I'm doing my first simulations with my model (with Zeke's rig adapted to it), simulating muscles and fascia together as you suggested and it works pretty well.
The knee area, where there are quite a few holes between the knee bone and the thin tendons of the other muscles, is the one that gives me the most trouble. I think my model would benefit from adding a tissue that would cover the knee and cover those holes, just like you have in the bear example.
Could you comment on the attachments (how many, what type and from which bones to which muscles do you make them) that you have in that piece so that it correctly follows the movement of the knee?
It would be very helpful if possible.