muscle building vs exercising

muscle building and exercising both are nouns.

Word	Noun	Adjective	Verb	Adverb
muscle building	Yes	No	No	No
exercising	Yes	No	No	No

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

muscle building: exercise that builds muscles through tension
exercising: the activity of exerting your muscles in various ways to keep fit

Other hypernyms of muscle building include exercise, physical exercise, physical exertion, workout.

muscle building (noun)	exercising (noun)
exercise that builds muscles through tension	the activity of exerting your muscles in various ways to keep fit