I’ve been getting down to the nitty gritty of scaled drawings on AutoCAD and Sketch-up this week, which has led me to the issue of exactly what forward sloping seat angle to use. There seems to be several pieces of research done into this and the conclusions vary widely. Cranz and the torso to thigh angle of 135o is widely quoted (her book actually states 120o to 135o.) Other researchers have shown much smaller angles are better suggesting just 4o to 6o, so the body doesn’t tend to slip forward so much and not so much weight is borne on the legs. One good comprehensive reference source I’ve found is a dissertation ‘Effects of seat angle on comfort and lower back pain at work’, by Margaret Catherine Graf.
I’ve decided in the end to tend towards the middle ground, and opt for a seat sloping forward 10 degrees. This angle reduces intradiscal pressure by approximately 30% in the lumbar region compared to standard sitting positions (Lelong et al 1992), yet is not too steep, so most of your weight is still borne on your sit bones rather than throwing a lot of your weight forward onto your shins.
http://e-collection.library.ethz.ch/eserv/eth:27554/eth-27554-02.pdf
F.E.WRIGHT - Autonomy Kneeling Chair 