[masraum]

I've done some reading in several areas that all pointed me to the same conclusion. I believe that the best seating position for the way we are built is with an angle between your spine and thighs that is slightly more than 90*, but most chairs are designed for angles a bit smaller than 90 degrees.

I did up a quick (very rough) sketch. Below in the position on the right (similar to the Japanese kneeling position 'seiza' (like sitting on your heels, used in Japan, martial arts, Zen meditation, etc...) the angle of the spine to the thighs is a bit more than 90* and allows the lower back to remain straight. If you can manage that position somehow without it bothering your knees, you'll find that it's easy to maintain for long periods and comfortable to your back (it's usually the knees that take the brunt of the position especially when not used to it).

In the other position which is common to lots of chairs, the rear ends up a bit lower than the knees which usually results in a position where the spine and thighs make a 90* angle or sometimes even an acute angle. In this position the lower back is often curved and doesn't support the weight of the upper body well. This usually results in a sore lower back and or shoulders.

I'm no doctor, this is just from reading bits of info in several places and experimenting with my seating position. I take aikido, so we spend lots of time in the seiza position. Once the feet and knees get used to it, it's really comfy. Unfortunately it's not common in the US and is REALLY hard to get used to.