system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why heart muscles can work 24/7 nonstop but other muscles like biceps can't and get tired to they point the can't contract more?
gpt
Cardiac muscle has a very different cellular architecture than skeletal muscle. 

Skeletal muscles get tired and lose contractility because you deplete the muscles' stores of glycogen (basically the stored form of sugar). They lose their easy to access energy source and stop working as well.

Cardiac muscle feeds primarily on circulating fats, and are designed to harness this very efficiently. Because they aren't relying on stored energy but instead circulating energy, they don't need to worry about depleting their stores.