American spelling vs muscle fiber

American spelling and muscle fiber both are nouns.

Word	Noun	Adjective	Verb	Adverb
American spelling	Yes	No	No	No
muscle fiber	Yes	No	No	No

American spelling (noun)	muscle fiber (noun)
English orthography used in the United States (characterized by 'analyze', 'center', 'usable')	an elongated contractile cell that forms the muscles of the body