I wouldn't recommend the knee or the shin. My shin is toughened up quite nicely, and it would take a lot to really hurt me there. As for the knee, one of the guys at my club is a pretty bulky guy, and not all muscle. I know my kicks are ok, and first time I sparred with him i figured taking out his knees was a good way to go. It was like kicking a tree, there was no give in his legs at all.