Disclaimer: Heat Adaptation requires stressing the body's thermoregulatory system. All information below is meant as general guidance. Please consult a medical professional and coach before undertaking any heat training. If at any point you experience any symptoms of heat-related illness, stop immediately and consult a medical professional.
It is well-known that frequent exposure to heat strain induces heat adaptations. You can induce heat strain by exercising in warm and/or humid weather, wearing warm clothing, or some combination of both (also see How to do a Heat Session).
To indicate how much your training session contributes to your heat adaptation, CORE has developed the Heat Training Load. It is based on how much time you've spent with an elevated Heat Strain Index, and ranges from 0 (no contribution to heat adaptation) to 10 (maximum contribution to heat adaptation).
Note that the calculation takes into account exercise data only, so, make sure to always pair your heart rate monitor! Without heart rate data, your training session will not contribute to your Heat Training Load. If multiple training sessions are done in a day, the total Heat Training Load for the day will be shown.
CORE’s Heat Zones can be used to guide heat training:
Heat Zone 1 (HSI 0.0–0.9): does not increase Heat Training Load.
Heat Zone 2 (HSI 1.0–2.9): increases Heat Training Load.
Heat Zone 3 (HSI 3.0–6.9): most effective to increase Heat Training Load.
Heat Zone 4 (HSI 7+): Not recommended for heat training. Heat strain is too high and may introduce health risks.
Frequent Heat Training Loads boost Heat Adaptation Score
The daily Heat Training Loads are used to calculate the Heat Adaptation Score. On days with a Heat Training Load higher than 2, your Heat Adaptation Score will increase. The higher the load, the larger the heat adaptation benefits. A couple of days with no Heat Training Load will cause your score to decline. Examples are shown in the article Boosting Your Heat Adaptation Score.
Planning your Heat Training Load
Heat Training Load depends on Heat Strain Index and duration. Note that you can reach a given Heat Training Load in many ways, depending on the type of training. During a typical heat training session, you aim to increase HSI in approximately 30 minutes, after which you maintain it for a given amount of time.
The table below shows the approximate HTL you can expect when you maintain a given HSI for a given duration. It assumes you take 20–40 minutes to reach this HSI, so each HTL value also includes the time you spent at lower HSI values. For example, to train for 1 hour at HSI 4.5 (yielding a HTL of 8.8), your total training duration should be approximately 1.5 hours (see example 1a).
Note that with a HSI below 2, HTL will never reach high values, no matter the duration. This is because heat strain is too low to adequately boost physiological adaptations.
Real-life Heat Training Load Examples
Disclaimer: the following are examples of different heat training scenarios meant for illustrative purposes. Please consult a medical professional before undertaking heat training.
You can reach a given Heat Training Load in many different ways, depending on the type of training. These real-life Heat Training Load examples are based on training data from our CORE-users.
Example 1a: Classic heat training with HSI 4.5 for 60 minutes
In scientific studies, the most common heat training includes at least 60 minutes at a HSI of at least 4.5. Including warm-up, total training duration is 90 minutes. This will yield a HTL of 8.7.
Example 1b: Classic heat training with HSI 4.5 for 75 minutes
Extending the time at HSI 4.5 to 75 minutes will yield a HTL of 10.
Example 2: Very intense heat training on the indoor bike trainer
Long and intense heat training session on the indoor bike trainer, while wearing extra clothing. Most of the time was spent in Heat Zone 3 (1.5 h), resulting in a Heat Training Load of 10.
Example 3: Long indoor training session with heat block at the end
Long training session on the indoor bike trainer, while wearing extra clothing. First part of the training was done at an easy to moderate pace. In the last hour, exercise intensity was increased to elevate heat strain and boost Heat Training Load to an 8.8.
Example 4: 1–hour heat training on the indoor bike trainer
Heat training session of approximately 1 hour on the indoor bike trainer, while wearing extra clothing. Most of the time was spent in Heat Zone 3 (44 min), resulting in a Heat Training Load of 6.1.
Example 5: Extremely long outdoor endurance training
This 8.5-hour outdoor training session at an ambient temperature of 18 °C resulted in a Heat Training Load of 4.7. Note that this CORE-user spent time at a HSI above 2 (i.e., upper half of Heat Zone 2), which is required to adequately elevate Heat Training Load.
Example 6: Interval running session outdoors
1.5-hour running training outdoors, including 1 hour of high-intensity interval training. With one hour spent in Heat Zone 2, this resulted in a Heat Training Load of 2.4.