According to http://news.bbc.co.uk/2/hi/health/6187080.stm, the optimal sitting position is not 90 degrees, but 135 degrees.
Disk movement was found to be most pronounced with a 90-degree upright sitting posture.
It was least pronounced with the 135-degree posture, suggesting less strain is placed on the spinal disks and associated muscles and tendons in a more relaxed sitting position.
The “slouch” position revealed a reduction in spinal disk height, signifying a high rate of wear and tear on the lowest two spinal levels.
