brawniness vs sinew

brawniness and sinew both are nouns.

Word	Noun	Adjective	Verb	Adverb
brawniness	Yes	No	No	No
sinew	Yes	No	No	No

As nouns, brawniness and sinew are synonyms defined as:

Other synonyms of brawniness include brawn, heftiness, muscle, muscularity.

brawniness (noun)	sinew (noun)
possessing muscular strength	possessing muscular strength
	a cord or band of inelastic tissue connecting a muscle with its bony attachment