Target heart rate—the heart rate range used to determine the desired intensity of an activity—will differ depending on the goal of the workout. You can calculate target heart rate using a percentage of your client’s heart rate maximum (HRmax), which can be predicted by subtracting your client’s age from 220, or by measuring your client’s heart rate while he or she performs a maximum exercise test. You can also calculate target heart rate using the Karvonen method, which takes into account your client’s resting heart rate (RHR). Subtract your client’s RHR from his or her age-predicted HRmax before multiplying the outcome by the desired percentage. Then add the RHR back onto that value. The difference between HRmax and RHR is called heart rate reserve (HRR).
Since RHR will decrease as cardiovascular fitness improves and HRmax can decrease with age, periodically recalculate target heart rate as your clients become more fit (or more sedentary) and get older. Age-predicted HRmax may be off by more than 10 to 15 beats per minute, since all people of the same age do not have the same HRmax. Therefore, it is much more accurate to directly determine HRmax with a maximum exercise test. Use HRmax, but don’t forget to consider subjective factors, such as how the client feels.
When the workout goal is to increase aerobic endurance, target heart rate should be 65 to 80 percent of HRmax (about 55%-70% of HRR). During interval training, which focuses on increasing cardiovascular performance, target heart rate should be greater than 80 percent of HRmax (70% of HRR). Source: http://www.ideafit.com/fitness-library/top-10-most-frequently-asked-questions-in-a-fitness-center-and-their-answers