You wrote
Changing cranks changes a lot of things. So while it could help with your medical issue it might change things for the worse on your overall position.
Obviously it changes other aspects of your fit. That has already been mentioned here.
I’m keen to understand how it changes things for the worse ? Or was that a general throw away statement ?
The vast majority of people will be using cleats and clipless pedals so their foot position is fixed. For those using flat pedals, well, they are in the minority and I’m not sure they’d be that interested in the biomechanics of shorter cranks given the limited repeatability of their foot placement, retention and repeatability.
You’ve now said it “obviously changes other aspects of your fit”, which was precisely my point.
Going from 172.5 mm to 165 mm changes the pedal circle. If the saddle is left where it is, the foot is 7.5 mm higher at the bottom of the stroke, changing knee extension there. If the saddle is raised to compensate, that increases saddle-to-bar drop and, because the saddle moves up along an angled seat tube, also moves it slightly rearward. That can affect effective reach, torso angle, hand pressure and weight distribution. Depending on the rider and which medical problem they are trying to solve, some of those changes could improve the position and some could make another aspect less comfortable. Hence my carefully chosen word “could”.
Being clipped in fixes the shoe’s attachment point to the pedal; it doesn’t make ankle angle, heel position or the movement of the foot through the stroke irrelevant. And dismissing everyone on flat pedals as a minority who presumably has no interest in bike fit is a rather strange argument and frankly elitist (my wife and daughter use flats).
So no, it wasn’t a throwaway statement. It was a brief internet-forum comment saying that changing one major fit variable can have knock-on effects and should be assessed as part of the whole position. But yes, apparently unless every use of the word “could” is accompanied by a full biomechanics lecture, it means the person knows nothing.
I'm not going to say another thing on the subject as this isn't the thread for it. I'll happily reply to a new thread if you make one to continue the discussion after this.