What is the big deal with sitting? Why can it cause such a problem? It seems contradictory that sitting can cause problems because it helps in relaxation and does not seem to require much from the energy systems of the body.
Generally, the main function of the intervertebral disc found in between two vertebrae is to act as a shock absorber. In the cause of movement, the position of the vertebra changes and this has a pressurizing effect on the disc. However, the pressure is constantly changing. These changes in pressure are important in maintaining the health of the intervertebral disc.
However, sitting in a sustained position for prolonged periods compresses the disc. The compression causes the fluid in the disc to squeeze out slowly over time. There is a constant change in the amount of fluid in the disc throughout the day and it is the reason why people could be up to an inch taller in the morning when the pressure has been taken off your spine at night.
To maintain the health of the disc, we need regular changes of position because sustained pressure on the disc squeezes out the disc fluid and the fibres in the disc become stressed and strained.
The disc is made up of two parts: nucleus pulposus (jelly-like fluid material) and surrounded by layers of ligamentous rings known as annulus fibrosus. During a sustained sitting, when the fluid content is squeezed out of the disc, the outer portions of the ligaments forming the annulus fibrosus starts to strain and stretch. If this persists for long, it may result in damage where the ligaments become elongated and inflammation can occur.
When this process is beginning, people often feel stiff and sore when standing up from a seated position. This can then progress to just feeling discomfort even while seated and eventually sitting becomes intolerable.
That is generally how the stages in the process occur. It does not necessarily have to involve pressure on the sciatic nerve. The pressure on the nerves only occurs when the outer casing of the disc is broken down. This results in bulging of the disc pushing towards the outer wall and pressing the sciatic nerve. That is where full-blown sciatica occurs.
It is dependent on the size of the bulge where you can get continuous pressure on the sciatic nerve and this will cause continuous pain in the leg. It may be more intermittent and could come and go depending on how long you sit and whether you have the opportunity to change your seated position.
It is important to note that even in an easy and relaxed position, scientific tests show that the pressure on the disc is still quite high. Obviously, spending long periods seated at work will not get us that regular change of position that will release pressure. This will lead to a gradual breakdown of the intervertebral disc. It is a slow process that takes long before it fully develops.
