The pressure inside the discs is higher with sitting than standing or lying. The sacral table rams up under the descending spine and the base is pinched in the middle as if caught in a vertical vice. Over time, fluid leaks out of the discs (they lose approximately 10 percent of their total fluid) and the stacked bony segments settle together, becoming a semi-rigid tubular mass. 

Most of the fluid escapes within the first hour or two of sitting, but the discs keep squeezing drier the longer they stay compressed.People who sit for long hours to work usually feel cast in the back when they get up. It can take several minutes before the base of the spine drops down and movement gets easier. If long hours of sitting are combined with low levels of activity the lowest discs never properly reflate.The discs puff up again with fluid faster than they expel it.

This is partly due to the concentration of electrolytes in the nuclear gel which drags in water under osmotic pressure, but also through the suction effect of the spine elongating when unweighted. 

The disc puffs up more quickly if you lie on your back and bring your knees to your chest which is why this simple action is such an important part of self-treatment. Bouncing your knees to chest and squatting exercises through the day help counteract the fluid loss from the discs, mainly by hinging open the back of your spine.

On the other hand, using a back block opens the front of your spine. It passively hyper-extends (over arches) your spine which not only opens the front of the discs and drags fluid in but also reverses the slumped posture of sitting. As part of yourself treatment program, you should use the block every day. I promise you will get addicted to the sensation of the segments pulling apart when you can almost feel the flattened mesh walls of the discs being tugged up and water being dragged in from the vertebral bodies through the sieve-like vertebral end-plates.

Many hours of sitting can also cause adaptive shortening of other structures around the low back which makes resuming a normal posture even more difficult. For example, the hip flexor muscles at the front of your hips are very powerful and tighten quickly when they spend long periods puckered in a shortened state.

Their tethering causes a sense of tightness across the front of the hips and down the thighs which makes it difficult to stand up straight. In the long term, this tightness has adverse consequences because it causes the front of the pelvis to tilt down at the front, which throws the spine out of balance and creates a typical ‘bottom out’ appearance.

Tight hip flexors also make you take much shorter steps because the legs cannot angle back properly at the hips. Again this is bad for the back because in making a decent stride, the spine must twist lefts and right to compensate for the poor hip mobility. If the sitting posture is especially slumped it causes adaptive contracture of the anterior longitudinal ligament which runs down the front of the vertebral bodies like a long elastic tape. 

In healthy circumstances its role is to limit the backward arching movement of the spine, but when it adaptively shortens it tethers the spine over in a hoop at the front, like an over-tight bowstring. People are often aware of their worsening posture and feel they are being kept stooped as if their shirt is tucked into their waistband.

Using the back block is the natural antidote for sitting. It helps undo all the problems acquired by hours spent crimped at the hips and knees with the shoulders curled forward over the task at hand. It opens out the bowed-over spine and chest; it pushes in the bottom and stretches out the front of the hips as the legs drop down to the floor. 

But most importantly, it decompresses the spine by pulling up the bunched down disc walls and sucking in the fluid. Bear in mind that sitting, or parking our pelvis on a chair, is a recent and fairly unnatural phenomenon. Many indigenous people still squat rather than use high-backed support. Even though their day may involve running or carrying heavy loads, both of which compress the base of the spine, they can easily dis impact it again comes nightfall by squatting to prepare food and eat.You would never see a Masai warrior slumped on a sofa. Frequent squatting exercises form a large part of the self-treatment program.Most sedentary jobs bring on a degree of segmental stiffness of the low back. 

Sitting hunched and scrunched on a chair-often with the legs crossed and the telephone glued to the ear-stiffness the whole body and dries out the base of the spine. Computer operators, machinists, or anybody who sit at a bench or desk working all day, are particularly susceptible. As the hours mount up, they get stiffer more quickly each time they sit, and take longer to straighten after getting up; a persistent broad band of immobility across the base of the spine eventually becomes over trouble.Long-distance drivers and taxi drivers also have a high incidence of back trouble.

 Apart from the sitting, the continuous vibration of the vehicle causes a greater loss of fluid from the lower discs. Trucks drivers are even more prone to back trouble if the cabin is high up and they have to jump a long way to the ground. The impact is particularly jarring when the spine is already compressed. Matters are made worse if they have to push heavy objects around in preparation for lifting on and off the tray.