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As you launch forward into the many HIIT workouts in this book, I really want you to understand how your body remarkably responds and adapts to these workouts. Even though I am an exercise physiologist, I am still totally amazed by the human body, particularly during exercise. As a result, I wish to share with you a glimpse of my world, my fellow HIIT enthusiasts (or soon-to-be enthusiasts!)—the world that studies what happens to the body when you exercise. In this short chapter, I hope you become just as enthralled with your body’s proficiency as I am.

Introducing ATP, the Fuel for Life and Exercise

To power your exercise and daily life activities, your body relies for energy on a chemical compound called “adenosine triphosphate,” or just ATP (discussed briefly in the previous chapter). ATP is produced when your body’s cells disassemble carbohydrates and fats from the foods you eat. To disassemble fat, our body must use quite a bit of oxygen, which is where we get the name “aerobic energy system.” In contrast, your cells can disassemble carbohydrates with or without oxygen present. When the cells do this without oxygen, we call it the “anaerobic energy system.”