From my training in Neurokinetic Therapy (NKT) with David Weinstock, David refers to the iliacus as a "LCF", which stands for Lower Cluster F**k, because of its common involvement as a facilitated compensator for so many other muscles throughout the body. For example, it is very common for the iliacus to compensate for a psoas or gluteus maximus or abdominal oblique muscles that aren't working as effectively as they can be.
So why is the iliacus commonly facilitated?
I don't know exactly, but my best guess could be it's location. The iliacus is a deep muscle that inserts to the upper two-thirds of the iliac fossa and inserts onto the lesser trochanter of the femur; it also has connections to the anterior aspect of the sacrum.
This means that it has its hands on several different joints: coxa-femoral, femoral-coxa, sacroiliac, and iliosacral. These joints are a part of the pelvic-hip complex which can also be viewed as a mid-point between the lower body and the upper body. Whenever there's a mid-point, there has to be a common ground, or "middle path" (as the Buddha would say), where a balance between the forces above and below take place. This balancing is a dynamic process and because the nervous system moves towards "stability" and avoids "instability", it tends to facilitate the deeper muscles (or muscles closer to the joint) in order to create a crude sense of stability through increasing joint compression when it senses an "unstable" process. From my experience, whenever I see a facilitated iliacus, it's common to see a compressed hip joint, an anteriorly rotated ilium, and a forward sacral base ipsilaterally. This can present itself clinically as femoral-acetabular impingement (FAI), forward sacral torsions, and/or iliosacral instability which presents symptomatically as pinching in the groin, hip pain, SI joint pain, low back pain, difficulty bending, walking, negotiating stairs, etc.
What happens if the iliacus is "inhibited"? No muscle is ever really fully inhibited; for lack of a better word, let's understand that when I say "inhibited", I mean a muscle that's not firing as effectively as it can be. David Weinstock mentions that it's rare to find an inhibited iliacus. But when you do, there's usually several layers of compensations going on. I usually see a multi-system overload and the "inhibited" iliacus is just the icing on the cake. Locally, it can be something as simple as positioning and functioning of the involved joints. For example, when there's a restriction in hip flexion and/or a restriction in anterior rotation of the ipsilateral ilium, the iliacus may test inhibited. To verify limited hip flexion involvement, one could muscle test the hip in less flexion after testing the involved iliacus in a shortened position and if it tests strong, limited hip flexion may be contributing to the inhibited iliacus due to arthrogenic inhibition. One can also test the psoas muscle and its inhibition is another way to verify hip flexion restriction. For iliacus inhibition due to limited anterior ilium rotation, one can test the quadratus lumborum and latissimus dorsi; these would test inhibited while the psoas would test strong.
Sometimes we assume just because we are a more sedentary society and that we tend to sit a lot that the iliacus is always in a shortened position and that it can more easily be recruited in times of instability and over time it can develop trigger points, be facilitated, and even after a while, get "tired"and become weak due to fatigue. We never really know unless we assess its role in relationship to the rest of the body and overall tie it together with its integrated function during movement. Without movement the iliacus would never be considered a "LCF".....
