Elegant and strong as the human back is, the job of bending over and straightening up again is a tall order. The trunk and spinal muscles which actively control the movement are discussed further on. 

However, several other anatomical features help make bending possible, by working as a physical brake to control the free fall of the spine when it tips forward. The strong fibrous wall of the disc which binds the cotton reels together. 

This contributes about 29 percent to the control of the segments going forward. As the segments glide forward, the still fibrous mesh of the wall retards the initial movement. When the spinal segments then tip forward and the back of the interspaces open up, the same diagonal mesh pulls up, like stretching up a garden lattice. However, even more, important in controlling bend are the various structure of the back compartment.

The facets contribute in two ways: a sloping stop-ramp, made by the joint surfaces, and extremely tough capsular ligaments. When viewed from the side, the lower facet surfaces taper upwards towards the front of the spine. As the spine bends this means the upper vertebrae must travel uphill as they go forward. 

They work like emergency stop-ramps beside steep downhill sections of highways, gradually bringing the vehicle to a halt as it nears the top of the ramp.In case of the back, the escalating tension of the soft tissues gradually brings the upper vertebrae to a halt, by which time the facet interfaces are firmly locked against each another and the ligamentum flavum and the facet capsule are tense at full stretch.

It is a marvellously ingenious system with both bone and soft tissues complementing the workings each other.As we go further into a bend the upper vertebrae then tips bodily forward by pivoting on the front edge, as the tail of the vertebrae attempts to lift away. This second part of the movement is restrained mainly by the facet joint capsule. They contribute a powerful 39 percent towards moderating bending. 

The ligamentum flavum contributes an initial 13 percent. All up, the facets contribute 52 percent of the ligamentous restraint on forward bending. No wonder they can suspend twice the body’s weight. It is very significant that none of these facet ligaments exerts control over the forward gliding action of segments, only forward tipping. 

Control of forwarding gliding is important because it is this action carried out to excess (when it is called forward shear) which constitutes the unstable element of a segment’s movement. All segments must avoid shear because it is potentially so devastating. 

And it is a failure to control the forward tipping which allows too much shear. The process works like this: As bending starts the initial forward glide is only about 2mm before the facets engage to stop it. As the bend continues, the facets disengage with the tipping forward action which makes the tail of the vertebrae lift up and away, leaving a gap between the two facet surfaces. 

However, once the tail of the vertebrae is away, the whole vertebrae can glide forward more, until the bony block engages once again. In this way, more tips allow more glide. Incidentally, the tipping action is what both the facet capsules and the ligamentum flavum are designed to resist, while multifidus, the deepest intrinsic muscle of the spine, is designed to control actively.