I can’t show the image, which would be helpful, but NDA stuff prevents me. What I’m trying to do is make the knee a perfect hinge, but none of the constraints seem to be the one. Basically, the knee has armored plates on it, so any sideways or twisting motion is bad. But I’m having difficulty getting it to play well with the IK system. If it’s a standard IK setup from knee to ankle, what’s the best way to constrain the joint?
Before you can put limitation on bone movement, you need to know each bone axis as it is rigged. Display the bone axis with the rig; in pose mode under Armature property button, subcategory Display in Pose mode, check Axes box.
In pose mode, select the shinbone you need to constraint its knee rotation. Under Bone property button select Inverse Kinematics subcategory to expand it. There is all the limitation setting you can assign to that bone. If your shinbone is rotating from knee joint about Z axis, lock X and Y axis. That’s it.
If you want, you can set a limit on how much knee joint can rotate in there too. Stiffness setting affects how much that bone get influenced by IK target movement as well. Each and every bone with in IK chain behavior can be manipulated from this panel.