Just found something on the UESP, in the highest level Dark Brotherhood quest, you are required to kill some "toughest fighter in the region." And here is the quest description...

Somehow I thought you might try for this one. Its your funereal. This mark is no arena gladiator. We're talking about a veteran of the Betony war. Killed 9 men, if the stories are right. Go to (weapons store) in (town). You'll be able to identify the mark easily because of the limp. A spear in the knee during the war caused it. Get back here in (time limit) days.


So yeah, apparently it is a tradition of Tamriel that the weakpoint of adventurers is the knee.

Kind of like how "Someone stole your sweetroll" is popular, even though sweetrolls have been a running joke since Arena (that is, older than dirt).

Some sort of stolen sweetroll reference has been in every single Elder Scrolls game to date.

And Fallout 3 http://fallout.wikia.com/wiki/Sweetroll. But it got cut from FNV.