The posterior abdominal wall is supported by the 12th thoracic (T12) and all five lumbar vertebrae (L1 - L5), as well as the intervertebral discs between them.

Three or four pairs of muscles are present in the posterior abdominal wall, depending on the individual:

• Psoas major
• Iliacus
• Quadratus lumborum
• Psoas minor

The psoas major together with the iliacus form the iliopsoas muscle.

Almost all of these muscles (except the quadratus lumborum) belong to the anterior group of the pelvic muscles.

The psoas minor is a variable muscle, being present in about 40% of the population.