In order to fully understand the changes in the position and function of the abdominal muscles during pregnancy and childbirth and their possible consequences, it is necessary to know the structure and function of the abdominal muscles.

The anterolateral abdominal wall is formed by the following muscles:

• rectus abdominis muscle, lying superficially
• external oblique muscles
• internal oblique muscles
• transverse abdominal muscle, belongs to the deepest layer of the lateral abdominal muscles
• pyramidal muscle

Rectus abdominis muscle

The rectus abdominis muscle forms the anterior abdominal wall. It connects the cartilages of the fifth through seventh ribs to the pubic tubercle. The rectus abdominis muscle is usually divided by three transverse tendon bands (tendon streaks), which divide it into three parts above the umbilicus and one larger part below the umbilicus. The tendon streaks are only found in the superficial layer. They are no longer present in the deep layer. The tendons of the rectus abdominis muscle run toward the linea alba, partly forward from the rectus abdominis muscle and partly behind, forming the sheath of the rectus muscle.

Linea alba is formed from the intersecting tendons of the oblique abdominal muscles and the transversus abdominis muscle (lateral abdominal muscles), forming the sheath of the rectus abdominis muscle. It is a tendinous band running from the gladius process of the sternum to the pubic symphysis, 10-25 mm wide, tapering inferiorly. The linea alba separates the right part of the rectus abdominis muscle from the left. The linea alba plays an important role in maintaining the stability of the abdominal wall from a mechanical point of view. The tension of the white crepe is regulated by the pyramidal muscle. The fibers of the white crepe run in three directions. There are transverse fibers, oblique fibers and a small number of irregular fibers. Differences in fiber architecture have been observed depending on gender. In women, there are a greater number of transverse fibers relative to oblique fibers in the subumbilical area and a smaller thickness and increased width of the linea alba in the subumbilical area as well. These differences in architecture may be a form of adaptation to the increasing pressure in the abdominal cavity during pregnancy.

The sheath of the rectus abdominis muscle consists of three layers:

• posterior lamina (posterior fascial layer)
• muscular layer
• anterior lamina (anterior fascial layer)

External oblique abdominal muscle

The external oblique abdominal muscle connects the lower ribs to the sheath of the rectus muscle, the inguinal ligament and the iliac crest. The fibers run diagonally from the top and side down and medially. The lower portions run vertically from the umbilicus. It intertwines with the internal oblique muscle on the opposite side. Belongs to the outer layer of the lateral abdominal muscles. It tightens the linea alba through a wide aponeurosis. Participates in the formation of the anterior sheath of the rectus abdominis muscle.

Internal oblique abdominal muscle

The internal oblique abdominal muscle lies between the iliac crest, the superior anterior iliac spine and the posterior parts of the inguinal ligament with the three lower ribs. The fibers run diagonally from the bottom and side, upward and medially. Posteriorly, it intertwines with the thoracolumbar fascia. It participates in the formation of the sheath of the rectus abdominis muscle.

Transverse abdominal muscle

The transverse abdominal muscle connects the six lower ribs, the thoracolumbar fascia, the iliac crest and part of the inguinal ligament to the sheath of the rectus abdominis muscle and the pubic symphysis. The upper part is positioned horizontally, while the lower part runs forward, medially and downward. It belongs to the deepest layer of the abdominal muscles. It participates in the formation of the sheath of the rectus abdominis muscle.

Pyramidal muscle

The pyramidal muscle lies forward and down from the rectus abdominis muscle. It runs from the pubic symphysis to the linea alba. Its function is to tighten the linea alba. In humans, it is a vestigial muscle.

Abdominal muscle functions

Abdominal muscle activity supports and protects the gut. Abdominal muscles are essential for maintaining proper posture, including stabilisation the pelvis and lumbar spine.

The rectus abdominis muscle, together with the other abdominal muscles, forms the central stabilising system. The rectus abdominis, the sheath of the rectus abdominis, the linea alba and the lower fibres of the external oblique muscle have a vertical orientation and provide vertical strengthening. Oblique strengthening is provided by the external oblique muscles, intertwined with the internal oblique muscles lying on the opposite side. Horizontal strengthening is provided by the transverse abdominal muscle and the fibres of the oblique muscles running transversely.

The rectus abdominis muscle is the strongest flexor of the trunk when the pelvis is stabilised. It is assisted by the oblique abdominal muscles. With stabilised thoracic attachments, the rectus abdominis muscle can pull the front of the pelvis upwards, which corresponds to pelvic straightening and leads to further flexion of the lumbar spine.

When rotating to the right, the left external oblique muscle and the right internal oblique muscle tense up. Lateral flexion is the result of tensing the internal and external oblique muscles and the quadratus lumborum muscle on the same side, i.e. with right lateral flexion, the muscles lying on the right side tense up. This movement is assisted by the rectus abdominis muscle on the same side.

The contraction of the transversus abdominis muscle and the diaphragm exerts pressure in the abdominal cavity and on the pelvic floor muscles, i.e. creates abdominal presser. This mechanism passively stretches the diaphragm and the pelvic floor. Abdominal presser stabilises the torso during weight lifting. It also occurs when coughing, laughing, urinating, defecating or giving birth.

Changes to the abdominal muscles during pregnancy

During pregnancy, the geometry of the abdominal muscles changes while still retaining muscle function. The developing uterus affects the shape of the abdomen and the alignment of the lumbar spine (deepening of the lumbar lordosis), which increases the distance between the attachments of the rectus abdominis muscle, i.e. causes the abdominal muscles to stretch and changes the angle of muscle attachment. Functionally, this manifests as a reduction in strength and other changes in the rectus abdominis muscle. It can lead to stretching and flaccidity of the linea alba, which can cause the right and left parts of the rectus abdominis muscle to separate along the linea alba, i.e. the development of diastasis recti abdominis.

You can read about the diastasis recti abdominis muscle in the following article entitled “Diastasis recti abdominis muscle – what is it?”.

Antonina Kaczorowska

References:

1. Jutta Hochschild. Anatomia funkcjonalna dla fizjoterapeutów. Wydanie I polskie pod red. Pawła Posłusznego. Wydawnictwo MedPharm Polska, Wrocław 2018.
2. Michalska A, Rokita W, Wolder D, Pogorzelska J, Kaczmarczyk K. Diastasis recti abdominis – a review of treatment methods. Ginekologia Polska. 2018;89(2):97-101.