system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is heart muscle so superior endurance wise compared to other muscles?

Walking up a few flights of stairs makes your leg muscles burn and fatigue. Doing something like squeezing a stress Ball 2000 times consecutively would render your forearm muscles almost unusable until they recover. Yet your heart beats 50-80 times per minute every single minute for your entire life and never needs a break, never gets sore even after beating at a higher rate during exercise. How does this work?
gpt
Because the body is very well optimised to provide the heart the resources it needs.

Muscle pain is due in considerable part to lactic acid build up. This occurs when energy is being produced faster than oxygen can be provided. The heart always has a strong supply of oxygen as long as there are no blockages and the lungs have oxygen, which means the heart never really runs out of the stuff. If the body is running a little low, then vessels are constricted to preserve blood oxygen at the heart and other vital organs.

Muscle pain is also in part due to the formation of tiny tears in muscle tissue. This happens when a muscle is exerted beyond what it's used to. The heart is always pumping, so it's completely accustomed to this motion and thus doesn't form tears. 

There's also quite a bit of redundancy in the heart. At any given point in time, 60-70% of your heart cells are resting. The heart has such a huge amount of mitochondria (bits in the cell where energy is produced) and such a ready supply of oxygen that even just very tiny rests for cells is enough to prevent fatigue.