knuckle down vs buckle down

knuckle down and buckle down both are verbs.

Word	Noun	Adjective	Verb	Adverb
knuckle down	No	No	Yes	No
buckle down	No	No	Yes	No

As verbs, knuckle down and buckle down are synonyms defined as:

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

knuckle down (verb)	buckle down (verb)
work very hard, like a slave	work very hard, like a slave