exercising vs exertion

exercising and exertion both are nouns.

Word	Noun	Adjective	Verb	Adverb
exercising	Yes	No	No	No
exertion	Yes	No	No	No

As nouns, exertion is a hypernym of exercising; that is, exertion is a word with a broader meaning than exercising:

exercising: the activity of exerting your muscles in various ways to keep fit
exertion: use of physical or mental energy; hard work

Other hypernyms of exercising include effort, elbow grease, sweat, travail.

exercising (noun)	exertion (noun)
the activity of exerting your muscles in various ways to keep fit	use of physical or mental energy; hard work