How hard you should push yourself depends on your level of fitness and your health history. Your doctor can recommend an optimal heart rate target for working out based on these factors.

On average, most people should aim for a target heart rate zone of 50 to 75 percent of their maximum heart rate. Maximum heart rate is computed by subtracting your age from the number 220. So if you are 40, your maximum heart rate would be 180, and your target heart rate zone would be between 90 and 135 beats per minute.

You should wear a digital or analog watch with a second hand or function to check your heart rate during exercise. You can calculate heart rate by placing your fingers at your wrist or neck pulse point and counting the number of beats for fifteen seconds. Multiply that number by four to get your heart rate. The number you get should be within your target zone.

If it's too high, take your intensity down a few notches. If you're exercising below your target zone, pick things up. If you are new to exercise, you should aim for the lower (50 percent) range of your target heart rate. As you become more fit, you can work toward the 75 percent maximum.