How to Prepare for a Hot Race: The Science of Heat Training

What heat does to an endurance athlete

During hard endurance work, only around a quarter or less of the energy you produce becomes mechanical work. The rest is released as heat, and the body has to dump it to keep core temperature in a safe range. It does this mainly in two ways: by moving warm blood to the skin, and by sweating so that evaporation can carry heat away.

Both of those defenses compete with performance. Blood sent to the skin is blood that is not returning to the heart or supplying working muscle. As sweat losses reduce plasma volume, the heart fills less with each beat, stroke volume falls, and heart rate climbs to compensate. This is often called cardiovascular drift. González Alonso and colleagues showed this pattern directly: as core temperature rose during prolonged cycling in the heat, heart rate rose and stroke volume fell in parallel [6]. In practical terms, the same pace costs a higher heart rate as the session goes on, and a given heart rate buys less speed.

The brain also plays a role. As core and skin temperature rise, effort feels harder and athletes tend to slow down, often before any dangerous temperature is reached. Researchers have proposed two main explanations, a critical core temperature and anticipatory regulation of pace, and both are discussed in the psychology section below.

The psychological side of heat

Heat does not only load the heart and the sweat glands. It also changes what an athlete feels, and those feelings shape pacing decisions. Two kinds of measures capture this. Perceived exertion is how hard the effort feels overall. Thermal sensation and thermal comfort describe how hot you feel and how much that bothers you. Both rise in the heat. Jenkins and colleagues found that warmer air raised thermal perceptions even when core temperature barely changed [7], which fits with skin temperature feeding directly into how hot we feel.

Research suggests that pacing in the heat is partly anticipatory. Tucker and colleagues had cyclists complete a time trial in hot and cool conditions and found that power output and quadriceps muscle activation began to fall early in the hot trial, while core temperature, heart rate and ratings of perceived exertion were still similar between conditions [17]. One interpretation is that the brain adjusts effort in advance to keep the athlete within a tolerable range. This sits alongside earlier work by González Alonso and colleagues showing that, in fixed intensity exercise to exhaustion, trained cyclists stopped at the same core temperature, about 40 C, whether they started the exercise cooler or warmer; starting warmer simply meant reaching exhaustion sooner [6]. Together these findings suggest that both a physical ceiling and anticipatory, perception based regulation are involved, with their relative weight depending on the type of effort.

This helps explain why heat acclimation feels so noticeable. The 2024 meta analysis by Tyler and colleagues found that heat adaptation lowered resting thermal sensation and thirst, and concluded that it reduces both physiological and perceptual strain [20]. Individual studies point the same way. Daily heat sessions lowered ratings of perceived exertion in rowers [5], and post exercise hot water immersion lowered perceived exertion and thermal sensation during later exercise in the heat [22]. Perception can also change before physiology does. Shaw and colleagues found that after four days of heat acclimation, perceived exertion, thermal sensation and thermal comfort had improved and performance capacity rose, while the physiological markers had not changed significantly [12]. Feeling better is encouraging, but it is not by itself proof of complete acclimation.

Psychological skills can also be trained directly. Wallace and colleagues gave 18 trained cyclists two weeks of motivational self talk practice specific to heat tolerance [21]. Compared with a control group, the self talk group extended time to exhaustion in the heat, tolerated a slightly higher core temperature at the end, and improved speed and accuracy on a test of executive function. The study was small and used a time to exhaustion test, which tends to exaggerate effects compared with a race, so the result should be read as promising rather than definitive. Still, it shows that how athletes talk to themselves can change how much heat they tolerate. A practical application is to rehearse a few short, specific phrases during heat sessions in training so they are familiar on race day.

Expectation setting matters as well. Because pace naturally falls in hot and humid conditions, athletes who hold onto cool weather pace targets can interpret a normal response to heat as failure. Planning race day around effort rather than a fixed pace, and estimating the likely cost of the conditions in advance, can reduce that mismatch. The heat calculator at angelanaethcoaching.com/pages/heat-and-humidity can help frame those expectations.

Finally, cooling appears to work partly through perception. In studies of dousing, a head cooling cap and neck cooling, performance improved while core temperature was unchanged, alongside lower skin temperature, better thermal comfort or lower thermal sensation [1, 14, 18]. In one neck cooling study, athletes stopped at the same perceived exertion and thermal sensation but at a higher core temperature and heart rate [19]. That is useful on race day, but it carries a caution: feeling cooler does not mean the body is cooler, so warning signs such as dizziness, confusion or a sudden drop in sweating should never be overridden.

What heat training changes

Repeated exposure to heat produces a set of adaptations known as heat acclimation. A 2024 meta analysis by Tyler, Reeve, Sieh and Cheung reviewed 135 studies and found that heat adaptation improved exercise performance and capacity in the heat, lowered resting core temperature and heart rate, raised sweat rate, lowered the core temperature at which sweating begins, and increased resting plasma volume [20]. The authors also noted that the effects were smaller than earlier estimates and varied considerably between studies.

