muscle vs heart muscle

muscle and heart muscle both are nouns.

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

As nouns, heart muscle is a hyponym of muscle; that is, heart muscle is a word with a more specific, narrower meaning than muscle:

muscle: animal tissue consisting predominantly of contractile cells
heart muscle: the muscle tissue of the heart; adapted to continued rhythmic contraction

Other hyponyms of muscle include striated muscle tissue, cardiac muscle, smooth muscle.

muscle (noun)	heart muscle (noun)
possessing muscular strength	the muscle tissue of the heart; adapted to continued rhythmic contraction
authority or power or force (especially when used in a coercive way)
one of the contractile organs of the body
animal tissue consisting predominantly of contractile cells
a bully employed as a thug or bodyguard

muscle (verb)	heart muscle (verb)
make one's way by force