Originally Posted by Jim_Gunn

The abdominal muscles can only contract so far, and the small amount of force and pressure is unlikely to cause internal injuries even to someone with a substantial belly. Some people, including myself, do these exercises called "stomach vacuums", which is essentially just that- sucking in your gut by trying to pull your belly button in towards your spine. It just trains those muscles slightly to strengthen one's core. It is supposed to strengthen those muscles a little, but frankly, I think that the positive is mostly psychological in my opinion, since the little bit of exercise this entails makes one more aware of one's mid-section and in tune with one's own core muscles, more than actually making them much stronger.