Injuries are another issue altogether. With knees, you don't mess around (I know because I have a chronically bad one) -- there are so many ways we can permanently screw them up. Sometimes even when you think you are concentrating on training and not the pain or injury, on an unconscious level your body really is "choking up" and you're not putting 100% into your intent and focus. That would also affect your training.
So go easy on that knee!