The rectus abdominis muscle, also known as the “abdominals or abs,” is a paired muscle running vertically on each side of the anterior wall of the human abdomen, as well as that of some other mammals. There are two parallel muscles, separated by a midline band of connective tissue called the linea alba. It extends from the pubic symphysis, pubic crest and pubic tubercle inferiorly, to the xiphoid process and costal cartilages of ribs V to VII superiorly.
The rectus abdominis muscle is contained in the rectus sheath, which consists of the aponeuroses of the lateral abdominal muscles. Three bands of connective tissue called the tendinous intersections traverse the rectus abdominus, which separates this parallel muscle into eight distinct muscle bellies. In the abdomens of people with low body fat these bellies can be viewed externally and are commonly referred to as a “four, six, or eight pack,” depending on how many are visible; although six is the most common.