muscle tone vs tonus

muscle tone and tonus both are nouns.

Word	Noun	Adjective	Verb	Adverb
muscle tone	Yes	No	No	No
tonus	Yes	No	No	No

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

muscle tone: normal tonicity of the muscles
tonus: the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli

Other hypernyms of muscle tone include tone, tonicity.

muscle tone (noun)	tonus (noun)
normal tonicity of the muscles	the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli