system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does building muscle work?

ELI5: How does building muscle work? 
How does this differ if your exercise is higher reps at lower weight vs. Tons of weight with a 1 rep max?
gpt
Your muscles are made of 2 types of fibers: fast twitch and slow twitch. Working out pulls some of the fibers apart a little, which makes your body add more fibers to repair them. More fiber means more strength or endurance. 

Low weight high reps is slow twitch fiber. They don’t fatigue as fast, but aren’t as strong. It builds endurance. High weight low reps is fast twitch fiber. They fatigue much faster. It builds strength. 

If your workout is running, then long distance is slow twitch, and sprinting is fast twitch.