Internal hip rotation and low back pain.
No brain surgery here if you have been on our station for the last several years. We pound home the critical importance of internal hip rotation all the time, here and in our clinic.
When the foot is on the ground, loading, the opposite leg is in swing. Part of this swing phase requires the hemipelvis on that swing side to also advance forward as well. This means that the stance phase leg will see the pelvis rotating atop of the static femoral head, this rotation is internal hip rotation. If one does not have sufficient internal hip rotation then the heel will be lifted prematurely, the foot might undergo an adductory twist (the heel moves medially into adduction which can look like the foot spinning "relatively" outward into external rotation) to name just a few (of many possible) pattern consequences. The loads can also move up into the lumbar spine, because, if the rotation is not there in the hip, or not buffered there, it either moves down into the limb or up into the pelvis and spine, or both. There are many strategies and patterns of loading responses available to the framework, it is your job to find them, source out the problem, and remedy. One must look for and understand the importance of sufficient internal hip rotation in your client, and the ramifications when it is not sufficiently present.
This study brings this principle to mind.