So now you know your HRmax and  heart rate for the various zones.  But how do you know what your heart rate is at any time - particularly if you are running, climbing steps or rowing?  You need a Heart Rate Monitor.    The consumer heart rate monitor was essentially invented by Polar 15 years ago.  They have progressively become more and more accurate and intelligent.  I don't see how anyone could safely train without one, or why anyone would want to - since they cost about the same as a pair of trainers, last much longer, and are much much better for health.

Using one is pretty easy.  A strap fixes round the chest (you become very used to it), you wear a special watch, and then you go!  The watch wirelessly picks up the heart beats recorded by the chest strap, and then applies some intelligence.   Depending on which watch you buy, you can track your heart rate, which zone you are in, how much fat you have burned, your level of fitness, your HRmax, and your VO2 max rating.  The first two or three items are fantastically useful - and the rest are for fitness or technology enthusiasts (or both, like me).

Here are some heartfelt testimonials to training at reduced intensities by using HRMs.

• A testimonial from a heart rate monitor user
  
• Precision Fat Loss - Heart Rate Monitor Training

Here is some good information about how heart rate monitors work

and Polar's own papers on the subject:

• Exercise and heart rate  
• Scientific development and evaluation of the Polar OwnZone feature 
• Scientific development and evaluation of Polar OwnCal/OwnCals feature