system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why does your heart never get muscle pains?

Seems like you can feel after-workout pain in all other muscles, so why not the heart?
gpt
All living cells need adenosine triphosphate (ATP) to function. ATP is made by biochemical reactions inside cells, mainly inside mitochondria. In the presence of oxygen (aerobic respiration) mitochondria churn out significantly more ATP and is more energy efficient than without oxygen (anaerobic respiration).

During intense activity, oxygen becomes scarcer and our bodies triage it. This is why our stomach cramps if we exercise after a full meal. Your body is redirecting blood and oxygen to skeletal muscles for the exercise, making your stomach's smooth muscles rely on more inefficient anaerobic reactions for their ATP. 

As activity continues and oxygen gets scarcer, more anaerobic reactions happen throughout your body and lactic acid accumulates. This makes muscles sore and needs to be cleared out to allow proper aeration.

Why are heart muscles spared from fatigue?
Three factors:

1) Cardiac muscle cells have significantly more mitochondria than other cells. Hearts never stop beating until we die. They need to constantly make ATP to power heart contractions.  This makes them VERY energy and oxygen hungry. Cardiac cells are literally furnaces. They can metabolize an assortment of foods (e.g. sugars, fats, proteins, they are not picky). Our bodies prefer energy efficiency, which is why not all our cells have as many mitochondria. If all our cells had cardiac appetites, we would probably have to eat all day without rest.

2) Like the brain, our heart has its own dedicated blood/oxygen supply. Our bodies ensure our heart always has the food and oxygen it needs. A heart attack is when our heart muscles do not receive enough oxygen.

3) Cardiac muscle cells have enzymes, chemical reactions, and other mechanisms that avoid creating lactic acid and quickly removes it when it does. 

Bottom line, our hearts are made of special muscle cells that have extraordinary adaptations to avoid fatigue, but are very energetically expensive.