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The abdominal wall is a complex structure that encases the abdominal cavity and viscera, connecting the thorax and pelvis. It is composed of several layers, including skin, fascia, muscle, and peritoneum. The abdominal muscles can be broadly divided into anterolateral and posterior walls, with the anterolateral wall further subdivided into flat and vertical muscles. The abdominal muscles play a crucial role in stabilising the trunk, protecting the spine, and facilitating essential bodily functions such as breathing, coughing, and vomiting. The deepest muscle layer, the transversus abdominis, is particularly important for stabilisation and maintaining internal abdominal pressure. Other muscles that contribute to abdominal wall stability include the rectus abdominis, external and internal obliques, and the pelvic floor muscles.
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What You'll Learn
- Transversus abdominis, the deepest muscle layer, stabilises the trunk
- Rectus abdominis, split into two by the linea alba, stabilises the pelvis
- External oblique muscles allow the trunk to twist
- Internal oblique muscles operate in the opposite way to external oblique muscles
- Quadratus lumborum stabilises the caudal connections of the diaphragm
Transversus abdominis, the deepest muscle layer, stabilises the trunk
The abdominal wall encloses the abdominal cavity, which holds the bulk of the gastrointestinal viscera. The abdominal muscles are strong bands of muscles lining the walls of the abdomen, or the trunk of the body. They are located towards the front of the body, between the ribs and the pelvis.
The transversus abdominis is the deepest of the flat muscles, layered on top of the internal obliques. 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.
The transversus abdominis helps to stabilise the trunk and maintain internal abdominal pressure. It serves to compress and retain the contents of the abdomen, as well as assisting in exhalation. The transversus abdominis contracts just after the brain has the initial thought for motion. For example, when an athlete raises an arm to strike a ball, the transversus abdominis and multifidus muscles must contract to stabilise the athlete while kinetic energy is transmitted to the ball.
The transversus abdominis is also vital for back and core health. It has the effect of pulling in what would otherwise be a protruding abdomen, giving it the nickname the "corset muscle". Training the transversus abdominis is critical for preventing and treating low back pain.
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Rectus abdominis, split into two by the linea alba, stabilises the pelvis
The rectus abdominis is a long, paired vertical muscle that runs on either side of the linea alba, on the anterior surface of the abdominal wall. It is commonly referred to as the "six-pack" muscle and is visible in individuals with low body fat. The rectus abdominis is split into two by the linea alba, a band of connective tissue that divides the muscle vertically.
The rectus abdominis is part of the abdominal muscles, which form the abdominal walls and connect the thorax and pelvis. These muscles are essential in stabilising the body during movement and protecting the spine. They also help hold organs in place and support the trunk. The abdominal muscles are divided into two sections: the anterolateral and posterior abdominal walls.
The rectus abdominis is one of the anterolateral abdominal wall muscles, along with the pyramidalis muscle. It acts to flex the spinal column, tense the anterior wall of the abdomen, and assist in compressing the contents of the abdomen. It also works with other abdominal muscles to stabilise and control the tilt of the pelvis.
Weak abdominal muscles can lead to serious problems, including spinal issues. Therefore, it is important to assess the strength of the rectus abdominis and perform exercises to strengthen it, such as the sit-up exercise, which involves raising and lowering the trunk against gravity.
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External oblique muscles allow the trunk to twist
The abdominal muscles are strong bands of muscles lining the walls of the abdomen, which is the portion of the trunk connecting the thorax and pelvis. They are located towards the front of the body, between the ribs and the pelvis. The abdominal muscles have many important roles, including supporting the trunk, helping with movement, and holding the organs in place.
The external oblique muscles are one of the outermost abdominal muscles, extending from the lower half of the ribs around and down to the pelvis. They are paired muscles located on the lateral sides of the abdominal wall. They cover the sides of the abdominal area, sitting on the top surface of the abdomen right below the subcutaneous fat and skin. The internal obliques are underneath the external obliques on each side of the trunk.
