc. 

Were it not for the toughness of the wall, the distortion would make it balloon out backwards. But because the wall is so strong, most of the force is taken up or absorbed by it. Thus the intradiscal pressure adds extra stiffening to the mesh. In this pre-stiffened state, it then retards the gapping of the two adjacent vertebrae as they try to pull apart with bending. The tensioned mesh slows the rate at which the back of the vertebrae can separate and steadies the bending.