Heart rate monitors can be a great tool. For example, we get many emails from our athletes when the weather is hot and they can't hit the correct paces in workouts. On days like those, we recommend that they measure their effort rather than be a slave to the watch.
Here are the ranges we recommend for heart rate training:
* Easy Runs: 65-79% of max heart rate (that's a big range. Stay higher or lower depending on how you are recovering from workouts or what races you have coming up.)
* Marathon Pace: 80-89% of max heart rate
* Longer Intervals or Threshold/Tempo: 88-92% of max heart rate
* 10K: 92.5-97% of max heart rate
* Short Intervals: 97.5-100% of max heart rate
To determine your max heart rate, wear your heart rate monitor for a short interval session or race. At the end of the race or workout, look at your heart rate monitor. The number you see is your max heart rate. Once you have the max figured out, you can work from there.
I like to mix things up - don't use the heart rate monitor for everything or the watch for everything. Some days you just have to let your body be your guide.