Sub-Lumbar Muscles

Full Title: Sub-Lumbar Muscles

Author: Lisa McPherson

Date of Publication: April 6, 2012


Research Paper Text:

The lumbar and pelvic areas of a dog are an integrated group of bones, muscles, tendons, blood vessels, nerves and fascia. There are many sub-sections of these areas but I would like to concentrate specifically the sub-lumbar muscles.

The sub-lumbar muscle structures are found under the last three thoracic and the lumbar vertebrae and are also considered pelvic limb muscles. This muscle structure is made up of four muscles: quadratus lumborum, minor psoas, major psoas and the iliacus muscles (the major psoas and iliacus together form the iliopsoas). All these muscles lie on one another in layers.

The Quadratus Lumborum is the most dorsal of the muscle group and has a thoracic as well as a lumbar portion to it. This muscle’s origin is from the last three thoracic vertebrae as well as the last rib and lumbar transverse process. It inserts into the lumbar transverse process or the lumbar vertebrae and the surface of the wing of the ilium. The lateral portion overhangs the transverse process of the lumbar vertebrae and lies on the ventral surface of the transverse abdominus origin tendon.

The thoracic portion of the muscle is made up of incomplete bundles of tendons and these bundles extend from the body to the last three thoracic vertebrae to the transverse process as far as the seventh lumbar vertebrae. It is covered by tendon leaves dorsally and ventrally.

The nerve supply of this muscle is the rami of the ventral branches of the lumbar nerves.

In the dog, this muscle is extremely strong and it flexes and fixes the vertebral column in the loins.

This muscle runs towards the pelvis under the middle of the spine. It lies between the iliac fascia and peritoneum ventrally and iliopsoas and quadratus lumborum dorsally. The psoas minor runs medial to the psoas major.

The psoas minor originates at the tendon of the quadratus lumborum under the last thoracic vertebrae and the first four or five lumbar vertebrae. The insertion is a corklike tendon on the ilium adjacent to the iliopubic eminence or inlet.

The nerve supply is the lateral branches of the rami ventrals of lumbar nerves one to four or five.

This muscle is responsible for steeping the pelvis and flexing the lumbar portion of the spine.

The iliopsoas is formed with the fusion of the psoas major and iliacus. It is a very deep narrow muscle ventral to the quadratus lumborum and dorsal to the psoas minor. It covers the psoas minor laterally and medially. It is easily isolated at the attachment.

The origin of the muscle (psoas major) is the transverse process of the second and third lumbar vertebrae and lies lateral to the quadratus lumborum. It also attaches by flat broad tendons (aponerosis) to lumbar three and four and lumbar four to seven ventral and laterally.

After the iliopsoas passes over the ilium as the psoas major, it joins to the iliacus from the ventral surface of the ilium between the arcuate line and lateral border of the ilium (hip bone). The two muscles insert at the minor trochanter of the femur.

The innervations of these muscles are the branches of the rami ventrales of the lumbar nerves.

The iliopsoas muscle is important in drawing the pelvic limb forward by flexion and rotation of the hip joint, flexes and fixes the vertebral column, draws the trunk backward and advances the hindlimb.

The sublumbar muscles are very important for the fixation and flexion of the spine and for ambulation. In extremely active and athletic dogs this area is prone to overuse and injury and rehabilitating the area can be problematic. It is thought that tears, specifically in the Iliopsoas, are relatively common but not often diagnosed in dogs. This can be because they are not shown on X-ray and ultrasound, while useful, often does not show microscopic tears. Usually the diagnosis of an iliopsoas strain is found clinically with specific testing on examination.


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