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The leg looks fine up to this part:

enter image description here

However when I go further up it just twists and flips backward like so:

enter image description here

I've scoured the internet and can't find a solution. The pole target offset is at 90 degrees.

Thanks. Blender file attached if you want to look at it.

Blender file

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When you move the foot up above a certain threshold then your pole target is below the line connecting the foot and hip joints. The knee joint will then orient down to the pole target as best as it can.

inverse kinematic

To fix this you have to move the pole target forwards and/or upwards so that it's above the line again.

About the pole target

The main part of the inverse kinematics constraint is to calculate the angles of upper and lower leg so that the leg points to the foot without there being any disconnected parts. Without a pole target there would be an infinite number of orientations that would achieve this. In this example this would leave the knee rotating around the axis from hip to foot – the red line drawn above.

The pole target works in a way that can be thought of as spanning a rubber band or spring between the pole target. The knee now rotates around the hip-foot-axis so that the distance between knee and pole target is minimized. In the screenshot above the pole target is very close to the axis, so a real rubber band probably wouldn't be very efficient. But nonetheless the distance between knee and pole target is lower here when the knee points backwards than it would be if the knee would be pointing forwards. If you pull the pole target around the axis – or, speaking for the side view, above the line and more far away – the knee will be pulled over to the other side of the axis as well.

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  • $\begingroup$ Thanks! this fixed the forward part although the backward leg movement still twists. But I think its ok because it goes back far enough for the movement I want before it starts to malform. I was just wondering, what is the explanation behind your answer? Like why does the pole target need to align with foot and hip joints? Thank you for helping me! $\endgroup$ – Adam J Jun 16 '18 at 11:54
  • $\begingroup$ I've extended the answer by an attempt to better explain the reason for the problem. Maybe keeping an eye out for the orientation of the hip-foot-axis and the pole target in relation to it will help you identify and resolve the leg twists. I have no experience with rigging so I can't provide reasonable suggestion about where the pole target for a knee should be in a model. $\endgroup$ – binweg Jun 16 '18 at 19:34
  • $\begingroup$ Thank you very much for your input. However honestly I still do not understand this, is there any source online you could point me to that would clear this up? It seems to be a pretty esoteric topic and I can't find much. $\endgroup$ – Adam J Jun 17 '18 at 5:48
  • $\begingroup$ I can highly recommend the “Humane Rigging” training videos by Nathan Vegdahl which is on YouTube. The part about the pole target starts in this video at about 13:10 $\endgroup$ – binweg Jun 17 '18 at 6:51

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