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The abdominal muscles are a group of muscles that line the walls of the abdomen, which is the trunk of the body. They are located at the front of the body, between the ribs and the pelvis. These muscles work alongside the back muscles to form the core muscles, which help to protect the spine and maintain balance. The abdominal muscles are composed of five main muscles: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis.
| Characteristics | Values |
|---|---|
| Number of Main Abdominal Muscles | 5 |
| Names of the Abdominal Muscles | Pyramidalis, Rectus Abdominis, External Obliques, Internal Obliques, Transversus Abdominis |
| Location | Walls of the Abdomen (Trunk of the Body) |
| Function | Holding Organs in Place, Supporting the Body During Movement, Protecting the Spine, Maintaining Balance, Maintaining Internal Pressure in the Abdomen |
| Common Injuries | Strains, Hernias |
| Treatment for Injuries | Avoid Exercise, Avoid Painful Activities, Gentle Stretching, Ice Injured Area, Apply Heat Before Activities |
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Rectus Abdominis
The rectus abdominis is a long, flat muscle that extends along the whole length of the front of the abdomen, from the rib cage to the pubic bone. It is separated from its fellow on the opposite side by the linea alba, a connective tissue. The rectus abdominis is part of the core muscles, which act like a natural weight belt to protect the lower back from injury.
The rectus abdominis is responsible for flexing the spinal column and tensing the anterior wall of the abdomen. It assists in compressing the contents of the abdomen and plays a role in respiration, particularly when forcefully exhaling. This muscle also helps to hold internal organs in place and keeps the body stable during movement.
The rectus abdominis is commonly referred to as the "'six-pack' muscle. When contracting, this muscle has the characteristic bumps or bulges that are associated with a "six-pack". The muscle is composed of two flat and parallel muscles, which are further subdivided into a series of smaller muscle bellies by tendinous intersections. The upper portion is attached to the cartilage of the fifth rib, and the muscle is typically around 10mm thick, although some athletes have a rectus abdominis up to 20mm thick.
The rectus abdominis can be targeted through exercises such as sit-ups and the abdominal draw-in exercise. To isolate the muscle, the exercise can be performed while bending the knees to minimise the engagement of hip flexors.
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External Obliques
The external oblique muscle is one of the largest parts of the trunk area. Each side of the body has an external oblique muscle, which is the outermost abdominal muscle. The external oblique muscles cover the sides of the abdominal area, extending from the lower half of the ribs around and down to the pelvis. Its lowest part connects to the top corner of the pelvis (called the crest of the ilium), the bottom-front of the pelvis (the pubis), and the linea alba, a band of fibres that runs vertically along the inside of the abdominal wall.
The external oblique is situated on the lateral and anterior parts of the abdomen. It is broad, thin, and irregularly quadrilateral, with its muscular portion occupying the side and its aponeurosis occupying the anterior wall of the abdomen. The external oblique muscle is supplied by the ventral branches of the lower six thoracoabdominal nerves and the subcostal nerve on each side. The upper two-thirds of the external oblique are supplied by the lower intercostal nerves T7-T11 and the subcostal nerve T12, while the lower third is supplied by the iliohypogastric L1 from the lumbar plexus. The upper two-thirds receive blood supply from the branches of the lower posterior intercostal and subcostal arteries, while the deep circumflex iliac artery supplies the lower third of the muscle.
The external oblique has a variety of functions, including supporting the rotation of the spine and trunk. It helps pull the chest downwards, which compresses the abdominal cavity and increases intra-abdominal pressure. It also performs ipsilateral (same-side) side-bending and contralateral (opposite-side) rotation: the right external oblique would side-bend to the right and rotate to the left, and vice versa.
The external oblique muscle is one of the outermost abdominal muscles, with the internal oblique muscle located underneath it. The external oblique is the largest and most superficial of the lateral abdominal muscles, lying beneath the thoracic and abdominal skin. Its muscular part contributes to the lateral part of the abdominal wall, while its aponeurotic part contributes to the anterior abdominal wall.
