For the last month my right knee hurts on and off.  It gets used often in Jiu-jitsu and Taekwondo as the right is my dominant leg.  I use the legs to push my opponents away on the mat or to hold them close to me as I choke them out.  When I walk around Catalyst Fitness it gets used on the stairs and on the cardio exercise machines when I test them.  Thursdays are leg days and of course the knees get a workout performing the exercises.