It isn't as though the medical community hasn't known this. I had a serious back injury over 10 years ago while still in high school. Semi-reclining to about 120 degrees is what my physical therapist suggested way back then.

Sitting upright puts a lot of pressure on your discs, but most people's back problems come from lack of strength in the abdomen, lower back, and "core" muscles connecting everything together. You can sit in whatever position you want so long as you keep in shape.