Quadratus lumborum muscle
In order to palpate the muscle one needs place the fingers above the posterior iliac crest at the level of the hip.
Origins & Insertions
It starts from the iliac crest of the ilium and runs craniomedially, attaching to the 12th rib and the transverse processes of the 1st to 4th lumbar vertebrae. All fibers together give the muscle a rectangular appearance.
The innervation is supplied by the subcostal nerve and branches of the lumbar plexus.
The quadratus lumborum fills a great amount of space within the abdomen and is therefore in close proximity to many structures. The colon, the kidneys and the diaphragm are located ventrally to the muscle, whereas the intrinsic back muscles lie dorsomedially. Both the iliohypogastric and ilioinguinal nerves course on the ventral surface of the quadratus lumborum after exiting the lumbar plexus and continue towards the lateral abdominal muscles.
Essentially, the quadratus lumborum contributes to the stabilization and movement of the spine and the pelvis. A bilateral contraction leads to an extension of the lumbar vertebral column. When the muscle is only activated on one side, the trunk is bent towards that direction (lateral flexion). In addition, the muscle fixes the 12th rib during movements of the thoracic cage and this way supports expiration (accessory muscle of expiration).
Overuse and strain of the quadratus lumborum are one of the major causes for chronic pain in the lower back. One typical cause is the habit of sitting at the desk using a reclined seat, which releases the intrinsic back muscles and weakens them in the long term. The weak back muscles must now be compensated by the quadratus lumborum leading to painful tension and stiffening of the muscle.
Other causes can be direct damage or any type of imbalance of the pelvis or spine which forces the quadratus lumborum to stabilize them. An example of that type of imbalance is unequal leg lengths.