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The abdominal muscles, commonly known as abs, are essential for maintaining core stability and supporting the spine. They consist of five main muscle groups: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis. These muscles work together to enable various movements and protect internal organs. The rectus abdominis, known for its role in creating the 'six-pack' appearance, runs down the centre of the abdomen, while the external and internal obliques allow for trunk rotation and side-to-side movement. The transversus abdominis, the deepest abdominal muscle, wraps around the midsection like a corset, providing additional stability. Together, these muscles contribute to everyday movements and athletic activities, demonstrating the crucial role of abdominal muscles in overall fitness and health.
| Characteristics | Values |
|---|---|
| Number of abdominal muscles | 5 |
| Location | Between the ribs and the pelvis on the front of the body |
| Function | Holding organs in place, supporting the body during movement, protecting the spine, keeping the body stable and balanced |
| Types | Pyramidalis, Rectus Abdominis, External Obliques, Internal Obliques, Transversus Abdominis |
| Appearance | Rectus Abdominis is a long, flat muscle, which can cause "six-pack" appearance |
| Muscle Strains | Caused by overstretching, overuse, or violent movement |
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Rectus abdominis
The rectus abdominis is a long, flat muscle that extends along the entire length of the front of the abdomen, from the ribs to the pelvis. It is separated from its fellow on the opposite side by the linea alba, a fibrous band of connective tissue that runs down the middle of the abdomen. The rectus abdominis is responsible for flexing the lumbar spine, as when doing a crunch, and plays a role in respiration during forceful exhalation.
The rectus abdominis is composed of paired sheets of muscle, separated by tendinous intersections that further subdivide it into smaller muscle bellies. When contracted, this muscle exhibits the characteristic bumps or bulges commonly referred to as a "six-pack." The upper portion of the rectus abdominis is attached to the cartilage of the fifth rib, with some fibres inserting into the rib itself. Its thickness can vary, typically ranging from 10 mm to 20 mm, with volumes of around 300 cm3 in non-active individuals and 500 cm3 in athletes.
The rectus abdominis is an important postural muscle, assisting in maintaining balance and protecting the spine. It helps to stabilise the trunk and regulate internal abdominal pressure, which is crucial for supporting the internal organs and facilitating functions such as breathing, coughing, and childbirth. The muscle works in conjunction with other core muscles, including the external and internal obliques, to enable movements like bending and twisting the trunk.
To effectively target and strengthen the rectus abdominis, exercises that focus on functional fitness and engage multiple muscle groups simultaneously are recommended. This integrated approach reflects how the muscles work together in daily activities, such as reaching, picking up objects, or bending down. By training the rectus abdominis alongside other abdominal and core muscles, individuals can improve their core strength, stability, and overall posture.
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Pyramidalis
The pyramidalis is a small, triangular muscle that is part of the abdominal wall. It is located at the base of the pubic bone, in front of the rectus abdominis, and is contained in the rectus sheath. The rectus sheath is a multilayered fascial compartment composed of the aponeuroses of the external abdominal oblique, internal abdominal oblique, and transversus abdominis muscles.
The pyramidalis muscle is present in 80% of the population, and its function is to tense the linea alba. It is innervated by the ventral portion of T12, and the inferior and superior epigastric arteries supply blood to the muscle. The linea alba is a fibrous structure that runs down the midline of the rectus abdominis, and the pyramidalis attaches to it midway between the umbilicus and the pubis.
The pyramidalis muscle is a vestigial muscle, which means it is tendinous or reduced in size compared to homologous muscles in other species, or frequently absent within or between populations. It is not always present, and about 20% of people do not have this muscle. When it is present, it may be unilateral and vary in size.
The pyramidalis muscle is used as a surgical landmark in classical caesarean sections to determine the midline and location of the linea alba. It has also been used as a source of striated muscle stem cells for the treatment of post-prostatectomy stress urinary incontinence.
