To get the most health benefits from aerobic activity, you should exercise at a level strenuous enough to raise your heart rate to your target zone. Your target heart rate zone is 50 to 75 percent of your maximum heart rate (the fastest your heart can beat). To find your target zone, look for the category closest to your age in the chart below and read across the line. For example, if you are 35 years old, your target heart rate zone is 93-138 beats per minute.

     To see if you are exercising within your target heart rate zone, count the number of pulse beats at your wrist or neck for 15 seconds, then multiply by four to get the beats per minute. Your heart should be beating within your target heart rate zone. If your heart is beating faster than your target heart rate, you are exercising too hard and should slow down. If your heart is beating slower than your target heart rate, you should exercise a little harder.
     When you begin your exercise program, aim for the lower part of your target zone (50 percent). As you get into better shape, slowly build up to the higher part of your target zone (75 percent). If exercising within your target zone seems too hard, exercise at a pace that is comfortable for you. You will find that, with time, you will feel more comfortable exercising and can slowly increase to your target zone.