break one's back vs buckle down

break one's back and buckle down both are verbs.

Word	Noun	Adjective	Verb	Adverb
break one's back	No	No	Yes	No
buckle down	No	No	Yes	No

As verbs, break one's back and buckle down are synonyms defined as:

Other synonyms of break one's back include knuckle down, slave.

break one's back (verb)	buckle down (verb)
work very hard, like a slave	work very hard, like a slave