Dominic Stone
The abdominal muscles, also known as the abs, are essential for supporting the trunk, allowing movement, and holding organs in place. There are five main abdominal muscles, each with distinct roles and functions. The rectus abdominis, often associated with the \six-pack\ look, is responsible for moving the body between the ribcage and pelvis. The transversus abdominis, the deepest muscle layer, stabilizes the trunk and maintains internal abdominal pressure. The external obliques, the largest of the flat muscles, enable the trunk to twist and turn. The internal obliques, thinner and smaller, work in conjunction with the external obliques to facilitate trunk movements. The pyramidalis, a small triangular muscle, helps maintain internal abdominal pressure. Understanding the unique characteristics of each abdominal muscle is crucial for developing effective workout routines and achieving core stability, strength, and improved posture.
| Characteristics | Values |
|---|---|
| Number of abdominal muscles | 5 main muscles |
| Names of abdominal muscles | pyramidalis, rectus abdominus, external obliques, internal obliques and transversus abdominis |
| Largest abdominal muscle | External obliques |
| Function | Hold organs in place, support the trunk, allow movement |
| Appearance | The rectus abdominis is the muscle that appears as a "six-pack" |
| Muscle strain | Caused by overstretching, coughing, laughing, etc. |
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Rectus Abdominis
The rectus abdominis is a long, flat muscle that extends along the entire length of the abdomen, from the ribs to the pelvis. It is separated from its fellow muscle on the opposite side by the linea alba, a connective tissue. The rectus abdominis is responsible for flexing the spinal column and assisting in respiration during forceful exhalation. It also plays a role in core stability, helping to keep the body stable and balanced, and protecting the spine.
The rectus abdominis is often referred to as the "'six-pack' muscle", as it can form bumps or a grid-iron 'six-pack' shape when someone has a trim, fit abdomen. This muscle is typically around 10 mm thick, but some athletes may have a rectus abdominis up to 20 mm thick. The muscle is composed of paired sheets of long, straight muscles that run vertically down the abdomen.
The rectus abdominis is innervated by the thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. It receives its blood supply from several vessels, predominantly the inferior and superior epigastric arteries. The muscle is attached by two tendons: one larger tendon connected to the pubic crest and a smaller, medial tendon attached to the pubic symphysis.
To isolate the rectus abdominis during exercise, it is recommended to perform movements with bent knees to minimise the engagement of hip flexors. This can include exercises such as sit-ups or raising the trunk from a supine position, which can also be used to assess the strength of the 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 the deepest muscle layer of the abs and wraps around the waist to support the spine. This muscle is at work when you compress your stomach toward your spine. It is also important for maintaining normal abdominal tension and increasing intra-abdominal pressure. The transversus abdominis is innervated by the lower intercostal nerves, the iliohypogastric nerve, and the ilioinguinal nerve.
The transversus abdominis has several origin points: the lateral one-third of the superior surface of the inguinal ligament and the associated iliac fascia; the anterior two-thirds of the inner lip of the iliac crest; the thoracolumbar fascia between the iliac crest and the 12th rib; 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 serves to compress and retain the contents of the abdomen, as well as assist in exhalation. It 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 also helps a pregnant person deliver a child.
The transversus abdominis is sometimes referred to as the "corset muscle" because it has the effect of pulling in what would otherwise be a protruding abdomen. Training this muscle can help to achieve a "flat belly". It is important to note that training the rectus abdominis muscles alone will not lead to a flat stomach; this is only achieved through training the transversus abdominis.
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External Obliques
The external oblique muscles are the largest and outermost of the three flat abdominal muscles of the lateral anterior abdomen. They are situated on the lateral and anterior parts of the abdomen. The external obliques are a pair of muscles, with one on each side of the rectus abdominis. They are the largest of the flat muscles and sit at the bottom of the stack. They run from the sides of the body towards the middle.
The external obliques are broad, thin, and irregularly quadrilateral, with their muscular portion occupying the side and their aponeurosis occupying the anterior wall of the abdomen. In most humans, the oblique is not visible due to subcutaneous fat deposits and the small size of the muscle. The external obliques arise from eight fleshy digitations, each from the external surfaces and inferior borders of the fifth to twelfth ribs (the lower eight ribs). These digitations are arranged in an oblique line that runs inferiorly and anteriorly, with the upper digitations attached close to the cartilages of the corresponding ribs, and the lowest attached to the apex of the cartilage of the last rib.
The external oblique muscles allow the trunk to twist, but 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 external obliques also help the body bend to the side and forward, and are involved in rotating the torso when putting on a seatbelt or swinging a baseball bat.
An oblique strain is a pulled or partially torn oblique muscle. It can occur when the muscle is overstretched or forced into a quick or forceful twisting motion. Oblique strains can be treated with over-the-counter pain relievers, external supports like abdominal braces, and targeted stretches and exercises.
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Internal Obliques
The internal abdominal oblique muscle is a thin, broad muscular sheet found on the lateral side of the abdomen. It is one of the three layers of the lateral abdominal wall, alongside the external abdominal oblique and transverse abdominis. The internal obliques are much thinner and smaller than the external obliques.
The internal abdominal oblique is an accessory muscle of respiration, acting as an antagonist to the diaphragm. When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing lung volume. Conversely, when the internal obliques contract, they compress the abdominal organs, pushing them up into the diaphragm, which then intrudes into the chest cavity, reducing lung volume and producing an exhalation.
The internal abdominal oblique is also involved in trunk movements, helping to maintain normal abdominal tension and increase intra-abdominal pressure. It flexes the trunk when contracted bilaterally and causes lateral flexion when contracted unilaterally.
The muscle fibres of the internal abdominal oblique can be divided into anterior, lateral, and posterior fibres. The anterior fibres arise from a deep structure known as the iliopectineal arch, passing inferomedially and merging with the tendinous fibres of the transversus abdominis to form the conjoint tendon.
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Pyramidalis
The pyramidalis muscle is a small, triangular-shaped muscle that is part of the anterior abdominal wall. It is located at the base of the pubic bone, originating 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 ligament that runs down the middle of the abdomen, from the sternum to the pubic bone. When the pyramidalis contracts, it tenses the linea alba, contributing to contracting the abdominal wall and increasing positive abdominal pressure. This has two main functions: protecting the abdominal organs and supporting physiological processes such as forced respiration, singing, micturition and defecation.
The pyramidalis muscle is innervated by the ventral portion of T12, with blood supplied by the inferior and superior epigastric arteries. It is present in 80% of the population, but in 20% of cases, it may be absent on one or both sides, or it may be doubled on one side. The pyramidalis muscle is also linked to the erection of the penis or assumption of upright posture in humans.
The pyramidalis is a vestigial muscle, meaning it is tendinous in larger part or reduced in size compared to homologous muscles in other species. Phylogenetically, the pyramidalis is connected to the pouch inside monotremes such as hedgehogs, platypuses, koalas and kangaroos.
The pyramidalis muscle is used as a surgical landmark, for example, to determine the midline and location of the linea alba during a classical caesarean section. It can also be 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 located between the ribs and the pelvis on the front of the body. There are five main abdominal muscles: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis.
The external obliques are the largest of the flat muscles. They are a pair of muscles, one on each side of the 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. They also help with overall posture, pulling the chest downwards, and protecting the organs by creating intra-abdominal pressure.
The abdominal muscles support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure.
