Heart Rate Zones

There are five heart rate zones used to determine the type and intensity of a given workout. They are referred to by number with one being very low intensity and 5 being very high intensity. Working out with your heart rate in a particular zone will give you specific benefits that vary from zone to zone. The zones are based on an individual's age and their current level of training. The easiest way to calculate the zones is to subtract your age from the number 220. This gives you a fair approximation of your heart rate max number. From there you can calculate zone five as 90 to 100% of your max. Zone four is 80 to 90% and so on.
As you run, you should keep a eye on what zone you are working in so you can control what type of workout you are getting. Here is an explanation of the zones:

Zone 1: Very Light. 50% to 60% of HRMax. All you need is an easy walk to get into this zone and the benefits are mostly recovery.

Zone 2: Light. 60% to 70% of HRMax. Depending on your age and fitness level, this may be like a brisk walk, a hike with a backpack, or a very easy jog. This is where low-intensity cardio workouts tend to take your heart rate. At this level you will develop endurance and you will train your body to utilize the fat burning process more efficiently.

Zone 3: Moderate. 70% to 80% of HRMax. Working out in heart rate zone 3 is especially effective for improving the efficiency of blood circulation in the heart and skeletal muscles. This is the zone in which that pesky lactic acid starts building up in your bloodstream. Training in this HR zone will make moderate efforts easier and improve your efficiency.

Zone 4: Hard. 80% to 90% of HRMax. Heart rate zone 4 is where the going gets tough. You’ll be breathing hard and working aerobically. If you train at this intensity, you’ll improve your speed endurance. Your body will get better at using carbohydrates for energy and you’ll be able to withstand higher levels of lactic acid in your blood for longer.

Zone 5: Maximum. 80% to 90% of HRMax. Heart rate zone 5 is your maximal effort. Your heart and your blood and respiratory system will be working at their maximal capacity. Lactic acid will build up in your blood and after a few minutes you won’t be able to continue at this intensity. If you’re just starting out or have only been training for some time, you probably won’t have to train at this intensity. If you’re a professional athlete, look into incorporating interval training into your training plan for peak performance.

Here is a table of heart zones for athletes aged 12 through 18:

Heart Rate Zones by Age