Heat Acclimation for Cyclists: Practical Strategies for Performance Enhancement
Competing in hot (e.g. ~30°C and above) and/or in humid environments can negatively impact performance. When the air temperature rises from 23°C to 32°C, it’s been shown to reduce power output by at least ~6-7% over a 30-minute time trial (Tatterson et al., 2000). Hot temperatures also increase the risk of cramping and pose a significant risk to health from dehydration, heat exhaustion or heat stroke.
Becoming ‘acclimatised’ or ‘acclimated’ to the heat can help reduce the impact of heat during competition*.
There is also emerging evidence that heat acclimation can enhance performance in milder temperatures (e.g. ~18°C).
In this article, we’ll look at the types of benefits you can get from heat acclimation, the different evidence-based strategies you can follow, and the potential risks posed.
Benefits of Heat Acclimation/Acclimatisation
Let’s look first at the benefits of heat acclimation.
It’s well evidenced that with sufficient exposure to hot environments, the body undergoes a series of adaptations that help it to tolerate higher temperatures. These adaptations include an increased sweat rate, more rapid onset of sweating at a lower body temperature, increased blood plasma volume, better regulation of ‘heat shock proteins’ (which protect cells from extreme heat), and improved reabsorption of electrolytes from the skin (Casadio et al., 2017; Heathcote et al., 2018).
Cumulatively, these adaptations help to reduce core body temperature and skin temperature, lower heart rate and perception of effort, and maintain better exercise performance at higher temperatures (Heathcote et al., 2018).
The performance benefits from heat acclimation when competing at extreme temperatures and/or humidity are very clear (Casadio et al., 2017; Heathcote et al., 2018).
However, there is also evidence that heat acclimation may be performance-enhancing at lower environmental temperatures.
For example, Kirby et al., 2021 found that a 3-week intervention involving training and heat acclimation protocols resulted in an average improvement in VO2max of 0.27L/min and a 0.6 km/hour increase in running speed at 4mmol/L of lactate (tested at 18°C). These were both significantly greater than the improvement seen among the subset of athletes who only followed the training program, with no heat acclimation. They are also very meaningful improvements from a competitive standpoint, especially considering the participants were already trained middle-distance runners. Similar results were found by Dalleck et al. (2019) among a group of 30 untrained participants.
Looking at actual performance improvements, Scoon et al. (2007) observed a 32% improvement in time to exhaustion among 6 male distance runners, when running at 5km pace following a heat acclimation protocol. In real-terms, this translates to approximately a 2% improvement over an actual 5km race, since ‘time to exhaustion’ tests substantially inflate performance improvements in races that involve pacing.
A small case study of two high-level runners recorded a 1.3% improvement in 5km time following a heat acclimation intervention, which resulted in both athletes achieving lifetime personal bests, when running in cool conditions. This study also showed a 5% improvement in 10km running time in a hot environment. These performance improvements were potentially at least partially attributable to the heat acclimation strategy used, though as the sample size was extremely small and there was no ‘control’ group, then we can’t rule out other causes of the performance improvement!
As well as physiological and performance changes, studies have also observed reductions in the perception of effort while exercising at the same absolute intensity following heat acclimation, even in cooler (18-20°C) environments (Heathcote et al., 2018).
It’s worth noting that several studies have failed to observe a performance improvement in cooler environments as a result of heat acclimation. However, these have either tested performance a long time after the heat acclimation period ended (Stanley et al., 2015), and thus adaptations may have been lost. Else, they used a time trial protocol, where small performance differences are hard to detect with a relatively small number of participants (Zurawlew et al., 2016).
How long does heat acclimation take?
This depends to some extent on how exactly heat acclimation is achieved.
However, it seems that broadly speaking 4-5 ‘doses’ of heat exposure on consecutive days is enough to bring about meaningful physiological changes, though for maximal results 9-14 doses seems to be optimal (Casadio et al., 2017; Heathcote et al., 2018; Kirby et al., 2021).
How to become heat acclimated for racing?
There are two broad approaches that have been shown scientifically to bring about heat-related adaptations and improve performance.
The first is ‘active heat acclimation’, which involves exercising in the heat. The other is ‘passive heat acclimation’, which involves just sitting or laying in the heat, usually (though not always) shortly after exercise.
We’ll look at each of these in turn.
