The iliopsoas refers to the joined psoas and the iliacus muscles. The two muscles are separate in the abdomen, but usually merge in the thigh. As such, they are usually given the common name iliopsoas. The iliopsoas muscle joins to the femur at the lesser trochanter, and acts as the strongest flexor of the hip.
The iliopsoas muscle is supplied by the lumbar spinal nerves L1-3 (psoas) and parts of the femoral nerve (iliacus).
It is a typical posture muscle dominated by slow-twitch type 1 fibers. Since it originates from the lumbar vertebrae and discs and then inserts onto the femur, any structure from the lumbar spine to the femur can be affected directly. A short and tight iliopsoas often presents as externally rotated legs and feet. It can cause pain in the low or mid back, SI joint, hip, groin, thigh, knee, or any combination.