Hence, when one does the crunches to shed flab off the lower abdomen, attention must be paid to these muscles. If one feels the muscles other than the lower ab muscles are worked, then it is a clear sign, that the exercise is not been done properly. Similarly, a large number of people suffer from lower back pain, when they do the lower abdominal crunches, it means that one is working the lower back muscles as opposed to the lower abdominal muscles.