The external oblique muscles allow the trunk to twist. When acting unilaterally and in synergy with the contralateral internal abdominal oblique, it rotates the trunk to the opposite side. For example, the right external oblique contracts to turn the body to the left. Twisting the trunk to the left requires the left side internal oblique and the right side external oblique to contract together.
The external abdominal oblique muscle originates 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. From here, 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.
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Internal oblique muscles operate in the opposite way to external oblique muscles
The abdominal muscles are strong bands of muscles lining the walls of the abdomen, which is the portion of the trunk connecting the thorax and pelvis. They are located towards the front of the body, between the ribs and the pelvis.
The abdominal wall can be divided into two sections: the anterolateral and posterior abdominal walls. The anterolateral abdominal wall consists of two vertical muscles located on the midline and three flat muscles on the anterolateral side, arranged from superficial to deep. The anterolateral abdominal wall spans the anterior and lateral sides of the abdomen. The three flat muscles are the external abdominal oblique, internal abdominal oblique, and transversus abdominis.
The internal oblique muscle is located just deep to the external oblique muscle. The external oblique muscle is the largest and outermost of the three flat abdominal muscles of the lateral anterior 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 functions to pull the chest downwards and compress the abdominal cavity, increasing intra-abdominal pressure. It also performs ipsilateral side-bending and contralateral rotation: the right external oblique would side-bend to the right and rotate to the left, and vice versa.
The internal oblique muscle functions similarly to the external oblique, except that it rotates ipsilaterally. In other words, it rotates to the same side. 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 obliques can also function unilaterally, meaning one-sided. In this case, they laterally flex the trunk and rotate it to the same side.
In summary, the internal oblique muscles operate in the opposite way to the external oblique muscles.
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Quadratus lumborum stabilises the caudal connections of the diaphragm
The quadratus lumborum is a muscle in the posterior abdominal wall, lying deep inside the abdomen and dorsal to the iliopsoas. It is the deepest muscle of the posterior abdominal wall and is often mistakenly referred to as one of the muscles of the back. The muscle is irregular in shape, but generally quadriangular, which is why it is described as 'quadratus' in Latin.
The quadratus lumborum is an integral part of the thoracolumbar fascia, a myofascial system that covers the posterior area of the human body, involving part of the lower and upper limbs. The thoracolumbar fascia is a continuation of the transverse abdominal muscle. The lateral arcuate ligament of the diaphragm rests on the quadratus lumborum, potentially enhancing respiratory function by allowing the muscle to act as a pivot.
The quadratus lumborum has three muscle fibre layers with different vectors. The thin anterior layer comprises iliocostal and iliothoracic muscle fibres, which terminate with a tendon or muscle. The middle layer comprises lumbocostal muscle fibres, which originate from the transverse processes of the lumbar vertebrae and insert into the 12th rib. The posterior layer consists of lateral iliocostal fibres and medial iliolumbar fibres, connecting the iliac crest to the lumbar vertebrae's transverse processes. The highly variable arrangement of the quadratus lumborum muscle fibres prevents the recognition of a precise orientation pattern.
The quadratus lumborum is involved in the stabilisation and movement of the spine and the pelvis. It is also important for the fixation of the 12th rib during movements of the thoracic cage, supporting expiration. The muscle likely serves as a junction for forces exerted by neighbouring muscles, influencing the vectors of various tensions due to its strategic position and fibre arrangement.
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Frequently asked questions
The abdominal muscles are strong bands of muscles lining the walls of your abdomen (the trunk of your body). They are located towards the front of your body, between your ribs and pelvis.
The abdominal muscles have many important roles, including supporting your trunk, helping with movement, and holding your organs in place. They also help protect your spine and keep your body stable and balanced.
The abdominal wall can be divided into two sections: anterolateral and posterior abdominal walls. The anterolateral abdominal wall consists of four main layers: skin, superficial fascia, muscles and associated fascia, and parietal peritoneum. The posterior abdominal wall contains three or four muscles, depending on the individual: psoas major, iliacus, quadratus lumborum, and psoas minor.
The deepest abdominal muscle is the transversus abdominis, which helps to stabilise the trunk and maintain internal abdominal pressure.
