We all have 4 layers of abdominal muscles. The only layer that actually spans all of your midsection is the deepest layer called the 'Transverse Abdominus'. This is the muscle responsible to flattening your belly, pulling your midsection in and supporting your lower back joints. Unfortunately, 90% of the common abdominal exercises, especially crunches, don't contract or strengthen this deepest layer and it stays weak. If you've ever given birth, by the 9th month of pregnancy, this muscle has stretched to twice its original length. If you don't re-strengthen it, your waistline stays wide and your belly will continue to protrude.