system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the average human heart beats 72 beats a minute. How can athletes have a resting heart rate of 40?

Please explain why/how does an athlete have to be calmer within?
gpt
Athletes have very high aerobic capacity, because they regularly work out at a very high exertion level.  As a result, their very fit hearts can pump all the blood their resting body needs at only 40 BPM.

It's not a calm/Zen thing, it's a fitness thing.