One of the problems with counting calories for a long duration activity is that the standard convention for counting calories is to include your underlying metabolism as well as the exercise itself. For a 30 minute workout, this is no big deal. But for an activity that lasts several hours, the double count can be substantial.
I'm not sure quite what happens to your heart rate at altitude, but it may well get quicker in trying to deliver oxygen to your muscles in that thinner air. So while your HRM may be accurate in counting heart beats, the calorie calculations may be off as the underlying algorithm is likely assuming a sea level atmosphere.
When using an online calculator for something like hiking, it is also worth remembering that it assumes continuous activity. So a 4 hour trip might only have 2.5-3 hours active hiking after allowing for rest breaks, navigation checks, etc.
I think that mapmyhike.com allows for elevation gains in its calorie calculations.
One useful rule of thumb is that each vertical foot gained is about equivalent in effort to 8 horizontal feet. So if you walked 6 miles and gained 5300 feet, that is equivalent to walking 14 miles. You would also need to allow for the weight of your pack, so something like www.caloriesperhour.com/index_burn.php may be better than Spark for this method.
So perhaps use several different methods, and see what they come up with, and perhaps average them, or discard the outlier(s) and use the methods that come up with similar figures.
I'm curious if anyone has input about using a heart rate monitor to calculate calories for hiking. I'm hiking my first 14er tomorrow and want to calculate the calories burned, I put it in the SP tracker and got a really high number that I am not sure can be correct. Do you guys think a HRM would be a better calculator for caloric expenditure for my little adventure?
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