I've learned who the perpetrator is who keeps hitting people in or around the knee. It was an Orc in the second Hobbit movie. One of the Dwarfs got hit just slightly above the knee. After that, he's out for the rest of the film and can't proceed with the others.
In order to become a guard, you must survive an arrow to the knee. Surviving the arrow without losing mobility is a great honor, and sign of strength. Only when you take an arrow to the knee are you strong enough to be a guard. Many start as adventurers, until the day when they are accepted by the guards after knee injuries. When a guard tells you about his arrow to the knee, he is not telling you a tale of woe, but one of triumph! That's what I like to think to maintain immersion, anyway XD.