What
happens to your engine at the end of a long car trip? It doesn’t require a
degree in automotive engineering to know that once you’ve reached your destination, your car’s engine stays warm as it gradually
cools to a resting temperature.
Here's a
cool fact: The same thing happens to your body after exercise. 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.
Your
metabolism is how your body converts the nutrients you consume in your diet to
adenosine triphosphate (ATP), the fuel your body uses for muscular activity.
ATP is produced either with oxygen using the aerobic pathways or without 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. Exercise that places a greater demand on the anaerobic
energy pathways during the workout can increase 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:
The body
expends approximately 5 calories of energy (a calorie is the amount of energy
required to heat 1 liter of water 1 degree centigrade) to consume 1 liter of
oxygen. Therefore, increasing the amount of oxygen consumed both during and
after a workout can increase the number of net calories burned.
Strength
training with compound, multijoint 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.
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 ATP is produced by the anaerobic pathways during
high-intensity exercise; once that ATP is 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.
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.
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% Max HR), circuit weight
training (4 sets/8 exercises/15 reps at 50% 1-RM) and heavy resistance exercise
(3 sets/8 exercises at 80-90% 1-RM to exhaustion) were compared, heavy
resistance exercise produced the biggest EPOC.
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 up 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.
AUTHOR
Pete
McCall, MS, CSCS, is an ACE Certified Personal Trainer and long-time player in
the fitness industry. He has been featured as an expert in the Washington Post, The New York Times, Los Angeles Times, Runner's World and Self. He holds a master's degree in exercise science and
health promotion, and several advanced certifications and specializations with
NSCA and NASM.