Before discussing the various exercise guidelines in the following chapters, here is an overview of how your body makes fuel for exercise. Your body uses the CHO, fats, and proteins you eat to make a chemical called adenosine triphosphate (ATP). You need ATP to contract your muscles during exercise. ATP can be made two ways. The first makes ATP without using oxygen and is called the anaerobic energy system. The second requires oxygen to make ATP and is called the aerobic energy system. Both of these systems are required during most activities but, depending on the duration and intensity of the activity, there is a greater reliance on one system over the other. Exercises lasting less than 5 minutes rely most on the anaerobic energy system, while exercises lasting more than 5 minutes rely most on the aerobic energy system.
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