I just want to add that if you stand up straight and look at your stomach. If you then flex the trunk forward, notice the lines that form.. if they did not form, you would not be able to bend at the waist.
A few years ago when I was new to training, I asked a friend what these lines were and why won't they go away! If they go away, my stomach would be so tight I wouldn't be able to bend at all.
BTW, those lines may also be a sign that when you are sitting you relax the abdominal/lower back muscles and let your body fall forward. Thus this puts the lines in the stomach as the skin folds and if done for prolonged periods of time, those lines will stay there until you start sitting up straight! Hopefully this helps!