The 4th abdominal muscle, the transverse abdominus is mostly responsible for forced expiration.

But, it is also helpful, along with your other abs in holding your stomach in and making it look flat. It has also been found to be in a weakened state in those who have chronic back pain / problems.

	Transverse Abdominus The Transversus originates on the outer third of the inguinal ligament, inner rim of the crest of the ilium, inner surface of cartilage of your lower six ribs, and the lumbar fascia.

Training Your Transverse Abdominus

Train this muscle by simply pulling your navel into your spine, and / or exhaling thoroughly, especially while crunching. (Psst...ladies, simply standing and drawing your navel in toward your spine can eradicate the lower ab pooch! Don't succumb to laziness. Do it. You'll look so, so hot! Men, if you're hanging a braggable beer belly, it's going to take a little more effort.)