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The abdominal muscles are a group of muscles located between the ribs 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. The abdominal muscles can be broadly divided into two categories: the anterolateral abdominal wall and the posterior abdominal wall. The anterolateral abdominal wall consists of four layers: skin, superficial fascia (connective tissue), muscles, and parietal peritoneum. The muscles of the anterolateral abdominal wall include flat and vertical muscles, with the most well-known being the rectus abdominis, commonly referred to as the six-pack muscle. The posterior abdominal wall is formed by the lumbar vertebrae, parts of the hip bones, and several muscles, including the quadratus lumborum. Together, these abdominal muscles work in harmony to control the movement of the spine, pelvis, and rib cage.
| Characteristics | Values |
|---|---|
| Number of abdominal muscle groups | 4 |
| Number of anterolateral abdominal wall muscles | 2 vertical muscles and 3 flat muscles |
| Anterolateral abdominal wall muscles | Rectus abdominis, pyramidalis, external abdominal oblique, internal abdominal oblique, transversus abdominis |
| Posterior abdominal wall muscles | Lumbar vertebrae, parts of the ilia of the hip bones, psoas major, iliacus muscles, quadratus lumborum muscle |
| Function | Support the trunk, allow movement, hold organs in place by regulating internal abdominal pressure, protect the spine, provide postural support, assist in forceful expiration, increase intra-abdominal pressure |
| Core muscles | Transversus abdominis, pelvic floor, oblique muscles, multifidus |
| Muscle causing "six-pack abs" appearance | Rectus abdominis |
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What You'll Learn
The four main abdominal muscle groups
The abdominal muscles are a complex set of muscles with many functions that contribute to a person's quality of life. They support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. The four main abdominal muscle groups that combine to completely cover the internal organs include:
Transversus Abdominis
The deepest muscle layer of the abdominal wall, the transversus abdominis is a thin layer of muscle that assists in supporting the lumbopelvic region (lower back and hips). Its main function is to increase intra-abdominal pressure to promote stability at the midsection and assist in breathing, coughing, and other essential movements. The transversus abdominis is arranged transversely around the abdomen, similar to the front of a belt on a pair of pants.
Internal Obliques
The internal obliques are a thin layer of muscle that spans the lateral side of the torso between the ribs and pelvis on each side of the abdomen. They are called obliques because of their odd shape. Their main role is to increase abdominal pressure. When both internal obliques are contracting, the upper body will flex forward. The internal oblique muscles allow the trunk to twist, but to the opposite side of whichever internal oblique is contracting. For example, the right internal oblique contracts to turn the body to the left.
External Obliques
The external oblique muscles are the closest to the surface of the abdomen. They extend inferiorly and medially, in the direction of sliding one's four fingers into pants pockets. The external obliques 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.
Rectus Abdominis
The rectus abdominis is slung between the ribs and the pubic bone at the front of the pelvis. When contracting, this muscle has the characteristic bumps or bulges that are commonly called "the six-pack". The rectus abdominis is segmented by three transverse bands of collagen fibres called the tendinous intersections. The main function of the rectus abdominis is to move the body between the ribcage and the pelvis.
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The anterolateral abdominal wall
The muscles of the anterolateral abdominal wall can be divided into two main groups: flat muscles and vertical muscles. There are three flat muscles located laterally in the abdominal wall, stacked upon one another. Their fibres run in differing directions and cross each other, strengthening the wall and decreasing the risk of abdominal contents herniating through the wall. The flat muscle group includes the external oblique, internal oblique, and transversus abdominis muscles. The external oblique is the largest and most superficial flat muscle in the abdominal wall, with its fibres running inferomedially. The internal oblique lies deep to the external oblique and has a smaller and thinner structure, with its fibres running superomedially.
The vertical muscle group includes the rectus abdominis and pyramidalis muscles. The rectus abdominis is a long, paired muscle found either side of the midline in the abdominal wall. It is split into two by the linea alba, a white, fibrous band made of the bilateral rectus sheaths that join at the anterior midline of the body. The lateral borders of the muscles create a surface marking known as the linea semilunaris. The rectus abdominis is commonly referred to as the “six-pack” muscle, as it is segmented by three transverse bands of collagen fibres, giving it a characteristic segmented appearance.
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The rectus abdominis muscle
The rectus abdominis makes up the top layer of the abdominal muscles and is separated from its fellow on the opposite side by the linea alba, a white, fibrous band. The muscle is further subdivided into a series of smaller muscle bellies by tendinous intersections, or intersectiones tendineae. The upper portion of the rectus abdominis is attached to the cartilage of the fifth rib, with some fibres inserting into the anterior extremity of the rib itself. The muscle is generally around 10 mm thick, but some athletes have a rectus abdominis up to 20 mm thick.
The rectus abdominis has a variety of functions, including moving the body between the ribcage and pelvis, flexing the spinal column, and tensing the anterior wall of the abdomen. It works in harmony with other muscles, such as the external obliques, to control the movement of the spine, pelvis, and rib cage during gait. The rectus abdominis is also involved in protecting the lower back from injury, acting like a natural weight belt together with the transversus abdominis, erector spinae, and oblique muscles.
Weak abdominal muscles can lead to a series of problems, including spinal issues. Assessing the strength of the rectus abdominis is important, and can be done by asking a patient to raise their trunk against gravity from a supine position. This exercise also helps to strengthen the muscle, along with sit-up exercises. To isolate the rectus abdominis during exercise, the knees should be bent to minimise the engagement of hip flexors.
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The external oblique muscles
The abdominal muscles are located between the ribs and the pelvis at the front of the body. They support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. The four main abdominal muscle groups that combine to completely cover the internal organs include the transversus abdominis, the deepest muscle layer.
The external abdominal oblique muscles originate from the external surfaces of ribs 5-12. The attaching fibres interdigitate with those of the serratus anterior and latissimus dorsi, forming a zig-zag oblique line on the lateral side of the thorax. The muscle fibres fan out towards the midline and inferior margins of the abdomen; the most posterior fibres course almost vertically, while the remainder passes anteromedially. The external oblique muscles have a variety of functions depending on whether they contract unilaterally or bilaterally. When contracting unilaterally and in synergy with the contralateral internal abdominal oblique, they rotate the trunk to the opposite side. When working together with the ipsilateral abdominal and back muscles, they contribute to the lateral flexion of the trunk on the same side.
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The internal oblique muscles
The lateral fibres of the internal oblique muscle are continuous with the rectus sheath, the large aponeurosis of the anterior abdominal wall. The rectus sheath largely encloses the rectus abdominis and pyramidalis muscles, as well as many neurovascular structures of the anterior abdominal wall. The internal oblique muscles also contribute to the formation of the inguinal ligament.
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Frequently asked questions
There are four main abdominal muscle groups that completely cover the internal organs. These are the transversus abdominis, the deepest muscle layer; the external obliques; the internal obliques; and the rectus abdominis.
The "six-pack abs" are the result of the three transverse bands of collagen fibres that segment each rectus abdominis muscle.
The abdominal muscles support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. They also help keep the body stable and balanced, and protect the spine.
