Heart rate? Age? Weight? How your stats factor in.
You knock out 45 minutes on the treadmill and the screen reads 480 calories burned. You crank out a solid one-hour spin class and check your fitness tracker: 670 calories. You check your phone after a fast-paced morning of running errands and your steps and energy expenditure app tells you you’ve burned 307, just from moving around to get things done.
We have a ton of devices and apps at our disposal to tell us how much we’re burning regardless of which activity we do. Burning calories means your body is turning the food you’ve consumed into usable fuel. We use a significant number of calories just to perform basic bodily functions–like breathing, digestion and growing cells–as well as to do more physically demanding tasks, like running, lifting, swimming and taking the stairs. The number of calories we need to meet our needs varies: your age, sex, weight, overall health and activity level will change your calorie needs and also impact how efficiently you use energy. As a very general rule, the number of calories you take in daily should not exceed the number of calories you burn.
Understanding what you’re burning during a workout can be a helpful tool if you’re interested in balancing your caloric intake with your caloric output, or if you’re looking for a way to assess how much your workouts are taking out of you. While you can use a formula to measure this for yourself, a device may be a more accurate way to know just what you’re putting out.
But how do these devices–the calorie burn meter on cardio machines or the fitness tracker you wear on your wrist – know how much you’re burning? Is this even the best data set to look at if you’re trying to boost your fitness or drop some weight?
Here’s what you need to know to get the most of your workouts and the information your devices provide.
What is your BMR?
Your resting or basal metabolic rate (BMR) is the amount of energy your body requires for its most basic functioning; basically, if you were laid up in bed, semi-conscious, the amount of calories you’d expend just staying alive is your BMR.
On top of this passive calorie burn you do without much thought, you also expend energy moving around: think spin class, barre workout, running after your kiddos or walking the dog.
Your body is unique in how it uses energy during activity
While the amount of energy we expend to survive remains pretty consistent, there are a few factors that impact how much you burn when you’re in motion (those extra, non-survival activities).
1. Body composition
Your body composition is a key component: the more lean muscle mass you have, the more you will burn at rest, as fat cells do not consume energy in the same way efficient way muscle cells do. (A pound of muscle burns an additional 1.5 calories daily; multiply this by your body weight and it’s a significant increase in burn across 24 hours.)
2. Body size and weight
Your body size impacts how much you burn as well: where two people weigh the same, the person with the larger frame is likely to burn more calories doing the same activity; however, a smaller person with more muscle mass may burn more (see above). Ultimately, the highest burn will come from a higher body weight made of a higher lean muscle mass percentage (the more muscle, the more you weigh, and the more you burn because of your active muscle cells and the act of moving more mass to complete a task.)
With age comes a slower metabolism, meaning it takes more time or intensity to reach the same calorie burn doing the same activity compared to when you were younger.
How you exercise also impacts your overall burn
Any amount of movement will burn calories, but how much and how efficiently depends a lot on how intense your workout is and its total duration.
Aerobic and endurance exercise
When your cardiovascular system is working aerobically, it’s burning steadily and efficiently (think 60-70% of your max effort, at a pace you can maintain for at least 30 minutes). Endurance training primarily involves the aerobic energy pathway to convert stored fat to energy, which requires a large amount of oxygen. Your cells are receiving oxygen throughout the exercise and caloric burn rate is pretty regular.
This type of exercise – think jogging, leisurely cycling or dance – is effective for calorie burn but may require a longer duration to meet a specific calorie-burn goal; luckily, because it’s lower intensity, this can be easier as you can go longer without fatiguing.
In these more anaerobic workouts, you’re doing short, fast, intense bursts of activity. Interval training combines short, high-intensity bursts of speed or power with periods of recovery built into one workout. Higher intensity interval requires the body to use both aerobic and anaerobic metabolism (glycogen and lactic acid) to help generate enough energy to fuel intense efforts.
Typically, a HIIT workout will be shorter in duration but higher in intensity than a low intensity, steady-state workout; basically, the difference between doing 15 stair sprints in a stadium versus 30 laps of the track at a more moderate pace.
Both of these types of exercise are valuable and burn calories: they draw on your system in different ways, however, so being adept at both is beneficial. To keep your body guessing (and efficient), it can be a good idea to aim for a combination of weight training, endurance work and hard interval sets across a week.
It’s not cardio, but you still burn a ton of calories lifting or doing bodyweight strength work, especially if you incorporate sets for speed and power. Additionally, you’re building muscle mass, which ultimately boosts how efficiently your body processes calories.
Fitness trackers take some of these factors into account when you work out…
The device you wear on your wrist will factor in all of these things – your body and the intensity and type of exercise you do – to give you a read of your calorie burn. According to fitbit, their devices take into account your BMR (those calories your body burns to maintain vital functions). When you set up your tracker account, you enter your age, height and weight: these are all factored into a BMR typical of your stats. In addition to this baseline burn, your tracker will estimate – again, using your vital statistics – how much you burn during a workout. You enter ‘run’ and it calculates an average burn per minute of exercise you performed, based on the average performance of that activity of someone of your age and gender.
… so do treadmills, kind of
When you hop on the treadmill and are prompted to enter your age, gender and weight, this is part of the calorie-burn experience: the machine uses this data to generate an idea of how much you should burn. Cardio machines use standard formulas similar to those of fitness trackers, but with limited information and no details pertaining to your body composition, fitness level or size, the number is at best a ballpark.
The formulas used across fitness brands vary, but many are based on the generic Compendium of Physical Activities, a guide that gives the average burn (relative to sitting still) for a variety of activities. While good as a baseline, it does not take into account exercisers’ personal details.
Another factor that a treadmill cannot account for, but nonetheless impacts your calorie burn is your form: the more efficient you are with your stride and your arm swing, the less you will burn, even if you’re working at the same intensity as a newer runner. In this situation, being more fit means burning less, as the extra burn for less-experienced runners comes from more ancillary motion, like bouncing up and down, not necessarily voluntary motion. The burn is higher but the run is less functional.
The limitations of tracker and treadmill calorie counts
While having an outline of your caloric expenditure workout to workout, the drawback to using your tracker or the readings on a machine are just estimates: without highly specialized, personalized formulae, a device can’t truly give you a perfect reading of what you’ve burned.
A study at Stanford University assessed different devices to check their accuracy. The results showed that the most accurate of them was off by an average of 27 percent and the least accurate by 93 percent.
Instead of using the number as a means to justify eating an additional meal every day, consider the calorie count a basic indicator of how hard you worked. You can use this to measure your progress and intensity across days and weeks. To get an even more personalized read of your intensity, gauge your workout using a heart rate monitor or a scale system for perceived exertion. If you’re really keen on knowing your caloric burn, you can use metrics like heart rate and body composition to get a clearer read.
Overall, the most useful indicator of how hard you’re working – and thus, your caloric burn – is how you feel during the workout. Push hard, take breaks and hydrate well to keep your heart rate and your endurance up. Overall, the more you move, the harder you work and the more efficiently you perform an exercise, the more you’ll burn… all things your tracker will have a tough time quantifying.