So I hit the gym this week and for the first time with an accurate HRM and in a controlled environment, tested myself a little.
I set the machine (a Precor C846i upright) to its watts control program, turned it up to 320 and spun at 115-120rpm for 25 minutes, then turned it up to 400W (the maximum setting) for about three to test my MaxHR, which started to bouncing off the peg at 208. For the 25 minutes of 320W my HR was floating between 180 and 190, mostly depending on how well I controlled my breathing, but it stayed below 185 for the most part. It felt as though this was a sustainable pace and didn't cause any extraordinary discomfort.
I'm 28, so the HR is a touch high for certain, but how accurate are these machines for gauging wattage?