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.

Kind of related phrase:

I WISH THAT THING WAS IN THE FER FORK OF HELL WITH ITS BACK BROKE - An old mountaineer "cussing his mowing machine." From "Mountaineers would 'cross hell on a rotten rail' to be colorful," a column by Byron Crawford, late 1981, The Courier Journal, Louisville, Ky.

Surely this is a reference to 'the last straw that broke the camel's back'. This certainly stopped the camel being even more loaded - his back was broken and his task was ended. By extension, all tasks coming to and end may be associated with a metaphorical 'broken back'.