Olav Aleksander Bu, world-renowned sports scientist and coach of Kristian Blummenfelt and Gustav Iden, is pulling back the curtain to give the CORE community an inside look at his heat training protocols and strategies. Kristian and Gustav have been implementing heat training for many years and the protocol below is designed as final prep ahead of big races like the 2025 IRONMAN World Championships in Nice.
Disclaimer: We do not accept responsibility for how individuals apply the principles of heat training or for any potential consequences arising from its use. Heat training can pose serious risks if not approached with caution. This documentation is intended solely for inspiration. See bottom of page for additional disclaimers.
Developed and written by Olav Aleksander Bu
Some Tips Before Starting
- Keep it simple. Some heat stimulus is better than none or too much. Too much heat will impair performance.
- We approach heat training the same way we approach other stressors, such as altitude training. Too much altitude is also “bad”.
- The immediate effect of heat training is that it adds additional stress to your body that it needs to cope with, in the same way as altitude. You’ll feel more tired before it normalizes.
- Don’t be discouraged if it goes slow: Kristian adopts quickly to altitude. Gustav does not. Gustav adopts quickly to heat. Kristian does not… We have each our uniqueness, and adopting slow just means there are bigger gains – but you have to start earlier and more gradual.
The Tools Needed
- You’ll need a CORE Sensor, heart rate monitor, and tools to measure power and pace.
- This protocol requires that you have used the CORE sensor for some time, so we can establish a core temp profile for training in normal conditions, and then advance from there.
- It is beneficial to have a Moxy Monitor, but not required.
Step 1: Establish a Core Temperature Profile
Before starting any heat training, it is important to establish a core temperature profile. Record inputs for each cell of the chart below where Avg C°/min represents the change in degrees Celcius per minute and the min and max temperature values are recorded for both core and skin temps.
More About Avg C°/min
Avg C°/min should be the change in temperature over a similar period for all sessions. This is because of inertia in temperature distribution in the body from where it is produced to where it is measured. Temperature distribution doesn’t behave the same way at low core temperatures compared to high core temperatures. At high temps, as the body has limited capacity left for buffering and dissipating heat to the surroundings.
Our proposal for a simplified measurement method is to take the 20 minutes of a session where the temperature increased the most. Note the start and end temperature of that segment and the duration of the segment, and divide the delta by the duration of the segment.
Why is this important? It is a gauge of intensity. How quickly your core temperature rises is critical information. If it rises 0.1 C°/min, it means that you would increase your core temperature ~1.0 C°/10 min. (e.g. from 38°C to 39°C). That leaves very little room for further increase in core temperature, especially if this happens early in a race. So pace yourself accordingly.
Why Are Both Intensity and Duration Important?
You can have a high intensity session that is short. This means you can achieve a moderate to high C°/min, but the absolute temperature will most likely be lower than, for example, a medium or even a low intensity session that is lasting long.
Thresholds
It doesn’t matter what thresholds definition you use, or how it is measured, as long as you divide your program basically into three intensity categories, where LIT is the lower intensity sessions, MIT is typically the “sweetspot/thresholds” sessions, and HIT is everything above “threshold” including VO2max, sprints etc. The sessions with medium intensity with high amount of kilojoule hours are also typically the sessions where you will see the highest core temperatures normally in training, and thus very effective for driving adaptation.
Step 2: Pre-Acclimatization
Start with passive heat adaptation. The reason for this is to minimally impair the training sessions at the start. Passive heat, means that the body temperature is not increased naturally due to activity (active heat adaptation).
Aim for 4-6 days with passive heat adaptation the first week with 2x 60-90 minutes sessions per day where you layer up, including thicker socks and hat. The goal is to sweat moderately at the end of the session. It should feel uncomfortable, and you might end up feeling drowsy.
Whether you do this before or at the end of your training session, or you do this as completely different time, should be based on what is the easiest for you. However, I would not recommend doing it too late in the evening, as it can impair the circadian rhythm.
The benefit of this approach is that it impairs minimally on training, while you get used to operating normally under basic heat. Typically when we are not used to heat at all, we get drowsy, tired, and under-perform both physically and mentally. After the pre-acclimatization, training with heat can start at a higher level.
Hydrate
Drink about 1 liter more per day, preferably during the heat session, to get used to taking in more liquid. There are some studies that recommend to reduce or not drink to dehydrate during heat interventions. This is not our recommendation. Nor is there any clear evidence that extra mineral intake is necessary, besides what you get from your food, unless your medical doctor says there is a deficiency that needs to be taken care of. Normally after 4-7 days of heat adaptation, the body will adapt to retain the minerals that are necessary. That is why we often see large salt patches on our clothes the first few sessions but then they gradually disappear. This is not because we become depleted of minerals, rather that the body is acclimatizing and retaining what is necessary.
