Heart Rate. It's not all about one number there many factors that come into play, heat, diet, stress to name a few. I get lots of questions about it., "I have a low heart rate is that bad ?" Well it could be or it could you've increaseed your heart stroke volume through training, or the other most common "I find it easy to hit my maximum heart rate" Give me a couch potato and a turbo and get his heart rate up quickly I might even get him to hit his maximum. You get my point.
Don't get me wrong it's great place to start but like most things there's more to it than just a number on screen.