Join our CAD community forums where over 25,000 users interact to solve day to day problems and share ideas. We encourage you to visit, invite you to participate and look forward to your input and opinions. Acrobat 3D, AutoCAD, Catia, Inventor, IronCAD, Creo, Pro/ENGINEER, Solid Edge, SolidWorks, and others.
Create a Point at the pivot of the thigh bone and then assign a position constraint to that point and use the thigh bone as a target, now assign an orientation constraint to the same point but this time use the pelvis bone as the target. You want the point to stay with the thigh bone but you do not want it to rotate with the thigh bone. Now create another point at the pivot of the thigh bone. Link that new point to the thigh bone. Create one more pioint at the thigh bone and Link this new point to the first point (in step 1), now add an orientation constraint to this point and use the First Point (step 1) and the second point (step 4) as targets. Finally, add the last point as a new bone in your skin modifier. Using this new point you will have an intermediary bone you can skin to that is always going to be half the rotation of the thigh bone. This trick works GREAT. It will also help to keep the joint from collapsing when bend the thigh bone.