Heat Mitigation Strategies For Endurance Runners
By Doug Stewart, Performance Coach, reviewed by Natalia Galan Lopez, Doctoral Researcher, Loughborough University
In order to try and reduce the negative impact of high temperatures on performance, athletes and coaches can work on implementing a number of different strategies. Some of these approaches are relevant prior to going to the warm-weather location, whilst others can occur at the event itself.
In a previous post, we discussed the impact that heat has on endurance performance. Additionally, even thermal perception, independent of any increases of the body temperature, can be detrimental to performance (Stevens et al., 2018). This is worth highlighting, as approaches that aim to improve an athlete’s perception of the heat and effectiveness of cooling strategies implemented could lead to performance benefits in competition. However, if applying these approaches (changing perceptions of body temperature, but not actually reducing it), ethical and safety considerations are paramount – as the core and/or skin temperature will remain elevated. So, their use should be carefully reviewed and considered before implementation (Stevens et al., 2018).
ETT Athlete Kris Jones during the Transgrancanaria 2024 Marathon race - ©️ @shitinthewoods
Heat Adaptation – Before Travel to An Event
This falls into two areas: heat acclimatisation (the natural environment – e.g. if racing somewhere hot, traveling there well in advance of the race) and heat acclimation (an artificial environment, such as using a sauna to elicit adaptations).
Research carried out with athletes competing in the 2015 IAAF World Athletic Championships in Beijing discovered that only 15% of the athletes competing had adopted a heat acclimatisation strategy or approach (typically around 20 days) prior to the competition (Périard et al., 2017), so this approach wasn’t widely used. This may be caused by the costs and inconvenience of travelling to locations ahead of the Championships. In contrast, at-home measures are easier and more practical to implement - so we will focus on heat acclimation protocols instead.
A study by McIntyre et al. (2022) compared the impact of 12 days of taking a hot bath (hot water immersion to give it the name they used – but it was sitting in a hot bath) after a run, compared to running in the heat, and a control group. They had 3 groups: a control group that ran in 19°C for up to 60 minutes, a group that ran in hot conditions of 33°C for up to 60 minutes, and the hot water group that ran in 19°C for up to 40 minutes, and then did up to 40 minutes following their run in a hot bath at 40°C.
Source: McIntyre et al., 2022
The researchers discovered that taking a hot bath after a run resulted in greater adaptations than simply running in the heat. Importantly, as well, although the experiment lasted 12 days, they tested the subjects after 3 and 6 days of the protocol. After 6 hot baths, most of the benefits had already been achieved. Additionally, the format of this protocol was 3 days of baths, a rest day, and then an assessment day. This was followed by 3 more days of the protocol, a rest day and another assessment day. The final block consisted of a further 3 days, a rest day and the final test. So, hot baths were not taken day after day, but included 2 days of breaks.
The study by McIntyre et al. had only male participants, whilst other research suggests that females may need longer protocols. For example, one study using female participants involved cycling in hot conditions and found that 4 days did not yield any benefits, but 9 days of the heat protocol did improve performance (Kirby et al., 2019). Another study exploring male and female participants, and using a running protocol to test heat acclimation after 5 and 10 days, discovered that, after 5 days, men’s body temperature and peak heart rate was lower, which was not the case in females. However, after 10 days the female participants displayed thermoregulatory and cardiovascular improvements (Mee et al., 2015). A 2023 review of the research into heat adaptation in females summarised that physiological changes were seen more consistently when the heat acclimation/acclimatisation approaches lasted between 8 and 14 days and were completed daily (Kelly et al., 2023).
There is no evidence that female athletes, when compared to males of a similar age and health status, are at a disadvantage with thermoregulation when training/racing in hot conditions (Yanovich et al., 2020), but variations of the heat adaptation protocol should be considered.
Heathcote et al. (2018) reviewed the available research on passive heat acclimation strategies and advised that athletes use hot baths and saunas, with research suggesting that around 30 minutes is appropriate. For optimal results, these sessions should occur directly after training. Sauna temperatures of 80°C and around 40°C for hot baths appear effective. If you are using a thermometer to assess core temperature, then achieving a temperature of around 38.5°C is effective.
Another element worth considering is the impact on the training itself. Following an ‘active’ protocol would involve completing training sessions in the heat, which would likely result in the intensity of the training sessions needing to be compromised. Passive strategies offer an alternative way to maintain intensity through all training sessions (obviously, the wider consideration of overall load will be important). However, if looking at active heating, you do not necessarily need to train in a specialist heat chamber, or travel to somewhere hot. Lundby et al., (2021) took 34 trained cyclists (4 female) and split them into three groups. Group one cycled in 35 degrees for 50 minutes per day, group 2 cycled in a thermal suit in an 18 degree room for 50 minutes per day, whilst group 3 replicated group 2, but added 25 minutes in a hot bath. The participants completed this for 10 days, and there were no significant differences between the three groups – all made similar adaptations to the heat, with no added benefit, when training in a thermal suit with hot water immersion (Lundby et al., 2021). So, heat interventions do no need to be expensive, it can be as simple as training wearing warm clothing!
