Broke the Back of It

Hi All,
I'm trying to find the origin of the phrase: Broke the Back of It, as in nearer completion of a task or getting the worst over and done with. I've asked where I work and have been given the following suggestions: linked to back breaking work; breaking the back of a book as you're reading through it, something to do with straws and camels; breaking someone's back as in rendering them impotent.
Does anyone know if any of these are correct or what the real answer is?
Any help and seuggestions very welcome.
Cheers
Mike

Well, if you are trying to kill something -- human or animal -- a good start is to break its back. (And I don't know that from personal experience.) That's a guess but I feel I'm on pretty firm ground.