What causes this deformation when switching to FK mode? I created a walk cycle where the person moves from point A to point B.

I added a circle in object mode, then I added the constraint in pose mode. But when I put in forward X up to Z I get this amazing deformation of the object!
What is the problem? It seems that the problem occurs during the switch between IK and FK modes. Until now, I could not understand these tools.

Why do I lose the motion? In fact, When I try to execute the walk cycle in FK mode using the snap buttons, I lose all motions in IK mode!
After you created a walk cycle as described in an earlier post: How should an animated character be moved while repeating a walk cycle? 

If only takes a few keyframes where you move the legs and arms



Note that the movement has to be in place, no keyframes must be added to the root bone.

Create a curve (This example uses a bezier circle) and add a Follow Path constraint to the root bone.



In the NLA editor add your action holding the walk cycle and set the repeat value that it matches the length of the animation.