system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When lifting weights why does muscle grow the best with heavyweights in a rep range of 4-6?
gpt
It has nothing to do with the reps, muscles can't count. It has to do with the amount of stress you put on the muscle and for how long. 

Best rep range for muscle growth is considered 8-12 reps though. The reason for that is that you can still use quite heavy weight and at the same time keep the muscle under tension for a decent time which uses most fibers in the muscle and therefore producing most overall results. 

4-6 is considered more strength training than muscle building.