Intervertebral Discs are found in the spaces between all the spinal vertebrae, apart from the Atlas (C1), Axis (C2) and the Coccyx. The discs connect the vertebrae and act as shock absorbers between them, protecting the spinal nerves, brain and the vertebrae themselves. Another function of the discs is to allow flexibility between the vertebrae and along the whole of the spinal column.
Each intervertebral disc is a few millimetres thick and, all together, the discs form a quarter of the spine's length. The outer covering of the discs is fibrous, with a pulpy centre which is semi-fluid. Although the pulp resists compression, the amount of fluid in the discs varies during the day according to how great a load is being put on them, squeezing out the fluid. So, for instance, if someone continually carries heavy objects and their mis-use makes their torso contract down as they work all day, the load on the spinal discs will be great. In extreme cases, the discs loose their capacity to maintain their fluid so that the vertebrae can grind together, crepitus occurs and nerves can end up by becoming pinched and damaged.
However, if that person were poised and using their body well, maintaining the length in the spine as they work, this good use will obviously put less pressure on the discs so that they lose less fluid. This will protect the discs, reducing the degree to which they degenerate and become brittle with age. It would also reduce the risk of having a back injury such as a torn muscle, or even a prolapsed disc.
It has been found that 'adult people are about 20 millimetres (1") taller when they get up in the morning, than when they go to bed at night'. (p 41)
R McNeill Alexander ~ The Human Machine.
This research was not done with people who study the Alexander Technique but it would seem likely that the degree to which Alexander pupils contract down each day and lose fluid from their discs, would be less than the amount shown in the general public. This effect would be stronger if people regularly use the Lying Down Procedure which gives an extra period of time each day, in which the intervertebral discs could re-absorb some fluid.