Rubber Guard Issues

Hey Guys,

I have been working on the Rubber Guard, based on the Mastering the Rubber Guard Book and DVD (seeing as how there is no school in MA or RI), and have been having a lot of success with it. However whenever I use the rubber guard (especially Crackhead Control and to a lesser extent Mission Control) it really tweaks my knees.

Am I not doing it right, because I really don't think that its a flexibility issue. I can do the double lotus stretch, the full butterfly stretch, and can even heel hook myself, and none of that bothers my knees at all...

Can anyone help me out, because while I love using the rubber guard (do to the successful submission rate I get from it), I might have to move away from it because whenever I use it in class my knees hurt for days afterward.

Thanks