I have leg IK constraint for Animating running dog. I need leg to bend in both direction, you will see on picture what I'm talking about. I know when making deforming bones you move head of the bone to the side you want that it to bend. But in my case, I need to bend it in both sides.enter image description here

This is straight leg, last one (purple) is IK constraint, so when I move it on Y axis, I want it to bend in wanted direction, is it possible to make some sort of switch?

  • $\begingroup$ I'm not sure this is possible so you probably need to use another method $\endgroup$
    – moonboots
    Commented Oct 14, 2021 at 14:11

1 Answer 1


When bones in an IK chain can bend both (or all) ways, the IK chain isn't fully determined-- there are at least two IK solutions, one bending one way and another bending a different way. Blender needs to just pick one of those solutions, and not change its mind, or else the IK bones will pop between either solution. So this isn't something that can be fully automated: if there are two solutions, the animator needs to pick between them. That's why it uses the existing bend to determine the direction IK should go.

However, it does not actually use the rest position of the bone to determine the direction of bend, but the posed position of the bone. That means that you can reverse the direction that an IK bone bends by rotating it, and you can do this in pose, with keyframes.

If you'd like this to happen according to automatic rules that you devise, that's possible. You can use drivers or constraints to bend the bone, which will affect the direction of the bend. One way you might do this is to use a limit rotation constraint, to limit the rotation in such a way that it reverses the bend. In Blender, constraints on IK-chain bones are always evaluated before the IK.

  • $\begingroup$ So basically, when leg fully sretch I can move Deformation bone in Edit mode to make it bend in wanted direction on first keyframe. Then, on other keyframe bend it again in other direction to move it on other direction than on that on first keyframe? About driver, do you have some formula or something how to make it, because I don't understand them very well. Also, what about FK maybe, IK to FK switch and trying that? I thought about that, but I am afraid my editor will become messy so I thought about some faster and cleaner way. $\endgroup$
    – Radoon
    Commented Oct 15, 2021 at 6:58

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