High Intensity Interval Training (HIIT) is one of the most popular forms of fitness today. These workouts are designed to exert a maximum amount of effort in a relatively short period of time. This can be as little as seconds to a couple minutes. While conducting my personal training sessions, I use 30 second exercise intervals as a baseline and sets of 8 - 16 based on muscle groups targeted. So, why is HIIT so popular?