There are many different philosophies regarding training zones and prescribing heart rate-based training for endurance athletes. The most accurate method to calculate training zones is based on an athlete’s maximum heart rate, VO₂ max, ventilation, and lactic acid levels over the course of a ramp incremental test.
After completing a ramp incremental test the end result is a set of calculated domains unique to the athlete which represents physiological boundaries for different intensity levels. The domains are referred to as the moderate, heavy and severe domains.
Now, most athletes do not undergo specific testing to accurately determine their different domains and instead need to rely on less accurate methods to find their training zones. These different methods include specific testing using a bike trainer, heart rate monitor or other wearable device; using a Rate of Perceived Exertion (RPE) scale; or a talk test.
Best practice is to use a combination of methods to achieve the closest approximation possible.
Methods of Calculating and Estimating Training Zones #
There are many ways of calculating or estimating training zones. Some of the methods include:
- Ramp Incremental Test
- Wearable device fitness testing
- Karvonen formula (maximum & minimum heart rate)
- Rate of Perceived Exhaustion (RPE)
- Talk test
Ramp Incremental Test #
A ramp incremental test is performed in a laboratory setting, and athletes wear a fitted mask to record oxygen intake, CO₂ outtake, and heart rate. The main purpose of the test is to find an individual’s VO2 Max and using that data it is possible to determine different training zones.
This test is considered the gold standard for establishing training zones. It is the most accurate method of calculating different physiological domains and will give very accurate heart rate thresholds and personalized zones athletes can use to guide their training.
The downside with this test is it is expensive and not easily accessible to the average person.
Wearable Device Testing #
There are a lot of tests offered by exercise and wearable device companies that will estimate thresholds and zones for training.
All the big companies (i.e. Polar, Garmin, Suunto, Zwift, etc.) offer their own testing protocols to help determine a user’s VO₂ Max, training zones, and threshold levels.
These tests can be fairly accurate and are useful in helping determine different training zones. A company’s website will sometimes detail how the tests are conducted and what they are measuring.
These tests are good options if you own a bike trainer, heart rate monitor or other wearable device that supports fitness testing and you would like to better estimate and understand your fitness levels.
Karvonen Formula #
The Karvonen formula calculates an individual’s heart rate reserve to establish training zones.
Heart rate reserve (HRR) is the difference between your maximum and resting heart rate, and it is a general measure of fitness that can be used to calculate training zones. The basic idea is, assuming equal maximum heart rate, an individual with a low resting heart rate is going to have different training zones than someone with a relatively high resting heart rate. By using max and minimum heart rate together, you can achieve a good estimate of individualized training zones.
There is a lot of literature pointing towards heart rate reserve as a useful estimate of heart rate zones and intensity levels for athletes, and it’s a very easy way for anyone to establish their own training zones for their workouts.
There is a great resource by Sport Fitness Advisor that breaks down the Karvonen formula. This formula is easy to calculate and applicable to everyone who knows their resting heart rate and a good estimate of maximum heart rate.
As a general rule, training zones and intensity targets are outlined below.
- Zone 1: 55-65% of HRR
- Zone 2: 66-79% of HRR
- Zone 3: 80-89% of HRR
- Zone 4: >90% of HRR
Rate of Perceived Exertion (RPE) #
The purpose of the RPE scale is to use your perception of how hard you are working to gauge your training zone. This is an excellent and reliable way to establish the correct level of intensity for your workouts.
There are two RPE scales:
Borg Scale #
The Borg scale measures RPE on a scale of 6-20, with a 6 being no exertion at all and 20 being maximal effort.
The scale is designed to correlate RPE with heart rate for a given intensity level, and this is done by multiplying your RPE by 10. In practice this would mean if you are exercising at a RPE of 12, your heart rate should be around 120 bpm. This works the other way too, where exercising at a heart rate of 120 bpm should equate to a RPE of 12.
There are 2 major drawbacks of the Borg Scale. First, the scale of 6-20 is not intuitive or easy to remember. Second, for many athletes heart rate and RPE aren’t very closely correlated. For example, someone who has a maximum heart rate of 170 bpm will rate maximal effort intervals a 20/20 on the scale, but they won’t be exercising at 200 bpm. There is a disconnect in that regard.
