Dominic Stone
The abdominal muscles are a group of muscles that form the abdominal walls, supporting the trunk, allowing movement, and holding organs in place by regulating internal abdominal pressure. There are five main abdominal muscles: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis. These muscles help to stabilise the trunk and maintain internal abdominal pressure, as well as providing support for the viscera and protecting them from injury. The abdominal muscles also play a role in breathing, urination, defecation, childbirth, vomiting, and coughing.
| Characteristics | Values |
|---|---|
| Number of abdominal muscles | 5 |
| Location | Between the ribcage and the pelvis on the front of the body |
| Function | Holding organs in place, supporting the body during movement, maintaining internal abdominal pressure |
| Muscle groups | Transversus abdominis, Rectus abdominis, External obliques, Internal obliques, Pyramidalis |
| Muscle fibres | Crisscrossing of fibres gives strength to the abdominal wall |
| Muscle layers | Three flat muscles (two obliques and the transversus) and two vertical muscles (the rectus abdominis and pyramidalis) |
| Muscle strains | Can be caused by overstretching, overuse, or violent/poorly performed movements |
| Muscle exercises | Pilates, Stability ball exercises, Abdominal draw-in exercise |
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Rectus abdominis
The rectus abdominis is a long, flat muscle that extends along the entire length of the front of the abdomen. It is one of the five main abdominal muscles, along with the pyramidalis, external obliques, internal obliques, and transversus abdominis. The rectus abdominis is responsible for holding internal organs in place and keeping the body stable during movement. It is also involved in flexing the spinal column and tensing the anterior wall of the abdomen.
The rectus abdominis is a paired muscle, with one on each side of the middle of the abdomen, running from the ribs to the front of the pelvis. They are separated by a connective tissue called the linea alba, which divides the muscle into two segments. The upper portion of the rectus abdominis is attached to the cartilage of the fifth rib, and it typically has a thickness of around 10 mm, although some athletes may have a thicker rectus abdominis of up to 20 mm.
The rectus abdominis is innervated by the thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. These nerves pierce the anterior layer of the rectus sheath, which is a multilayered fibrous compartment that contains the rectus abdominis and pyramidalis muscles. The blood supply to the rectus abdominis comes predominantly from the inferior and superior epigastric arteries, along with contributions from other vessels.
The rectus abdominis plays a crucial role in core stability, acting like a natural weight belt to protect the lower back from injury. It is also responsible for flexing the lumbar spine, as seen in exercises such as crunches or leg-hip raises. Additionally, the rectus abdominis assists with breathing and is important for respiration during forceful exhalation, such as after exercise or in certain medical conditions.
Weak abdominal muscles, including the rectus abdominis, can lead to spinal problems and other issues. Abdominal muscle strains, or pulled abdominal muscles, can occur when the muscle is stretched too far, resulting in microscopic tears or, in severe cases, a rupture from its attachment. To prevent strains, it is important to incorporate regular stretching, warm-up routines, and cooling-down periods into your exercise regimen.
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External obliques
The external oblique muscles are one of the five main abdominal muscles. They are a pair of muscles, with one on each side of the rectus abdominis. They are the largest of the three flat abdominal muscles, situated on the lateral and anterior parts of the abdomen. The external obliques are broad, thin, and irregularly quadrilateral, with their muscular portion occupying the side of the abdomen, and their aponeurosis occupying the anterior wall.
The external obliques have a variety of functions. They allow the trunk to twist from side to side, and they work with the internal oblique muscles to enable this twisting and turning movement. The external obliques also function to pull the chest downwards, compressing the abdominal cavity and increasing intra-abdominal pressure. This increase in pressure is useful in processes such as breathing, singing, and defecation. The external oblique muscles also perform ipsilateral side-bending and contralateral rotation: the right external oblique would side-bend to the right and rotate to the left, and vice versa.
The external oblique muscles are innervated by the anterior rami of the thoracic spinal nerves T7-T12. The superior-most part of the muscle is supplied by the intercostal nerves T7-T11, while the subcostal nerve T12 innervates the lower part. The muscle receives sensory innervation from the lumbar plexus, via the iliohypogastric nerve (L1). The upper two-thirds of the external oblique receive blood supply from the branches of the lower posterior intercostal and subcostal arteries, while the deep circumflex iliac artery supplies the lower third.
The external oblique muscles can be targeted and strengthened through specific exercises. For example, bilateral contraction of the external oblique, along with the rectus abdominis and internal oblique, flexes the trunk by drawing the pubis towards the xiphoid, as in crunches or sit-ups.
