By: Barbell Logic Team
Measuring the caloric cost of an activity is an attractive prospect for many people. In the constant pursuit of fitness, popular culture has ascribed value to certain activities based on how many calories you burn while doing them. Exactly this prospect—”Burn More Calories with This One Weird Trick”—has launched fitness fads and has been used to justify endurance or conditioning types of exercise above strength training as more useful.
But if we only ask “how many calories will I burn” while lifting, we would probably be disappointed. Strength training doesn’t burn a lot of calories, it uses them. And it’s important to know why.
Energy Balance: How Many Calories Does Lifting Burn?
Everything you do requires energy, and your body is in a constant state of regulating energy intake and balancing that with energy expenditure. This process of energy homeostasis is important in all forms of physical exercise. You convert and expend energy to do work and that energy has to be replaced. But that’s not the whole story. Certain activities also spur physical changes and these changes can require additional energy. In particular, strength training produces energy needs beyond those required just to move the barbell around in your squat rack. Because energy comes from calories and calories come from food, we tend to become hyperfocused on how many calories an activity burns. When, really, when we are training for strength, we should consider not how we are burning calories but how our body is using that energy to get stronger.
Energy in Exercise
Energy in bioenergetics is the capacity to do work. “Work” in this case is the Newtonian definition—the product of a given force acting through a given distance: Work = Force x Distance. Energy, then, is the capacity to produce force through muscular contractions. Every interaction we have with our environment, from getting out of bed to lifting weights, requires energy.
How we use energy is complicated, but there are two important points to keep in mind. The first is that energy is never created. It only converts from one form to another. This is true as far as our understanding of the Universe goes, and it is true in the microcosm of the human body. When you take in energy (measured as the calories you consume in food and drink), store it (in various forms such as fat and glycogen), and convert it to useable forms (like ATP) the energy is undergoing various changes. The second thing to keep in mind is that these changes are inefficient. They produce excess amounts of unusable energy in the form of heat.
Scientists, then, can measure the amount of energy used at rest or during different activities in a few different ways. Direct measurement of heat production, the amount of physical work produced, and through an indirect measure of O2 consumption. When you are at complete rest, heat produced within your body will be the same as that leaving your body. So if we were to rig up an accurate measure of the heat leaving your body while you lie down, unmoving, we could get an accurate estimate of the amount of energy you are expending during rest.
When you are active, however, say, during exercise, some of your energy expenditure becomes movement—work. During exercise, direct heat measurements are not accurate measures of energy expenditure because not all the heat you produce is lost during exercise (your body temperature rises), and you have other changes occurring such as sweat causing you to lose body mass. Instead, it’s possible to indirectly estimate energy expenditure by measuring the amount of work you do or in the measurement of O2 consumption, which shows the product of the aerobic oxidation process.
O2 consumption, measuring the V02 during exercise, can give an accurate picture of the metabolic processes occurring during that activity. There is a certain amount of metabolic C02 produced from fuel oxidation during exercise that can help show the transition from food to ready energy in the form of ATP. This method is plus a measure of heat production and a measure of the amount of work produced is often used to estimate the number of calories you burn during different activities. They produce tables like the one below:
Estimates of Caloric Expenditures*
Notably, however, indirect measurements of caloric expenditure will not give an accurate representation of the metabolic processes when the body uses other sources of energy, energy conversions that do not use oxygen. As an unfairly brief overview, aerobic oxidation occurs at relatively low intensities, high volume events that we think of as “endurance” activities. These are repeated muscle contractions that you can continue to perform for several minutes or even several hours. But the body has immediate stores of energy for short-burst, high-intensity bouts of work in the form of stored ATP and can produce energy without oxidation through anaerobic glycolysis. When these other energy sources are being used, the indirect method of measuring O2 consumption does not give an accurate picture of the metabolic process that goes into the amount of work being performed.
Activities where the ability to do work relies heavily on anaerobic processes—such as the short duration but high-intensity event of a heavy set of five squats—are more difficult to quantify, which may be why weightlifting type activities often do not appear on these types of tables. The anaerobic portion of the exercise can only be estimated, creating significant room for error. For that reason, all estimates of strength training (at least that performed at high-intensities) contain some estimation of the caloric cost of the work performed. (As opposed to direct laboratory measurement of the exergy expended during aerobic activity.) (Reis, Júnior, Zajac, and Oliveira, “Energy Cost of Resistance Exercises: an Update,” J Hum Kinet. 2011 Sep; 29A: 33–39 (Published online 2011 Oct 4. doi: 10.2478/v10078-011-0056-3.)
