That's something you should do after you've used a good one quite a bit. In A:M. Maybe you already have. Ok then. But if you haven't, then you are starting out with some mistaken goals.
Without knowing how they are rigged, impossible to say.
I'm going to guess it's the lack of a knee control. Have you watched the "Simplest IK leg" video on my tuts page? That shows a knee pointer or knee target. Which is something already built into the rigs I mentioned.
BTW, there are zero animation posing situations in which you leave the feet targets away from the feet or let them stretch the legs absolutely straight. Never get into that situation and you'll never have to undo weird rig behavior because of it.