Asked this in /fit/ but I know we have some /elite/ athletes here:

I'm trying to work on developing more varied mobility I guess, and am trying to follow this routine: 
> https://www.youtube.com/watch?v=N9Oq1YX75EE
Pic rel, I'm having trouble in this position, with the whole bodyweight supported on the folded leg. I find this painful in the back of my knee and cannot fully rest in this position or move around. Anything to develop/work on this?