When you exercise, your body must burn some sort of fuel to provide the energy for your activity. The fuel source for this activity is the same, regardless of the exercises you perform. However, the method used to produce this fuel varies with the type and intensity of your chosen activity.
Adenosine Triphosphate
Whatever the intensity of your exercise, the energy for your activity comes from a substance called adenosine triphosphate, or ATP. Every cell in your body contains this substance, which is produced inside cellular structures called mitochondria. To create energy, Georgia State University explains, your mitochondria split ATP molecules apart; this process leaves behind a residual molecule called adenosine diphosphate, or ADP. To create more energy and sustain your physical activity, your body breaks down the foods in your diet, then uses these breakdown products to convert ADP molecules into more ATP molecules. Depending on the type of exercise you perform, your body can sustain this conversion process with or without the need for oxygen.
ATP Production Pathways
Your body can produce ATP through any one of three separate conversion reactions, or pathways. In the first conversion pathway, your body gets ATP by breaking down a naturally occurring compound in your muscles called creatine phosphate. In the second pathway, your body gets ATP by burning a carbohydrate-based fuel source called glycogen without the presence of oxygen. In the third pathway, your body uses oxygen to burn glycogen, fat and protein. Because the first two conversion pathways don't require oxygen, they're called anaerobic pathways. Because the third pathway requires oxygen, it's called the aerobic pathway.
Pathways During Exercise
Regardless of the intensity of your exercise, you initially rely on the breakdown of creatine phosphate to get energy, according to the Sports Fitness Advisor website. However, your body's supplies of this substance are limited, and you will burn through them in a matter of seconds. To continue your activity, your body will turn either to the anaerobic burning of glycogen or the aerobic burning of glycogen, fat and protein. In reality, your body draws on both of these energy pathways simultaneously; the proportion of energy coming from each pathway depends on the type and intensity of your activity.
High-Intensity Examples
If you perform a short-duration, high-intensity exercise like sprinting, you will mostly rely on the anaerobic burning of glycogen to sustain ATP production, the Topend Sports website explains. If you participate in a moderate-duration, high-intensity activity like middle-distance running, you will rely on both anaerobic glycogen burning and aerobic burning of glycogen, fat and protein. If you participate in a lengthy, high-intensity activity like long-distance running, you will rely primarily on your aerobic energy pathway. Consult your doctor and a certified fitness instructor for more information on exercise intensity and the ways your body produces ATP.
