The difference between steady state exercise and high intensity interval exercise often boils down, in many arguments between the two, to EPOC. In this article, we will discuss what EPOC is, what causes it to occur, and the physiology of its occurrence.
What is EPOC?
EPOC, or excess post-exercise oxygen consumption, is exactly as the long hand explains – an increase in oxygen consumption right after ending an exercise or stressful bout . It is also synonymously called “after burn” .
What causes EPOC?
This term is most often attributed to intense, short burst exercise bouts like high intensity interval training (HIIT) as the consumption for oxygen continues to be high disproportionate to the current exercise situation (post-exercise resting). We can experience this rather poignantly by simply moving quickly for a short period of time (30 seconds) until breathing increases in speed to attenuate this increased movement, then abruptly stop; then, we will notice our breathing does not immediately downregulate back to resting levels, but continues to be disproportionally high as it slowly tapers back down to resting levels. This tail end of oxygen use, breathing, is EPOC.
EPOC can be caused by exercise, danger, or any situation in which stress is put on the body that leads to sympathetic nervous system excitation (typically attributed to exercise, however), but the effect of EPOC will depend on the duration focus (>50 minutes at >70%VO2 intensity), intensity focus (>6 minutes at 105%VO2 intensity), and just overall difficulty of the stress – typically, the more stress, the more EPOC (especially short burst, high intensity) .
How long does EPOC last?
The length of EPOC duration is, of course, dependent on intensity and duration, but can last up to 48 hours (although any noticeable labored breathing would only last a few minutes, at most) . Interestingly, trained individuals tend to have a shorter EPOC than untrained, likely due to adaptations; this is a key reason why high intensity interval training can be a great practice in high intensity sports to establish someone’s conditioning in relation to EPOC – faster recovery, faster a person can perform near peak levels again (think football or basketball, for example) .
How many calories used during EPOC?
This is a difficult topic to answer, but we can give some idea of how much EPOC increases caloric expenditure beyond that expended during exercise, because we know that for every liter of oxygen, we expend about 5 calories . We can say that the higher the intensity of exercise, the higher the EPOC, and therefore the higher the calories used post exercise; one study looked at two groups that matched use of 500 calories at a particular intensity and duration (one group at low intensity, aka 50%VO2 and the other at vigorous intensity, aka 75%VO2) and found those in the vigorous group expended an additional 45 calories in post exercise recovery compared to only 24 calories in the low intensity group . Typically, EPOC is higher in men than women at similar intensity levels .
Bottom line, if you can measure increased oxygen use over basal/resting levels, you can find a good estimate of how much energy we use in EPOC.
Understanding the Physiology
For us to understand why EPOC occurs, we need to understand some basic metabolism and how our body operates.
First, at rest, our body uses, largely, fat for energy, and it is able to use said fat molecules for energy if, and only if, oxygen is present . Obviously, we need energy to function, to be alive, so we need fat and we need oxygen for us to be alive. However, as our demand for energy increases, due to increased movement (exercise, for example), our body’s need for oxygen and fat increases. However, eventually, our body reaches a point at which it cannot produce enough energy fast enough using fat and oxygen if that movement is at an intensity that is not sustainable via lipid (fat) metabolism; this prompts a shift to a different metabolism system using no oxygen and no fat, but instead uses carbohydrates . The drawback of this new metabolic system is that, due to its lack of oxygen use, it is not sustainable beyond a few minutes before the body forces us to slow down our movement to a point that oxygen and fat metabolism can take over again. However, although we may not realize it, going from a standstill to even something as small as walking requires an even quicker energy delivery method, as we cannot wait for our oxygen dependent metabolism to offer us enough energy to begin movement, and this is where a “jumpstart” metabolism system takes over to bridge the gap between consistent energy delivery and immediate energy delivery – the name of that jumpstart metabolism is the phosphocreatine (PCr) system .
This PCr system gives instantaneous energy to allow the lipid metabolism to catch up and set itself in a position to continue movement, or if that movement is too abrupt and/or intense, the carbohydrate metabolism can take over for a few minutes, and then lipid metabolism (with oxygen) can finish the job when fatigue sets in .
Okay, so that is a basic understanding of metabolism, but how does this relate to EPOC?
Well, EPOC exists solely because of our metabolic situation at any given time. So, imagine you are walking and suddenly break out into a sprint; your body is switching seamlessly between all three of these metabolic systems to supply the exact amount of energy for you to perform your sprint. So, let us break this down, simply.
You are walking, and your body is under minimal stress, so it is easily consuming enough oxygen to use, largely, lipid metabolism. Then, you suddenly break out into a sprint, and now your body is under a larger amount of stress and because of the immediate nature of your movement, it quickly consumes energy from your PCr system. Then, you sprint for 30 seconds to a minute, so your body runs out of energy via the PCr system and is required to use carbohydrates, without oxygen dependence, to supply enough energy. This whole time, you are breathing faster and faster to consume maximal amounts of oxygen, because your lipid metabolism is trying as hard as it can to supply as much energy as it can muster. Eventually, you tire, your metabolism can no longer keep up, so you slow down or stop, but your breathing is still heavy and forced – this is EPOC.
EPOC is necessary in this situation for several reasons. First, understand that when we break out into a sprint, our body is immediately put in an energy deficit (not calorically, talking ATP) as the PCr system is depleted, and as we continue through the carbohydrate system, we are impeded, eventually, by the production of lactic acid . So, this deficit in oxygen is indicative of the energy needs of the body’s muscle cells as energy is depleted intracellularly, hence the continued gasping for air although exercise/movement has stopped (or slowed significantly); we continue to breathe heavily to introduce as much oxygen into the cells to allow the cells to revert back to lipid metabolism and replenish the ATP (energy) stores in the PCr system and dissociate lactic acid produced from non-oxygen dependent carbohydrate metabolism to resynthesize glycogen .
However, while this is one of the main reasons for oxygen debt and continued, intense breathing, it is not the sole reason. Also, as evidenced by heat production during and post-exercise, energy is continuously being used as enzymatic activity is still heightened to get the cells of the body back to a homeostatic position – as well as the synthesis of further ATP molecules in the mitochondria of these muscle cells .
Now, while these are the two primary reasons for EPOC, the mechanisms of EPOC are still debated and the likelihood that more than these two explanations play a role is almost certain, although as of yet unknown.
All in all, excess post-exercise oxygen consumption (EPOC) is the chief reason for our intense breathing immediately after exercise as the body attempts to “pay” its oxygen debt and attenuate the heat release during and after exercise. The body undergoes EPOC anywhere from 5 minutes to several hours, although any conscious appearance will happen within the first few minutes’ post exercise. EPOC is more severe as intensity of exercise increases, and is responsible for 20 – 50 calories of calories used (roughly), depending on exercise duration, intensity, among other factors.
Writer: Nicolas Verhoeven
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