'Back' in the Bible

On the contrary, who are you, O man, who answers [arrogantly] back to God and dares to defy Him? Will the thing which is formed say to him who formed it, “Why have you made me like this?”

“Let their eyes be darkened so that they do not see,And make their backs bend [under their burden] forever.”

For if you were cut off from what is by nature a wild olive tree, and against nature were grafted into a cultivated olive tree, how much easier will it be to graft these who are the natural branches back into [the original parent stock of] their own olive tree?

Or who has first given to Him that it would be paid back to him?