yikers!
Just doing that test myself- I could feel a real difference just from having my head up or bent down! It really pulled in the hamstring area, almost zingers, would be zingers if someone was putting pressure down on my head.
http://www.innerbody.com/image/musbov.html

that whole site is kind of fun- not fully detailed though
but good for quick overviews
http://www.innerbody.com/htm/body.html