The rectus abdominis muscle, also known as the “abs and lower abdominals,” 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.
more about Rectus abdominis muscle
There are two parallel muscles, separated by a midline band of connective tissue called the linea alba (“white line”). 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.
It 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 abdominal region of well conditioned athletes, the three superior bellies on the left and right side of the umbilicus can be easily viewed externally resulting in the rectus abdominus being commonly referred to as the “six-pack.”
The rectus abdominis is a long flat muscle, which extends along the whole length of the front of the abdomen, and is separated from its fellow of the opposite side by the linea alba.
The upper portion, attached principally to the cartilage of the fifth rib, usually has some fibers of insertion into the anterior extremity of the rib itself.
Blood supply: The rectus abdominis has several sources of arterial blood supply. In reconstructive surgery terms, it is a Mathes and Nahai Type III muscle with two dominant pedicles. First, the inferior epigastric artery and vein (or veins) run superiorly on the posterior surface of the rectus abdominis, enter the rectus fascia at the arcuate line, and serve the lower part of the muscle. Second, the superior epigastric artery, a terminal branch of the internal thoracic artery, supplies blood to the upper portion. Finally, numerous small segmental contributions come from the lower six intercostal arteries as well.
Innervation: The muscle is inserted by three portions of unequal size into the cartilages of the fifth, sixth, and seventh ribs. The muscles are innervated by thoracoabdominal nerves, which pierce the anterior layer of the rectus sheath.
Variation: The sternalis muscle may be a variant form of the pectoralis major or the rectus abdominis. Some fibers are occasionally connected with the costoxiphoid ligaments, and the side of the xiphoid process.
The rectus abdominis is an important postural muscle. It is responsible for flexing the lumbar spine, as when doing a so-called “crunch” sit up. The rib cage is brought up to where the pelvis is when the pelvis is fixed, or the pelvis can be brought towards the rib cage (posterior pelvic tilt) when the rib cage is fixed, such as in a leg-hip raise. The two can also be brought together simultaneously when neither is fixed in space.
The rectus abdominis assists with breathing and plays an important role in respiration when forcefully exhaling, as seen after exercise as well as in conditions where exhalation is difficult such as emphysema. It also helps in keeping the internal organs intact and in creating intra-abdominal pressure, such as when exercising or lifting heavy weights, during forceful defecation or parturition (childbirth).