In reply to biscuit:
My partner and I have used poles since 1997. Usually carrying pretty big loads by UK standards.
Almost 30 years later one of us has pretty debilitating knee issues, and the other, pretty painful knees on big descents.
(I'm resting today after two 1600-1700m ascents/descents over 3 days.)
We both now manage our knees (as best we can) by no longer using poles on descents.
I think 30 years of pole use on descents has meant the supporting muscles were not engaged properly, and the forces of every downward step have slammed into the joints.
When your knees are knackered, you can really feel the different dynamics when using or not using poles, even for a single downward step.
With poles, your legs feel stiff and jarring. Without, they feel fluid.
We can't wear mountain boots anymore, only soft shoes, and stay slightly flexed and stepping down onto the ball of the foot. Like barefoot running, I imagine. I call it trotting down hills.
So, yeah, poles have enabled us to power up and down hills, but most probably at the expense of our poor knees.
I used to swear by them (people would joke that I was a Leki rep), now I am more cautious.
Of course there are many ways of doing anything, and we have probably just been a bit too reliant on the poles.
Best of health to everyone's knees