It is important to consider safety and the additional stress for heat protocols. Going straight into 30 minutes in an 80°C sauna is potentially not safe, especially if not checking core temperature. So, building up to these times is a more sensible option. Another element to consider is that this will add additional ‘stress’ on your body, so, if adding this protocol into a heavy training load, you must be mindful and adapt training as required.
Furthermore, hydration will be important, as you will be sweating in the bath if the protocol is done properly.
ETT Athlete Robert Hajnal rehydrating during the 2023 Doi Inthanon by UTMB 175-mile race ©️ @running.insider
Race Cooling
A 2023 review of the existing research of pre-race and within race cooling discovered that these actions do help reduce the decline in sporting performance in hot conditions. Pre-cooling seems to be more effective when test subjects are asked to produce a constant workload (e.g. cycle at a certain power output) versus self-paced protocols, with no difference when cooling during the different protocols (van de Kerkhof, 2023).
To give an example of the sort of pre-cooling strategies that were included in the research review: Coelho et al. (2021) discovered that pre-race head cooling (so wearing a cold compress / bandana / cap on the head) improved 5km run times. The subjects were not highly trained athletes, but the group that had 20 minutes of pre-race head cooling (the rest of the body was at an ambient temperature) ran the 5km faster than when sat in the same ambient temperature for 20 minutes prior to the run without head cooling. Interestingly, both core and head temperatures were lower when the head had been cooled.
Another study explored the impact of head cooling during a 5km time trial. Like the pre-cooling results, when the subjects ran with a cooling hat, they recorded a lower temperature at their head level, as well as improved thermal comfort, and ran faster (Spannagl et al., 2023).
Ice vests are another popular option for cooling. A 2023 review exploring the use of cooling vests in various sport settings found that they impacted the perception of heat and performance. This essentially means that the athletes had a lower perception of effort and improved their performances as a result of wearing the ice vest (Fernández-Lázaro et al., 2023).
There are also studies exploring a combination of cooling techniques. For example, a 2022 study explored the use of a cooling hat and ice ingestion (Mazalan et al., 2022). There were three scenarios:
1. Head cooling during exercise (HC)
2. Ice ingestion prior and then head cooling during (Mix)
3. A control group.
The HC and Mix approaches both saw improvements in cognitive function during the exercise, but the Mix group saw a reduction in core temperature not seen in the HC group. This suggests that a pre-cooling strategy coupled with in-exercise cooling offers greater opportunities for mitigating the impact of the heat.
Why do it?
When it comes to heat adaptation and cooling, it is interesting to consider the athletes’ knowledge and also that of coaches and other practitioners. If advising athletes to undertake various protocols, like the ones mentioned above, then understanding ‘why’ is important.
Alabdulwahed et al. (2022) captured insight from 55 athletes and 99 practitioners as to their beliefs on heat acclimation/acclimatisation and discovered that the practitioners ranked improved exercise capacity/performance, exercising core temperature, enhanced sweat rate and improved RPE as the most important reasons for carrying out heat acclimation. Athletes mentioned the enhanced sweat rate and earlier sweat onset the most, while exercising core temperature and RPE did not score as highly.
The researchers discovered that the barriers for heat adaptation in the eyes of the practitioners was the athlete preference followed by cost and access to facilities/equipment/expertise. Athletes most frequently referenced encountering no barriers (35%), compared to 33% referencing issues with access to facilities. These barriers were partially solved by highlighting lower cost non lab-based interventions, such as thermal clothing and hot baths (Alabdulwahed et al., 2022).
ETT Athlete Meryl Cooper after finishing 7th place at the Transgrancanaria 2024 Marathon © @runningtofeel
Practical Application
Tyler et al., (2024) meta-analysis of the literature to date provided the following recommendations:
1. Spending as ‘much time as possible exposed’ to warm temperatures will enhance psychological and physiological adaptations and performance. Expose yourself for ideally 14 days to the high temperatures prior to competition. However, beneficial changes can occur within 5 days.
2. If an athlete has undertaken acclimation previously, re-acclimation may happen faster than if doing it for the 1st time. So, a short term heat acclimation prior to an event may serve as a ‘top up’.
3. The research to date does not provide an optimal heat acclimation approach, however, the main stimulus looks to be achieving a sustained increase of core body temperature, so an approach to heat acclimation that achieves this is likely advantageous.
4. Core body temperature can be influenced by the ambient temperature, the clothes worn and the intensity of the training. These components can be adjusted to influence core temperature and provide a suitable increase to achieve adaptations.
5. If using active heat acclimation (such as running in warm clothing), then monitoring the exercise intensity and having a progressive plan, is advised to prevent overloading and ensure that adaptations continue to take place.
6. When monitoring core temperature, it is important to keep the approach to measuring consistent, as different sites on the body yield different results.
7. The impact of heat acclimation on core body temperature and heart rate are similar, and one could use heart rate as a practical approach, to achieve a suitable magnitude of strain – e.g. setting a heart rate target and then once hit, adjusting exercise intensity to maintain the elevated heart rate rather than letting it drift up. However, core temperature measurements are required to identify any heat illness.
