Heat acclimatisation (HA) is a technique elite athletes use to enhance performance in both hot and cold conditions. This process, achievable through increased environmental temperature, acclimatised chambers, or overdressing, offers a range of physiological benefits.
Physiological Adaptations
During heat adaptation, the body undergoes significant changes to handle heat stress. Initially, exercise in heat leads to an increased heart rate, which gradually decreases over 3-4 days, improving stroke volume and cardiac output [1]. Sweat production increases, aiding in thermoregulation, while sweat becomes less diluted, allowing for more efficient cooling [2]. Plasma volume expands due to increased water retention, which helps lower body temperature [3,4].
This increased water retention is driven by elevated aldosterone levels, a hormone that causes the kidneys to retain more water and salt in the bloodstream [4]. This results in a lower concentration of haemoglobin in the plasma, enabling the heart to pump more blood to the skin for cooling while ensuring muscles receive adequate blood flow [2]. The redistribution of blood to the skin for cooling results in a lack of oxygen in the muscles, stimulating an increase in haemoglobin mass [2]. This increase in haemoglobin mass allows for improved oxygen transport to muscles, which has been suggested to improve performance [1,5].
Heat stress also stimulates the anaerobic system [6], increasing the need for muscle glycogen, crucial for improving sprinting performance. Additionally, heat training triggers the expression of heat-shock proteins [7], which repair and protect cells from damage, enhancing resilience to stress.
2. Research Findings
Several studies have investigated HA in well-trained athletes, though limitations exist. Ely et al. (2018) studied the effects of overdressing (CLO) versus training in a hot chamber (HOT, 40°C) in 13 runners [7]. They found significant increases in rectal temperature, sweating, and heat-shock protein 72 (HSP72) in the HOT group, with higher increases in rectal temperature among women [7]. However, the study lacked a control group without HA and had a short intervention period, which may not have been sufficient for full acclimatisation.
Lorenzo et al. (2010) examined 12 endurance cyclists performing HA in a climatic chamber (40°C) over 10 days [1]. They observed reductions in rectal temperature and heart rate and increases in VO2 max, power output at lactate threshold, and time-trial performance [1]. These improvements were not seen in the control group, suggesting the effectiveness of HA [1]. However, the study's design had potential confounding factors, such as participants maintaining their regular workout routines. In a systematic review, Chalmers and colleagues (2014) summarised data collected from 8 studies showing that HA improved performance in hot conditions; however, VO2 max and peak power did not show a significant increase [6].
Philip et al. (2022) investigated HA in professional rowers, with 12 participants cycling or rowing in control or hot conditions (34°C) over 10 days [5]. They found significant increases in plasma volume and power output, with trends towards improvements in blood lactate and VO2 peak in the HA group [5]. The study lacked time-trial performance data, making it difficult to fully assess performance improvements.
3. Optimising Heat Acclimatisation
Effective HA protocols require careful consideration of duration, frequency, intensity, and number of heat exposures. To prevent performance interference and minimise health risks, heat exposure should be integrated into periodised training plans. Short-term acclimatisation is best scheduled during tapering periods to avoid excessive fatigue. Competition should occur shortly after adaptation, as the benefits of HA are acute and degrade by approximately 2.5% a day without continued exposure [8]. For long-term adaptations, athletes should start the intervention during a low-volume cycle and maintain at least three weekly exposures to heat.
Conclusion
Heat acclimatisation offers significant benefits for athletic performance, but careful planning and individualisation are crucial to maximise its effectiveness. Addressing research limitations and refining protocols can enhance our understanding and application of this valuable training strategy.
REFERENCES
1 Lorenzo, S., Halliwill, J. R., Sawka, M. N. & Minson, C. T. Heat acclimation improves exercise performance. Journal of Applied Physiology 109, 1140-1147 (2010). https://doi.org/10.1152/japplphysiol.00495.2010
2 Périard, J. D., Racinais, S. & Sawka, M. N. Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. Scandinavian Journal of Medicine & Science in Sports 25, 20-38 (2015). https://doi.org/https://doi.org/10.1111/sms.12408
3 Nose, H., Mack, G. W., Shi, X. R. & Nadel, E. R. Shift in body fluid compartments after dehydration in humans. Journal of Applied Physiology 65, 318-324 (1988). https://doi.org/10.1152/jappl.1988.65.1.318
4 Kirby, C. R. & Convertino, V. A. Plasma aldosterone and sweat sodium concentrations after exercise and heat acclimation. J Appl Physiol (1985) 61, 967-970 (1986). https://doi.org/10.1152/jappl.1986.61.3.967
5 Philp, C. P. et al. Can ten days of heat acclimation training improve temperate-condition rowing performance in national-level rowers? PLoS One 17, e0273909 (2022). https://doi.org/10.1371/journal.pone.0273909
6 Chalmers, S., Esterman, A., Eston, R., Bowering, K. J. & Norton, K. Short-term heat acclimation training improves physical performance: a systematic review, and exploration of physiological adaptations and application for team sports. Sports Med 44, 971-988 (2014). https://doi.org/10.1007/s40279-014-0178-6
7 Ely, B. R. et al. Physiological Responses to Overdressing and Exercise-Heat Stress in Trained Runners. Med Sci Sports Exerc 50, 1285-1296 (2018). https://doi.org/10.1249/mss.0000000000001550
8 Daanen, H. A. M., Racinais, S. & Périard, J. D. Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis. Sports Med 48, 409-430 (2018). https://doi.org/10.1007/s40279-017-0808-x
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