sit-up vs tummy crunch

sit-up and tummy crunch both are nouns.

Word	Noun	Adjective	Verb	Adverb
sit-up	Yes	No	No	No
tummy crunch	Yes	No	No	No

As nouns, tummy crunch is a hypernym of sit-up; that is, tummy crunch is a word with a broader meaning than sit-up:

sit-up: a stomach exercise in which a person sits up from a supine position without using the arms for leverage
tummy crunch: an exercise designed to strengthen the abdominal muscles

Other hypernyms of sit-up include stomach exercise.

sit-up (noun)	tummy crunch (noun)
a stomach exercise in which a person sits up from a supine position without using the arms for leverage	an exercise designed to strengthen the abdominal muscles