Hi, The muscles of the back work eccentrically during forward bending (similar to crunches) of the body. You should understand that eccentric work of the muscles is very demanding, and if your muscles are weak, you are bound to expereice pain. It is very much suggested that if you feel pain in back while doing the crunches, then you should stop the crunches and concentrate on strengthening the back muscles. Check ou which muscles of your back are weak. Very high chances of having latissimus dorsi muscle weakness along with the bacvk extensors. Once these muscle are strong, then you can start the crunches again.