myotonia vs tonicity

myotonia and tonicity both are nouns.

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

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

myotonia: abnormally long muscular contractions; slow relaxation of a muscle after a contraction
tonicity: the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli

Other hypernyms of myotonia include tone, tonus.

myotonia (noun)	tonicity (noun)
abnormally long muscular contractions; slow relaxation of a muscle after a contraction	the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli