I don't know if it was a top answer when you looked 8 hours ago, but a cardiovascular physiologist answered (14 hours ago) a very thorough, detailed answer with second-order explanations. It's now _the_ top comment, here is a direct link with tracking removed:
I was thinking of that answer too. It talks about the ways heart muscle is different from skeletal muscle, which is cool, but I don't see it carry those differences through into explaining about fatigue...
https://www.reddit.com/r/askscience/comments/fm2z8x/if_a_hea...