The posterior abdominal wall is a complex area of the body made up of muscles, vessels and nerves that work together to hold abdominal organs in place, adjust our posture, stabilize the hip joint and even flex our thighs on leg day!

The main paired muscles of the posterior abdominal wall include psoas major, iliacus and quadratus lumborum; and naturally, for those muscles to work properly, they receive innervation from various nerves.

Okay, now let’s take a closer look at these muscles! First up is the psoas major, which is a long, thick muscle that lies lateral to the lumbar vertebrae. It originates superiorly on the transverse processes of the bodies, and adjacent intervertebral discs of the T12 through L5 vertebrae.

The psoas major passes inferolaterally, deep to the inguinal ligament in order to insert on the lesser trochanter of the femur. Finally, approximately half the population has a psoas minor muscle that sits on the anterior aspect of each psoas major muscle.

Now, the psoas major muscle is covered by the psoas fascia or the psoas sheath, which is attached medially to the lumbar vertebrae and pelvic brim. The psoas sheath is thickened superiorly forming the medial arcuate ligament.

Inferior to the iliac crest, the psoas fascia is continuous with the part of the iliac fascia that covers the iliacus muscle.

When acting inferiorly with the iliacus muscle, the psoas muscle flexes and laterally rotates the thigh and when it acts superiorly it causes lateral flexion of the vertebral column.

When sitting, the psoas muscle acts with the iliacus muscle to flex the trunk. The innervation of the psoas major muscle comes from the anterior rami of the L1, L2 and L3 spinal nerves.