The iliocostalis lumborum is the muscle which attaches to the iliac crest and the angles of the ribs in the back of the body. It is part of the ilocostalis 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 inferior 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, ensures correct posture, and bends the body if active on one side. It is relaxed during full flexion and transfers the load to the ligaments. When it returns from full flexion, it remains relaxed and transfers the tension to the hamstrings and gluteus maximus. 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.