system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is the “second breath” phenomenon that runners sometimes experience?

Is it real or just a placebo effect? And if it’s real, what exactly is happening in your body at that point?
gpt
All of the technical stuff gets complicated but basically the drop in energy is you’re body running out of immediately available fuel sources. Once they are completely depleted your body resorts to braking down glycogen you have stored in your muscles. Once the glycogen is broken down into glucose, your body can utilize it and thus you feel that “second wind”