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The abdominal muscles, commonly known as abs, are strong bands of muscles lining the walls of the abdomen, which is the trunk of the body. They are located at the front of the body, between the rib cage and the pelvis. The abdominal muscles are made up of five pairs of muscles, with two being vertical and three being flat muscles stacked on top of each other. The four distinct muscles that make up the abs are the rectus abdominis, pyramidalis, external obliques, and internal obliques. The deepest muscle of the abs is the transverse abdominis, which wraps around the torso to support the spine.
| Characteristics | Values |
|---|---|
| Location | Between the ribcage and pelvis at the front of the body |
| Composition | Five main muscles: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis |
| Function | Hold organs in place, support the trunk, enable movement, and provide spine stability |
| Appearance | "Six-pack" appearance due to low belly fat revealing muscle detail |
| Exercise | Targeted exercises include bicycle crunches, mountain climbers, twisting hanging knee raises, and planks |
| Strain Prevention | Regular stretching, warming up, cooling down, and maintaining good form during sports |
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What You'll Learn
External oblique muscles
The external oblique muscles are one of the five main abdominal muscles, along with the pyramidalis, rectus abdominus, internal obliques, and transversus abdominis. They are located on each side of the rectus abdominis, which is the muscle that runs down the middle of the abdomen. The external obliques are the largest and most superficial of the flat abdominal muscles. They extend from the lower half of the ribs around and down to the pelvis, covering the sides of the abdominal area. They sit beneath the skin and subcutaneous fat of the abdomen, with the internal obliques underneath them.
The external oblique muscles are responsible for allowing the trunk to twist. When one side of the external oblique contracts, the trunk twists to the opposite side. For example, if the right external oblique contracts, the body turns to the left. They work together with the internal oblique muscles to allow the trunk to twist and turn. The internal obliques are located just inside the hipbones and operate in the opposite way to the external obliques. So, to twist the trunk to the left, the left side internal oblique and the right side external oblique contract together.
The external oblique muscles are paired muscles located on the lateral sides of the abdominal wall. They originate from the external surfaces of the lower ribs and insert onto the iliac crest and pubic tubercle. The muscle fibres fan out towards the midline and inferior margins of the abdomen, with the most posterior fibres running almost vertically. The external oblique muscles contribute to the lateral part of the abdominal wall, with their aponeurotic part contributing to the anterior abdominal wall.
The external oblique muscles are innervated by the anterior rami of the thoracic spinal nerves T7-T12. The intercostal nerves T7-T11 supply the uppermost part of the muscle, 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 of the muscle.
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Internal oblique muscles
The internal oblique muscles are one of the five main abdominal muscles, the others being the pyramidalis, rectus abdominis, external oblique, and transversus abdominis. The internal oblique muscles are a pair of muscles that lie on top of the external oblique muscles, just inside the hip bones. They are much thinner and smaller than the external oblique muscles. The internal oblique muscles flank the rectus abdominis and are located just inside the hip bones.
The internal oblique muscles work together with the external oblique muscles to allow the trunk to twist and turn. However, they operate in the opposite way to the external oblique muscles. For example, twisting the trunk to the left requires the left side internal oblique and the right side external oblique to contract together. The internal oblique muscles are involved in forced breathing, acting as an antagonist to the diaphragm and 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 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.
The internal oblique muscles are also important for movements of the trunk, maintaining normal abdominal tension, and increasing intra-abdominal pressure. They have multiple sites of origin, which are 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 deeper structure known as the iliopectineal arch, while lateral fibres originate from the anterior two-thirds of the iliac crest and then diverge superiorly and medially. Posterior fibres originate from the posterior end of the iliac crest and the thoracolumbar fascia.
The internal oblique muscles, along with the external oblique and transversus abdominis muscles, form the three distinct layers of the lateral abdominal wall.
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Transversus abdominis
The transversus abdominis is a deep abdominal muscle, layered below the internal oblique muscle. It is the innermost of the flat muscles of the abdomen. It is a broad paired muscular sheet found on the lateral sides of the abdominal wall.