Active Heat Acclimation (Training in the Heat)
The most heavily-researched strategy involves training in hot environments, either outdoors or in a heat chamber. These environments should be sufficiently hot to substantially increase core body temperature, skin temperature and sweating for a period of time - usually this is between 30-40°C and a relative humidity of 20-60% (Casadio et al., 2017).
In much of the research, athletes have trained daily (or sometimes twice-daily) in the heat for 1-2 weeks, with each session lasting ~60-minutes (Casadio et al., 2017). As mentioned above, meaningful results can be achieved with as little as 4-5 days, but the biggest adaptations occur after ~14 days.
Adaptations seem to be intensity-specific, so low-intensity training in the heat tends to help with sustained, aerobic races or events, whereas higher-intensity training in the heat tends to help most with improving stochastic or intermittent efforts (Casadio et al., 2017; Heathcote et al., 2018). Since cycling is usually a combination of these two (i.e. events are long, but involve repeated bursts or high intensity riding), then a mixture of low- and high-intensity training is probably optimal.
Indeed, it’s been suggested that a good strategy might be to begin with low-intensity training initially (e.g. over the first 4-5 days), while the early adaptations begin to take place, and then introduce higher-intensity training in the second half of the adaptation cycle (Casadio et al., 2017). This might allow higher-intensities to be tolerated in training, and thus for training quality to be higher while heat acclimation is being undertaken.
Passive Heat Acclimation (Sauna and Hot Water Exposure)
Two further well-evidenced strategies for acclimating to the heat involve ‘passive’ heat exposure.
The first uses immersion in a hot water bath, which is a practical strategy for many athletes, since this can usually be done at home. The bath water should be kept between 40-45°C, and each immersion session should last for roughly 30-45 minutes.
Research shows that 6-7 hot water baths, either on consecutive days, or spread over several weeks induces similar adaptations to active heat acclimation strategies (Casadio et al., 2017).
A second strategy involves sauna exposure, with temperatures between ~80-100°C. Between 6-9x 30-minute exposures spread over 1-3 weeks (at least 3 sessions per week) is enough to induce meaningful adaptations (Kirby et al., 2021; Casadio et al., 2017; Heathcote et al., 2018)
In both cases, there seems to be a benefit to including exercise immediately before the heat exposure. This helps to increase core body temperature to a greater extent. The heat exposure may also enhance some of the adaptations stimulated by the exercise (specifically increased mitochondrial function via improved citrate synthase activity) and may increase heat shock protein synthesis (Casadio et al., 2017; Heathcote et al., 2018).
These passive strategies will, in most cases, be more practical for athletes who cannot afford the time and/or financial cost of travelling abroad to train in hotter climates.
Risks
Subjecting the body to heat, whether via active or passive strategies imposes a stress on the body, and may impact your capacity to train and recover.
The impact of active heat acclimation strategies may be particularly notable, since the capacity to train in the heat will certainly be reduced, and the travel involved in getting to a hot climate takes time and imposes an additional stress. Exercising in the heat also places additional stress on the sympathetic nervous system, heart and on fuel availability, which exercise in the heat having a higher energy cost (Casadio et al., 2017).
This training impact needs to be borne in mind when decided when and how to achieve heat acclimation, and we consider this more extensively below when looking at how long heat acclimation adaptations last.
When considering the stress imposed, you should consider your body size. Athletes who are ‘wide’ relative to their height (e.g. high muscle and/or body fat, meaning the volume of the body is high relative to the skin surface area) will find it harder to dissipate heat, and therefore will have a higher stress imposed by a given environmental temperature.
The stress imposed by heat acclimation can be reduced by making sure you hydrate well during and after each heat exposure, and any session should be stopped if you feel nauseous, faint or have a headache.
The impact on training may also be reduced by spreading exposures out over a longer period, which may allow for better recovery between heat-related stress, and for harder ‘key’ sessions to be integrated on days where the athlete has had several days to recover from heat exposures. Three sauna exposures per week over a period of 3-weeks has recently been shown to be effective in improving VO2max and pace at 4mmol/L of lactate, which are key markers of endurance performance (Kirby et al., 2021). Three sessions per week seems, in our view, to present a practical balance between allowing high-quality training, recovery and heat exposures.
It’s also worth noting that once heat acclimated, sweat rates will be higher, and so staying on top of hydration can be more challenging. In some cases (such as long ultra-distance events), where a cumulative mismatch between sweat rates and fluid intake over a large number of hours may lead to very severe dehydration, then heat acclimation may not actually be advantageous. To our knowledge, this hasn’t been studied however.
