sinew vs heftiness

sinew and heftiness both are nouns.

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

As nouns, sinew and heftiness are synonyms defined as:

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

sinew (noun)	heftiness (noun)
possessing muscular strength	possessing muscular strength
a cord or band of inelastic tissue connecting a muscle with its bony attachment	the property of being large in mass