There are special chairs which don't weaken the back. I remember my father having one such without a back to it and the stool facing forwards to the computer, a weird, futuristic contraption. I like leaning forward in my chairs so don't worry re this issue.
My one concern has been that ever since going rawpalaeo, if I lie in bed more than a couple of minutes after waking up, I quickly develop muscle-cramps. I think I didn't have this isssue before rawpalaeodiet because my muscles and bones at the time were very weak - somehow, as my muscles/bones strengthened, beds became less healthy for me, I think - though I don't have any plans to sleep on the floor as yet as I only have this issue if I'm lazy and continue lying in bed for ages after waking.