The external oblique and the internal oblique are the abdominal muscles that contour and shape your waistline. Their primary function is to rotate the spine. The external oblique is the outermost abdominal muscle. It is a broad, thin muscle that originates at the borders of the lower ribs. Its fibers run downward and inward in a V pattern. Running underneath the external oblique is the internal oblique. Its fibers start at the hips and run upward and inward in an inverted V pattern to meet the lower ribs. Together, the oblique muscles function like the corsets that women wore a century ago. These days, you can create your own perfectly defined waistline with a corset of strong, firm oblique muscles. These muscles work hardest in exercises that require you to turn your upper body during movement, as in the Oblique Curl and the Waistline Cruncher.
Lying beneath the obliques is the rectus abdominis, which helps to flex (bend) the spine. Its fibers run vertically from the top of the pubic bone to the lower ribs. An exercise can focus the muscle contraction on either the top or bottom portion of this muscle. In a standard bent-knee sit-up, the upper body is lifted, causing the top part of the muscle to contract with greater force. To work on the pouching area below the waistline, you must increase the contraction in the lower portion of the rectus abdominis. You can do this by making the muscle lift your legs and buttocks, as in the Reverse Hip Lift.
The fibers of the innermost abdominal muscle, the transversus abdominis, run across the abdomen from side to side. The transversus abdominis helps to keep the abdominal organs in place. To maintain proper form, you need to tighten this muscle, which is done by pulling in your stomach.
Ab exercises can help improve back pain.