Animation Issue Between Keyframes

Hello all,
I’ve been looking for an answer to this question for quite some time now with no luck, so I figured I would submit it to all the wonderful experts here. I’m working on animating a simple character, and I haven’t run into any trouble until now. I’m working on a front-flip action in which he leaps off the ground, curls up, rolls, uncurls, and lands. Your basic ninja somersault/dive/roll/thing. The issue I’m encountering is that as he rotates, not all of the bones follow the rotation smoothly. Specifically, his leg twists around quite unnaturally in between the first and second keyframes, as well as between the second and third keyframes (though it then twists differently) and so on. As best I can tell, the issue is either with the IK constraint on the leg causing odd deformations (though I’m not sure why that would be since it works fine in every other scenario that I’ve tried), or else it’s just a problem with the way Blender is calculating the rotation of the rig. If that’s the case, is there a way that I can fix it without having to manually add in keyframes at every point where it doesn’t want to cooperate? I’m just not fond of the idea of spending a lot of time doing that. It seems to me that there ought to be some way to simply recalculate the rotation between keyframes to smooth it out, but a few hours of playing with the Dope Sheet, the NLA editor, and the Graph editor don’t seem to be helping. Does that all make sense? I appreciate your help!