Anterior abdominal muscles
The anterior abdominal muscles are part of the musculature that contributes to the anterolateral abdominal wall, along with the lateral abdominal muscles on either side. They collectively form part of the boundaries of the abdominal cavity. The muscles of the anterior abdominal wall are located near the midline between the costal margin superiorly and the pubis inferiorly.
There are two pairs of muscles, each located immediately lateral to the linea alba. The majority of the anterior abdominal wall is formed by the rectus abdominis muscle. The pyramidalis muscle is also present, but only in around 80% of individuals, and is thus of somewhat less significance to the overall integrity of the anterior abdominal wall. The muscles of the anterior abdominal wall are largely involved in protecting the contents of the abdominal cavity, but also function to move the trunk and assist in other bodily functions.
|Rectus abdominis muscle||
Origin - Pubic symphysis, Pubic crest
Origin - Pubic crest, Pubic symphysis
Insertion - Linea alba
Innervation - Subcostal nerve (T12)
Function - Tenses linea alba
This article will outline the abdominal muscle anatomy, including their attachments, function, innervation, blood supply, and related fascia and aponeuroses.
- Fascia and aponeuroses
- Rectus abdominis muscle
- Pyramidalis muscle
- Clinical notes
Fascia and aponeuroses
Superficial to the muscles is the subcutaneous fascia, Camper’s fascia and Scarpa’s fascia. Camper’s fascia is the superficial, fatty layer. This layer can have a variable degree of thickness, for example, a greatly increased thickness in obese individuals. In males, this layer continues over the penis and outer layer of the spermatic cord into the scrotum.
Here, it has very little adipose tissue and also contains the dartos muscle (smooth muscle fibres). Posterior to the scrotum, Camper’s fascia is continuous with the superficial perineal fascia, or Colles fascia. In females, it continues from the suprapubic skin to the labia majora and perineum. We have mentioned a lot of structures, haven't we? But don't worry, if you want to recall their anatomy, we have some articles that you can go through and reinforce your anatomy knowledge.
Scarpa’s fascia is the deeper, membranous layer of fascia. It is tightly attached to the linea alba and pubic symphysis in the midline, and loosely attached to the external oblique aponeurosis more laterally. Superiorly, it is continuous with the superficial fascia covering the remainder of the trunk. Inferiorly, it blends with Camper’s fascia superficially, and the underlying fascia lata of the thigh distal to the inguinal ligament.
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In males, it becomes the superficial or fundiform ligament of the penis as it extends inferiorly from the the anterior abdominal wall onto the penis. It also continues over the spermatic cord into the scrotum, becoming continuous with the fascia of the perineum. In females, it continues onto the labia majora and into the perineum.
The rectus abdominis and pyramidalis muscles are enveloped by a thick sheath, the rectus sheath. The rectus sheath is formed by the aponeuroses of the lateral abdominal muscles enveloping the rectus abdominis and pyramidalis muscles as they converge on the midline tendinous structure, the linea alba.
Immediately deep to the rectus abdominis and pyramidalis muscles, and enclosed in the rectus sheath is a layer of fascia called the transversalis fascia, which separates the anterior abdominal wall from the extraperitoneal fat. Posteriorly, the transversalis fascia is continuous with the thoracolumbar fascia, and forms a continuous sheath anteriorly. Inferiorly, it is continuous with the iliac and pelvic fasciae, while superiorly, it blends with the fascia of the diaphragm. It is attached to the entirety of the iliac crest and posterior of the inguinal ligament lateral to the femoral vessels. It also forms the posterior boundary of the inguinal canal.
Rectus abdominis muscle
Origin and insertion
The rectus abdominis muscle is composed of a pair of vertically oriented muscles. They are one of the two pairs of muscles that contribute to the anterior abdominal wall. The name ‘rectus abdominis’ means ‘straight abdominal’, and is indicative of the parallel direction the fibres of the muscle take as they pass from their origin to their point of insertion. The muscle has a strap-like shape, and is narrower inferiorly, widening as it passes superiorly.
|Origin||Pubic symphysis, Pubic crest|
|Insertion||Xiphoid process, Costal cartilages of ribs 5-7|
|Innervation||Intercostal nerves (T6-T11), Subcostal nerve (T12)|
|Blood supply||Inferior and superior epigastric vessels|
|Function||Trunk flexion, Compresses abdominal viscera, Expiration|
The rectus abdominis has two points of origin. The lateral head originates from the crest of the pubis, between the pubic symphysis and the pubic tubercle. The medial head originates from the pubic symphysis, interlacing with the fibres of the muscle on the contralateral side. It inserts onto the xiphoid process of the sternum and onto the costal cartilages of ribs 5-7.
