Somebody asked MetaFilter (via Cynical-C): 

Do you think a super strong person could yank off his own head?

Like one of those strong-man competition guys (the ones who lift those big boulders and carry around telephone poles), or some other freakishly [strong] person, could one of them actually kill himself by ripping off his own head? Or would other physiological factors make this feat impossible?

I personally don't think it would be possible for a few reasons: 

1.  Leverage.  It would be nearly impossible to get enough leverage to yank your own head off with your own arms.  You'd basically be using your triceps and a few other arm and back muscles, and no angle would make it any easier.  If you were flexible enough to use your legs, I highly doubt it would be possible to get a tight enough grip.  It would be much easier with a pulley system or other mechanical device, but that's totally different. 

2.  Tension.  As you exert effort into performing an action, you tense up.  Try it.  The harder you pull on your own head, the tighter your own body keeps it in place. 

3.  Pain.  If you made any progress in removing your head, you'd most likely be forced to stop once you caused any amount of significant damage, such as pulled ligaments or even broken bones.  The pain you'd be in and the emotional shock you'd be experiencing would essentially disable all other body mechanics.