The quadratus femoris is one of the six hip muscles that provide external rotation. It is the most inferior of all of them. The QF is a rectangular shaped muscle that attaches to the anterior ischium (sit bone) and the posterior greater trochanter. The sciatic nerve often lies on top of this muscle.
The function of the hip rotators is to provide stability to the hip joint. If they present as continually tight in an athlete or patient, this is a sign that the hip joint isn't being centered and the hip rotators are always "on."
The QF is also a lateral rotator in flexion and extension.
Because of its attachment points there is a fascial connection to the biceps femoris, sacrotuberous ligament and adductor magnus.
I have found that it can significantly limit hip abduction. After the QF is treated, hip centration improves and better function out of the glute medius and minimus is achieved. It can also significantly improve active straight leg raise.