I don't know why but I really don't think it's a good idea. I used to get my back cracked all the time from friends and stuff, they had no idea what they were doing but I knew what felt good and told them how to do it. I have a lot of back problems now, I do not blame it on that but it obviously didn't help in the long run, so why bother. Ask your doctor, he/she will be able to tell you why. I guess there's a reason doctors, phyisotherapists and chiropractors have to go through so much school. I certainly wouldn't walk into a physiotherapists office when he/she hadn't had any schooling and wasn't certified. Take care and think about it carefully. kristy.
cracking your back??? 
