Calculating your target heart-rate zone is done by subtracting your age from 220. This will be your maximum heart rate. Then multiplying that number by 0.7. If you would like a little higher end, then use 0.85. Here is an example. For a 55-year-old the maximum heart rate would be 165 BPM, and the range during exercise is 116 and 140 beats per minute. Of course these numbers are approximate and it is important to note that individual targets might vary.
Aggressive exercise might yield greater health benefits than moderate workouts. But working out too vigorously isn't a great idea. A heart rate monitor can help you stay in your safety zone while helping you reach your target heart rate.
There are many different types of heart monitors including wrist and chest strap models. The Rhythm+ is accurate like the chest strap monitors, but it is much more comfortable like the wrist versions. The key to a quality heart rate monitor is the consistent reading of the user's heart-rate and the Rhythm+ delivers.