i actually have chronic tendoitits in both of my knees and took therapy for it, my physical therapist told me that i have to strengthen every other muscle so that the strain is transferred and not focused on the knees when i am doing anything, so point being you just have to work your legs from hip to toe, ur gluets, quads, calves etc...any above excesices are awesome, squats, lunges, leg raises, and calve pressups(don't know what its really called but u lean against a wall in a slant and raise up and down from your tippy toe to flat as fast as you can)