The iliopsoas muscle group is made up of three muscles— psoas major, psoas minor and iliacus. The iliacus starts along the rim of the pelvis lying behind the psoas. The iliacus and psoas then meet to share a common tendon that attaches on the lesser trochanter a small knob of bone on the back half of the inner thigh. This is the only tendon that attaches to this bone in contrast to the greater trochanter on the outer thigh that has many muscles attaching to it including the piriformis which, along with the psoas are the only two muscles in the body that connect the legs to the spine.
When standing the psoas and iliacus lift the leg forward for walking among other things. When lying down the psoas and iliacus can lift the trunk from the floor like a sit up. The iliacus because of its attachment to the pelvis and leg acts only on the hip joint and controls hip movement when involved in twisting the hips for things like kicking.
It is a short and strong muscle creating a good deal of the power required for flexion but like the psoas it is susceptible to becoming short and tight especially due to a sedentary lifestyle. The psoas and iliacus are among the most important muscles in the body but the iliacus is often left out of the discussion when focusing on the all mighty psoas major.