Using hrm to measure calories during Insanity - accuracy

I do the pure cardio session quite often and I'm wondering if my hrm is underestimating the calorie burn. I find myself much hungrier after compared to running although running appears to burn more calories in same timeframe.

I get about a 370cal burn during this session.

Because insanity isn't steady state cardio but certainly demanding physically on top of the cardio involved, is it possible that hrm's might underestimate in this case?

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