The rectus abdominis is the large muscle in the mid-section of the abdomen. It enables the tilt of the pelvis and the curvature of the lower spine.
Located next to the rectus abdominis (on each side of the body) and beneath the external oblique is the internal oblique muscle. Originating at the lumbar fascia (a connective tissue that covers the lower back), the outer portion of the inguinal ligament (a ligament located on the bottom-outer edge of the pelvis), and back of the iliac crest (the upper-outside portion of the pelvis), the internal oblique muscles end at the bottom edge of the rib cage, the rectus sheath (fibrous tissue that covers the abdominal muscles) and the pubic crest (an area in the lower-front of the pelvis). Located closer to the skin than the transverse abdominal muscle, the internal obliques support the abdominal wall, assist in forced respiration, aid in raising pressure in the abdominal area, and help rotate the trunk.
Also situated on each side of the body, are the largest and the most superficial (outermost) of the three flat muscles of the lateral anterior abdomen: the external obliques. Extending from the lower half of the ribs down to the pelvis, the external oblique muscles (together) cover the sides of the abdominal area, help rotate the trunk, pull the chest (as a whole) downwards (which compresses the abdominal cavity), and supports the rotation of the spine.
The transverse abdominal muscle (TVA), also known as the transverse abdominis, transversalis muscle and transversus abdominis muscle, is a muscle layer of the anterior and lateral (front and side) abdominal wall which is deep to (layered below) the internal oblique muscle. Named for the direction of its fibers, the transverse abdominal is the innermost of the flat muscles of the abdomen, helps to compress the ribs and viscera (the internal organs in the abdominal cavity), and provides thoracic and pelvic stability.