buckle down vs knuckle down

buckle down and knuckle down both are verbs.

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

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

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

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