The iliocostalis lumborum is the muscle that attaches to the iliac crest and the back of the ribs.

It is part of the iliocostalis column of muscles, which are responsible for the primary movement of back extension. The iliocostalis lumborum provides resistance when the body bends forward and provides the force necessary to bring the body back into an upright position.

It is connected to the iliocostalis thoracis, which handles the six bottom ribs and the iliocostalis cervicis, which handles ribs three to six.

The iliocostalis lumborum, along with the thoracis and the cervicis, extends the vertebral column, facilitates good posture, and bends the body if active on one side. When the body is fully bent forward, backward, or to either side, the iliocostalis lumborum is relaxed and transfers the load to the ligaments. When a person is standing upright, it is also relaxed because the hamstrings in the upper leg and gluteus maximus help hold the body in place.

The nerve supply of the iliocostalis lumborum comes from the posterior rami of the spinal nerves. Its vascular supply is from the intercostal and lumbar arteries.