Anterior Abdominal Muscles

The anterior abdominal muscles are part of the abdominal wall musculature. They tense the anterior abdominal wall together with the lateral abdominal muscles. There are two muscles in the anterior abdominal wall, as follows:

• rectus abdominis
• pyramidalis

Out of the two muscles, the rectus abdominis is the only active one in humans, helping to tense the abdominal wall and moving the trunk, while the pyramidalis is rudimentary. This article will describe the two muscles of the anterior abdominal wall, together with their innervation and functions.

Anatomy

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).

Function

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.