The rectus abdominis muscles are located immediately lateral to the linea alba that extends along the midline of the anterior abdominal wall. The lateral borders of the muscle create a semicircular border, the linea semilunaris, which extends from the tip of the 9th costal cartilage to the pubic tubercle. They are enclosed in the rectus sheath, and enveloped by the aponeuroses of the lateral abdominal muscles as they pass to insert onto the linea alba.
Located along the anterior length of the rectus abdominis muscle are three horizontal bands of connective tissue called tendinous intersections. The tendinous intersections attach to the rectus sheath as it envelopes the muscle, separating it into individual bellies. They are typically located at the level of the xiphoid process, the umbilicus, and mid way between the two. They are responsible for the ‘six-pack’ appearance of the abdomen in some individuals with a highly developed rectus abdominis muscle, and low amount of subcutaneous abdominal fat.
Contraction of the rectus abdominis muscles produces flexion of the trunk. This movement occurs at the level of the lumbar spine. Flexion of the trunk toward the pelvis occurs where the pelvis is fixed; alternatively, flexion of the pelvis on the trunk occurs where the trunk is fixed (such as in the leg raise exercise). The muscles are also involved in stabilising the pelvis and controlling the neutral position of the pelvis, and preventing abnormalities in the pelvic position, such as anterior or posterior pelvic tilt. Contraction of the rectus abdominis muscles also compresses abdominal viscera and contributes to forced expiration.
Innervation and blood supply
The muscles are innervated by the terminal branches of the anterior rami of spinal nerves T6-T12, which begin as intercostal nerves in the intercostal spaces. The anterior rami of the T6-T11 spinal nerves are called the thoracoabdominal nerves, while the T12 anterior ramus becomes the subcostal nerve. The thoracoabdominal nerves continue into the abdomen to supply the rectus abdominis muscles after having supplied the thoracic wall by passing through the intercostal space. The subcostal nerve does not travel in an intercostal space, but instead is located in the subcostal groove of the twelfth rib.
The rectus abdominis muscle has its arterial blood supply mainly by the superior epigastric (continuation of the internal thoracic) and inferior epigastric arteries (internal iliac branch) which run along the internal surface of the muscle. The inferior epigastric artery is the main artery supplying the rectus abdominis muscle. There are sometimes contributions from the lower intercostal and subcostal arteries, posterior lumbar, and deep circumflex iliac arteries. The venous drainage of the muscles mirrors the arterial supply.
Origin and insertion
The pyramidalis muscles are a pair of triangular shaped muscles that are located within the rectus sheath superficial to the rectus abdominis muscles.
|Origin||Pubic crest, Pubic symphysis|
|Innervation||Subcostal nerve (T12)|
|Blood supply||Inferior epigastric vessels|
|Function||Tenses linea alba|
Each pyramidalis muscle originates from the pubic crest and the pubic symphysis. They decrease in size as they ascend, ending as a pointed apex that inserts medially onto the linea alba. Each muscle typically inserts onto the linea alba midway between the pubis and the umbilicus. The muscle is highly variable, and is absent in roughly 20% of individuals.
The paired pyramidalis muscles tense the linea alba when they contract. Collectively the muscles of the anterior abdominal wall, in conjunction with the muscles of the lateral abdominal wall, form a sturdy but flexible boundary for the abdominal cavity which protects the abdominal viscera. They also compress the viscera to maintain and increase intra-abdominal pressure.
This is important in a number of different mechanisms. Increasing intra-abdominal pressure elevates the diaphragm, which forcefully expels air from the lungs. Contraction of the anterolateral abdominal wall also produces the force required for defecation, micturition (urination), vomiting, and parturition (childbirth).
Innervation and blood supply
The pyramidalis is innervated by the terminal branches of the subcostal nerve, which is the anterior ramus of spinal nerve T12. It receives its arterial blood supply from the inferior epigastric artery, and to a lesser extent the deep circumflex iliac artery. Its venous drainage follows the arterial supply.
