This is an excerpt from Heart Rate Training by Roy T. Benson & Declan Connolly.
Heart rate monitor capacities have advanced dramatically in the last several years. They have improved in function, appearance, reliability, and accuracy. The new generation of wrist-reading monitors has proven reliable for the most part, and so much more practical than the chest strap systems. Modern monitors provide basic information on calorie expenditure, oxygen consumption, time in target zones, average and maximum heart rates achieved during the workout, and time in heart rate recovery zones, as well as offering audible signals, 24-hour heart rate measurements, and a whole lot more. All this information can be easily recorded and stored on your smartphone for later analysis, allowing you to fine-tune and individualize your program to a greater degree than ever before. This info can be downloaded and stored on your computer as well. You can also compare your heart rate response from day to day. If these workouts are at the same speed, you will be able to see your training heart rate response over time (i.e., Your heart rate should decrease when working at the same speed as you get fitter). The recordable and downloadable features allow you to see your daily data on a spreadsheet. When viewing your changes over time on this single spreadsheet, the data can greatly reinforce your determination to train smartly.
Data on calories expended, which is indirectly calculated from heart rate, is a helpful feature because it expands the use of the monitor to people who are less performance oriented and more interested in issues such as health improvement, weight loss, and cardiac rehabilitation. Often people will have a distinct caloric expenditure target per day or week that translates into weight or fat loss. For example, it is common to use the basic value that you must burn 3,500 calories to lose one pound (0.5 kg) of fat. Therefore, burning 500 additional calories per day through exercise equates to roughly one pound of fat loss per week.
For athletes, the 24-hour monitoring option provides an abundance of new information they can use to fine-tune their programs and monitor their intensity levels. This fairly convenient system tracks not only exercise heart rates but also recovery rates. This provides vital information related to adaptation and recovery. By tracking the heart rate over 24 hours, athletes get thousands of data points that are downloadable to a computer to create a graph or spreadsheet. These data can reveal not only the slightest signs of illness, overtraining, and fatigue, but also improvement in fitness levels. Having this data can allow an athlete to compare morning resting heart rates over time and potentially even detect early signs of overtraining or illness before it progresses too far.