A diastasis is a separation of the abdominals. It can be caused by pregnancy, obesity, or age.
The most common symptom is a bulge in the abdominal area.
Crunches create a diastasis or make a diastasis larger if you have one. A diastasis is caused by a forward forceful movement on the outermost abdominal muscles (rectus abdominis). When the shoulders come off the floor it is impossible to engage the transverse muscle. If you cannot engage the transverse muscle then it is going forward forcefully making the diastasis larger or creating one. Besides working the muscles in the wrong direction they also work the discs of the spine in the wrong direction. Solution to safely increase the intensity of your abdominal work: When doing the head lifts, bring your feet away from the buttocks. This brings the small of the back off the floor. The higher the small of the back is off the floor, the harder it is to use your abdominals to put it on the floor.