Re: Crunches and sit-ups
either your technique is flawed or your back has some problem. Let someone observe your technique and if it is ok then definitely go see a doctor. 8 reps per set for a person of average shape is nothing and if your back is hurting in the second set then there is something wrong. Try folding your hands over your chest instead of behind the neck. the only stretch or pain you should feel is in your abs and nowhere else.