8. Heat acclimation has been shown to result in the commencement of more diluted sweat at an earlier time than otherwise. This may heighten dehydration, and subsequently hydration status should be closely watched.
9. Heat acclimation can decrease the perception of thermal strain felt by the athlete, so it is very important to measure factors like core temperature and/or heart rate to reduce the risk of heat illness.
10. There are inter-individual differences, meaning that the benefits of heat acclimation may not be seen in all individuals/variables monitored. So, results will be person specific.
All in all, there is a lot of recent evidence validating many of the approaches we would naturally use. A combination of heat adaptation prior to travel and cooling prior to and during the race will likely go some way to mitigate the negative impact of the heat on your performance.
References:
Alabdulwahed, S., Galán-López, N., Hill, T., James, L. J., Chrismas, B. C. R., Racinais, S., ... & Taylor, L. (2022). Heat adaptation and nutrition practices: athlete and practitioner knowledge and use. International Journal of Sports Physiology and Performance, 17(7), 1011-1024.
Coelho, L. G., Ferreira‐Júnior, J. B., Williams, T. B., Maia‐Lima, A., Borba, D. A., Silva, C. D., ... & Silami‐Garcia, E. (2021). Head pre‐cooling improves 5‐km time‐trial performance in male amateur runners in the heat. Scandinavian Journal of Medicine & Science in Sports, 31(9), 1753-1763.
Fernández-Lázaro, D., García, J. F., Corchete, L. A., Del Valle Soto, M., Santamaría, G., & Seco-Calvo, J. (2023). Is the Cooling Vest an Ergogenic Tool for Physically Active Individuals? Assessment of Perceptual Response, Thermo-Physiological Behavior, and Sports Performance: A Systematic Review and Meta-Analysis. Bioengineering, 10(2), 132.
Heathcote, S. L., Hassmén, P., Zhou, S., & Stevens, C. J. (2018). Passive heating: reviewing practical heat acclimation strategies for endurance athletes. Frontiers in physiology, 1851.
Kelly, M. K., Bowe, S. J., Jardine, W. T., Condo, D., Guy, J. H., Snow, R. J., & Carr, A. J. (2023). Heat Adaptation for Females: A Systematic Review and Meta-Analysis of Physiological Adaptations and Exercise Performance in the Heat. Sports Medicine, 1-27.
Kirby, N. V., Lucas, S. J., & Lucas, R. A. (2019). Nine-, but not four-days heat acclimation improves self-paced endurance performance in females. Frontiers in physiology, 10, 539.
Lundby, C., Svendsen, I. S., Urianstad, T., Hansen, J., & Rønnestad, B. R. (2021). Training wearing thermal clothing and training in hot ambient conditions are equally effective methods of heat acclimation. Journal of Science and Medicine in Sport, 24(8), 763-767.
Mazalan, N. S., Landers, G. J., Wallman, K. E., & Ecker, U. (2022). A combination of ice ingestion and head cooling enhances cognitive performance during endurance exercise in the heat. Journal of Sports Science & Medicine, 21(1), 23.
McIntyre, R. D., Zurawlew, M. J., Mee, J. A., Walsh, N. P., & Oliver, S. J. (2022). A comparison of medium-term heat acclimation by post-exercise hot water immersion or exercise in the heat: adaptations, overreaching, and thyroid hormones. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 323(5), R601-R615.
Mee, J. A., Gibson, O. R., Doust, J., & Maxwell, N. S. (2015). A comparison of males and females' temporal patterning to short‐and long‐term heat acclimation. Scandinavian journal of medicine & science in sports, 25, 250-258.
Périard, J. D., Racinais, S., Timpka, T., Dahlström, Ö., Spreco, A., Jacobsson, J., ... & Alonso, J. M. (2017). Strategies and factors associated with preparing for competing in the heat: a cohort study at the 2015 IAAF World Athletics Championships. British journal of sports medicine, 51(4), 264-270.
Spannagl, B. J., Willems, M. E., & West, A. T. (2023). Effects Of A Head-Cooling Cap On 5-Km Running Performance In The Heat. International journal of exercise science, 16(6), 193.
Stevens, C. J., Mauger, A. R., Hassmèn, P., & Taylor, L. (2018). Endurance performance is influenced by perceptions of pain and temperature: theory, applications and safety considerations. Sports medicine, 48, 525-537.
Tyler, C. J., Reeve, T., Sieh, N., & Cheung, S. S. (2024). Effects of Heat Adaptation on Physiology, Perception, and Exercise Performance in the Heat: An Updated Meta-Analysis. Journal of Science in Sport and Exercise, 1-23.
van de Kerkhof, T. M., Bongers, C. C., Périard, J. D., & Eijsvogels, T. M. (2023). Performance Benefits of Pre-and Per-cooling on Self-paced Versus Constant Workload Exercise: A Systematic Review and Meta-analysis. Sports Medicine, 1-25.
Yanovich, R., Ketko, I., & Charkoudian, N. (2020). Sex differences in human thermoregulation: relevance for 2020 and beyond. Physiology, 35(3), 177-184.