How long does heat acclimation last?
Adaptations following heat acclimation seem to decline within around 2-4 weeks, depending on the length of the initial acclimation. It’s been suggested that approximately 1 day of acclimation is lost with every 2 days without heat exposure (Heathcote et al., 2018) - though recent evidence may suggest that adaptations last a little longer than this (Kirby et al., 2021). In any event, this suggests that after the initial acclimation period, the dose of heat exposure can be cut down by roughly 50% while maintaining adaptations.
Bearing in mind the negative impact that a heat acclimation protocol may have on training and recovery, it may make sense to perform the initial heat acclimation period several weeks or months out from the target event or competition, during a period where training is less critical. Then a lower dose of heat exposure can be used to maintain heat acclimation up to the target event/race. Acclimation strategies that don’t involve excessive travel are clearly best in this regard - i.e. sauna and hot bath strategies!
Final Considerations
It’s important to note that not everyone responds the same to heat exposure. Women may require longer to acclimate. Ethnicity can also impact the time taken to acclimate, with athletes genetically originating from hotter climates acclimating faster. Some evidence also suggests that athletes with a higher VO2max and/or longer training history (and thus a greater level of exercise-induced heat exposure) may adapt faster, and have a higher baseline heat tolerance (Casadio et al., 2017)
Overall, heat acclimation strategies may be useful for a wide range of cyclists - not just those who are competing in hot environments, since performance benefits carry over to cooler environments too. There is a risk of compromising training and/or recovery, and in our view passive acclimation strategies (sauna or hot water exposure) will have a lesser impact on training quality, especially if exposures are limited to 3-4 times per week, thus allowing the chance to recover from heat exposures within the week.
*It’s worth noting that strictly speaking the term acclimatisation generally refers to the process of adapting to heat in a natural environment (such as spending time living and training in a hot environment), whereas the term acclimation refers to specific artificial strategies such as use of heat chambers, saunas and hot water baths. However, we’ll refer to these terms interchangeably, since the adaptations we’re looking to achieve in each case are the same!
References
Casadio, J. R., Kilding, A. E., Cotter, J. D., & Laursen, P. B. (2017). From lab to real world: heat acclimation considerations for elite athletes. Sports Medicine, 47, 1467-1476.Casadio, J. R., Kilding, A. E., Cotter, J. D., & Laursen, P. B. (2017). From lab to real world: heat acclimation considerations for elite athletes. Sports Medicine, 47, 1467-1476.
Dahlquist, D. T., Dieter, B., Brotherhood, J., & Koehle, M. (2023). Case Study The Effects of Post-Exercise Sauna Bathing on 5-and 10-km Performance in University Level Track Athletes: Heat Acclimation and Running Performance. Journal of Sport and Human Performance, 11(1), 13-22.
Dalleck, L. C., Byrd, B. R., Specht, J. W., & Valenciana, A. K. (2019). Post-Exercise Passive Heating Strategies with Hot Water Immersion and Sauna Suits Improve VO2max, Running Economy, and Lactate Threshold. Int J Res Ex Phys, 15(1), 87-96.
Heathcote, S. L., Hassmén, P., Zhou, S., & Stevens, C. J. (2018). Passive heating: reviewing practical heat acclimation strategies for endurance athletes. Frontiers in physiology, 9, 404566.
Kirby, N. V., Lucas, S. J., Armstrong, O. J., Weaver, S. R., & Lucas, R. A. (2021). Intermittent post-exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle-distance runners. European journal of applied physiology, 121(2), 621-635.
Scoon, G. S., Hopkins, W. G., Mayhew, S., & Cotter, J. D. (2007). Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of Science and Medicine in Sport, 10(4), 259-262.
Stanley, J., Halliday, A., D’Auria, S., Buchheit, M., & Leicht, A. S. (2015). Effect of sauna-based heat acclimation on plasma volume and heart rate variability. European journal of applied physiology, 115, 785-794.
Tatterson, A. J., Hahn, A. G., Martini, D. T., & Febbraio, M. A. (2000). Effects of heat stress on physiological responses and exercise performance in elite cyclists. Journal of science and medicine in sport, 3(2), 186-193.
Zurawlew, M. J., Walsh, N. P., Fortes, M. B., & Potter, C. (2016). Post‐exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scandinavian journal of medicine & science in sports, 26(7), 745-754.