The Lateral Abdominal Muscles
The lateral abdominal muscles comprise the following three muscles:
- Abdominal external oblique muscle: courses from the 5th to 12th rib ventromedially to the anterior layer of the rectus sheath. At its origin it is tightly connected with the serratus anterior and latissimus dorsi muscle. Ventrally it builds a large aponeurosis which extends medially to the linea alba and caudally to the iliac crest, the pubis bone and the inguinal ligament. The innervation is carried by the lower intercostal nerves.
- Abdominal internal oblique muscle: originates from the thoracolumbar fascia, the iliac crest and the inguinal ligament and inserts cranially at the lower costal cartilages and ventrally at the linea alba. In men, caudal fibers extend to the spermatic cord merging to form the cremaster muscle. The internal oblique is innervated by both the lower intercostal nerves as well as branches of the lumbar plexus (iliohypogastric nerve and ilioinguinal nerve).
- Transversus abdominis muscle: runs from the inner surface of the lower costal cartilages, the thoracolumbar fascia and iliac crest horizontally to the linea alba. Caudal fibers are involved in the formation of the cremaster muscle. The innervation is analogous to that of the internal oblique.
The aponeuroses of the lateral abdominal muscles build the rectus sheath, a tendinous envelope for the rectus abdominis muscle.
It is divided into an anterior and posterior layer which interdigate in the median line (linea alba). Above the arcuate line (about 4-5 cm caudal of the umbilicus) the anterior layer consists of the aponeuroses of the internal and external oblique and the posterior layer is made up the aponeuroses of the internal oblique and transverse abdominis. Below the arcuate line the anterior layer of the rectus sheath is build by the aponeuroses of all three lateral abdominal muscles whereas the posterior layer is only covered by the transverse fascia and the peritoneum.
The abdominal muscles are responsible for the tension of the anterior body wall. In collaboration with the back musculature they stabilize the vertebral column and move the trunk. Through abdominal press they increase the intra-abdominal pressure and thereby support emptying processes (e.g. defecation, micturition) and the exhalation (expiratory breathing muscles).
In detail, a bilateral contraction of the lateral abdominal muscles causes an abdominal press and ventral flexion (internal and external oblique). Depending on the particular muscle the unilateral contraction leads either to an ipsilateral lateral flexion (internal and external oblique), ipsilateral rotation (internal oblique and transverse abdominis) or contralateral rotation (external oblique).
The abdominal muscles are important antagonists of the back musculature. If they are not exercised the back muscles prevail and the body takes a passive and slack posture. In the long term this threatens a hyperlordosis of the lumbar vertebral column.
An insufficiency of the lateral abdominal muscles and their aponeuroses (e.g. in obese people or physiologically during pregnancy) may cause a diastasis recti where the rectus sheaths spread apart from the linea alba. The thereby resulting gap is filled by the abdominal fascia which becomes thinner but still remains intact. In comparison to an abdominal wall hernia there is no hernial orifice because of which patients with diastasis recti often stay free of complaints.