CORE’s Passive Heat Training Load Calculator

CORE has developed a tool enabling you to calculate your passive Heat Training Load based on the details of your session. In the CORE app, tap on the “Heat Training Load” card. Click on the button “Add passive heat training”, complete the form, and add the calculated Heat Training Load to your training data.

You must not use your CORE sensor in any passive heat training environment (i.e. sauna, steam room, hot bath) as there is a likely potential of exceeding the maximum device operating temperature. This can result in a damaged sensor, fire hazard, or skin burns – not to mention inaccurate data.

How is the Passive Heat Training Load calculated?

CORE’s thermal experts created a model to predict the behaviour of core and skin temperatures during passive heating, based on thermal data reported by 28 scientific studies. Based on your manual inputs, the model estimates the heat strain during the session and calculates the Heat Training Load. Some important parameters of the model, including their effect on Heat Training Load, are listed below.

An increase in the following parameters increases Heat Training Load (+):

• Core and skin temperatures at the start of the passive session

• Air/water temperature & humidity

• Session duration

An increase in the following parameters decreases the Heat Training Load (-):

• Number and duration of breaks

• Accelerated cool-down after the session, for example taking a cold plunge or shower

• Your Heat Adaptation Score

Passive heat training examples

The examples below reflect passive Heat Training Loads calculated for the average non-heat adapted CORE user. If you want to estimate possible Passive Heat Training Loads, you can play around with the calculator in your app.

Influence of exercise prior to passive heating

As explained above, post-exercise passive heating is more effective than passive heating alone. For example, starting a sauna session of 30 min (90°C/194°F and 10% relative humidity) from baseline (core temperature 37°C/98.6°F) results in a passive Heat Training Load of 4.3. The same sauna session post-exercise (core temperature 38.3°C/100.9°F) results in a passive Heat Training Load of 7.2.

Hot water vs. sauna bathing

The effectiveness of hot water and sauna sessions largely depends on the conditions. For example, a 30-min passive heating session with initial core temperature 37°C/98.6°F results in a Heat Training Load of 2.1 in all of the following settings:

• Hot water bath at 40°C/104°F
• Sauna at 73°C/163°F and 10% relative humidity
• Sauna at 55°C/131°F and 55% relative humidity
• Steam room at 48°C/118°F and 100% relative humidity

Influence of cool-down method

It is common after a sauna to take a cold shower or plunge. However, be aware that this accelerates the recovery of core and skin temperatures, and therefore slightly reduces the passive Heat Training Load. For example, a post-exercise sauna session of 20 min at 80°C/176°F and 15% relative humidity can result in a passive Heat Training Load of 3.3 when taking a cold shower or plunge and 3.6 for a natural cool-down.

Influence of breaks

Even though breaks cause core and skin temperature to decline, it may be effective to introduce short breaks when it allows you to increase the total exposure time. For example, a session of 2x15 min with a break of 5 min will provide a higher Heat Training Load than a 20-min continuous session. A post-exercise sauna session at 55°C/131°F and 60% relative humidity can result in a Heat Training Load of 4.8 for 2x15 min and 3.2 for 1x20 min.

Sessions tested by scientists

The studies below observed physiological adaptations and/or performance improvements after a passive heat training intervention. We estimated the passive Heat Training Loads of the sessions they implemented. You may want to use this as inspiration for your own training.

Boosting your Heat Adaptation Score: Passive versus active heat training

To gain heat adaptations, you will need to do multiple heat training sessions. Read more about this in the article Boosting Your Heat Adaptation Score. Both passive and active heat training sessions will boost your Heat Adaptation Score, but what is most effective?

There is limited research comparing passive and active heat training directly. The scarce data that is available does not show a notable difference between 5–6 days of active or passive heat training when the thermal stimulus (a combination of heat strain and duration) is similar. Note that, during passive heating without prior exercise, it is difficult to achieve the same thermal stimulus as during active heat training, likely making it less effective. However, when combined with prior exercise, passive heating may offer similar adaptation benefits to active heat training.

Even though post-exercise passive heating seems very effective, we recommend to always add active sessions to your heat training plan. Active heat training in hot and/or humid conditions, ideally similar to the anticipated race environment, helps you fine-tune your pacing and hydration strategies. It also better prepares you for the perceptual strain you may experience during the race.

Therefore, the maximum Heat Training Load you can achieve with passive heating is limited to 8 (instead of 10). Note that a total daily Heat Training Load of 10 can still be achieved when combined with exercise. We strongly discourage a combined Heat Training Load (active + passive) higher than 10, and therefore the total Heat Training Load cannot exceed this value. Excessive heat training can be harmful and requires prolonged recovery, limiting heat adaptation benefits.

References

Ashworth E, Cotter J, Kilding A. Post-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military context. Ergonomics. 2023;66(1):49-60. https://doi.org/10.1080/00140139.2022.2058096.

Heathcote SL, Hassmén P, Zhou S, Stevens CJ. Passive Heating: Reviewing Practical Heat Acclimation Strategies for Endurance Athletes. Front Physiol. 2018;20;9:1851. https://doi.org/10.3389/fphys.2018.01851.

Kissling LS, Akerman AP, Campbell HA, Prout JR, Gibbons TD, Thomas KN, Cotter JD. A crossover control study of three methods of heat acclimation on the magnitude and kinetics of adaptation. Exp Physiol. 2022;107(4):337-349. https://doi.org/10.1113/EP089993.

McIntyre RD, Zurawlew MJ, Oliver SJ, Cox AT, Mee JA, Walsh NP. A comparison of heat acclimation by post-exercise hot water immersion and exercise in the heat. J Sci Med Sport. 2021;24(8):729-734. https://doi.org/10.1016/j.jsams.2021.05.008.

Scoon GS, Hopkins WG, Mayhew S, Cotter JD. Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. J Sci Med Sport. 2007;10(4):259-62. https://doi.org/10.1016/j.jsams.2006.06.009.

Zurawlew MJ, Mee JA, Walsh NP. Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations in Endurance Trained and Recreationally Active Individuals. Front Physiol. 2018;9:1824. https://doi.org/10.3389/fphys.2018.01824.