What is the difference between the psoas and iliacus muscles?

Your hip flexor is made up of the iliacus and psoas, which are intimately connected near the hip, but are still two separate muscles. They perform similar, yet different functions in the body, have different attachment points, and are innervated by different nerve roots.

If there is tension in one, there is likely tension in both. For the best and longest-lasting results, release both your iliacus and psoas muscles.