Random uninformed idea… this is surely wrong but… birds ‘knee’ joints are the opposite of humans, have their ‘butt’ muscles have evolved alongside their wing/breast muscles?
https://en.m.wikipedia.org/wiki/Bird_feet_and_legs
In a lot of mammals too the knee is "hidden"
https://duckduckgo.com/?q=horse+hind+legs+anatomy&t=fpas&iax...