The transversus abdominis is important for maintaining normal abdominal tension and increasing intra-abdominal pressure. It 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 assisting in exhalation.
The transversus abdominis is involved in forced breathing exhalation, contracting with the internal intercostal muscles and other abdominal muscles. It also helps to compress the ribs and viscera, providing thoracic and pelvic stability.
The transversus abdominis is vital for back and core health, and is sometimes referred to as the "corset muscle" as it has the effect of pulling in what would otherwise be a protruding abdomen. Training this muscle is key to achieving a "flat belly".
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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 the abdomen. The rectus abdominis muscle is contained within the rectus sheath, which is formed by the merging of the aponeurosis of transversus abdominis, external and internal oblique abdominal muscles. The rectus abdominis muscle is separated at the midline by a band of dense connective tissue called the linea alba. The connective tissue defining each lateral margin of the rectus abdominis is the linea semilunaris.
The rectus abdominis muscle extends from the pubic symphysis, pubic crest, and pubic tubercle inferiorly, to the xiphoid process and costal cartilages of the 5th–7th ribs superiorly. The muscle is traversed by bands of connective tissue called the tendinous intersections, which divide it into distinct muscle bellies, resulting in a grid-iron 'six-pack' shape in those with low body fat. There are three tendinous intersections in the rectus abdominis muscle: one at the level of the umbilicus, another at the level of the xiphoid process, and the third midway between them.
The rectus abdominis muscle has several sources of arterial blood supply. The inferior epigastric artery and vein, or veins, run superiorly on the posterior surface of the rectus abdominis, enter the rectus fascia at the arcuate line, and serve the lower part of the muscle. The superior epigastric artery, a terminal branch of the internal thoracic artery, supplies blood to the upper portion of the rectus abdominis muscle. Additionally, small segmental contributions come from the lower six intercostal arteries. The superior and inferior epigastric arteries form anastomoses with each other, allowing for collateral flow along the midline abdominal wall.
The rectus abdominis muscle assists with breathing and plays a crucial role in respiration during forceful exhalation, such as after exercise or in conditions like emphysema. It also helps maintain the internal organs in place and creates intra-abdominal pressure, which is essential during activities such as exercising, lifting heavy weights, forceful defecation, or parturition (childbirth). The rectus abdominis muscle works alongside other abdominal muscles to compress the abdominal viscera and increase intra-abdominal pressure, facilitating processes like forced breathing, labour, defecation, and micturition.
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Pyramidalis
The abdominal muscles are strong bands of muscles lining the walls of the abdomen, which is the trunk of the body. They are located between the rib cage and the pelvis at the front of the body. The abdominal muscles support the trunk, allow movement, and hold organs in place.
The pyramidalis is one of the five main abdominal muscles. It is a small, triangular vertical muscle, located at the base of the pubic bone. It is part of the anterior abdominal wall and is found on each side of the linea alba. The linea alba is a ligament that runs down the midline of the body, from the xiphoid process to the pubic symphysis. The pyramidalis muscle tenses the linea alba, contributing to contracting the abdominal wall and increasing positive abdominal pressure.
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 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.
About 20% of people do not have pyramidalis muscles. The pyramidalis muscle is present in 80% of the human population and may be absent on one or both sides. When absent, the lower end of the rectus abdominis muscle becomes proportionately increased in size.
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Frequently asked questions
The abdominal muscles are located between the ribcage and the pelvis on the front of the body. They form the abdominal walls, supporting the trunk and holding organs in place.
There are five main abdominal muscles, consisting of two pairs of vertical muscles and three flat muscles stacked on top of each other. The vertical muscles are the rectus abdominis and the pyramidalis. The flat muscles are the external obliques, internal obliques, and transversus abdominis.
The rectus abdominis is a pair of long, straight muscles that run down the middle of the abdomen. They are responsible for flexing the spine and holding internal organs in place. They are also the muscles that create the ""six-pack" look when well-defined.
The deepest abdominal muscle is the transversus abdominis, which wraps around the torso to support the spine and create internal abdominal pressure.
