Body building is sore, slow, and needs rest, but the heart doesn't. Why?

Question

I have a question about heart muscle rebuild after high intensity cardio. I wonder if it is a proper question in this forum. If you think not, please kindly tell me where it should be.

Building muscle process like biceps is to slightly break muscle fiber first with weight training, and let it rest and rebuild for the next three days with high protein and without using it. If used, it will cause pain, or increase existing pain. Pain is a body natural mechanism to tell one not to disturb it during recovery.

But, cardio isn't the case. We cannot feel the heart muscle get sore. And, we cannot have a rest heart or we die. So, how does heart get strong differently?

Years ago I do sledgehammer almost everyday. During my annual health inspection my doctor told me I have a large and strong heart, which is equal to an professional athlete's. He actually asked me if I am an athlete. I am not. I am an engineer working with computer in front of desk in office. If I remember correctly it grows 1.5 or 2 times heart muscle thickness within half an year. This growth speed is crazy. If a body builder can increase his biceps size twice within half a year, we will call him king. But it seems pretty easy for a heart to achieve it, and I was not young. I was 37. Doctor also tell me if I indeed exercise a lot and it doesn't have any pain, then it is normal. But, I still worry about the rapid growth speed so I stop for years until recently. The next year my heart is back to regular size again. The point is that my story indicates that heart muscle building definitely has a different growth mechanism, it can grow fast without pain and rest. That's why I post a question here.

Answer 1

I find some answers in wikipedia.

This explains that heart doesn't or rarely have a sore and don't need rest even after a high intensity cardio exercise. But I cannot find a explanation about how growing fast.

Cardiomyocytes have a high mitochondrial density, which allows them to produce adenosine triphosphate (ATP) quickly, making them highly resistant to fatigue.

Also it turns out that it even has a medical term to describe it: Athletic heart syndrome

Athletic heart syndrome (AHS) is a non-pathological condition commonly seen in sports medicine in which the human heart is enlarged, and the resting heart rate is lower than normal.

The athlete's heart is associated with physiological remodeling as a consequence of repetitive cardiac loading.[3] Athlete's heart is common in athletes who routinely exercise more than an hour a day, and occurs primarily in endurance athletes, though it can occasionally arise in heavy weight trainers. The condition is generally considered benign, but may occasionally hide a serious medical condition, or may even be mistaken for one.[4]

Athlete's heart most often does not have any physical symptoms, although an indicator would be a consistently low resting heart rate. Athletes with AHS often do not realize they have the condition unless they undergo specific medical tests, because athlete's heart is a normal, physiological adaptation of the body to the stresses of physical conditioning and aerobic exercise.[5]