The most solid, certain, and necessary portion of hip extension must be the bony skeletal base. Wait, why "must"? What about the soft tissue (muscles, tendons, ligaments, fascia etc) or the nervous system's control over hip extension?
If the bones do not extend properly then surely the soft tissue or nervous system wont allow extension, right? Perhaps without clear and obvious bony extension the other two systems must fight to create a form of extension. Perhaps we can call that "hyper-extension"? Let's leave that up in the air for now and just focus on what the skeletal structure could be doing to unlock hip extension.
What are the pieces of hip extension? Simple. The femur, the ilium and the sacrum.
Wait, the sacrum is a bit different than the femur and ilium being that it's all by itself. It seems that with any deviation of the spine, any torque from either side of the ilium and certainly either femur must both be perfectly and harmoniously extended for me to say the sacrum has extended symmetrically. Obviously, the sacrum extends, but I am having a hard time understanding how that happens without some sort of torque or deviation brought on by all of its neighbors. Maybe ideal sacral extension is a myth or perhaps it doesn't matter?
Certainly an ilium extends though. I am a little hesitant to say the pelvis as a whole purely extends because like the sacrum that is implying that all the other structures around the pelvis like the spine, thorax, or even below the femurs, the tibias, ankles and feet are all perfectly and symmetrically balanced.
The pelvis being a central fixture in the skeleton seems to have its drawbacks, or perhaps those aren't drawbacks but can be seen as advantages?
(The Thomas test)
It seems to me that the easiest and most clear extension in the hip is done by the femurs. When you see the femur move back beyond the mid-line of the body you have hip extension, right? I guess the femur could move beyond the mid-line if the same ilium is abducted, or essentially turned towards that same side to give the illusion it's extended when really it's over extension or rotation of the ilium. With that said, couldn't over extension of the thoracic and lumbar spine essentially force the pelvis and femurs to hyper-extend to compensate?
Ill have to think about this a bit more it seems. Perhaps next time we can move past the skeletal pieces of extension and add to it the soft tissue portion.