Weird feeling in the knee doing natural leg curls+ which rings should I buy?

Well title sums it up, doing the NLC at home, on a rug, with 4 towels, and 2 soft pants folded it still feels awkward to the knee ( like something is moving in there )

 

are you guys having the same thing or should I add even more support bellow my knees??

 

 

 

Also, could anyone recommend which rings I should buy online??

 

Thanks.

Squeeze your quad. This will lock the kneecap in place.

I put my knees on a foam pad that's a couple inches thick.  Helps a lot.  Dan's suggestion is also important.  Pain behind the knee is just the hamstring insertion from straining on this movement.