The Iliacus and Psoas (called iliopsoas when taken together) serve a unique role in the body. They work as both hip flexors and low back stabilizers. When deeper injuries occur, such as to the lumbar disc, their spasm can be the source of a great deal of pain and loss of function.
Once the muscle group is sufficiently irritated, a "tug of war" appears to occur between these muscles that live in front of the spine, with the larger muscle groups (such as the erector spinae) that we more easily feel in spasm on the back of the spine. A typical referral pattern for the iliacus and psoas (called iliopsoas when taken together) is as follows.
This muscle group appears to be a commonly misunderstood and missed muscle during typical medical evaluations where diagnosis seems to be overly reliant on MRI imaging and the like. Yet for overall recovery, it is very important to evaluate and treat this muscle for full recovery from most low back injuries.