Heat & Endurance Runners: The Impact of Heat on Athletic Performance
By Doug Stewart, Performance Coach, reviewed by Natalia Galan Lopez, Doctoral Researcher, Loughborough University
The 2023 International Olympic Committee’s (IOC) consensus statement on Recommendations and Regulations for Sport Events in the Heat states that:
“The athlete should specifically prepare for the expected environmental conditions (ie, heat acclimation), manage their health status before the event and plan their hydration, cooling, warm-up and clothing according to the risks associated with the forecasted environmental conditions.”
Research conducted at the IAAF World Championships in 2019 showed that 63% of athletes undertook heat acclimation/acclimatisation prior to the championship. Moreover, those that had undertaken the heat acclimation/acclimatisation protocols performed better and had fewer incidents of issues caused by the heat (Racinais, 2022). More recently, at the 2022 World Race Walking Team Championships in Oman, researchers discovered that 57% of competitors completed some form of heat acclimation/acclimatisation, again with them outperforming those that had not. Moreover, all the 4 medallists surveyed by the researchers had adopted heat acclimation/acclimatisation in the build-up to the Oman event (Galan-Lopez et al., 2023).
The IOC recommends heat preparation and the research suggests it is beneficial for both performance and mitigating against issues, such as heat stroke, linked to competing in hot weather. But what is the reason for the benefits of heat acclimation/acclimatisation and how do you implement a heat acclimation/acclimatisation protocol?
ETT Athlete Robert Hajnal rehydrating during the 2023 Doi Inthanon by UTMB 175-mile race
In this first of a series of articles on the topic, we’ll analyse the impact of heat on athletic performance, to see whether this is a factor you need to be focusing on in order to improve results.
Source: Tatterson et al. (2000). Comparison of elite cyclist power output over 30 minute time trial performance. HT = 32 degrees, NT = 23 degrees. Time is displayed on the X-axis, with power output on the y-axis
A number of studies researching cyclists have shown the impact of heat on performance. Tatterson et al. (2000) compared a 30-minute cycling time trial in cool conditions (23°C and 60% relative humidity) and hot conditions (32 °C and 60% relative humidity). With the elite cyclists studied, the skin temperature was significantly higher throughout the time trial, which resulted in a cardiovascular response, displayed by a higher heart rate. Despite the higher heart rate, the power output was lower in the hot conditions.
In addition to the cyclists’ heart rate being higher, the Rating of Perceived Exertion was generally higher over the course of the 30 minutes.
Source: Tatterson et al. (2000). Comparison of elite cyclist heart rate in beats per minute and rating of perceived exertion (Borg scale) over 30 minute time trial performance. Hot = 32 degrees, Cool = 23 degrees. Time is displayed on the X-axis, with heart rate in beats per minute and rating of perceived exertion using the Borg scale on the y-axis
It is worth highlighting that, whilst the RPE and HR merge between the conditions in the above graph at the 30-minute time check, cyclists were producing significantly more power in the cool conditions (as shown on the previous graph where the power outputs are diverging).
Source: Periard et al. (2011). Average power output at 5 minute intervals and the final kilometre at maximum effort during a simulated 40km time trial. Note – the cool condition session resulted in a faster completion time, hence 1 less point
In a self-paced 40km time trial, the impact of these elements on performance was clearly shown in research by Periard et al. (2011). With participants asked to complete a simulated time trial at the highest sustainable work rate (blinded to time, power output or their heart rate). They completed this twice, once in hot conditions (35°C and 60% relative humidity) and once in cool conditions (20°C and 40% relative humidity).
The power output was similar at the start of the trial, but began to diverge, with there being a significant difference, from a statistical perspective, after 20 minutes. Across each reading the average heart rate of the participants was around 8 beats higher in the hot conditions. Their core temperature and skin temperatures were also higher in the hot environment, with RPE also higher in the hot conditions.
