muscle tone vs tonicity

muscle tone and tonicity both are nouns.

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

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

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

Other hypernyms of muscle tone include tone, tonus.

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