muscle fiber vs American spelling

muscle fiber and American spelling both are nouns.

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

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