Why are saddle chairs and stools good for back pain?

curves
Curves of the spine

The spinal column is not completely straight. It curves from front to back. It does not normally curve side to side. The way it curves from front to back is fixed. In the neck and in the lower back the spine curves inwards or into the body. Medically the inward curve is called lordosis. Hence there is a lordosis in the neck and in the lower back. It is called the cervical lordosis and the lumbar lordosis. However behind the chest wall and in the pelvis the spine curves outwards or to outside the body. The curve to the outside is medically called kyphosis. Hence the thoracic kyphosis. To keep the lower back in lordosis is very crucial. If the inward curve is not maintained, then the discs are not kept in a functional position. This will lead to back pain. This curve is so important that surgeons who operate on the spine take a lot of care in maintaining this curvature when the spine is fused

In normal life the lumbar lordosis is affected by the position of the hip. When one is standing the muscles in the front is stretched and it pulls he lumbar spine forward to slightly increase the lumbar lordosis.

LordosisObliterated

In the images shown above Position 1, 2, 4 & 5 helps to recreate these natural curves. Position 3 makes you lose the curve.

Is there research to support saddle seats?

Dr. Jay J Keegan from the Neurosurgical deparment, University of Nebraska studied 4 healthy volunteers . He repeatedly X-rayed the lower back of these volunteers in different positions. A tracing of the bones in the lower back helped to calculate the angle of lumbar lordosis. Dr. Keegan stated that the most common cause of low back pain related to seating, was the bulging of the disks. He identified that the curve in the lower back is dependent on the trunk – thigh angle. A trunk thigh angle of 135 degrees maintained the normal lumbar lordosis.
Journal of Bone & Joint Surgery American Volume . 35(3):589-603, July 1953.

Dr. AC Mandal from Copenhagen, Denmark published his work in 1981. His research identified that the seat should be tilted forward to keep the spine in a functional position.
Appl Ergon. 1981 Mar;12(1):19-26.

Prof Jorgen Eklund published his work in 1986 from the University of Nottingham. His research showed that when the seats sloped by 30 degrees, there was less shrinkage of the discs and less load on the spine.
Thesis submitted to the University of Nottingham for the degree of Doctor of Philosophy, October, 1986. Industrial seating and Spinal loading

Sitting on a saddle seat is not easy!

Sitting on a saddle seat / stool is not easy. The seat places the spine in the most functional position. But it can cause discomfort in the groin and upper thighs. The appropriate recommendation is that the saddle seat is used with a sit-stand table. This will allow one to stand for varying periods during the day. Please note that comfort is not equal to correct posture. Too much comfort adds the risk of static positions. This is even worse when sat still over long periods of time. The posture on a saddle seat is a dynamic posture. Sitting on a saddle seat is like going to the gym. One cannot sit still on a saddle seat, but need to move. Moving the muscles surrounding the lower back increases the core strength. A normal 8 hour working day should be spit into sitting for 5 hours and standing for 3 hours. Standing also increases the metabolic rate. This increased metabolism over a period of one year equals to running 10 marathons.