Aerobic exercise is great for your body, but it's easy to over exert yourself. It's also possible that your workout may not be as intense as it needs to be. You can check your heart rate to determine whether you're working too hard or not hard enough. To estimate your aerobic training zone, subtract your age from 220 (two hundred twenty). If you take that answer and multiply it by 0.6 (zero-point-six), you'll get the lower end of your aerobic heart rate. This means that your heart should beat at least this many times per minute during your workout. You should also make sure that you're not working too hard by keeping your heart rate under a certain level. To determine your maximum aerobic heart rate, subtract your age from 220 and multiply the answer by 0.8 (zero-point-eight). If your heart rate exceeds this number while you're working out, slow down until your heart rate is within the proper range. This range may vary according to your fitness level. If you don't currently exercise, your training zone will likely be lower. If you already exercise four or more times a week, it will likely be higher. To make the most of your workout and to exercise safely evaluate your heart rate at least once during your workout. If you're above the maximum or below the minimum, adjust your workout and take your pulse again in a few minutes. Remember to listen to your body and stop if you feel pain. You should always consult a doctor before beginning any exercise program.