[ashpelham]

Most good HRM's have the ability for you to input specific information about yourself, such as age, weight, height, maxHR, resting HR, even zones. If all of this information is put in appropriately, I think the numbers they return can be very close to approximate. Still, your HR can vary dramatically, and may have little to do with your level of exertion. For instance, the first time I watched "Speed", my HR was probably quite high a lot of it. But I was sitting on my ass the whole time .