Measuring the caloric cost of an activity is an attractive prospect for many people. In the constant pursuit of fitness, popular culture has ascribed value to certain activities based on how many calories you burn while doing them. Exactly this prospect—”Burn More Calories with This One Weird Trick”—has launched fitness fads and has been used to justify endurance or conditioning types of exercise above strength training as more useful, usually because it will “blast your fat away.”
Feeling the Burn
Narrowly focusing on calorie burn, however, ignores some important points about fitness, sport, or exercise in general that are worth reviewing. The first is that intentional change to your body requires intentional adaptations to physical stress and/or nutritional intake. Rarely is the definable goal of an individual tied primarily to a negative energy balance, a calorie burn that exceeds expenditure. This may be a shorter term goal when a person needs to lose body fat, but even then negative energy expenditure is going to be combined with intentional nutritional intake and the right kind of activity. Because the goal is (usually) not just to burn tissue but to improve body composition.
Beyond the relatively narrow adaptation to negative energy expenditure, few functional adaptations come from “calorie burn” alone. If you are engaged in an endurance sport, you will get better at oxidization—you will actually burn fewer calories at the same intensity level as you adapt. If you are engaged in HIIT-type training, you will likely improve your general conditioning by stressing and adapting your aerobic and anaerobic energy pathways and the specific muscular endurance required for that type of training. These adaptations are fine, but if your goal is simply the calorie burn, you are missing the point of the activity from an adaptive perspective.
No physical adaptations are more generally useful than strength. But if we only ask “how many calories will I burn” while lifting, we would probably be disappointed. A regular novice training day only has about 31 repetitions, or 7 sets of work, each lasting about a minute; lots of rest between sets; and low-intensity warmups. Compared to jogging for ninety minutes, the caloric expenditure during lifting is minimal. But that ignores what happens after training.
The problem from a “calorie burn” perspective is this: Strength training doesn’t burn a lot of calories, it uses them. The functional response to strength training is an anabolic process, a building up process. And this process requires a lot of energy.
First, you have to replace the energy stores you used during the high-intensity sets, your stored ATP and glycogen. These are replenished through the aerobic oxidative process during something known as excess post-exercise oxygen consumption (EPOC). EPOC is part of the restorative process, to bring you back to a resting state. There is a particular tie between a long-lasting EPOC and high-intensity anaerobic exercise:
“High intensity activities result in a greater activation of the sympathetic nervous system(56), which in turn results in a post-exercise increase in lipid metabolism in response to changes in the substrate predominantly used for energy production . . . [O]ther aspects to be considered are glycogen resynthesis, tissue injury and the effects that lead to muscle hypertrophy as a result of resistance training, which may also cause greater energy expenditure.” (de Mello Meirelles and Gomes, “Acute effects of resistance exercise on energy expenditure: revisiting the impact of the training variables,” Rev Bras Med Esporte Vol. 10, No 2 (2004).)
Though only estimates, it’s possible that over half the caloric cost of strength training occurs in EPOC. (Scott, Luchini, Knausenberger, Steitz, “Total Energy Costs—Aerobic and Anaerobic, Exercise and Recovery—of Five Resistance Exercises,” Central European Journal of Sport Sciences and Medicine, Vol. 8, No. 4: 53–59 (2014).)
We have been writing a lot about the body’s need to return to “normal,” a balanced state called homeostasis. This concept of energy balance at its core is exactly that. When you engage in any activity that is significant enough to disrupt homeostasis, your body wants to restore the balance. Some forms of exercise use energy at a rate that the body actually keeps up with pretty well. This is why a conditioned runner can run for hours on end without crashing. These types of exercises are often lauded for their energy expenditure and the number of calories you will burn doing them. But that energy is mostly going into heat and movement. When you strength train, you burn calories, then you use your body’s energy to rest, recover, and build. How you can optimally fuel that process, give your body the right building blocks, and be precise with your energy intake is another rabbit-hole (a deep one, called “nutrition”), but we should start with the idea that our goal when lifting isn’t primarily to burn calories. Our goal is to cause and fuel change—stronger muscles and stronger, more useful bodies.