Abdominal Muscles

Key Point:

The muscles of the anterior abdominal wall comprise thin sheets that compress and protect the abdominal contents, and, therefore, are involved in a range of actions, including defecation and micturition, parturition, and forceful expiration of air (as in heavy breathing, coughing, or yelling).

External oblique:

Originates from the external surfaces of ribs 5-12
Inserts on the ilium (the anterior ½ of the iliac crest and the anterior superior iliac spine), the pubic tubercle, and, via its aponeurosis, the linea alba.
The inferior border of this aponeurosis forms the inguinal ligament, which is a strong cord of connective tissue to which other muscles attach.
The fibers of the external oblique extend inferomedially from their posterior origins.
The external obliques compress the abdomen and flexes and rotates the trunk.

Internal oblique:

Originates on the lateral 2/3rds of the inguinal ligament, the iliac crest, and the thoracolumbar fascia.
It inserts at the linea alba, pubic crest and pectineal line, and external surfaces of ribs 9-12; it gives rise to an aponeurosis, which inserts at the midline.
Its fibers run superomedially at an oblique angle.
The internal oblique works with the external oblique to compress the abdomen and flex and rotate the trunk.

Rectus abdominis:

Originates on the pubic crest and pubic symphysis of the pelvis.
Inserts into the xiphoid process of the sternum and on the external surfaces of the costal cartilages of ribs 5-7.
Rectus abdominis depresses the ribs and flexes the trunk.
Rectus sheath covers anterior and posterior surfaces of rectus abdominis; it is formed by the aponeuroses of the other abdominal muscles.
Its fibers run vertically in four segments that are separated by horizontal tendinous intersections.

Transversus abdominis:

Originates on the lateral 1/3rd of the inguinal ligament, the iliac crest, the thoracolumbar fascia, and the external surfaces of the costal cartilages of ribs 7-12.
Inserts along the linea alba, pubic crest, and pectineal line.
Transversus abdominis compresses the abdomen and depresses the ribs.