Determining appropriate exercise intensity is essential. After all, consistently exercising at too high of an intensity can create metabolic damage that results in overtraining, which can keep you from reaching your goals. On the other hand, exercising at too low of an intensity may not burn enough calories for weight loss or provide the necessary overload to improve aerobic capacity. Identifying the most effective training intensity for your needs requires understanding the role that heart rate plays in measuring the physiological changes happening in your body during exercise. Knowing how to measure and use your heart rate is essential if you want to get the best results from your exercise program. Here are a few considerations for how to use heart rate to identify the most appropriate exercise intensity for your needs and goals.

First, invest in a heart-rate monitor. If you are serious about your fitness, wearing a monitor can help ensure that you’re training at the right intensity to reach your goals. The most affordable monitors measure your heart rate only during exercise, while the most expensive models include a GPS system to track your distance covered, measure calories burned during a workout (it’s an estimate determined via an algorithm based on age, weight, gender and heart beat) or sync with your computer to store all of your training information.

There are different types of monitors, as well. Some use a strap around your chest to read the frequency of your heart rate and that information is transmitted to a watch, which allows you to see your heart beat in real time. Another type of monitor is worn on the wrist, eliminating the need for a strap. There is a slightly larger margin of error for these types of monitors, but wearing a strap around the chest can be uncomfortable, so it is nice to have that option. Many commercial cardiovascular machines will sync with your chest unit to show your heart beat on the display of the machine. 

Once you’ve invested in a monitor you can use it to identify the most effective training zone(s) to use for your goals. A training zone is a range of heart beats and represents specific levels of exercise intensity. There are various models of heart-rate training zones that can help you determine the appropriate intensity for your needs. ACE recommends a three-zone model based on identifying the heart rate at specific metabolic markers, namely the first and second ventilatory thresholds (VT).

The first VT (VT₁) is the point at which the body transitions from aerobic metabolism, which uses oxygen and fats to create adenosine triphosphate (ATP)—the chemical that fuels muscle contractions—to anaerobic metabolism. The talk test can be used to identify the heart rate at VT₁ and Zone 1 becomes the training intensity where the body is most efficient at using aerobic metabolism. Wearing a heart-rate monitor allows you to perform steady-state (SS) exercise in Zone 1 at an intensity just below VT₁, which will help you to improve your aerobic capacity and utilize fat as a fuel source at higher exercise intensities. The talk test is easy to perform and can be done on any piece of exercise equipment that allows you to gradually adjust intensity.

The second VT (VT₂) identifies the heart rate at the onset of blood lactate (OBLA) and is often referred to as the lactate threshold. VT2 can be determined using a field test. Zone 2 becomes the heart rates above VT₁, but below VT₂ and is the intensity where your body most effectively uses anaerobic glycolysis—the breakdown of glycogen—to create ATP. 

Zone 3 is at or above the OBLA and is where the body relies primarily on ATP stored in muscle. Two factors limit the ability to train in Zone 3 for any extended period of time. First, there is a limited amount of ATP stored in muscle that is quickly depleted and, second, there is a rapid accumulation of metabolic waste that quickly changes blood acidity. Training in Zone 3 should be followed by a period of time in Zone 1 to allow the removal of metabolic waste and the replenishment of ATP in the muscle cells. 

High-intensity interval training (HIIT) is popular because it can help burn a large amount of calories in a short period of time. HIIT programs feature work intensities at or above OBLA (in Zone 3) with recovery periods below VT₁ (in Zone 1). Even though you are exercising at a lower intensity, you are still burning calories in a Zone 1 active recovery interval because you are not only still exercising, but your body is expending energy as it works to remove waste and produce new ATP. Wearing a heart-rate monitor can help you determine that you are at the necessary exercise intensity to allow for an appropriate recovery after a high-intensity interval. 

Steady-state (SS) aerobic training in Zone 1, SS anaerobic training in Zone 2 or HIIT in Zone 3 produces specific outcomes. In Zone 1, you can sustain low-to-moderate physical activity for an extended period of time. In Zones 2 and 3, the by-product of anaerobic metabolism accumulates quickly, which limits the length of time you can sustain exercise at those intensities. Training in all three zones will help you burn calories, but having the ability to measure your heart rate and knowing how to identify your training zones provides you with a customized workout plan based on how your body responds to exercise. Your specific training goals will determine which zones to use and how long to exercise in each. Keep in mind that exercising hard is only part of the equation. Knowing how to exercise smart by monitoring your training intensity can help you achieve long-lasting results.