7 Interesting Facts About What Happens To Your Body After An Intense Workout
Before we consider what goes on in our bodies during and after a workout, let’s first compare it to starting and running a car engine.
Ever noticed how warm your car engine is by the end of a long road trip and how long it takes to cool back down to normal temperature?
Here’s a fun fact: The same thing happens to your body after an intense workout. Similar to how a car’s engine remains warm after being turned off, once a workout is over and you’re back in your daily routine, your body’s metabolism can continue to burn more calories then when at complete rest. This physiological effect is called excess post-exercise oxygen consumption, or EPOC. Also known as oxygen debt, EPOC is the amount of oxygen required to restore your body to its normal, resting level of metabolic function (called homeostasis). It also explains how your body can continue to burn calories long after you’ve finished your workout.
Without getting too technical, your metabolism is your body’s mechanism of converting the nutrients you consume in your diet to energy in the form of adenosine triphosphate (ATP) molecules.
ATP is the primary source of fuel that your body uses for muscular activity. ATP is produced either with oxygen using the aerobic pathways or in the absence of oxygen relying on the anaerobic pathways. When you first start to exercise, your body uses the anaerobic energy pathways and stored ATP to fuel that activity. A proper warm-up is important because it can take about five to eight minutes to be able to efficiently use aerobic metabolism to produce the ATP necessary to sustain physical activity. Once a steady-state of oxygen consumption is achieved, the aerobic energy pathways are able to provide most of the ATP needed for the workout. As intensity of exercise increases, a greater demand is placed on the need for oxygen and if the body cannot sustain enough oxygen delivery to the muscles, the anaerobic energy pathways kick back in, increasing the need for oxygen after the workout, thereby enhancing the EPOC effect.
Here are seven things you should know about EPOC and how it can help you achieve optimal levels of calorie burning from your workouts:
1. During the immediate post-exercise recovery period, oxygen is used for the following functions:
- Production of ATP to replace the ATP used during the workout
- Resynthesis of muscle glycogen from lactate
- Restore oxygen levels in venous blood, skeletal muscle blood and myoglobin
- Work with protein for the repair of muscle tissue damaged tissue during the workout
- Restore body temperature to resting levels
2. Exercise that consumes more oxygen burns more calories.
The body expends approximately 5 calories of energy to consume 1 litre of oxygen. Therefore, increasing the amount of oxygen consumed both during and after a workout, can increase the amount of net calories burned.
3. Circuit training and heavy resistance training with short rest intervals require ATP from the anaerobic pathways, leading to a significant EPOC effect.
Strength training with compound, multi-joint weightlifting exercises or doing a weightlifting circuit that alternates between upper- and lower-body movements places a greater demand on the involved muscles for ATP from the anaerobic pathways. Increased need for anaerobic ATP also creates a greater demand on the aerobic system to replenish that ATP during the rest intervals and the post-exercise recovery process. Heavy training loads or shorter recovery intervals increase the demand on the anaerobic energy pathways during exercise, which yields a greater EPOC effect during the post-exercise recovery period.
4. High-intensity interval training (HIIT) is the most effective way to stimulate the EPOC effect.
The body is most efficient at producing ATP through aerobic metabolism; however, at higher intensities when energy is needed immediately, the anaerobic pathways can provide the necessary ATP much more quickly. This is why we can only sustain high-intensity activity for a brief period of time—we simply run out of energy. HIIT works because during high-intensity exercise ATP is produced by the anaerobic pathways; once that ATP exhausted, it is necessary to allow ATP to be replenished. The rest interval or active-recovery period during an anaerobic workout allows aerobic metabolism to produce and replace ATP in the involved muscles. The oxygen deficit is the difference between the volume of oxygen consumed during exercise and the amount that would be consumed if energy demands were met through only the aerobic energy pathway.
5. EPOC is influenced by the intensity, not the duration of exercise.
Higher intensities require ATP from anaerobic pathways. If the ATP required to exercise at a particular intensity was not obtained aerobically, it must come from the anaerobic pathways. During EPOC, the body uses oxygen to restore muscle glycogen and rebuild muscle proteins damaged during exercise. Even after a HIIT workout is over, the body will continue to use the aerobic energy pathway to replace the ATP consumed during the workout, thus enhancing the EPOC effect.
6. Research has shown that resistance training can provide a greater EPOC effect than running at a steady speed.
In an extensive review of the research literature on EPOC, Bersheim and Bahr (2003) concluded that “studies in which similar estimated energy cost or similar exercising VO2 have been used to equate continuous aerobic exercise and intermittent resistance exercise, have indicated that resistance exercise produces a greater EPOC response.” For example, one study found that when aerobic cycling (40 minutes at 80 percent Max HR), circuit weight training (4 sets/8 exercises/15 reps at 50 percent 1-RM) and heavy resistance exercise (3 sets/8 exercises at 80-90 percent 1-RM to exhaustion) were compared, heavy resistance exercise produced the biggest EPOC.
7. The EPOC effect from a HIIT or high-intensity strength-training workout can add 6 to 15 percent of the total energy cost of the exercise session.
High-intensity workouts require more energy from the anaerobic pathways and can generate a greater EPOC effect, leading to extended post-exercise energy expenditure. Heavy weight training and HIIT workouts appear to be superior to steady-state running or lower-intensity circuit training in creating EPOC (LaForgia, Withers and Gore, 2006).
Admittedly there is some debate about the significance of the EPOC effect for the average exercise participant because the high-intensity exercise required for EPOC can be extremely challenging. However, if you want results and are ready for the challenge, increasing the intensity of your workouts by using heavier weights, shorter rest intervals or high-intensity cardio intervals may be worth the effort. While HIIT or heavy resistance training is effective and beneficial, remember to allow at least 48 hours of recovery time between high-intensity exercise sessions and try to limit yourself to no more than three strenuous workouts per week. If you do start increasing the intensity of your workouts to boost EPOC, consider adding these recovery strategies.
Bersheim, E. and Bahr, R. (2003). Effect of exercise intensity, duration and mode on post-exercise oxygen consumption. Sports Medicine, 33, 14, 1037-1060
LaForgia, J., Withers, R. and Gore, C. (2006). Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. Journal of Sport Sciences, 24, 12, 1247-1264.