We do an awful lot of sitting these days, even though the human body was clearly not designed for it (sitting shortens lives).

In order to compensate for this sedentary behavior, we can promote a healthy back by sitting on a 135 degree angle to lessen pain and strain. Check out this BBC report and study.

"As to what is the best angle between thigh and torso when seated, reclining at 135 degrees can make sitting more difficult as there is a tendency to slide off the seat: 120 degrees or less may be better."