The manner in which the human body is formed allows for certain joints and appendages to move or rotate within specific, tight parameters. When said appendages are forced in a direction beyond a certain allowable movement, they can cause horrific injuries.
Edmonton Eskimos (CFL) quarterback Matt Nichols learned this human anatomy lesson the hard way after taking a hit from a rushing defender.
It's not for the faint of heart (it comes at about :40 into the video).
Of course, if Al Michaels were calling the game he'd say merely that Nichols was out with "a leg"...no comments