That’s one thing we don’t have to worry about with Noir Kitty. I suspect that when he was a very young street cat, he was unable to get enough nutrition to develop his back leg muscles properly. The muscles run mainly in a ridge along the front of the legs, leaving a kind of flabby depression over the rest of the haunch. From above, he has a definite waist, too. There is another depression between his midsection and his hips that is quite noticeable, especially now that he has a bit of a pot belly.
Our vet isn’t concerned about this. Noir gets along just fine, though I’ve noticed his back feet splay outward a bit when he runs. But he doesn’t jump onto counters or furniture, (the exception being the couch but he hasn’t done that in about a year) and he can’t stand on his back legs unsupported. When I swish one of his toys around, he’ll try to stand up to get it but can only get his front feet a few inches above the floor.
He is a happy cat and gets more loving every day. So, other than the odd throw-up mess, he’s just about perfect.