Do you know how tackling works? It's generally people trying to knock you over, and they generally go for the lower body because of truly bizarre and obscure physical concepts like center of gravity.
If you have a pre-existing knee injury, that's a shame for you. It does not mean that people are not allowed to look cross-wise at your knees. The rules do not change.
Like Rodney Harrison said, take off that skirt and put on some slacks, Tom Brady. You wuss.