Validation of upper thermal thresholds for outdoor sports using thermal physiology modelling

ABSTRACT

Thermal safety guidelines with upper thresholds aim to protect athletes’ health, yet evidence-based sport-specific thresholds remain unestablished. Experimenting with athletes in severely hot conditions raises ethical concerns, so we used a thermo-physiological model to validate the thresholds of guidelines for outdoor sports. First, the reproducibility of the joint system thermoregulation model (JOS-3) of core temperature has been validated for 18 sports experiments (n = 213) and 11 general exercise experiments (n = 121) using the Bland – Altman analysis. Then, core temperatures were predicted using the JOS-3 in conditions corresponding to the upper thresholds, and if the 90^th–99.7^th percentile core temperature value (corresponding to 0.3%–10% of the participants) exceeded 40°C, the thresholds were judged as potentially hazardous. Finally, we proposed revisions for sports with potentially hazardous thresholds. As a result, the JOS-3 could simulate core temperature increases in most experiments (27/29) for six sports and general exercises with an accuracy of 0.5°C. The current upper thresholds for marathons, triathlons, and football are potentially hazardous. Suggested revisions, based on specified percentiles, include: Football: revise from wet bulb globe temperature (WBGT) 32°C to 29–31°C or not revise. Marathon: revise from WBGT 28°C to 24–27°C. Triathlon: revise from WBGT 32.2°C to 23–26°C. If conducting sports events under the revised upper thresholds proves difficult, taking measures for a possible high incidence of heat illness becomes crucial, such as placing additional medical resources, assisting heat acclimatization and cooling strategies for participants, and rule changes such as shorter match times and increased breaks.

KEYWORDS:

• Body temperature
• Computer simulation
• Heat stroke
• Practice guidelines
• Safety
• Sports
• Thermal physiology modeling

Abbreviations

EHS	=	Exertional heat stroke
JOS-3	=	Joint system thermoregulation model
PCTL	=	Percentile
RH	=	Relative humidity
SD	=	Standard deviation
Ta	=	Ambient temperature
TREE	=	UTokyo REsource Explorer
WBGT	=	Wet bulb globe temperature

Acknowledgments

The authors would like to thank Barbara Baldwin, MPH of the International Institute for Race Medicine, for providing information on the IIRM Medical Care Manual, and Mr Yoshito Takahashi for advising the use of thermal physiology models, including the JOS-3.

Disclosure statement

TO is working full-time at the Mitsubishi Research Institute, Inc., which is associated with research, consulting, and ICT solutions and has potential competing interests. The other authors declared no conflicts of interest.

Contributors

TO contributed to the conception of the study, devising of the methods, data acquisition and analysis, and drafting of the manuscript; JT contributed to devising of the methods and data analysis; MF, KN and YH supervised the study. All authors contributed to interpreting, and discussing the results, and to drafting and revising of the manuscript.

Data availability statement

The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/23328940.2023.2210477.

Additional information

Funding

This study was partially supported by the Climate Change Adaptation Research Program of the National Institute for Environmental Studies, Japan, and was performed by the Environment Research and Technology Development Fund (JPMEERF20222G01) of the Environmental Restoration and Conservation Agency provided by Ministry of the Environment of Japan.