a crunch for me is when your shoulder blade is off the ground... i heard if you go all the way up to your knees it could hurt your back in the long run. I do about 100-200 crunchs a day... not all at once... i would start off doing a few a day.. and work up to it.... you should feel it in the morning... if you don't feel your stomach tightening.. you may want to do more... hope this helped.... later