Heat Prep for Ultra Runners – UTMB Mont Blanc

Heat adaptation is achieved by repeatedly exposing the body to elevated core and skin temperatures, typically for 45–90 minutes over 7–14 sessions. Physiological changes include earlier onset and higher rate of sweating, reduced heart rate at a given workload, increased plasma volume, and improved skin blood flow, together enhancing the body’s ability to dissipate heat and sustain performance. In a race like UTMB Mont Blanc, heat adaptations offer a critical safeguard against the varying microclimates and inevitable climb in heat stress that comes with hours upon hours of intense racing.

Judith is racing the OCC at the UTMB World Series Finals – a 55 km course with 3,425 m of positive elevation gain. To prepare for potential heat stress, she began structured heat training on June 30th, completing around 12–14 sessions so far, alongside training in naturally hot environments.

Each heat session was designed to sustain significant thermal load: she typically spent 40–60 minutes in Heat Zone 3, taking 20–35 minutes to reach that zone. All sessions were performed on the indoor bike trainer, fully overdressed in thermal underwear, a rain jacket, and the CORE suit, sometimes even with gloves, to limit heat loss. Judith finds morning sessions more effective, as she starts fresh and can reach target heat strain more easily. Over her preparation period, she observed increased sweat production and a lower heart rate during heat sessions, clear indicators of effective heat adaptation.

One example from her training: 25 minutes of easy pedaling to gradually reach Heat Zone 3, followed by 1 hour of steady, easy pedaling to maintain that zone. During the cooldown, she still spent 7 minutes in Heat Zone 3, as it takes time until core temperature decreases. Throughout the session, she was properly overdressed, with skin temperature remaining high and stable between 37.0°C and 36.6°C. Looking ahead, Judith plans 4–5 shorter sessions before race day, with the final heat session scheduled at least 5 days before the start to allow full recovery.

Petter is also lining up for the at the OCC at the UTMB World Series Finals – a 55 km course with 3,425 m of positive elevation gain. While many athletes focus on heat adaptation well before race day, Petter’s approach combines both adaptation and targeted race-day cooling strategies to perform at his best.

After focusing on heat prep throughout the off-season (heat zone 3 for 45–60 minutes, 4–7 heat sessions per week, 3-5 weeks per block), Petter turned his attention towards acclimating to the altitude this summer.

"When the summer season arrived and we travelled down to the Alps, we trained at altitude," he explains."I did not want the extra strain that heat training adds to the body, so I focused on normal training instead. However, when I returned home to Norway, where the temperatures are lower and we live at sea level, I tried to maintain the adaptations I had gained in the Alps by resuming a few heat sessions."

Petter is not alone in this strategy. Professional cycling teams, including Lidl-Trek ahead of the 2025 Tour de France, have adopted the same model. Research backs it up as well: a 2024 study by Oberholzer et al. found that heat training can effectively preserve 100% of hemoglobin gains from altitude.

The benefits have been clear in Petter’s training. “It has been interesting to observe how my body has adapted to the heat. Normally, I would feel like I was overheating when pushing in warm conditions, but now I feel more in control and able to manage the strain,” he says — a strong indicator that his adaptations are paying off.

For race day, Petter will also use a cooling vest before the start: Pre-cooling will help lower his skin and core temperature before the gun goes off, delaying the onset of heat strain and preserving cardiovascular capacity. Combined with other cooling strategies, like pouring water over his head, using ice in a cap or neck area, and pacing climbs strategically, this will help him maintain a higher sustainable effort over the demanding course.

Research consistently shows that the combination of heat adaptation and pre-cooling delivers powerful advantages: improved heat tolerance, a cooler starting point that delays thermal strain, reduced perceived exertion, and more efficient hydration. For Petter, this means one thing: better performance when it matters most, deep into UTMB.

Training for trail running often already includes very high running volume, so many trail runners opt for heat training on the bike, since it reduces impact stress while still delivering the desired thermal load. Passive methods (i.e. sauna sessions, steam rooms, or hot baths) can also be layered into a training plan to boost heat exposure without adding muscular strain. No matter which approach you choose, the Heat Training Load and Heat Adaptation Score in the CORE App make it simple to measure progress and track your adaptations over time. Check out this article on how to do a heat training session if you are interested in learning more.