Do you see these people walking around all day long, who appear not to be that fat, but whose bellies stick out like they are preggo? This happens because their internal abdominal muscles are weak and loose. Don’t let this happen to you.
The best way to flatten your belly is to work your invisible abdominal muscles. The transverse abdominis lies beneath your rectus abdominus; the rectus abdominus of course being the muscle that most people refer to as “the six-pack”.
When you suck in your stomach and your waist appears to be flatter, the muscle responsible for this is indeed the transverse abdominis. The good news is that the transverse abdominis can be trained with exercise.