If you already have back problems, there are classes you can take that specifically deal with practicing BDSM as some one with a disability/physical limitations. The key is to get your instruction from some one who knows their shit, and then to not to take it too fast when you try it yourself. Always, always listen to your body, and don't forget that a good stretch before a play session can do wonders to help get your body prepared.

EDIT: I guess I should say this has some one who has been suspended multiple times and deals with joint issues. I have never been hurt (in the bad way) when being suspended, mostly because the person suspending me was really knowledgeable about their craft and could do it safely.