knuckle down vs break one's back

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

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

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

Other synonyms of knuckle down include buckle down, slave.

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