Have you tried yoga or serious stretching? (Not just those you do after a jog.)
I am finding it harder to sit on my heels. I about have to keep a pillow under my butt and another under my knees and it is does not remain comfortable for long. Knee discomfort for sure.
I need to get more flexible! I am working on this. I should have been more diligent and done more to remain more limber.
Chrissy