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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 a pair of muscles, one on each side of the rectus abdominis, and are situated on the lateral and anterior parts of the abdomen. They run diagonally downward from the lower portion of the ribs down to the pelvis, covering the sides of the abdominal area. 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 obliques have a variety of functions, including ipsilateral lateral flexion of the trunk and contralateral rotation of the trunk when they contract unilaterally. Bilateral contraction flexes the trunk anteriorly, increasing intra-abdominal pressure, which is useful in processes such as breathing, singing, and defecation. The external obliques also help pull the chest downwards and compress the abdominal cavity, increasing intra-abdominal pressure. This compression of the abdominal cavity is useful for processes such as the Valsalva maneuver.
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. They work together with the internal abdominal oblique and transversus abdominis to make up the lateral abdominal muscles. In a broader context, these muscles, along with the rectus abdominis and pyramidalis, form the anterolateral abdominal wall.
The external abdominal oblique is innervated by the anterior rami of the thoracic spinal nerves T7-T12. The intercostal nerves T7-T11 supply the superior-most part of the muscle, while the subcostal nerve (T12) innervates the lower part. The muscle receives sensory innervation from the lumbar plexus, specifically the iliohypogastric nerve (L1).
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Internal obliques
The internal obliques are a pair of abdominal muscles found underneath the external obliques and on top of the transversus abdominis. They are thin, broad muscular sheets that run diagonally up the sides of the body from the pelvis to the lower ribs, just inside the hip bones. They are located on the lateral abdominal wall, which they form one layer of, along with the external obliques and the transversus abdominis.
The internal obliques help with movement of the trunk, allowing the body to rotate and bend from side to side. They also help to maintain normal abdominal tension and increase intra-abdominal pressure. They are accessory muscles of respiration, acting as antagonists to the diaphragm by reducing the volume of the chest cavity during exhalation. When the internal obliques contract, they compress the abdominal organs, pushing them upwards into the diaphragm, which then intrudes back into the chest cavity, reducing the volume of the lungs and producing an exhalation.
The internal obliques are also referred to as "same-side rotators" because they operate in the opposite way to the external obliques. For example, to twist the trunk to the left, the left side internal oblique and the right side external oblique must contract together.
Weakness of the internal obliques, along with other abdominal muscles, can increase the risk of abdominal hernias.
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Transversus abdominis
The transversus abdominis is one of the five main abdominal muscles. It is a deep abdominal muscle and one of the most important core muscles. It is the deepest of all the abdominal muscles, wrapping around the midsection like a corset and pulling it in.
The transversus abdominis is a broad paired muscular sheet found on the lateral sides of the abdominal wall. It is innervated by the lower intercostal nerves (thoracoabdominal, nerve roots T7-T11), as well as the iliohypogastric nerve and the ilioinguinal nerve. It is positioned deep to the internal oblique muscle and comprises the deepest layer of the lateral abdominal wall.
The transversus abdominis has several origin points:
- Lateral one-third of the superior surface of the inguinal ligament and the associated iliac fascia
- Anterior two-thirds of the inner lip of the iliac crest
- Thoracolumbar fascia between the iliac crest and the 12th rib
- 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, perpendicular to the linea alba.
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. It also assists in exhalation and helps to compress the ribs and viscera, providing thoracic and pelvic stability.
Weak transversus abdominis muscles can result in an inability to hold in the stomach after a large meal or when gassy, and low back fatigue after prolonged standing or walking. Training the transversus abdominis can help to improve core stability and prevent low back pain.
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Frequently asked questions
Abdominal muscles, or abs, are strong bands of muscles lining the walls of your abdomen (the trunk of your body). They support your trunk, help with movement, and hold your organs in place.
There are five main abdominal muscles: pyramidalis, rectus abdominis, external obliques, internal obliques, and transversus abdominis.
The rectus abdominis is a very long, flat muscle that extends along the whole length of the front of the abdomen. It is the muscle that people refer to when they talk about having a six-pack.
The external obliques are a pair of muscles, one on each side of the rectus abdominis. They are the largest of the flat muscles and allow the trunk to twist from side to side. The internal obliques are a pair of muscles that sit on top of the external obliques, just inside the hip bones. They are much thinner and smaller than the external obliques, but they also help the body twist and bend from side to side.
The pyramidalis is a small, triangular muscle located at the base of the pubic bone. It helps maintain internal pressure in the abdomen. Interestingly, about 20% of people don't have this muscle.
