Rectus abdominis muscle
Rectus abdominis, informally known as the abs muscle, is a long muscle of the anterior abdominal wall. In those with low body fat, it is clearly visible beneath the skin forming the ‘six pack’. It extends from the rib cage all the way to the pubic bone.
Rectus abdominis belongs to the anterior abdominal muscles together with pyramidalis muscle. But taking the functional anatomy into account, these two muscles comprise the anterolateral abdominal wall along with the three lateral abdominal muscles; external oblique, internal oblique and transversus abdominis.
|Origin||Pubic symphysis, pubic crest|
|Insertion||Xiphoid process, costal cartilages of ribs 5-7|
|Innervation||Intercostal nerves (T7-T11), subcostal nerve (T12)|
Inferior epigastric and superior epigastric arteries; contributions from posterior intercostal, subcostal and deep circumflex arteries
|Function||Trunk flexion, compresses abdominal viscera, expiration|
This article will discuss the anatomy and function of the rectus abdominis muscle.
- Origin and insertion
- Blood supply
- Clinical points
Origin and insertion
Inferiorly the rectus abdominis muscle is attached by two tendons; the larger one is attached to the pubic crest, from the pubic tubercle to the pectineal line, while the small, medial tendon is attached to the pubic symphysis.
The fibres of the rectus abdominis then extend vertically superiorly and insert into the xiphoid process of sternum and costal cartilages of the 5th, 6th and 7th ribs. Generally speaking, the most lateral fibres attach to the anterior end of the 5th rib, however in some cases it extends to the 3rd and 4th ribs, or this slip is absent altogether.
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The rectus abdominis muscle is paired muscle that runs vertically, either side of the linea alba, on the anterior surface of the abdominal wall. The linea alba is a band of connective tissue that divides the two halves of the muscle vertically.
The linea semilunaris is the tendinous intersection that separates the lateral edge of the muscle from the external oblique and internal oblique muscles that lie on the lateral surface of the anterior abdominal wall. It usually extends from tip of the ninth costal cartilage to the pubic tubercle.
Finally, there are three tendinous intersections in the rectus abdominis muscle. One of these horizontal intersections is present at the level of umbilicus, another at the level of xiphoid process and the third mid way between them. These fibrous bands divide the muscle into segments, resulting in a grid iron ‘six pack’ shape in those with low body fat. The intersections are believed to be representations of myosepta which delineate the muscle forming myotomes.
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The rectus abdominis muscle itself lies within the rectus sheath, which is formed by the merging of the aponeurosis of transversus abdominis, external and internal oblique abdominal muscles.
The rectus abdominis muscle is innervated by the thoracoabdominal nerves, which enter the rectus sheath by piercing its anterior surface.
They pass between the transversus abdominis and internal oblique muscle layers, and pierce the sheath of the rectus abdominis muscle.
The nerves are simply the anterior divisions of the 7th to 11th lower intercostal nerves, that continue to supply the abdominal wall after the intercostal spaces they supplied end medially.
In addition, small terminal branches of the lower three posterior intercostal arteries, subcostal and deep circumflex artery also provide some contribution.
Rectus abdominis flexes the trunk anteriorly. Moreover, working together with other abdominal muscles, this muscle compresses the abdominal viscera and increases the intra-abdominal pressure, which has an important function in processes such as forced breathing, labor, defecation and micturition.
The rectus abdominis also stabilizes and controls tilt of the pelvis (antilordosis).
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The transverse rectus abdominius muscle flap is a free flap used for breast reconstruction surgery. A segment of the muscle is incised and raised along with the perforating inferior epigastric artery and anastomosed with branches of the internal mammary/internal thoracic artery. Nowadays, the deep inferior epigastric artery perforator flap (skin, fascia and fat only i.e. no muscle) has replaced the TRAM flap as it requires less recovery time.
A hernia is an abnormal protrusion of an organ beyond its usual cavity. The anterior abdominal wall and inguinal region are common sites for hernias. A hernia can be irreducible (cannot be returned to the abdominal cavity by pressure), obstructed (the intestine becomes blocked, resulting in frank constipation and abdominal pain), incarcerated (stuck in its cavity) or strangulated (loss of blood supply due to compression of the vessels).
Spigelian Hernia - This occurs when the fascia or musculature of the linea semilunaris is weak, allowing bowel to herniate through. The linea semilunaris separates the lateral margin of the rectus abdominius from the lateral abdominal muscles.
Umbilical Hernia - This is when the posterior surface of the umbilicus is weak or becomes weakened, and it causes an outpouching of bowel. It occurs in children (most commonly African Caribbean children), where it usually disappears itself. It also occurs in conditions where the intra-abdominal pressure is raised e.g. ascites from liver disease, or pregnancy.
Paraumbilical hernia - This occurs when the fascia or musculature around the umbilicus is weak, allowing bowel to herniate through.
Epigastric hernia - This occurs when the fascia or musculature of the epigastric region is weak, allowing bowel to herniate through the linea alba.
Incisional hernia - This occurs when the fascia or musculature after a surgical incision is weak, allowing bowel to herniate through.
Direct Inguinal hernias - It is an acquired hernia. It involves the bowel herniating through a region known as Hasselbach’s triangle, which is bordered by the lateral margin of the rectus abdominis muscle medially, the inferior epigastric artery and vein laterally, and the inguinal ligament inferiorly. Hernia exits through the superficial inguinal ring.
Indirect inguinal hernias - This occurs if there is a patent processus vaginalis, or a congenital weakened area around the region of the processus. This allows bowel to herniate through. The bowel herniates medially to the inferior epigastric artery and vein, and hence enters the deep inguinal ring. Over time, or acutely, this may traverse the length of the inguinal canal, and pass through the superficial inguinal ring (at the superolateral aspect of the scrotum), and may enter the scrotum. This is termed an inguinoscrotal hernia.
This is a rare defect of the anterior abdominal wall, when the muscular layers fail to close, resulting in the abdominal content being held outside the abdominal cavity in a sac. It is associated with chromosomal abnormalities such as Edward’s syndrome (trisomy 18) and Patau syndrome (trisomy 13).
If the anterior abdominal wall fails to close during foetal development, the contents of the abdominal cavity may herniate out. This is similar to Omphalocele, but it usually does not involve the umbilical cord, and is to the right of the umbilical site. Some parts of organs may be suspended in amniotic fluid. The disorder is less associated with other defects than omphalocele. Surgical treatment is difficult as the abdominal cavity shrinks and the abdominal organs swell as they develop.