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The abdominal muscles, also known as the abs, are essential for supporting the trunk, facilitating movement, and holding organs in place by regulating internal abdominal pressure. There are five main abdominal muscle groups: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis. The rectus abdominis is the muscle responsible for the “six-pack appearance often associated with a trim and fit abdomen. However, achieving visible abs is not solely dependent on muscle development but also on reducing subcutaneous and visceral fat through weight training, cardio, and dietary changes.
| Characteristics | Values |
|---|---|
| Number of abdominal muscles | 5 |
| Location | Between the ribcage and pelvis on the front of the body |
| Composition | 2 vertical muscles and 3 flat muscles stacked on top of each other |
| External obliques | Run diagonally downward from the lower ribs to the pelvis |
| Internal obliques | Run diagonally upward from the pelvis to the lower ribs |
| Rectus abdominis | Extends from the pubis upward to the ribs; may form a "six-pack" |
| Pyramidalis | Small, triangle-shaped vertical muscle located at the base of the pubic bone |
| Transversus abdominis | Deepest muscle layer that wraps around the waist to support the spine |
| Appearance of "six-pack" abs | Low belly fat and toned abdominal muscles |
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Transversus abdominis
The transversus abdominis is a deep abdominal muscle and one of the most important core muscles. It is the deepest muscle layer of the abdomen and is often referred to as the “corset muscle” due to its ability to pull in what would otherwise be a protruding abdomen.
The transversus abdominis wraps around the waist to support the spine and prevent instability, as well as supporting the lower back. It is the innermost of the flat muscles of the abdomen and is positioned deep to the internal oblique muscle. It is a broad paired muscular sheet found on the lateral sides of the abdominal wall. The transversus abdominis has several origin points, including the lateral one-third of the superior surface of the inguinal ligament and the anterior two-thirds of the inner lip of the iliac crest. From these origin points, the transversus abdominis fibres course horizontally over the lateral abdominal wall towards the midline.
The transversus abdominis is important for maintaining normal abdominal tension and increasing intra-abdominal pressure. It helps to stabilise the trunk and maintain consistent internal pressure in the abdomen. During essential bodily functions such as breathing, coughing, vomiting, and childbirth, the transversus abdominis regulates internal pressure. It also helps with movement between the ribcage and pelvis, allowing the trunk to be in constant motion and supporting the spine during walking, sitting, standing, and twisting.
The transversus abdominis also helps to hold internal organs in place and protect them. This includes organs such as the stomach, intestines, pancreas, liver, and gallbladder. Additionally, it helps to maintain posture and provide core support.
Training the transversus abdominis can be beneficial for improving core stability and preventing low back pain. The “drawing-in” technique is an effective way to activate and strengthen this muscle.
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Rectus abdominis
The rectus abdominis is a pair of long, flat, straight muscles that are located in the middle of the abdomen, from the ribs to the pelvis. They are divided by a muscle called the linea alba, which is a connective tissue. The rectus abdominis is the muscle that is often referred to as the "'six-pack'".
The rectus abdominis is the most superficial of the abdominal muscles. It is responsible for flexing the spinal column, tensing the anterior wall of the abdomen, and assisting in compressing the contents of the abdomen. It also assists with breathing and plays an important role in respiration when forcefully exhaling, as seen after exercise. The muscle is innervated by the thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. The sensory supply is from the 7-12 thoracic nerves.
The rectus abdominis is attached by two tendons; the larger one is attached to the pubic crest, from the pubic tubercle to the pectineal line, while the small, medial tendon is attached to the pubic symphysis. The fibres of the rectus abdominis then extend vertically superiorly and insert into the xiphoid process of the sternum and the costal cartilages of the 5th, 6th, and 7th ribs. The muscle is paired and runs vertically, either side of the linea alba, on the anterior surface of the abdominal wall.
The rectus abdominis is an important postural muscle. It moves the body between the ribcage and the pelvis. When contracting, this muscle has the characteristic bumps or bulges that are commonly called the "six-pack". This muscle is clearly visible in people with low body fat.
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External obliques
The external obliques 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, extending from the lower half of the ribs around and down to the pelvis. The external obliques are broad, thin, and irregularly quadrilateral, with their muscular portion occupying the side and their aponeurosis the anterior wall of the abdomen.
