Anterior Abdominal Muscles
The anterior abdominal muscles consist of:
- Rectus abdominis muscle: originates from the pubis (between the pubic tubercle and symphysis) and inserts at the 5th to 7th costal cartilage and the xiphoid process of the sternum. Thereby it passes through the rectus sheath, a tendinous muscle envelope built by the aponeurosis of the lateral abdominal muscles. The rectus abdominis muscle has 3-4 horizontal tendinous intersections which are adhered to the anterior layer of the rectus sheath giving it its multi-bellied appearance (“washboard stomach”). These are traces of the segmental arrangement during embryogenesis (metamerism). The innervation is carried by the lower intercostal nerves.
- Pyramidalis muscle: runs diagonally from the pubis (ventral to the insertion of the rectus abdominis muscle) within the rectus sheath to the linea alba. It is innervated by the subcostal nerve (12th intercostal nerve).
The main tasks of the rectus abdominis muscle and the lateral abdominal muscles are the:
- movement of the trunk
- stabilization of the vertebral column
- tensioning of the abdominal wall
They are responsible for the abdominal press, increasing the intra-abdominal pressure (e.g. during defecation or vomiting). Furthermore, they support the exhalation by pulling down the thorax through their contraction (expiratory breathing muscles). In particular, the rectus abdominis muscle does a ventral flexion (flexion of the trunk or erection of the pelvis) and thus is an important antagonist of the intrinsic back muscles.
The pyramidalis is a rudimentary muscle and relates to the pouch muscle of monotremes (e.g. hedgehog, duckbill) and marsupials (e.g. koala, kangaroo). Its original function, tensioning of the linea alba, does not play a role in higher mammals anymore.
Abdominal muscles stabilize the vertebral column together with the back musculature. If they are poorly developed they cannot keep up to the antagonistic force of the back muscles. In the long term, this leads to hyperlordosis of the lumbar vertebral column and anterior pelvic tilt.
The tension of the abdominal muscles counters the weight of the internal abdominal organs. In case of overload (e.g. obesity, pregnancy) the increased intra-abdominal pressure can cause a hernial opening of the abdominal wall through which viscera (guts) may protrude. Predisposed weaknesses are often areas with few muscles such as the linea alba (epigastric hernia), the navel (umbilical hernia) and the inguinal ligament (inguinal hernia). On the contrary there is no hernial opening in diastasis recti, a condition where the rectus sheaths spread apart from the linea alba.