*Have* their been any medical studies done, though?
I mean...at the clinic, we push dogs' butts down in to a sit (while simultaneously lifting their head up to rock them back) all of the time when we need them in a certain position for blood draws (although we are 15x happier when the dog will sit itself with a command or light pressure).
Now, behaviorally it isn't a sound practice, but medically, i don't know if it matters?
Maybe there are veterinary behavior journals you could search through?