I'm not sure how they originally attached, but I have elastic glued into my knees with snaps on them. My knees are
heavy and the snaps hold pretty well. I used 2 part epoxy that came as a gel to hold the elastic on. It wraps all the way around the inside of the knee from end to end, and it has never come undone, even when I had to kneel for a picture (I thought the guy in front of me was gonna get a piece of knee armour in the back of the head

) they only tend to slip down because of the weight. I know Bobafettish also put velcro on the inside of the knees and the front of his jumpsuit to prevent his from slipping. I hope this helps.