Knees bending backward

I have a humanoid character who has armature problems. My main problem is this: The leg armature has 3 bones, thigh, chin and foot. I’m using bone constraints to have the leg automatically bend when the foot and toe empties are moved. The ‘foot’ bone has two constraints, IK Resolving at the ankle empty, and Track To to the toe empty.
My problem is this… it works fine until I bring the foot empties too high, or to various positions, and the armature will flip around so the knee of the leg is essentially bending backward… sort of like in Football accidents. Is there any way to keep the leg bending in a constant natural fashion?


Unfortunately that is a problem with using IK,

I don’t recall if there is a way around this…I don’t believe so. Maybe someone else has an idea. I would get around it by using FK instead. Maybe create a second skeleton with FK for problem poses, and use the IK skeleton for walking, ect.

Hope that helps!


I’m not entire sure what FK is… Forward Kinematics?

You’re reply did give me an idea. I took my IK resolving a step further… to involve 3 empties. I have the foot with resolving and tracking, and the chin. So I have the knee, heel and toe empties to worry about, but it does seem to do the trick… even though it’s more complicated than I would have liked.

Yes, FK means Forward Kinematics.

IK is nice, but when it comes to animating characters sometimes the computations that bend the joints bend them in ways that are awkward for a human form. FK solves this problem but requires more effort to position and animate well. In short, in IK you can move the children and the parents follow. In FK, the parents move and the kids follow.

I hope your solution works for you!

Blend on,

sorry if its already been answered/figured out to your satisfaction, but here’s two solutions.
1- pre bend the joints in the model and the armature in edit mode in the direction they bend, at least slightly, if they’re ik joints. i.e knees/elbows. This is sortof a must in blender (my experience backed up by more advanced animators.)
2- to control the bend further, add an extra target for the knee (ik target for the calf bone) or the elbow (ditto for forearm) you can now control your knee/elbow more precisely. depending on your preference, you can make the new targets childeren of the foot (or arm) target, or not.

wow…that’s a good idea, bending them in edit mode… I wish I would have tried that before spending dozens of man-hours figuring it out. What has worked best for me is like everyone says the knees and elbow empty’s being Tracked to…but I suggest making the knee the parent of the ankle and the ankle the parent of the foot… that way you don’t have to screw with the position of the foot, just rotate the knee…

I actually have everything parented to something else, so I move the “CENTER” empty and all the empty’s move… it has it’s pros and cons, but it might work for you, I’ve found that you can have good control like this.

Does anyone know how to parent things temporarily? That would be cool.


The pre-bending idea makes some level of sense, and I’ll see if I can deploy the thought into action.
The TrackTo with the knee, being the third empty, is actually working quite well. At least, much better than before. I don’t parent the limbs to the Central empty, since I’d like to move the torso and have the legs stay in one spot. Of course, this means every time I move or scale the character, I have to gather together all the empties first.