Every time that I workout at the gym I am disturbed by the number of people that continue to perform sit-ups. They are done in classes, on the mats and even on a slant board with added weights. The damage that is being done could lead to permanent problems (disc, bowel/bladder) as well as low back pain.  According to Stuart McGill, one traditional sit-up imposes about 730 pounds of compressive load on the spine.  So do this 20,30 or 40 times and imagine the damage that can be done.  Sit-ups should not be performed at all by most people.