This is why we prefer to use the more intuitive and easy to understand Modified Borg 10-point scale to measure RPE.
Modified Borg Scale #
The modified Borg scale measures RPE on a scale of 1-10, with a 1 being little to no physical exertion and 10 being maximal effort.
This is the scale NSL coaches uses to prescribe effort levels for all workouts, and a method for athletes to gauge their exertion during exercise to ensure they are training in the appropriate zone.
RPE levels correspond to the Training Zone format we use as follows:
| Zones | RPE |
| Zone 1 | 2-4 |
| Zone 2 | 4-5 |
| Zone 3 | 5-7 |
| Zone 4 | 8-10 |
RPE is one method of estimating training zones without needing to conduct any testing or calculations. If you know your training zones, then RPE is a great way to confirm that your heart rate and intensity level during a workout is appropriate to your perceived exertion.
Talk Test #
The talk test is one of the most basic methods to discern different intensity levels. It’s a little less subjective than RPE.
The talk test works by gauging intensity levels based on how easy it is to hold a conversation. If you are exercising and you can easily hold a normal conversation with no laboured breathing then you are in a very easy intensity zone.
If you can converse but there are short pauses to catch your breath or your breathing is audibly more laboured, then you are in a moderate intensity zone.
At higher intensity, it should be very difficult to speak while exercising. This will range from brief fragmented sentences to only a few spoken words at a time.
A general breakdown below:
- Zone 1 training: individuals should be able to hold a conversation somewhat comfortably somewhere between the moderate intensity level and low intensity level. If you are exercising with another person it should be possible to hold a conversation for the duration of a workout.
- Zone 2 training: individuals should be at the moderate intensity level. They can hold a conversation, but breathing is somewhat labored and you have to pause to catch your breath periodically.
- Zone 3 training: individuals should be able to speak in fragmented sentences. It is not possible to hold a normal conversation and breathing will be laboured.
- Zone 4 training: individuals will only be able to say a few words with very heavy breathing. There is no opportunity to hold a conversation and even saying a few words is very difficult.
- Maximal Sprints: individuals cannot talk at all. This is during maximum efforts where it is almost impossible to say anything and breathing is extremely labored i.e. sprinting.
This blog post does a good job highlighting how the talk test works and how to apply it.
Training Domains #
Physiological domains are marked by major physiological changes in breathing and lactic acid production during exercise.
Training domains are most accurately calculated by completing a Ramp Incremental Test (RIT) in a laboratory setting where individuals wear a gas exchange mask, heart rate monitor and provide lactic acid samples.
After completing a RIT, data is compiled and three different domains will be established. The three domains are: moderate, heavy and severe. These domains are marked by distinct physiological changes at specific intensity levels which correspond to heart rate values and ranges.
Moderate Domain #
The moderate domain is your easy, long endurance zone which can be sustained for hours; this is also what is referred to as zone 1.
There is minimal lactic acid production and an increase in exercise intensity does not result in a significant increase in measured lactic acid levels. The bulk of an athlete’s endurance training is within this domain, and it is the training intensity responsible for building endurance, improving oxygen consumption, and increasing efficiency of slow-twitch muscle fibers.
Heavy Domain #
The heavy domain is a range encompassing zone 2 and zone 3 intensity levels.
The difference between the moderate and heavy domain is an increase in exercise intensity will result in an increase in lactic acid production.
The heavy domain sees a linear increase in lactic acid levels as intensity level increases, and the body exists in an equilibrium where lactic acid levels will remain stable if intensity levels are held constant.
The top half of this domain is where athletes want to be when exercising at a prescribed zone 3 intensity, and it should be possible to maintain this intensity level between 45 minutes to hours.
This domain is good for endurance and can help increase VO₂.
Severe Domain #
The severe domain ranges from the top heavy domain threshold to maximal effort.
The severe domain is an anaerobic zone where lactic acid production is unstable and will continually increase even intensity levels remain constant.
The low end of this domain can be maintained for around 45 minutes, whereas the top end may only be sustained for seconds.
This intensity level is great for improving VO₂ and fast-twitch muscle fibers, both of which contribute to increased pace and maximal force production.