Each of these changes addresses a part of the problem described above. A larger plasma volume helps the heart stay filled, which limits cardiovascular drift. Sweating starts earlier and more freely, so cooling begins sooner. A lower starting core temperature gives more room before heat begins to limit effort.

A 2025 quantitative review by McDonald and colleagues looked at how protocol details shape these adaptations [9]. Across 211 papers, the average protocol was about eight heat sessions of about 90 minutes. On average, heat acclimation lowered end exercise heart rate and core temperature, expanded plasma volume, raised sweat rate and improved time trial performance by around three percent, a modest but meaningful change.

Timing deserves a careful note. As described above, four days can improve how heat feels and how long athletes last before the physiology has clearly changed [12]. Longer exposure reaches further. Cubel and colleagues followed 20 elite male cyclists, half of whom completed five weeks of daily one hour heat sessions. Sweating adaptations and heat tolerance developed mainly in the first three weeks, while hemoglobin mass rose by about three percent at three weeks and four percent at five weeks [2]. Rønnestad and colleagues also reported increased hemoglobin mass with five weeks of heat training in elite cyclists and in male cross country skiers [10, 11]. In the skiers, however, the increase in hemoglobin mass did not translate into a clear performance difference, so this benefit should be treated as promising rather than proven for racing.

Sauna and hot water immersion

Passive heat, meaning a sauna or hot bath after training, is attractive because it adds heat without adding training load. Zurawlew and colleagues had non acclimated men run for 40 minutes in temperate conditions and then sit in 40 C water for 40 minutes, on six consecutive days. This produced heat acclimation and improved 5 km time trial performance in the heat, though not in cool conditions [22]. Across the studies reviewed by Solomon and Laye, hot water immersion used water around 40 C, and sauna studies used air temperatures of 90 to 108 C after exercise [13].

The evidence here is more mixed. Tyler and colleagues described passive heat as a practical, effective alternative to active heat acclimation [20]. A narrower 2025 systematic review by Solomon and Laye, limited to controlled trials of post exercise heat exposure, found only a trivial and imprecise effect on performance in the heat, a moderate effect on thermal sensation, and low to very low certainty overall [13]. A fair reading is that passive heat can help, particularly when combined with active heat work, but it should be treated as a useful extra rather than the foundation.

If you use it, keep it conservative: start with shorter sessions, take breaks, hydrate, and step out if you feel dizzy or unwell.

Keeping what you built

Heat adaptations fade. A 2018 systematic review and meta analysis by Daanen and colleagues found that heart rate and core temperature adaptations decline by roughly two to three percent of the gained adaptation for each day without heat, and that re acclimation happens considerably faster than the original block [3]. Cubel and colleagues found that two weeks after five weeks of heat training ended, both performance in the heat and hemoglobin mass had returned to starting levels [2]. Decay is not uniform, however: in the team sport study above, performance was well maintained over two weeks without heat [4].

Less work is needed to hold adaptations than to build them. Rønnestad and colleagues found that three heat sessions per week for three weeks kept hemoglobin mass higher in elite cyclists than stopping heat training [10]. A light, regular dose in the weeks before the race helps protect the work you have done.

Cooling on race day

Cooling strategies can reduce heat strain during the race itself. In 13 trained runners at about 30 C, Anderson and colleagues found that water dousing made a 10 km time trial faster and lowered sweat rate and thermal sensation, without changing core temperature [1]. Spannagl and colleagues found that an ice filled cap worn continuously made a 5 km run in 32 C faster in ten triathletes and improved thermal comfort [14]. Tyler and colleagues found that a neck cooling collar improved a 15 minute time trial after a 75 minute steady run in the heat, but not a 15 minute time trial on its own [18], and extended running time to exhaustion in the heat [19]. Because the benefit seems to come largely from reduced perceived strain, cooling is useful but should not be used to override warning signs.

Practice ice, cold sponges and dousing in training so they feel routine on race day. To estimate how heat and humidity may affect your race pace, use the heat calculator at angelanaethcoaching.com/pages/heat-and-humidity.

Women and masters athletes

Women remain underrepresented in heat research. In the trials reviewed by Solomon and Laye, 43 of 199 participants were women [13]. Kirby and colleagues studied eight female endurance athletes and found that four days of heat acclimation did not improve a 15 minute cycling time trial in the heat, while nine days did [8]. Female athletes may benefit from planning a longer block. Two of the hemoglobin mass studies cited here were conducted only in men [2, 11].

None of the studies cited here looked specifically at masters athletes. Physiology research in general shows that sweating and skin blood flow responses tend to decline with age, although training status matters a great deal. Older athletes may want to progress gradually and pay close attention to hydration.

Limits of the evidence

Many heat studies are small, involve mostly male participants, and use short laboratory tests. In the McDonald review, average gains in time to exhaustion were far larger than gains in time trial performance, and time trials better reflect racing [9]. The overall direction is consistent, but expect modest, meaningful improvements rather than large ones.

