muscle building vs musclebuilding

muscle building and musclebuilding both are nouns.

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

As nouns, muscle building and musclebuilding are synonyms defined as:

muscle building and musclebuilding: exercise that builds muscles through tension

Other synonyms of muscle building include anaerobic exercise, bodybuilding.

muscle building (noun)	musclebuilding (noun)
exercise that builds muscles through tension	exercise that builds muscles through tension