I'd probably have a bad kink in my neck after a sprint/ jog. Doing up my shoe laces would be a challenge. ☺
I actually had a friend in junior high school who could bend her knees backward. It was a little creepy, but she did it a lot to amuse us. If I could do that, I think I would try to join Cirque du Soleil. :)
That might make sitting down kind of difficult.
One of the aliens in Men in Black could do that.
And horses can sleep standing up because their knees do that (to a very limited degree).
I guess we would walk like flamingos.