muscular tonus vs tonicity

muscular tonus and tonicity both are nouns.

Word	Noun	Adjective	Verb	Adverb
muscular tonus	Yes	No	No	No
tonicity	Yes	No	No	No

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

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

Other hypernyms of muscular tonus include tone, tonus.

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