The external oblique muscles are paired and located on the lateral sides of the abdominal wall. They work together with the internal obliques, which lie underneath them, and the transversus abdominis to make up the lateral abdominal muscles. The external obliques contribute to the lateral part of the abdominal wall, while their aponeurotic part contributes to the anterior abdominal wall. The superior, medial, and inferior margins of the muscle relate to their respective attachment points, while the posterior margin is free.
The external oblique muscles have a variety of functions, including stabilising the core, rotating the spine and trunk, and bending the body from side to side. They also help pull the chest downwards and compress the abdominal cavity, increasing intra-abdominal pressure. This increase in pressure is useful in processes such as breathing, singing, and defecation.
The external oblique muscles are innervated by the anterior rami of the thoracic spinal nerves T7-T12, with the intercostal nerves T7-T11 supplying the uppermost part of the muscle and the subcostal nerve (T12) innervating 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 inferior third.
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Internal obliques
The internal obliques are one of the five main abdominal muscles, along with the pyramidalis, rectus abdominis, external obliques, and transversus abdominis. They are found on the lateral side of the abdomen, underneath the external obliques, and run diagonally up from the pelvis to the lower ribs.
The internal obliques are important for maintaining normal abdominal tension and increasing intra-abdominal pressure. They also help with movements of the trunk, allowing you to rotate your spine and bend your body from side to side.
As an accessory muscle of respiration, the internal obliques act as an antagonist to the diaphragm, helping to reduce the volume of the chest cavity during exhalation. When the internal obliques contract, they compress the organs of the abdomen, pushing them up into the diaphragm, which then intrudes back into the chest cavity, reducing the volume of the air-filled lungs and producing an exhalation.
The internal obliques are referred to as "same-side rotators" because their contraction causes ipsilateral rotation and side-bending. They work together with the external oblique muscle on 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.
Weakness in the internal obliques, along with other abdominal muscles, can increase the risk of abdominal hernias.
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Pyramidalis
The pyramidalis is a small, triangular-shaped muscle that is part of the 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 ends by inserting into the linea alba, halfway between the umbilicus and pubis. The linea alba is a ligament that runs down the midline of the abdomen, separating the left and right abdominal muscles. When the pyramidalis contracts, it tenses the linea alba, contributing to abdominal wall functions such as increasing intra-abdominal pressure during labour, forced expiration, and defecation.
The pyramidalis muscle is innervated by the ventral portion of the T12 spinal nerve and receives its blood supply from the inferior and superior epigastric arteries. It lies within the rectus sheath, a multilayered fascial compartment composed of the aponeuroses of the external abdominal oblique, internal abdominal oblique, and transversus abdominis muscles. The pyramidalis is positioned superficially to the inferior part of the rectus abdominis muscle.
The pyramidalis muscle is present in about 80% of the human population and may be absent or vary in size in some individuals. Its specific function is somewhat vague, but it is thought to work in conjunction with the other abdominal muscles to support various physiological processes and protect the abdominal organs.
In medicine, the pyramidalis muscle is used as a surgical landmark to determine the midline and location of the linea alba during procedures such as classical caesarean sections. Additionally, it has been utilised as a source of striated muscle stem cells for treating post-prostatectomy stress urinary incontinence.
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Frequently asked questions
The abdominal muscles, or abs, are located between the ribcage and pelvis on the front of the body. There are five main abdominal muscles: pyramidalis, rectus abdominus, external obliques, internal obliques, and transversus abdominis. These muscles make up the core, along with the back muscles, and help to support the spine and keep the body stable and balanced.
Visible abdominal muscles, often referred to as a "six-pack," are a result of low belly fat and toned muscles. Losing weight and burning off body fat can be achieved through diet and exercise, with a focus on toning the abdominal muscles. Strength training and weight lifting have been shown to be effective methods for reducing visceral fat.
The abdominal muscles have several important functions. They help to support the trunk, enable movement, and hold organs in place by regulating internal abdominal pressure. Additionally, they provide protection for the viscera and assist in breathing, urination, defecation, coughing, and vomiting.