As the anterior abdominal wall muscles are the main flexors of the vertebral column, weakness in or underdevelopment of these muscles can result in hyperlordosis of the lumbar vertebral column and anterior tilt of the pelvis. Anatomically, lordosis is the normal inward (anterior) curvature of cervical and lumbar regions of the vertebral column. The curvatures of the spine allow the weight of the trunk to be carried correctly over the pelvis for a stable gait and posture. Hyperlordosis occurs where there is excessive lordosis of the lumbar spine. It is caused by incorrect action of the extensor muscles of the vertebral column, such as a spasm brought on by lifting heavy objects incorrectly. Insufficient contraction of the anterior abdominal wall muscles to oppose hyperlordosis is a contributing factor. Observation of an exaggerated lumbar lordosis is a visual sign of hyperlordosis. X-ray can be used to measure the angle of lordosis in the lumbar spine. Hyperlordosis is treated mainly by corrective exercises to strengthen the muscle groups contributing to posture, including the gluteal region, the abdominal wall muscles, the hip flexor muscles, the quadriceps and the hamstrings.
Anterior pelvic tilt
Anterior pelvic tilt is the abnormal tilting of the pelvis so that the pelvic inlet is oriented anteriorly. Weakness of the anterior abdominal wall muscles is a contributing factor in anterior pelvic tilt, along with other groups such as the gluteal muscles and hamstrings. Weakness of these muscle groups results in an inability to oppose the action of the quadriceps and hip flexors. Treatment is by strengthening the rectus abdominis and the pyramidalis muscles to counteract the action of the quadriceps and hip flexor muscles which are causing the anterior tilt.
In abdominal surgery, a paramedian incision is the typical surgical approach used to decrease the risk of inguinal herniation post surgery. The abdomen is incised over the belly of the rectus abdominis muscle. While the fascia is incised, the muscle, however, remains intact and it can be pushed laterally without its neurovascular supply being interrupted. This is because the nerves and vessels enter the muscle on the lateral aspect. During a cesarean section, a horizontal incision is typically made across the abdomen, and the rectus abdominis muscles are separated and moved laterally to gain access to the pelvic cavity.
Divarication of the Recti
Divarication of the Recti occurs where increased strain is placed on the anterior abdominal wall, such as in the case of obesity, pregnancy, or chronic straining. This leads to a widening of the space between the rectus abdominis muscles, meaning that the force being transmitted by the anterior abdominal wall muscles is not directed all the way across the midline through the linea alba and is instead directed through the separate rectus abdominis muscles either side of the linea alba. As a result, the abdominal contents bulge against the wall in the midline as the muscles contract on either side, without actually extending out through the wall as occurs in a herniation.
Divarication of the recti usually presents as a midline ridge beneath the skin anywhere between the xiphoid process and the pubis, becoming more prominent with contraction of rectus abdominis. Diagnosis is usually made by physical examination, but radiological imaging such as ultrasound or CT can be used to confirm the diagnosis. Treatment of the condition itself is not usually necessary. However, treatment of complications, such as umbilical hernias, may be required.
Umbilical hernias occur when some of the abdominal contents protrude through the anterior abdominal wall, specifically through the linea alba, above or below the umbilicus. They present as a bulge near the umbilicus that may or may not be obvious depending on whether or not the abdomen is relaxed. True congenital umbilical hernias occur where there is usually an abnormality present from birth. In some instances, the developing gut tube (midgut), which protrudes into the umbilical cord during weeks 6-10 of development, does not fully re-enter the abdomen and partially remains present as a sac enclosed by peritoneum outside of the anterior abdominal wall. This is called an omphalocele. Infantile umbilical hernias are present in the newborn and are mostly associated with increased intra-abdominal pressure. Umbilical hernias can also occur in the adult. These are acquired umbilical or paraumbilical hernias, and are usually the result of a weakening at a point along the linea alba.
Diagnosing an umbilical hernia is typically done by physical examination. Ultrasound or CT can also be used to confirm the diagnosis. Umbilical hernias can be dangerous as strangulation of the intestines can occur, leading to a blockage of the gut. The part of the intestines that protrudes through the abdominal wall might also be deprived of adequate blood supply, which can cause necrosis of that section of the gut. Should complications present, surgery may be required to repair the herniation. The surgical procedure is relatively straightforward. It involves a small incision being made near the umbilicus through which the intestines can be safely pushed back into the abdomen followed by closure with stitches. The abdominal wall is usually reinforced by a mesh to prevent further herniation before the incision is closed.
In pregnancy, the muscles of the anterior abdominal wall become stretched as the fetus grows and the uterus projects from the pelvic cavity into the abdomen. Pregnant women are susceptible to divarication of the recti because of the increase in intra-abdominal pressure the developing fetus places on the anterior abdominal wall. Postpartum, divarication of the recti can also occur due to residual weakness in the muscles.