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The abdominal muscles are a group of muscles that form the abdominal wall, enclosing the abdominal cavity and holding the gastrointestinal viscera. These muscles support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. The abdominal muscles can be divided into two broad categories: the anterolateral abdominal wall muscles and the posterior abdominal wall muscles. The anterolateral abdominal wall muscles include the rectus abdominis, pyramidalis, external oblique, internal oblique, and transversus abdominis. These muscles can compress the abdomen, creating higher pressures in the abdomen and thorax, which is essential for various bodily functions such as defecation, urination, vomiting, and childbirth.
| Characteristics | Values |
|---|---|
| Abdominal wall | Encloses the abdominal cavity, which holds the gastrointestinal viscera |
| Abdominal muscles | Transversus abdominis, Rectus abdominis, External oblique, Internal oblique, Pyramidalis |
| Functions | Support the trunk, allow movement, hold organs in place, stabilise the trunk, maintain internal abdominal pressure, facilitate sneezing, coughing, vomiting, defecating, regulate internal abdominal pressure, protect the spine |
| Compression of abdominal contents | Compression of abdominal contents is caused by the contraction of the transversus abdominis |
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What You'll Learn
External oblique muscles
The external oblique muscle is one of the outermost abdominal muscles, extending from the lower half of the ribs around and down to the pelvis. The external oblique muscles cover the sides of the abdominal area, sitting on the top surface of the abdomen, right below the subcutaneous fat and skin. The internal obliques are underneath the external obliques on each side of the trunk.
The external abdominal oblique is the largest and most superficial of the lateral abdominal muscles. It lies beneath the thoracic and abdominal skin, covering the internal abdominal oblique and anterior halves of the ribs and intercostal muscles. The muscle is innervated by the anterior rami of the thoracic spinal nerves T7-T12, with the intercostal nerves T7-T11 supplying the superior-most part and the subcostal nerve T12 innervating the lower part. The muscle receives sensory innervation from the lumbar plexus via the iliohypogastric nerve (L1).
The external oblique muscle originates from the external surfaces of ribs 5-12. The muscle fibres fan out towards the midline and inferior margins of the abdomen. At the midclavicular line medially and spinoumbilical line inferiorly, the external oblique continues as an aponeurosis, inserting to the linea alba, pubic tubercle, and anterior half of the iliac crest. The linea alba is a fibrous line that splits the rectus abdominis into two, creating a vertical groove extending inferiorly from the xiphoid process.
The abdominal muscles support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. They work together to produce movements of the spine and to compress the abdominal viscera. The deep abdominal muscles, together with muscles in the back, make up the core muscles, helping to keep the body stable and balanced, and protecting the spine.
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Internal oblique muscles
The internal oblique muscles are one of the three layers of muscles that make up the lateral abdominal wall, the other two being the external abdominal oblique and the transversus abdominis. The internal oblique muscles are located deep to the external abdominal oblique and superficial to the transverse abdominis.
The internal oblique muscles are a broad, thin muscular sheet found on the lateral side of the abdomen. The direction of their fibres is obliquely oriented, perpendicular to those of the external abdominal oblique. The internal oblique muscles originate on the inguinal ligament, which is a ligament that runs from the anterior iliac spine to the pubic bone. They also originate on the anterior iliac crest. The fibres then pass inferomedially, arching over the inguinal canal and merging with the tendinous fibres of the transversus abdominis muscle to form the conjoint tendon. The internal oblique muscles insert onto the costal cartilages of the lower four ribs and the abdominal aponeurosis, which is a superficial sheet of connective tissue over the abdomen. They also insert onto the linea alba, a fibrous band of connective tissue that runs from the xiphoid process to the pubic symphysis.
The internal oblique muscles are important for several functions. Firstly, they are accessory muscles of respiration, acting as antagonists to the diaphragm. When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing the volume of the lungs and allowing them to fill with air. Conversely, when the internal oblique muscles contract, they compress the organs of the abdomen, pushing them up into the diaphragm, which intrudes back into the chest cavity, reducing the volume of the air-filled lungs and producing an exhalation. Secondly, the contraction of the internal oblique muscles causes ipsilateral rotation and side-bending. They work together with the external oblique muscles of the opposite side to achieve this torsional movement of the trunk. For example, the right internal oblique and the left external oblique contract as the torso flexes and rotates to bring the left shoulder towards the right hip.
In males, some of the anterior fibres of the internal oblique muscles extend into the spermatic cord and form the cremaster muscle.