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Internal Obliques
The internal oblique is one of the five main abdominal muscles, alongside the pyramidalis, rectus abdominis, external oblique, and transversus abdominis. These muscles form strong bands of muscles lining the walls of the abdomen (the trunk of the body).
The internal abdominal oblique is a broad, thin muscular sheet found on the lateral side of the abdomen. It forms one of the three layers of the lateral abdominal wall, along with the external oblique on the outer side and the transversus abdominis on the inner side. The internal oblique's fibres are obliquely oriented, perpendicular to those of the external abdominal oblique.
The internal abdominal oblique has multiple sites of origin, distributed along the anterolateral side of the trunk. According to their origin, the muscle fibres can be divided into the anterior, lateral, and posterior fibres. Anterior fibres arise from a deep structure known as the iliopectineal arch, passing inferomedially and arching over the inguinal canal to merge with the tendinous fibres of the transversus abdominis muscle.
The internal oblique is important for maintaining abdominal pressure and movements of the trunk. It helps to regulate pressure during essential bodily functions such as breathing, coughing, and childbirth. The internal oblique acts as an antagonist to the diaphragm during respiration, helping to reduce the volume of the chest cavity during exhalation. 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 lung volume and producing an exhalation.
The internal oblique also plays a role in trunk flexion and rotation. It works with the external oblique muscle of the opposite side to achieve 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.
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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 the deepest of the flat muscles, layered on top of the internal obliques. The transversus abdominis is one of the abdominal muscles that make up the core muscles, alongside the back muscles. These muscles help protect the spine and keep the body stable and balanced.
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.
The transversus abdominis helps to stabilise the trunk and maintain internal abdominal pressure. It also serves to compress and retain the contents of the abdomen, as well as assist in exhalation. The transversus abdominis is important for maintaining normal abdominal tension and increasing intra-abdominal pressure.
Weak transversus abdominis muscles can result in an inability to hold in the stomach after a large meal or when gassy, as well as low back fatigue after prolonged standing or walking. Abdominal draw-in exercises are an effective way to activate and strengthen the transversus abdominis.
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Pyramidalis
The pyramidalis is one of the five main abdominal muscles, alongside the rectus abdominis, external obliques, internal obliques, and transversus abdominis. It is a small, triangular muscle, located at the base of the pubic bone, in front of the rectus abdominis. The pyramidalis is part of the anterior abdominal wall and is contained within the rectus sheath.
The pyramidalis muscle is attached to the pelvis at the pubic symphysis and pubic crest, with tendinous fibres arising from the anterior part of the pubis and the anterior pubic ligament. The muscle then narrows and moves upward, ending in a pointed extremity that is inserted into the linea alba, between the umbilicus and pubis. The linea alba is a band of connective tissue that runs down the centre of the rectus abdominis muscle. The pyramidalis muscle tenses the linea alba when it contracts, contributing to the abdominal wall's ability to increase intra-abdominal pressure.
The pyramidalis is innervated by the subcostal nerve, the anterior ramus of spinal nerve T12. The inferior and superior epigastric arteries supply blood to the muscle. The pyramidalis is present in 80% of the population, but it may be absent on one or both sides, or be of unequal size. It is a vestigial muscle, meaning it is tendinous or reduced in size compared to homologous muscles in other species, or frequently absent within or between populations.
The pyramidalis muscle is used to determine the midline and location of the linea alba during a classical caesarean section. It has also been used as a source of striated muscle stem cells for the treatment of post-prostatectomy stress urinary incontinence.
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Frequently asked questions
The abdominal muscles are bands of muscles lining the walls of the abdomen (the trunk of the body). They are located at the front of the body, between the ribs and the pelvis.
There are five main abdominal muscles: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis.
The rectus abdominis is a pair of muscles that run down the middle of the abdomen, from the ribs to the pelvis. They hold internal organs in place, help keep the body stable during movement, and are also known as the “sit-up” muscles.
The external oblique muscles are a pair of muscles located on each side of the rectus abdominis. They are the largest of the flat abdominal muscles and allow the trunk to twist.
