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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.”

Students always ask me, what about the proteins? Don’t proteins provide a lot of energy for exercise? Actually, proteins are used only sparingly for fuel during exercise. Here’s why: the proteins you eat are digested and broken down into specific amino acids. Most of these amino acids are used to make new proteins, which build and repair muscles, tissues, skin, and bones; other amino acids help make hormones and messenger chemicals for the body. What amino acids don’t do, however, is power a workout.

The deep involvement of the body’s two energy systems, aerobic and anaerobic, is part of what makes HIIT workouts so unique. Let’s journey a little deeper into these systems.

The Aerobic Energy System: The Long-Term Energy Source

The aerobic energy system is the chief energy production system of the body, and the most efficient one. The term aerobic literally means “with oxygen.” As the name implies, this energy system cannot work unless there is sufficient oxygen available in the muscles during exercise. It takes about 5 minutes to fully turn on the pathways for the aerobic energy system to provide enough ATP for the workout. In other words, your aerobic system needs a brief start-up period before it provides enough ATP for your muscles to complete a sustained HIIT workout.

Your aerobic energy system capacity is primarily dependent on your training status. Perhaps this is best illustrated during the Olympics, when we see world champion athletes accomplishing magnificent endurance feats at the world’s most competitive level. This is a spot-on example of the colossal potential of the human body to adapt to high-quality training. I tell my students that your aerobic energy system is analogous to a large double-decker bus with a hefty gas tank. Modern buses can travel at relatively moderate speeds for great lengths of time. Similarly, via the aerobic energy system, we can tap into our carbohydrate and fat reserves for fuel, and with the help of oxygen your body can exercise for long periods at light, moderate, or hard intensities. During prolonged workouts such as cycling, cross-country skiing, and distance running, your muscles are dependent on the aerobic energy system’s ability to continuously produce ATP. In my exercise science classes we call the aerobic energy system “mitochondrial respiration.”

Let’s break down the term, mitochondrial respiration. First, as you may recall from your earlier reading in the book, mitochondria are small, bean-shaped organelles in your cells. They are the energy factories that completely disassemble fats and carbohydrates, and with the help of oxygen, produce the ATP needed for sustained aerobic exercise (and life). The term “respiration” refers simply to the use of oxygen in the aerobic energy system.

My students often ask me why our bodies don’t exclusively use fats for fuel in mitochondrial respiration. They pose their question like this: we store so much more fat than carbohydrates, so why don’t we use 100% fat for fuel for exercise? Here’s my answer: during exercise, our need for ATP is massive; we need it to keep us moving. And, as discussed above, the source of ATP is fats and carbohydrates. Our cells break down fats for fuel much more slowly than they break down carbohydrates, so, as our need for energy increases during high-intensity workouts such as HIIT, our muscle cells call on carbohydrates to come to the rescue. We are always burning fats during exercise, but as our intensity increases we use quite a bit of carbohydrates too, because they can produce ATP quite fast. The great news is, the more regularly you do HIIT workouts, the faster your body learns how to burn fat to make ATP. So, one of the greatest benefits of regular aerobic exercise is that it trains your muscles to become very efficient fat-burning engines.

The Anaerobic Energy System: The Source of Rapid Energy

Anaerobic literally means “without oxygen.” The anaerobic energy system is activated during activities requiring quick bursts of energy, such as lifting weights, running short races, jumping, and throwing. The anaerobic energy system will also be called on to help the aerobic system during the work intervals in your HIIT workouts. Essentially, you are going to power your challenging HIIT work intervals with your aerobic and anaerobic energy systems working as a team. Together they will give you the ATP fuel you need to exercise.

The Muscles Are the Engines that Do the Work

Finally, we’ll complete the story of how the body powers your HIIT workouts with an overview of muscles. Muscles are of great interest to exercise enthusiasts and scientists (and beachgoers, too). Your muscles attach to the skeleton and contract continuously during exercise. Muscles are powered by ATP, which comes from the aerobic and anaerobic energy systems. Your bones provide the structure for the muscles to do their work, enabling your body to move. Think of muscles as the engines driving your body’s movements and the skeleton as a complex arrangement of levers, fulcrums, and force arms that carry out the movements. As you continue to challenge the muscles of your body, they rapidly learn and adapt to work harder and function better. Regularly doing HIIT workouts is going to lead to a healthier, stronger you. Let’s keep going!