Maintaining good posture is vital for getting nice and firm abdominal muscles. Having a distinctive hollow back contributes to the appearance of a protruding abdomen and can even emerge in very thin people.
The hip flexor muscles have to be stretched and the abdominal muscles strengthened in order to counteract a protruding abdomen. It is also likely that weak gluteal muscles contribute to the development of protruding abdomens.
The abdominal muscles together with the back and bottom muscles keep the spinal column upright.
The pelvis is supported by abdominal, bottom and hamstring muscles.
The pelvis must be upright and the natural curvature of the lumbar spine flattened for a straight posture.
The Men's Health Big Book: Getting Abs: Get a Flat, Ripped Stomach and Your Strongest Body Ever--in Four Weeks, Adam Bornstein
A sway back occurs when the muscles which keep the pelvis upright are too weak and the muscles which tilt the pelvis are too strong or too short.
Weak abdominal muscles tilt the pelvis forwards and cause an accentuated hollow back. As a result, the back muscles are shortened and the abdomen protrudes forward.
The New Abs Diet: The Six-Week Plan to Flatten Your Stomach and Keep You Lean for Life
The New Abs Diet Cookbook: Hundreds of Delicious Meals That Automatically Strip Away Belly Fat!, David Zinczenko, Jeff Csatari