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Can you trust cardio machine calorie counters?

By Jennifer Sage On September 17, 2012 Under General Fitness and Health, Heart Rate Training

Another of my regular resources for articles and information is Angry Trainer Fitness. The blogger, Alfonso, analyzes fitness claims and discusses whether they are “fact or fiction.” While I don’t agree with him 100% of the time (he is more forgiving about Soul Cycle than I am, though he does question the effectiveness of their methods), he usually is quite accurate in his assessments, and I really appreciate anyone who is a fitness “myth buster.” His goal is to highlight and promote the more realistic and effective methods and expectations for fitness (based on science) instead of the fluff that is so prevalent in the fitness world.

In this article he discusses whether the calorie counters on cardio machines or heart rate monitors are accurate. A 110 lb client of his claimed she had burned “740 calories in a 45-minute spin class” and because of that, loves that particular class. He, like me, knows that her numbers were likely very inaccurate.

Alfonso’s answer is in line with what I tell students and instructors. You must realize that calories burned on a machine or HR monitor are not accurate—the machine has no way of knowing your size, weight, fitness level, or other relevant information that affects how many calories you burn. It is only going by gender and heart rate, and it is basing absolute heart rate on your age (220–age = max HR), so before it even registers a single beat the device is starting from a place of inaccuracy.

Let’s look at a scenario. A 110 lb (50 kg) 40-year-old woman takes a 45 min spin class and averages a heart rate of 150 bpm during class. A 180 lb (82 kg) man of the same age takes the same class and averages the same 150 bpm during class. The computer says she burned 750 calories and he burned 800 calories (it would most likely adjust the estimate a little higher because he is a male).

How could it possibly know what effort they put out? How could it know how fit each one is, how much muscle mass each one has (which affects caloric consumption), their training history, what they did prior to the workout (coffee? stress? medicines?), or how tired they are? How would it know what their lactate threshold is? Suppose for one of them, that represents a higher average than he or she normally has because of a stressful meeting at work, they were running late, missed a subway, and ran into class at the very last minute. In this case, the average heart rate would most likely be higher from the very beginning, regardless of who you are.

So can you see how unreliable calorie counters are that are based on heart rate, because of the fact that heart rate is so variable?

However, there is a way to use the calorie counter, as Alfonso suggested, as a general comparison from one workout to the next—as long as you are comparing the calorie counter on the same piece of equipment, the same computer on a bike, or your same heart rate monitor each time. While it is possible that a higher average heart rate, and the resulting higher calorie reading, can be caused by what I described above (stress, caffeine, fatigue, etc.) and may not mean a harder workout, as a general indicator, if you used to burn 400 so-called calories (according to the machine or to your HR monitor) in a class, but now you’re burning 500, then you are in general working harder. If you are working harder, then in general you are burning more calories.

We all know those people who come to class, or to the gym, who don’t put out much effort. They move their legs, they pretend to push hard in intervals, they sweat a little, but they hesitate (or are afraid) to push hard. It is these types of people who this method would work for. They are the ones who need to push their heart rates a little higher. If we can get them to “burn” 100 calories more on average for a specific period of time, then indeed, they are probably working harder and will hve a greater benefit.

But for your stronger students, who are absolutely fixated with how many calories they burn at every moment, who already work out a lot, the fight for a few extra calories burned in Bobby’s class over Paula’s class is a little bit silly. There is just no way to know for sure.

Actually…let me rephrase that, there IS a way to be accurate with your calories burned, but it’s not going to happen on most cardio machines or a heart rate monitor. The answer is to measure your work with a power meter. For most accurate results, you want a real one, one that measures actual power, such as on the CycleOps bikes. The power meters that estimate power are the next best options, which you can find on the Keiser M3 or the Schwinn power bike. For more information on real caloric burn, listen to the discussion I had with Danielle Foster about kjoules at the Indoor Cycling Association.

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