Heart rate is widely accepted as a good method for measuring intensity during running, swimming, cycling, and other aerobic activities. Exercise that doesn’t raise your heart rate to a certain level and keep it there for 20 minutes won’t contribute significantly to cardiovascular fitness.
The heart rate you should maintain is called your target heart rate. There are several ways of arriving at this figure. One of the simplest is: maximum heart rate (220 - age) x 70%. Thus, the target heart rate for a 40 year-old would be 126.
Some methods for figuring the target rate take individual differences into consideration. Here is one of them:
Resting heart rate should be determined by taking your pulse after sitting quietly for five minutes. When checking heart rate during a workout, take your pulse within five seconds after interrupting exercise because it starts to go down once you stop moving. Count pulse for 10 seconds and multiply by six to get the per-minute rate.