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Transversus abdominis
The transversus abdominis is a broad, paired muscular sheet found on the lateral sides of the abdominal wall. It is also referred to as the transverse abdominal muscle (TVA), transversalis muscle, or transversus abdominis muscle. The transversus abdominis is the deepest muscle layer of the abdomen, lying deep to the internal oblique muscle.
The transversus abdominis is innervated by the lower intercostal nerves (thoracoabdominal nerve roots T7-T11), the iliohypogastric nerve, and the ilioinguinal nerve. It serves to compress and retain the contents of the abdomen, providing thoracic and pelvic stability. Additionally, it assists in exhalation by contracting during the exhalation phase of respiration to force air out of the thorax.
The transversus abdominis has several origin points, including the lateral one-third of the superior surface of the inguinal ligament, the anterior two-thirds of the inner lip of the iliac crest, the thoracolumbar fascia between the iliac crest and the 12th rib, and the internal aspects of the lower six ribs and their costal cartilages. From these origin points, the transversus abdominis fibres course horizontally over the lateral abdominal wall towards the midline, oriented perpendicular to the linea alba.
The transversus abdominis is an important muscle for maintaining normal abdominal tension and increasing intra-abdominal pressure. It helps to pull in what would otherwise be a protruding abdomen, earning it the nickname the "corset muscle". Training this muscle is vital for achieving a "flat" belly, as it provides bracing for the human core during heavy lifts.
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Rectus abdominis
The rectus abdominis muscle, also known as the "abdominal muscle" or "abs", is a pair of segmented skeletal muscles found on the ventral aspect of a person's abdomen. The rectus abdominis is a very long, flat muscle that extends along the whole length of the front of the abdomen. It is contained in the rectus sheath, which is a multilayered aponeurosis—a durable, resilient, fibrous compartment. The rectus abdominis is an important postural muscle, responsible for flexing the lumbar spine and assisting with breathing and respiration during forceful exhalation.
The rectus abdominis is split into two by the linea alba, a band of dense connective tissue. The lateral borders of the muscles create a surface marking known as the linea semilunaris. The muscle is intersected by fibrous strips, known as tendinous intersections, which divide it into distinct muscle bellies. These fibrous bands give rise to the "six-pack" appearance in individuals with a well-developed rectus abdominis and low body fat. The muscle is innervated by thoraco-abdominal nerves, which enter the rectus sheath by piercing its anterior surface. The sensory supply is from the 7-12 thoracic nerves.
The rectus abdominis originates from the pubic crest at the lower end and inserts with the help of fibres into the 5th to 7th costal cartilage and the xiphoid process of the sternum. The muscle is paired and runs vertically, either side of the linea alba, on the anterior surface of the abdominal wall. The rectus abdominis belongs to the anterior abdominal muscles, together with the pyramidalis muscle. However, functionally, these two muscles comprise the anterolateral abdominal wall along with the three lateral abdominal muscles: external oblique, internal oblique, and transversus abdominis.
The strength of the rectus abdominis can be assessed by asking the patient to bring their trunk against gravity from a supine position. The therapist will then grade the patient's performance and ability to follow instructions. To isolate the rectus abdominis during exercise, the exercise can be performed while bending the knees to minimise the engagement of hip flexors.
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Pyramidalis
The pyramidalis is a small, triangular muscle of the anterior abdominal wall, found on each side of the linea alba. It is present in 80% of the human population, but its defined function is vague. It may be absent on one or both sides, or occasionally doubled on one side. The muscles on the two sides are sometimes of unequal size.
The pyramidalis muscle is part of the rectus sheath and is located anterior to the rectus abdominis muscle. The rectus sheath is a multilayered fascial compartment composed of the aponeuroses of the external abdominal oblique, internal abdominal oblique, and transversus abdominis muscles.
The pyramidalis muscle originates from the pubic symphysis and pubic crest. The muscle belly narrows down as it courses superiorly and inserts into the linea alba, halfway between the umbilicus and pubis. The linea alba is a fibrous line that splits the rectus abdominis into two, creating a surface marking known as the linea semilunaris.
The pyramidalis muscle is innervated by the subcostal nerve, which is the anterior ramus of spinal nerve T12. The inferior and superior epigastric arteries supply blood to the pyramidalis muscle.
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Frequently asked questions
The muscles that compress the abdomen are the external oblique, internal oblique, transversus abdominis, and rectus abdominis.
The external oblique muscles allow the trunk to twist to the opposite side of whichever external oblique is contracting. For example, the right external oblique contracts to turn the body to the left.
The internal obliques work in the opposite way to the external obliques. They can be used to compress the abdomen for exhalation.
The transversus abdominis is the deepest muscle layer. Its main roles are to stabilise the trunk and maintain internal abdominal pressure.
