hypotonia vs tonicity

hypotonia and tonicity both are nouns.

Word	Noun	Adjective	Verb	Adverb
hypotonia	Yes	No	No	No
tonicity	Yes	No	No	No

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

hypotonia: (of muscular tissue) the state of being hypotonic
tonicity: the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli

Other hypernyms of hypotonia include tone, tonus.

hypotonia (noun)	tonicity (noun)
(of muscular tissue) the state of being hypotonic	the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli