Commonly, when looking for the calories expended in a given exercise or activity, such as lifting weights, you would turn to a calorie listing of common activities and the calories burned for an allotted period, and you would see things like, "kayaking for one hour burns 350 calories." The problem with these tables is that they make certain assumption about the person doing the activity. A better method to finding the calories used in weight lifting is to use an individualized calculation process.
The MET
The modern, scientific method of figuring calories burned during a workout involves first calculating your resting metabolic rate (RMR), and then multiplying it by the MET, or metabolic equivalent task, of the activity in question. METs are numbers like 1.3, 6.5, or 10. When multiplied by your hourly RMR, they give you an individual estimate of the number of calories you use in that exercise.
Weight Lifting METs
You can find a list of the METs for hundreds of common activities at the University of South Carolina School of Public Health website, where the research was done to produce the most recent "Compendium of Physical Activities" in 2000. These are the values that the Compendium gives for weight lifting at different levels of effort:
Weight lifting, power lifting, or body building, vigorous effort, MET = 6.0
Weight lifting, power lifting, or body building, light or moderate effort, MET = 3.0
Calculate your RMR
The next step in the process is to get your resting metabolic rate, which is the number of calories you use when completely at rest, as if sitting quietly. Without going to a testing site to have your RMR tested by a process called "indirect calorimetry." the next best thing is to figure your RMR manually, or use an online RMR calculator.
One way to calculate it yourself is with the John J. Cunningham equation, a simple, yet accurate equation that reads: RMR = 370 + 21.6 x FFM, where FFM is fat-free mass, also known as lean body mass (LBM). For example, assume you are 94 kilograms and have a body fat percentage of 8 percent. Your fat-free mass in kilograms would be 94 x (1 - 0.08) = 86.5 kilograms, and your RMR would be 370 + 21.6 x 86.5 = 1,868. This is the number of calories you burn at rest in 24 hours. Your hourly RMR would simply be 1,868 / 24 = 77.8 calories.
Weight Lifting Calories Burned
So now, to determine the number of calories that you use when lifting weights, you would take your hourly RMR and multiply it by the MET value above. This gives you the hourly calorie burning rate for weight lifting. For example, suppose you were pumping iron for 90 minutes with vigorous effort. Your hourly calorie burn would be your hourly RMR x MET = 77.8 x 6.0 = 467. An hour and a half at that rate would be 700.5 calories.
Final Words
In considering this process, you should realize that all of these concepts, equations, and tables are based on statistical averages and confidence intervals, so they don't apply to everyone. If you suffer from a chronic illness, autoimmune disorder or metabolic disease, there may be some difference in your results. For the majority, however, this method can be an excellent means for figuring the calories expended in weight lifting or any other activity.
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