A heart rate monitor is only going to give an accurate estimate of calories burned during cardio exercise, not strength training. So I would not recommend using one for strength exercises.
The tracker doesn't know how intense your strength training routine is, so it's difficult to compare 20 minutes of this kind of workout to 20 minutes of another. If you're doing a really challenging strength routine, I'd agree that you'll burn more. It's just very difficult to know how much more.
In the grand scheme of things, it probably won't have a big impact on your weight loss progress one way or the other, so I would just consider any extra calories burned to be a bonus to your weekly goals.
The calorie values assigned to strength training are fairly small because while you are using large muscle groups it is a very short amount of time working. I am not saying that I agree with it but if you really want to compare your best bet would be to get a heart rate monitor.
I've been a bit baffled by the results on the exercise tracker lately, as apparently 20 minutes of 'yogilates' (basically stretching) burns more calories than 20 minutes of strengthening exercises such as squats, lunges and press ups, whilst using dumbbells. Plus, the information on how many calories cycling burns is inconsistent. Is it their mistake or mine?
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