According to the American Heart Association, heart rates help you evaluate initial fitness levels, measure exercise intensity and monitor your progress.
The intensity of a workout can be measured as a percentage of your maximum heart rate. To calculate your max heart rate (MHR), subtract your age from 220 as follows:
MHR= 220 ¿ age
For example, the max heart rate for the average 40-year-old is:
220 ¿ 40 = 180
Maximum heart rate generally declines with age, from about 220 beats per minute in childhood to about 160 beats per minute at age 60. This fall in heart rate decreases by approximately one beat per minute per year. There is no strong evidence to suggest that training influences the decline in maximum heart rate. Even so, people of the same age may have quite different maximum heart rates, so it's more accurate to calculate your MHR using a stress test rather than using an age-related formula. Check with your gym or doctor about taking a stress test. (Source: Heart-Rate Zones)