Whilst these lab studies provide valuable insight, for runners, the real-world analysis is maybe even more interesting. At the Athletics World Championships in hot conditions (over 25°C) an average decrease of around 2% in performance was seen in races over the 5,000m distance, with it being 3% in the men’s marathon (Guy et al., 2015). In trail and ultrarunning, the difference in finish times at the Western States Endurance Run between cooler and hotter years is even greater. The 2006 edition was a hot year, with temperatures ranging from 7.2 to 38°C, whilst 2007 was a cool year (2.2 to 30.6°C). The performance decrease was around 8% in the hotter year (Parise and Hoffman, 2011).
In road marathons, the slower runners appear to be impacted more by the hotter conditions than the faster runners (Ely et al., 2007, 2008; Vihma, 2010). Over the 100-mile distance it’s the faster runners that are impacted to a greater extent than the slower runners (Parise and Hoffman, 2011). With an event like Western States, there are obviously elements of running through the night for many runners where temperatures are cooler, which may in part explain why there is a variance from the marathon distance.
Overall, there is a clear impact of the heat on performance in endurance sports. It leads to an increase in body temperature, which the results tell us leads to a decrease in Vo2Max and an increase in perceived exertion by comparison to cooler conditions (Periard et al., 2021). This results in a decrease in running speed, including in ultra runners (Parise and Hoffman, 2011), likely due to the athlete making the decision to stop (in constant work rate studies) or decrease their power output/speed, as seen in the cycling studies and running research respectively (Periard et al., 2021).
Our next blog will focus on mitigation strategies to help minimise the impact of heat on your performance, including how to stop heat exhaustion and how to increase your stamina in the heat.
References:
Ely, M. R., Cheuvront, S. N., Roberts, W. O., & Montain, S. J. (2007). Impact of weather on marathon-running performance. Medicine and science in sports and exercise, 39(3), 487-493.
Ely, M. R., Martin, D. E., Cheuvront, S. N., & Montain, S. J. (2008). Effect of ambient temperature on marathon pacing is dependent on runner ability. Medicine & Science in Sports & Exercise, 40(9), 1675-1680.
Galan-Lopez, N., Esh, C. J., Leal, D. V., Gandini, S., Lucas, R., Garrandes, F., ... & Taylor, L. (2023). Heat Preparation and Knowledge at the World Athletics Race Walking Team Championships Muscat 2022. International journal of sports physiology and performance, 1(aop), 1-12.
Guy, J. H., Deakin, G. B., Edwards, A. M., Miller, C. M., & Pyne, D. B. (2015). Adaptation to hot environmental conditions: an exploration of the performance basis, procedures and future directions to optimise opportunities for elite athletes. Sports medicine, 45, 303-311.
Périard, J. D., Cramer, M. N., Chapman, P. G., Caillaud, C., & Thompson, M. W. (2011). Cardiovascular strain impairs prolonged self‐paced exercise in the heat. Experimental physiology, 96(2), 134-144.
Périard, J. D., Eijsvogels, T. M., & Daanen, H. A. (2021). Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological reviews.
Racinais, S., Havenith, G., Aylwin, P., Ihsan, M., Taylor, L., Adami, P. E., ... & Bermon, S. (2022). Association between thermal responses, medical events, performance, heat acclimation and health status in male and female elite athletes during the 2019 Doha World Athletics Championships. British Journal of Sports Medicine, 56(8), 439-445.
Racinais, S., Hosokawa, Y., Akama, T., Bermon, S., Bigard, X., Casa, D. J., ... & Budgett, R. (2023). IOC consensus statement on recommendations and regulations for sport events in the heat. British journal of sports medicine, 57(1), 8-25.
Parise, C. A., & Hoffman, M. D. (2011). Influence of temperature and performance level on pacing a 161 km trail ultramarathon. International journal of sports physiology and performance, 6(2), 243-251.
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.
Vihma, T. (2010). Effects of weather on the performance of marathon runners. International journal of biometeorology, 54, 297-306.
Photo credits: Montri Boonyasat (https://www.instagram.com/running.insider/ )