The “Afterburn Effect”: Excess Post-exercise Oxygen Consumption (EPOC)

Laying on the ground in a pool of my own sweat, grasping for air like I just completed an Ironman triathlon. I’m probably being semi-dramatic, but I had just finished a 30-minute bike for total calories, which might as well have been a 100-mile run through the desert. As I’m slowly able to peel my weak, overheated body off the ground, I take one last glance at the screen I had just been peering at for what felt like three days. 481 calories. I felt good about that, but the more I got to thinking about it, did I really accomplish that much? I mean 481 calories, basically I burned off enough calories to eat a couple hot dogs, or a king size candy bar… THAT is how hard I have to work just to burn 481 calories?! I think we’ve all felt that way at one time or another, well I’m here to explain the process behind the “afterburn effect”. Otherwise know as excess post-exercise oxygen consumption, or EPOC.

To understand how EPOC works, you first need to understand what homeostasis means. Homeostasis is the tendency of an organism or cell to regulate its internal conditions, such as the chemical composition of its body fluids, to maintain health and functioning, regardless of outside conditions. As you can imagine certain exercise techniques, such as CrossFit, or Olympic lifting take the body completely away from homeostasis. The heart rate elevates, the body temperature changes, the ph-levels in your blood change. So, to fight that, the body expends energy to return itself to a resting state. It’s counter-intuitive to think the body would expend energy to slow your heart rate down, or lower your body temperature, but that’s exactly what happens, and that’s where EPOC comes in. To expend the energy to return your body to homeostasis it needs increased oxygen uptake.

To reiterate, after cardiovascular exercise or weight training, the body continues to need oxygen at a higher rate than before the exercise began. This sustained oxygen consumption is known as excess post-exercise oxygen consumption (EPOC). This has also been referred to as an “oxygen debt” (1). So, you just got off the bike, you’ve burned 481 calories, but that’s just the start of your caloric expenditure. Now your body needs to get itself back to neutral, let’s look at the steps it takes…

1.       Your body needs to replenish its energy systems, without getting into too high of detail it’ll need to replenish its creatine or ATP which is used for high intensity immediate energy (think 10 second max effort bike). It also needs to replenish muscle glycogen which is a stored form of glucose (sugar), also used for energy during your workout.

2.       Next, it needs to re-oxygenate the blood and restore circulatory hormones. During exercise, large amounts of oxygen are used to break down food substrates (protein, carbs, fats) for energy. Therefore, the body continues to expend energy after exercise to re-oxygenate the blood. In addition, in the post-exercise period, the body restores the levels of circulatory hormones, which increased during exercise, to normal.

3.       The next, and probably first thing you physically notice after exercise is your core body temperature returning to normal. As energy is expended in the body and muscles are burning fuel it’s no secret that your body temperature rises, the pace of that rise can be increased due to things like dehydration and hormone imbalance. Much like the energy expended when your muscles are used, there’s also energy expended when your body temperature is returned to homeostasis.

4.       Lastly, energy is greatly expended as your body returns its heart rate and breathing back to neutral. Sometimes the heart rate may take hours to return to its normal resting rate, based on the amount and intensity of exercise you’ve participated in (2).

What does that mean for me?

Because the body continues to expend energy after exercise, EPOC plays a supplemental role to an exercise program in weight management. Currently, researchers are interested in the effect different forms of exercise have on EPOC. The evidence suggests that a high-intensity, intermittent-type of training (interval training) has a more marked effect on EPOC (3). Also, it appears that resistance training produces greater EPOC responses than aerobic exercise (4). The research suggests that high-intensity resistance exercise disturbs the body’s homeostasis to a greater degree than aerobic exercise. That doesn’t mean that both don’t have a positive impact on EPOC, just that one is greater than the other. The underlying devices that cause the higher EPOC observed in resistance exercise include elevated blood lactate, and an increase in circulating anabolic hormones (4).

Inspecting the data from a few different studies, it appears that EPOC accounts for post-exercise expenditure of 51 to 127 (4) kilocalories. Since a pound of fat is equal to 3,500 kilocalories, the effect of EPOC on weight control must be regarded in terms of a cumulative effect over time. So, as you can see, the impact is moderate, but it’s surely worth being considered as an additional factor.

The body is certainly an amazing mechanism, and I hope this helps you understand how the body stays in overdrive by consuming higher amounts of oxygen long after you finish your workout. It’s also worth considering that you’ll burn more calories post workout if your routine includes high intensity interval training or resistance training than if you hop on the treadmill or go for a long moderate bike. The moral of the story, the bike screen was wrong… I can eat THREE hot dogs after my bike ride J .

Sources

1.       Bahr, R. & Sejersted, O. M. 1991. Effect of intensity of exercise on excess post-exercise oxygen consumption. Metabolism, 40, (8), 836-41.

2.       Melby, C.L. et al. 1993. Effects of acute resistance exercise on post-exercise energy expenditure and resting metabolic rate. Journal of Applied Physiology, 75, 1847-53.

3.       Haltom, R.W. et al. 1999. Circuit weight training and its effects on excess postexercise oxygen consumption. Medicine & Science in Sports & Exercise, 31, 1613-8.

4.       Burleson, M.A. et al. 1998. Effect of weight training exercise and treadmill exercise on elevated post-exercise oxygen consumption. Medicine & Science in Sports & Exercise, 30, 518-22.