We don’t use anything besides pure carbs and water in training and racing.
Step 3: Acclimatization
The last step is to begin the active heat training. This is where we are going to start to use the data from the core temperature profile in step 1 above. The goal is to reach the same values and eventually go slightly above. As a result power and pace have to be reduced. This period should ideally last 2 weeks leading into your travel to Kona (or the warmer climate you plan to go for training and racing).
Week 1 Plan
- Target indoors sessions involve heating your training room to approximately the same conditions as your target hot race environment.
- Introduce heat on 2x bike and 2x run LIT short and LIT medium sessions first. Aim to overshoot the max core temperature you normally do on these sessions, but adjust actively.
- Pace yourself so that the C°/min that you found for these sessions stays the same or slightly higher than normal by actively adjusting power or pace to maintain your C°/min target. It will feel more stressed to “chase” this metric in the start, but as you get used to it, it becomes fairly relaxed. It is enough to give it a glance every second minute or so if you pace fairly steady.
- After 4 LIT sessions, you can introduce heat on your short MIT sessions.
- The fatigue you should target on the sessions should be so that they are not too far from what you normally would experience, but the difference is that they are more heat / central fatigue than muscular fatigue. The goal is to minimally affect the next session you have, which is without heat.
- If you feel “smashed” going into the next session without heat, then you are overdoing the heat.
Bringing Up Power and Pace Towards Normal
Again, referencing the table, the goal is now to gradually bring up the pace and power towards normal. The temperature is priority 1, and humidity is priority 2. Temperature should be simulated by training indoors with a heater in your room, and should be pre-warmed and kept stable throughout the session.
The humidity you simulate by wearing the CORE Heat Training Suit (or other warm clothing) will build during the session. This is perfectly fine.
Target a temperature close to the highest average conditions you will see in your target race. If it is a triathlon, you want to simulate the highest average temperature you are going to experience during the run.
Be aware that in a closed room in order to be able to raise the temperature, the CO2 will build during the session, and can exceed 2000 PPM. This will have a secondary effect which is similar to slight altitude as the CO2 will now be 4x higher than normal. This is normally not dangerous for healthy individuals limited to a training session, but can be uncomfortable.
Week 2 Plan
- Pace yourself similarly to week 1, but it is ok to aim for slightly higher temperatures and deltas again – not exhaustion.
- Heat should now be on most of your LIT sessions and 1-2 short MIT sessions.
- The goal in the coming sessions is to go hard in the beginning of the session to raise temperature delta slightly more aggressive than the table for a similar session, so that you aim for the max temperature towards 2/3 of your session. Then gradually back off, bringing the delta close to 0 before you go progressive on the last intervals aiming for normal power and pace – not higher. Maintain heat until finish cool down. Introduce heat on your HIT sessions this week (if any) first. They will normally not induce high core temperatures because the nature of sessions normally limits the kilojoules hours possible to achieve, and thus the heat that is possible to produce.
- Finally bring heat into your medium and long MIT sessions. The goal now is to again aim for max core temp that has been observed before, but aim to reach it around half way to 2/3 into the session, then back off on the power and pace to maintain core temperature there – it is ok if it still rises a bit, before you go for target power and pace again towards the end of the session. Finish hot – but not exhausted – and drink plenty of water and carbs.
Bonus: Sweat and Hydration Rates
- Measure your weight naked before you start the session
- Prepare and measure all bottles with fluids before the session starts
- Include carbs – they are crucial for performance, especially during heat
- Try to avoid having to urinate during the session. If you do, measure your weight just before and after you urinate
- After the session is finished and before showering
- Swab of any excess sweat on the skin until you are fairly dry
- Measure your weight naked
- Measure all the same bottles again
- Your sweat rate for this kind of effort is now:
- Pre-Post weight + Pre-Post fluid and divide it by the duration of the session
- This would normally, for an average person on MIT medium session fall in the range of 1-3 l/h
- That means that if during a race simulation in heat at home you lost 2 l/h, you should aim to consume 2 l/h with water and carbohydrates
Disclaimers
- This plan is designed to be implemented as "final prep" leading up to a big race. If you are interested in a more basic introductory heat training protocol, see here.
- With over a decade of advanced research on heat training, its effects, and potential causes, we understand the need for a personalized approach for each athlete. Your training program should also be individualized, as it is a continuous and evolving process.
- It is important to note that there are still areas within training—especially heat training—that remain under-explored. While some effects and correlations have been measured, the underlying causes are not yet fully understood.
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This documentation does not delve into the detailed effects, correlations, or causes of heat training. Instead, it aims to share observations regarding performance during races in warmer-than-usual climates, providing inspiration for potential adaptations. However, you are solely responsible for how you choose to implement these strategies and are encouraged to seek professional guidance