cardiac muscle vs heart muscle

cardiac muscle and heart muscle both are nouns.

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

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

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

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