succumb vs knuckle under

succumb and knuckle under both are verbs.

Word	Noun	Adjective	Verb	Adverb
succumb	No	No	Yes	No
knuckle under	No	No	Yes	No

As verbs, succumb and knuckle under are synonyms defined as:

Other synonyms of succumb include buckle under, give in, yield.

succumb (verb)	knuckle under (verb)
consent reluctantly	consent reluctantly
be fatally overwhelmed