for the knee target, I just declared the target bone (the little ball in front of the knee) as the pole target of the ik constraint of the “tibia” bone (Bone.001). The chain length is two.
There might be a better way to do it.
I just realised when you use the knee target, it also rotates the foot, which might be bad, so I fixed it by declaring the top parent of the foot bones, Bone.003 (sorry for not renaming), as a hinge (in Editing, Armature Bones). That seems to solve the problem.
The new file is attached.
What I like in this rig is it solves the problem of slippery foot. You don’t have to move the foot to keep it in place when you rotate at the ball or the tiptoes.
Hope this is helpful.
footrig_v03.blend (147 KB)