References

1. Anderson M, et al. The Effect of Water Dousing on Heat Strain and Performance During Endurance Running in the Heat. Int J Sports Physiol Perform. 2024;19(11):1291 to 1298. doi.org/10.1123/ijspp.2024-0044
2. Cubel C, et al. Time course for onset and decay of physiological adaptations in endurance trained athletes undertaking prolonged heat acclimation training. Temperature. 2024;11(4):350 to 362. doi.org/10.1080/23328940.2024.2383505
3. Daanen HAM, et al. Heat Acclimation Decay and Re Induction: A Systematic Review and Meta Analysis. Sports Med. 2018;48(2):409 to 430. doi.org/10.1007/s40279-017-0808-x
4. Duvnjak Zaknich DM, et al. Continuous and intermittent heat acclimation and decay in team sport athletes. Eur J Sport Sci. 2019;19(3):295 to 304. doi.org/10.1080/17461391.2018.1512653
5. Gill N, Sleivert G. Effect of daily versus intermittent exposure on heat acclimation. Aviat Space Environ Med. 2001;72(4):385 to 390. https://pubmed.ncbi.nlm.nih.gov/11318020/
6. González Alonso J, Teller C, Andersen SL, Jensen FB, Hyldig T, Nielsen B. Influence of body temperature on the development of fatigue during prolonged exercise in the heat. J Appl Physiol. 1999;86(3):1032 to 1039. doi.org/10.1152/jappl.1999.86.3.1032
7. Jenkins EJ, et al. Delineating the impacts of air temperature and humidity for endurance exercise. Exp Physiol. 2023;108(2):207 to 220. doi.org/10.1113/EP090969
8. Kirby NV, et al. Nine, but Not Four, Days Heat Acclimation Improves Self Paced Endurance Performance in Females. Front Physiol. 2019;10:539. doi.org/10.3389/fphys.2019.00539
9. McDonald P, et al. Influence of Exercise Heat Acclimation Protocol Characteristics on Adaptation Kinetics: A Quantitative Review With Bayesian Meta Regressions. Compr Physiol. 2025;15(3):e70017. doi.org/10.1002/cph4.70017
10. Rønnestad BR, et al. Heat Training Efficiently Increases and Maintains Hemoglobin Mass and Temperate Endurance Performance in Elite Cyclists. Med Sci Sports Exerc. 2022;54(9):1515 to 1526. doi.org/10.1249/MSS.0000000000002928
11. Rønnestad BR, et al. Heat suit training increases hemoglobin mass in elite cross country skiers. Scand J Med Sci Sports. 2022;32(7):1089 to 1098. doi.org/10.1111/sms.14156
12. Shaw J, et al. Effectiveness of short term isothermic heat acclimation (4 days) on physical performance in moderately trained males. PLoS One. 2022;17(11):e0270093. doi.org/10.1371/journal.pone.0270093
13. Solomon TPJ, Laye MJ. The effect of post exercise heat exposure (passive heat acclimation) on endurance exercise performance: a systematic review and meta analysis. BMC Sports Sci Med Rehabil. 2025;17(1):4. doi.org/10.1186/s13102-024-01038-6
14. Spannagl BJ, et al. Effects of a Head Cooling Cap on 5 Km Running Performance in the Heat. Int J Exerc Sci. 2023;16(6):193 to 204. doi.org/10.70252/TKAR7672
15. Stephenson BT, et al. Mixed Active and Passive, Heart Rate Controlled Heat Acclimation Is Effective for Paralympic and Able Bodied Triathletes. Front Physiol. 2019;10:1214. doi.org/10.3389/fphys.2019.01214
16. Stephenson BT, et al. High Thermoregulatory Strain During Competitive Paratriathlon Racing in the Heat. Int J Sports Physiol Perform. 2020;15(2):231 to 237. doi.org/10.1123/ijspp.2019-0116
17. Tucker R, Rauch L, Harley YX, Noakes TD. Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment. Pflugers Arch. 2004;448(4):422 to 430. doi.org/10.1007/s00424-004-1267-4
18. Tyler CJ, et al. Practical neck cooling and time trial running performance in a hot environment. Eur J Appl Physiol. 2010;110(5):1063 to 1074. doi.org/10.1007/s00421-010-1567-7
19. Tyler CJ, et al. Cooling the neck region during exercise in the heat. J Athl Train. 2011;46(1):61 to 68. doi.org/10.4085/1062-6050-46.1.61
20. Tyler CJ, Reeve T, Sieh N, Cheung SS. Effects of Heat Adaptation on Physiology, Perception, and Exercise Performance in the Heat: An Updated Meta Analysis. J Sci Sport Exerc. 2024;6(3):195 to 217. doi.org/10.1007/s42978-023-00263-8
21. Wallace PJ, McKinlay BJ, Coletta NA, Vlaar JI, Taber MJ, Wilson PM, Cheung SS. Effects of Motivational Self Talk on Endurance and Cognitive Performance in the Heat. Med Sci Sports Exerc. 2017;49(1):191 to 199. doi.org/10.1249/MSS.0000000000001087
22. Zurawlew MJ, et al. Post exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scand J Med Sci Sports. 2016;26(7):745 to 754. doi.org/10.1111/sms.12638

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