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.

Next is the iliacus muscle, which is a large triangular muscle that lies lateral to the inferior portion of the psoas major muscle. It originates superiorly on the superior two thirds of the iliac fossa, ala of sacrum, and anterior sacroiliac ligaments. It also inserts at the lesser trochanter of femur.

In fact, most of its fibers join the tendon of the psoas major muscle to form the iliopsoas muscle, which is the main flexor of the thigh and a stabilizer of the hip joint, and it also helps maintain erect posture.

The innervation of iliacus comes from the femoral nerve, which is derived from the anterior rami of spinal nerves L2 through L4.

Now let’s look at the quadratus lumborum muscle, which forms a thick muscular sheet in the posterior abdominal wall, along the transverse processes of the lumbar vertebrae. It originates inferiorly on the iliac crest and iliolumbar ligament and inserts superiorly on the medial half of the inferior border of the 12th rib and the tips of the lumbar transverse processes.

The quadratus lumborum muscle is enclosed within two layers of the thoracolumbar fascia. The anterior layer of the thoracolumbar fascia - also called the quadratus lumborum fascia - thickens in the superior aspect to form the lateral arcuate ligament.

Bilaterally, the quadratus lumborum muscles extend the trunk and also fix the 12th pair of ribs during inspiration. Unilaterally, it causes lateral flexion of the vertebral column. Its innervation comes from the subcostal nerve and anterior rami of the L1–L4 nerves.

So far so good! Time for a quick quiz. What are the main actions of the composite iliopsoas muscle and the quadratus lumborum muscle?

Now let’s have a quick look at the nerves of the posterior abdominal wall, which provide somatic innervation through the subcostal nerves and the lumbar plexus; and autonomic innervation through the sympathetic trunks and associated ganglia.

First up, the subcostal nerves are paired nerves that represent the anterior rami of T12. They arise in the thorax and pass posterior to the lateral arcuate ligaments into the abdomen.

Then they run inferolaterally on the anterior surface of the quadratus lumborum to eventually pass through the transversus abdominis, internal oblique, and external oblique muscles innervating them and the skin of the anterolateral abdominal wall.

Next up is the lumbar plexus, which forms within the psoas major muscle. This is a network of nerves that is composed of the anterior rami of the L1 through L4 spinal nerves.

The nerves that arise from the lumbar plexus include: the iliohypogastric nerve; the ilioinguinal nerve; the genitofemoral nerve; the lateral cutaneous nerve of the thigh; the femoral nerve; the obturator nerve; and finally, the lumbosacral trunk.

First, let’s look at the ilioinguinal and the iliohypogastric nerves, both of which arise from the anterior ramus of L1. They enter the abdomen posterior to the medial arcuate ligament and then pass inferolaterally, anterior to the quadratus lumborum.

Then they run superior and parallel to the iliac crest with the iliohypogastric nerve travelling more superiorly than the ilioinguinal nerve. Then they pierce the transversus abdominis and internal oblique muscles near the anterior superior iliac spine supplying them with motor innervation.