When you exercise you stimulate your metabolism to burn more calories. Your body temperate, heart rate, and breathing rate all increase and can be measured to determine what level of intensity you are working at. Once you complete your bout of exercise your body begins to return to homeostasis. Depending on what you did during your workout, this process can take up to 48 hours. During this time your body burns calories at a higher rate. This “after burn” effect is known as excess post-exercise oxygen consumption, or EPOC.
The amount of calories burned during EPOC is determined by the intensity of your workout, and more specifically the amount of oxygen debt you create. Imagine you start sprinting as fast as you can for a ¼ mile. Once you stop you will be breathing heavily, you will be sweating, and your heart will be racing. Think about how long it would take to return your breathing and heart rate, and body temperature back to normal. Much longer than if you walked, or jogged the same distance. Your body is hard at work trying to get back to homeostasis even after you stop exercising.
This phenomenon has a short and a long-term effect on the amount of calories you burn. Intensity is most accurately measured in VO2, or the volume of oxygen consumed by exercise. Exercising at a rate higher than the body’s ability to use oxygen to burn energy creates oxygen debt. The debt must be “repaid” once you stop exercising, and oxygen will continue to be consumed at a greater rate. You can measure this by analyzing the ratio of CO2 to O2 in a person’s breath, as well as heart rate, body temperature, and breathing rate. The area under the curve of the fast component of EPOC is typically proportional to the area above the curve for oxygen debt. The slow component is related to the steady decline in body temperature and other homeostatic processes and can last up to 48 hours post exercise. Simply put the higher the intensity, the longer it takes to get back to normal, and the longer the amount of time you are burning excess calories.
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