Is my Polar FT7 underestimating calories burned, or have I been doing it wrong?

thatonerunnergirl

I recently got a Polar FT7 heart rate monitor to help track as I lose 15 pounds. I used to have a nicer model Polar watch (it was a nice, GPS compatible one with a "fit test") but unfortunately it stopped working and I couldn't afford to splurge for the more expensive one this time. Anyway, today I did a pretty high-intensity 75 minute workout, for which my heart rate average was 170 and maximum was 184. The calories burned over that time, however, was only listed as 671. This seems a little low to me (it was nearly 400 under the machine's reading, but I know the machine way overestimates). When I plugged the heart rate averages into an online calculator, I got that I should have burned 884 (I weigh 135 pounds).

On my old watch, there was an option for a "fit test" which would change your VO2 max settings. To my knowledge, the higher your VO2 max, the more energy you can expend at a lower heart rate--is that correct? As I am a runner mine is quite high and I would burn over 700 on my hour-long runs, according to the old watch. This newer model does not have that option, and I am wondering if that is the problem.

Anyway, if anyone could provide any insight into this it would be greatly appreciated. I know I am on the shorter and lighter side of people here, but I see all these crazy high burns and it puzzles me how I could be working at an average of 85% of my max heart rate for that long and not even burn 10 cal/min. (This was on the arc trainer.) Is there some other setting I need to change?

Thanks.