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When I use a Concept2 indoor rower, I also start a Workout on my Apple Watch. Both devices have a heart rate monitor, and both are measuring my calories. Yet there is a wide difference between the two, and I wonder why it is so large. (I understand that Apple Watch's measurement is a rough approximation, but is there more to it?)

Here are three workouts I did today:

Workout                Rower     Watch                            % diff
20 min steady / HIIT   259 cal   144 cal active / 173 cal total   80%  /50%
35 min HIIT            448 cal   267 cal active / 320 cal total   68% / 40%
10 min low intensity   111 cal   65 cal active / 80 cal total     71% / 39%

My thinking is that the rower is the ultimate authority on calories -- it is an ergometer, designed to accurately measure work, which means energy, which means calories.

So, is the Apple Watch simply wildly inaccurate, or are the two devices measuring different things under the umbrella of calories?

  • Your body isn't 100% efficient. You don't just burn exactly the energy put into rowing, which is why this is not that simple. Please pay attention when doing a workout the next time: your heart will pump faster, you will breathe differently, your body will get warmer and then there is the heightened metabolism in general - you can't just say measure the output. This is why your argument for the rower isn't correct. – Raditz_35 Sep 30 at 10:36
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(...) it is an ergometer, designed to accurately measure work, which means energy, which means calories.

Trying to estimate the calories burned from the effective work done is remarkably inaccurate. When walking or running, the effective work being done is zero, because the resulting horizontal force is zero. You can still get a rough estimate for some exercises like cycling by considering that the human body is roughly 20% efficient, but you can't extrapolate this result for other types of exercises. To get a more reliable estimate, people usually calculate the calories burned based on pre-measured MET values for a particular exercise and adjust the result based on the subject's sex, age, height, and weight.

Another complication is that the machine might be giving you 3 different kinds of calories burned: only the extra calories from the exercise; the total including your BMR; or the total including the afterburn. I'm not sure how the Apple Watch works, but it's likely that it takes into account your heart rate (rest, max, and everything during the exercise) and a bunch of personal information. It might take a few days to learn your rest and max HRs.

As to which one is more accurate, if your watch is set up correctly, I'd probably trust it more than the rower. But the difference is too big. It might be that both are slightly off. Try different types of exercises to see if the watch is consistently giving strange results. There are a bunch of calories calculators online for you to compare the results with.

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@Wood gave a good answer, but I disagree with the conclusion: I'd trust the machine over the watch. It simply has more information, such as the resistance level and the amplitude of motion.

I doubt that the machine uses your HR in its estimates. This is because your HR is not particularly useful in calorie estimation: a weaker guy may do the same exercise, and his HR will go up more, but that doesn't mean he's burning more calories.

The watch, on the other hand, has to use HR (and an accelerometer) as a crude measure of intensity.

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