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Internal obliques
The internal obliques are one of the five main abdominal muscles. They are a pair of muscles that lie on the lateral abdominal wall, just inside the hip bones. They are thin, flat muscles, which sit on top of the external oblique muscles. Together, the internal and external obliques make up two of the three layers of the lateral abdominal wall. The third layer is formed by the transverse abdominis.
The internal obliques are important for maintaining normal abdominal tension and increasing intra-abdominal pressure. They work with the external oblique muscles to allow the trunk to twist and turn. The internal obliques are also 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, which then fill with air. Conversely, when the internal obliques 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.
The internal obliques also act with the external oblique muscles to achieve torsional movement of the trunk. For example, twisting the trunk to the left requires the left internal oblique and the right external oblique to contract together. Similarly, 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.
The internal obliques are supplied by the lower six thoracic spinal nerves (T7-T12), namely the terminal branches of the lower five intercostal nerves and the subcostal nerve. The iliohypogastric and ilioinguinal nerves (L1) also provide minor contributions to the nervous supply of this 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 one of the five main abdominal muscles, which support the trunk, allow movement, and hold organs in place. The transversus abdominis is the deepest muscle layer and its main roles are to stabilise the trunk and maintain internal abdominal pressure.
The transversus abdominis is also known as the transverse abdominis, transversalis muscle, and transversus abdominis muscle. It is a muscle layer of the anterior and lateral abdominal wall, deep to the internal oblique muscle. It serves to compress and retain the contents of the abdomen as well as assist in exhalation. The transverse abdominal helps to compress the ribs and viscera, providing thoracic and pelvic stability. It can contract during the exhalation phase of respiration to force air out of the thorax.
The transversus abdominis has several origin points:
- Lateral one-third of the superior surface of the inguinal ligament and the associated iliac fascia
- Anterior two-thirds of the inner lip of the iliac crest
- Thoracolumbar fascia between the iliac crest and the 12th rib
- 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 aponeurosis makes up the posterior wall of the rectus sheath which covers the posterior upper three-quarters of the rectus abdominis, together with the aponeurosis of the internal abdominal oblique muscle.
Abdominal exercises such as the abdominal draw-in exercise target the transversus abdominis.
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Pyramidalis
The abdominal muscles are strong bands of muscles lining the abdominal walls. They are located between the ribcage and the pelvis on the front of the body. They support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. There are five main abdominal muscles: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis.
The pyramidalis muscle is a small triangular muscle located anterior to the lower part of the rectus abdominis muscle within the rectus sheath. It is attached by tendinous fibres to the anterosuperior margin of the pubis and by ligamentous fibres in front of the pubic symphysis. The muscle decreases in size as it runs upwards and ends as a pointed apex that is attached to the linea alba midway between the umbilicus and pubis. It is innervated by a small branch of the subcostal nerve, which is the anterior ramus of spinal nerve T12. The inferior and superior epigastric arteries supply blood to the pyramidalis muscle.
The pyramidalis muscle is present in 80% of the human population. It may be absent on one or both sides, and the lower end of the rectus may be proportionately larger in size in such cases. Occasionally, the muscle is doubled on one side, and the muscles of the two sides are sometimes of unequal size. It may also extend higher than the usual level.
The pyramidalis muscle tenses the linea alba when contracting. It usually contracts together with the other abdominal muscles, contributing to contracting the abdominal wall and increasing positive abdominal pressure. These actions play a dual role. Firstly, the contraction of the abdominal muscles physically protects the abdominal organs. Secondly, it supports certain physiological processes such as forced respiration, singing, micturition, and defecation.
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Frequently asked questions
The abdominal muscles are the muscles forming the abdominal walls, the abdomen being the portion of the trunk connecting the thorax and pelvis. There are five main abdominal muscles: pyramidalis, rectus abdominus, external obliques, internal obliques and transversus abdominis.
The abdominal muscles support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. They also help with breathing, urination, defecation, childbirth, vomiting, and coughing.
The four main abdominal muscle groups that combine to completely cover the internal organs are: transversus abdominis, rectus abdominis, external obliques, and internal obliques.
Common upper abdominal injuries include muscle strains and hernias. Strains or pulls are common and can be caused by overstretching, overuse, or violent or poorly performed movements of the trunk. Hernias are a bulging of abdominal contents, usually intestines, through a weakness or gap in the abdominal muscles.
Abdominal exercises should be carefully selected and progressed gradually. It is important to incorporate exercises that train your core muscle group rather than individual muscles. Some effective methods include Pilates and using a stability ball.
