High Chairs
A.C. Mandal
Since it is school children who have to sit on school chairs for the greater part of the day, it would seem reasonable to find out what chair height they prefer.
I have tried to answer this by testing a group of 80 children and grownups. I used a chair with a tilting seat and a table with an adjustable sloping top. Both the chair and the table were mounted on hydraulic pistons so that their height could be adjusted up and down.
When testing this group, I asked them to sit on the chair at the table when both were at the heights recommended by the ISO - i.e. 18" high chair and 27.5" high table. The seat tipped 5° backwards, and the tabletop was horizontal.
Under these conditions, a 68.5 inches tall girl had to bend right over in order to read a book at a distance of 12" (fig. 45).
When the seat was tipped forward, the desktop was inclined and the chair and the table were pumped up and down, everyone found it much easier to sit and work at heights much greater than the ISO standard. The position they all preferred was one in which they were sitting with an almost perfectly straight back (fig. 45 B) - exactly Keegan's normal position, that is, the rider's position. They nearly all wanted chairs and tables 6 to 8 inches higher than the ISO standard.
It is much easier to breathe properly with the diaphragm in this position. It is only possible to breathe with the upper part of the chest in the bent over position. In the high-sitting position, the pressure in the abdomen is less, and this in turn leads to better blood circulation from the legs.
It is important for this test that all people tested are sitting down all time, and that the changes in height are produced by a hydraulic pump. This means that they can feel the differences in tension and pain as the chair and table are moved up and down without being aware of exactly how high they are sitting. In the high-sitting position it is necessary to move forward on the seat to achieve maximal forward slope of the seat. In this position there is no need for the lumbar support and of course it has no effect.
I usually ended the test with the chair and table lowered to the ISO recommended height. There were many that felt both uncomfortable and aware of pain in their backs.
Excerpt from:
The Seated Man: Homo Sedens, Dafnia Publications, 1985, p. 63-65