buckle down vs break one's back

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

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

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

Other synonyms of buckle down include knuckle down, slave.

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