Most conventional chairs have a seat height of 18”, a height which is designed to ensure a “90 degree, 90 degree, 90 degree” (ankle, hip, knee) posture for most people. While the ankles and knees are fine at 90 degrees for prolonged periods, sitting with the hips at 90 degrees has the unfortunate effect of causing most people to slump forward at the low back, collapsing the “triple curve” architecture of the spine into a simple “C” shape. However, this problem is solved by sitting with the hips at an angle that is greater than 90 degrees:
Changes in the curves of the back:
a. Typical Seated Posture (“90/90/90”)—note “C” shaped spine
b. Perch Posture—curves more like standing posture
c. Standing Posture—note normal “S” shaped spine
Drawing from The Chair by Galen Cranz 1998 WW Norton Publishers
Although the lumbar spine is quite flexible, the sustained loss of the normal convex forward curve of the low back (“lumbar lordosis”) quickly makes sitting uncomfortable and has been implicated as the origin of low back pain.
To avoid the loss of the lumbar spine’s normal lordosis it’s important to adjust sitting height to ensure that the hip joints are 3 or 4 inches higher than the knees. This requires that taller people have taller chairs and that shorter people have shorter chairs, something that seems obvious in retrospect. For this reason, QOR360 chairs are not only adjustable but come in two size ranges: 17” – 21” and 20” – 27”. The QOR360 chair that is not adjustable (Juno) comes in three sizes.