Time out--you are not comparing the same joint here at all. The
equine "knee" is the carpus (your wrist), and the human knee corresponds to
the horse's stifle.
There is considerably more play in the femoropatellar joint of
either species than in the carpus.
(I have been sitting on my hands in this thread to avoid sounding
pedantic, but if you're going to make human/equine comparisons, you need to
try and make them asaccurately as possible.)
--CMNewell, DVM