system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: interval workouts are often recommended. Why is a 20 minute interval run peaking at say 80% better than running at 70-80% intensity for the full 20 minutes, which will probably result in a greater distance covered? What is the benefit of the short breaks?
gpt
HIIT (High-Intensity Interval Training) takes advantage of how our bodies use energy. When we keep our heart rate high for too long, we begin to use glycogen as our main source of energy. Utilizing the rest period fluctuates your heart rate up and down, so we can continue to use stored fat as our main source of energy. This kind of exercise is super good for burning calories and creating a strong cardiovascular system, but not very good at building muscle mass. 

HIT (High-Intensity Training) is better for overall strength training. However, it is a LOT harder on the body, and a LOT harder to accomplish for most people. It isn't so great for burning calories, and it puts a lot of stress on the cardiovascular system, so if you haven't done your due diligence in working up to that point, it's probably not that good for you.