Expression: "The human heart is a muscle ..."

[Tom]

Hi

Could you please tell me which of the following sentences is most natural and correct? Please tell me about the rest also.

[Molly]

For me, the first one. The generic use of the definite article is common in the Academic register. The word "atrophy" is also mostly found in that register. The collocation of "The + noun" (generic reference) and "atrophy" sound better than the collocation of "A + noun", IMO.

The third sentence is not grammatical.