agonist vs antagonistic muscle

agonist and antagonistic muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
agonist	Yes	No	No	No
antagonistic muscle	Yes	No	No	No

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

agonist: a muscle that contracts while another relaxes
antagonistic muscle: (physiology) a muscle that opposes the action of another

agonist (noun)	antagonistic muscle (noun)
(biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction	(physiology) a muscle that opposes the action of another
a muscle that contracts while another relaxes
someone involved in a contest or battle (as in an agon)
the principal character in a work of fiction