Dominic Stone
The abdominal muscles, commonly known as abs, are bands of muscles lining the walls of the abdomen. They are located towards the front of the body, between the ribs and the pelvis. There are four distinct muscles that make up the abs: rectus abdominis, external obliques, internal obliques, and transversus abdominis. Together with the back muscles, they form the core muscles, which help protect the spine and maintain balance. The abs are involved in a wide range of movements, from biking and running to swinging a golf club. They also help hold organs in place and provide stability to the body.
| Characteristics | Values |
|---|---|
| Number of muscles | 5 |
| Names of muscles | Rectus abdominis, pyramidalis, external obliques, internal obliques, transversus abdominis |
| Muscle function | Holding organs in place, supporting the body during movement, stabilising the trunk, maintaining internal pressure in the abdomen, protecting the spine, keeping the body stable and balanced |
| Muscle appearance | "Six-pack" abs are caused by contracting of the rectus abdominis muscle, creating bumps or bulges |
| Muscle injuries | Abdominal strains, hernias, rectus sheath hematoma |
| Muscle exercises | Crunches, leg raises, oblique workouts, Pilates, rehabilitation programs |
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Rectus abdominis
The rectus abdominis is a long, flat muscle that extends along the whole length of the abdomen, from the rib cage to the pubic bone. It is separated from its fellow on the opposite side by the linea alba, a connective tissue. The rectus abdominis is the straight-down-the-centre muscle that is responsible for the "'six-pack"' look in people with low body fat.
The rectus abdominis is paired and runs vertically on either side of the linea alba, on the anterior surface of the abdominal wall. It is divided into two segments by the linea alba, and further subdivided into smaller muscle bellies by tendinous intersections. The muscle fibres extend vertically and insert into the xiphoid process of the sternum and the costal cartilages of the 5th, 6th and 7th ribs.
The rectus abdominis is an important postural muscle. It flexes the lumbar spine, tenses the anterior wall of the abdomen, and assists in compressing the contents of the abdomen. It also assists with breathing, playing a role in respiration when forcefully exhaling, such as after exercise.
The rectus abdominis is one of the abdominal muscles that make up the core muscles, along with the transversus abdominis, erector spinae and obliques. These muscles act like a natural weight belt to protect the lower back from injury and keep the body stable and balanced.
Abdominal exercises should target all the abdominal muscles together, rather than isolating the rectus abdominis. However, to isolate the rectus abdominis, exercises can be performed with bent knees to minimise the engagement of hip flexors.
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External obliques
The external oblique is one of the outermost abdominal muscles, situated on the lateral and anterior parts of the abdomen. It is the largest and most superficial of the three flat abdominal muscles of the lateral anterior abdomen. The external oblique is a paired muscle, with one on each side of the rectus abdominis, and sits at the bottom of the stack. 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 muscle originates from the external surfaces of ribs 5-12. The muscle fibres fan out towards the midline and inferior margins of the abdomen, with the most posterior fibres coursing almost vertically and the remainder passing anteromedially. The external oblique contributes to the lateral part of the abdominal wall. Its aponeurotic part contributes to the anterior abdominal wall, forming the anterior layer of the rectus sheath. The superior, medial, and inferior margins of the muscle relate to their respective attachment points, while the posterior margin is free.
The external oblique functions to pull the chest downwards and compress the abdominal cavity, increasing intra-abdominal pressure. It 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. When contracting unilaterally, it causes ipsilateral lateral flexion of the trunk and contralateral rotation of the trunk. Bilateral contraction flexes the trunk anteriorly, increasing intra-abdominal pressure, which is useful in processes such as breathing, singing, and defecation.
The external oblique is supplied by the ventral branches of the lower six thoracoabdominal nerves and the subcostal nerve on each side. The cranial portion of the muscle is supplied by the lower intercostal arteries, while the caudal portion is supplied by branches of either the deep circumflex iliac artery or the iliolumbar artery. The upper two-thirds of the external oblique is supplied by the lower intercostal nerves T7-T11 and the subcostal nerve T12, while the lower third is supplied by the iliohypogastric L1 from the lumbar plexus.
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Internal obliques
The internal oblique muscles are found on the lateral abdominal wall, comprising one of its three layers. They are located just inside the hip bones and flank the rectus abdominis. The internal obliques are underneath the external oblique muscles, which are V-shaped and run diagonally down the sides.
The internal obliques are a broad, thin muscular sheet. They are perpendicular to the external oblique, with obliquely oriented fibres. The internal oblique muscles are important for movements of the trunk, helping to maintain normal abdominal tension and increase intra-abdominal pressure. They are also accessory muscles of respiration, acting as an antagonist to the diaphragm and helping to reduce the volume of the chest cavity during exhalation.
The contraction of the internal oblique muscles causes ipsilateral rotation and side-bending. They act with the external oblique muscles 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.
The internal oblique muscles are supplied by the lower six thoracic spinal nerves (T7-T12), namely the terminal branches of the lower five intercostal nerves and the subcostal nerve.
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Transversus abdominis
The transversus abdominis is a broad paired muscular sheet found on the lateral sides of the abdominal wall. It is also referred to as the transverse abdominal muscle (TVA), transversalis muscle, or transverse abdominis muscle. It is the innermost of the flat muscles of the abdomen, and it is positioned deep to the internal oblique muscle. The transversus abdominis is one of the five main abdominal muscles, along with the pyramidalis, rectus abdominis, external obliques, and internal obliques.
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 muscle 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 compress the ribs and viscera, providing thoracic and pelvic stability.
The transversus abdominis is also involved in respiration, contracting during the exhalation phase to force air out of the thorax. Additionally, it assists in childbirth by helping to compress the abdomen. The muscle is vital for back and core health, pulling in what would otherwise be a protruding abdomen, earning it the nickname the "corset muscle". Training this muscle is key to achieving a "flat belly".
Abdominal exercises that target the transversus abdominis include the abdominal draw-in manoeuvre, which is a useful technique to activate and strengthen the muscle. This exercise can be progressed by adding external resistance or upper and lower limb movement while holding the abdomen drawing in.
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Pyramidalis
The pyramidalis muscle is a small, triangular-shaped muscle that is part of the anterior abdominal wall. It is found on each side of the linea alba, a fibrous band of tissue that runs down the midline of the abdomen. The linea alba connects the right and left rectus abdominis muscles, which are the muscles that form the "six-pack".
The pyramidalis muscle originates from the pubic symphysis and pubic crest of the pelvis and attaches to the linea alba midway between the umbilicus and pubis. 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. Within the sheath, the pyramidalis muscle is located superficially to the inferior part of the rectus abdominis muscle.
The nerve supply to the pyramidalis muscle is from the ventral portion of spinal nerve T12, although this has been reported to vary in some cases. The blood supply to the muscle comes from branches of the inferior epigastric artery, with additional supply from the deep circumflex iliac artery in some instances.
The defined function of the pyramidalis muscle is somewhat vague, but it is primarily associated with tensing the linea alba. When the pyramidalis muscle contracts, it works together with other abdominal muscles to increase intra-abdominal pressure. This dual action serves as a defence mechanism, protecting the abdominal organs, and supporting various physiological processes such as forced respiration, singing, micturition, and defecation.
The pyramidalis muscle is present in about 80% of the human population. In some individuals, it may be absent on one or both sides, or it can be doubled on one side with varying sizes between the muscles.
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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 are located towards the front of your body, between your ribs and pelvis.
There are four distinct abdominal muscles: rectus abdominis, external obliques, internal obliques, and transversus abdominis.
Abdominal muscles have many functions, including holding organs in place, supporting the body during movement, and helping with breathing. They also help protect your spine and keep your body stable and balanced.
There are many exercises that can help strengthen your abdominal muscles, such as crunches, leg raises, and oblique workouts. It is important to work on all types of abdominal muscles instead of isolating them for individual workouts.
