Kristian Blummenfelt ran a 2:39:21 marathon at the 2022 Ironman World Championship on his way to a third-place finish. Thanks to data released by Entalpi, we can see his core temperature, skin temperature, and heart rate throughout the run. The core and skin temp data was collected by a CORE sensor worn throughout the entire race.
The Hawaii Ironman is known for its brutal weather conditions. While the 2022 edition was slightly milder than in many years, air temperatures still reached 30.6°C/87.1°F, with relative humidity up to 65%. The data reviewed here gives a unique insight into the behaviours and physiological response of an elite endurance athlete whose core body temperature peaked at 40.8°C/105.5°F and who needed medical attention at the race finish.
Progression of core temp
The data here shows only the marathon portion of the race; CORE data for the swim and bike portions have not been released.
The first thing to note about the chart below is the progression of Kristian’s core temp (green line). He started the marathon with his core temp at a comfortable 38.3°C/100.9°F. But within 45 minutes it had climbed to 39°C/102.2°F, where it remained stable for the next 80 minutes.
Soon after eventual race-winner Gustav Iden pulled away and Kristian was fighting to remain in 3rd place, Blummenfelt’s core temperature began to quickly rise. Over the next 25 minutes it climbed to 40.7°C/105.3°F, while his heart rate increased by 10 beats per minute.
Kristian’s core temp remained at 40.7°C/105.3°F for the last 10 minutes of the race and stayed very high for an additional 20 minutes after the finish, even as he was being attended to by medics. His core temp actually peaked at 40.8°C/105.5°F several minutes after the race.
Blummenfelt’s core temp started declining steeply 20 minutes after the finish, and within another 20 minutes had dropped to 38.3°C/100.9°F, the same temperature he started the marathon at.
Throughout the first two hours of the marathon, Blummenfelt’s skin temperature (orange line on the chart) repeatedly dropped abruptly by several degrees. These are good indications of his cooling efforts. Dousing the head and torso with water are great ways to cool the skin and, subsequently, the core temperature. In addition to receiving water at aid stations, Kristian could be seen carrying a water bottle during the early parts of the run.
These cooling measures, along with adequate hydration and smart pacing, likely contributed to his core temp plateauing at 39°C/102.2°F for 80 minutes.
After 2 hours, we see very large and sudden drops in Kristian’s skin temperature. At this point in the race, at aid stations athletes were taking ice and huge jugs of water. The ice went into the tri suit and the jugs of water were poured over the head.
Kristian’s core temp was starting to climb here, but after each drop in skin temperature, we can see a temporary plateau in core temp. Even so, his core temp continued to rise in between aid stations.
Threshold core temps
Many athletes have identified their threshold core temperatures–the maximum temperature which they can endure for a limited amount of time. Here, we can see Kristian’s core temp was over 39.5°C/103.1°F for the last 30 minutes of the race, and over 40°C/104°F for the last 20 minutes of the race. At the finish line, he received immediate medical attention.
Blummenfelt’s data shows patterns that we’ve seen in other athletes in other races. A few highlights worth noting:
• Blummenfelt ended the bike with a reasonable core temperature. A triathlete’s core temp will almost always rise during the run, whatever its starting point. If that starting point is too high, the core temp will quickly become unsustainable. Athletes on the bike have the benefit of better wind cooling. Coupled with sustainable pacing, maximising cooling at the end of the bike leg can help stabilise or drop the core temperature ahead of the run.
• Cooling measures were critical. For two hours, hydration, smart pacing, and dousing with water kept Kristian’s core temp in check. During the last 40 minutes, ice and extreme water dousing kept his core temp from rising even faster than it did as he pushed hard to the finish.
• For much of the marathon, Blummenfelt’s core temp was in a range that would be considered a “red zone” for many athletes. We know that Kristian does season-long heat training and is extremely well heat adapted. Such adaptions can raise the core temp at which an athlete can sustainably race without performance degradation.
• Thermoregulation is highly individual, and it is not atypical for an elite athlete to reach the core temp of 40°C/104°F that Kristian did for the last 20 minutes of the race. However, it is important to realise that he has trained and raced at such thresholds and understands their implications. These core temperatures can be extremely dangerous for unprepared athletes.
• Once core temp reaches a very elevated level, lowering it becomes exceedingly difficult. Blummenfelt’s core temp reached 40.7°C/105.3°F for the last 10 minutes of the race and stayed extremely high for an additional 20 minutes after the finish, even while receiving medical care.
The Kona 2022 data presented here is provided courtesy of Entalpi under the Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) license.
Data for core body temperature and skin temperature was collected with a CORE sensor worn by Blummenfelt during the entire triathlon.