heart muscle vs cardiac muscle

heart muscle and cardiac muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
heart muscle	Yes	No	No	No
cardiac muscle	Yes	No	No	No

As nouns, heart muscle and cardiac muscle are synonyms defined as:

heart muscle and cardiac muscle: the muscle tissue of the heart; adapted to continued rhythmic contraction

heart muscle (noun)	cardiac muscle (noun)
the muscle tissue of the heart; adapted to continued rhythmic contraction	the muscle tissue of the heart; adapted to continued rhythmic contraction