Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android


As far as you will be having the race in Tokyo in mid summer, none of the measures, even a pile of all measures, can ensure safety.

Professor Yokohari discussing heat stress risks facing runners at the 2020 Tokyo Olympics where temperatures could reach 39°C

In April 2018, temperatures at the London Marathon reached 24.1°C, with more than 100 people taken to hospital suffering heat-related illness. This pales in comparison to the projected temperatures for athletes competing at the Tokyo 2020 Olympics where in August 2018 temperatures reached a sizzling 39°C. Temperature has been recognised as a critical indicator of health for centuries and is still routinely measured as one of the four primary vital signs. Despite our improvements in monitoring body temperature, incidences of heat stroke have increased over past decades, and it accounts for more deaths than all other natural disasters combined.1 Heat stroke is the second highest cause of death in sport after cardiac conditions.2 In addition to associated health issues, heat stress has a large effect on exercise performance. Yes, warming up enhances performance during high-intensity efforts, but an increase in whole-body core temperature during prolonged exercise in hot and/or humid environments limits performance by taxing the circulatory system and may lead to the development of heat-related illness.3 In this chapter we explore the current scientific knowledge regarding acute and chronic responses to exercising in the heat, and provide practical recommendations and guidelines for clinicians, medical teams, event organisers and athletes.


Core temperature fluctuates during the day between ∼36.5 and ∼37.5°C. When exposed to a cold or hot environment, humans maintain homeostasis through both behavioural thermoregulation (e.g. clothing, sheltering) and autonomic thermoregulation (e.g. sweating, vasodilation/constriction). Skin temperature and environmental conditions can induce both positive and negative heat exchange (i.e. heat gain and loss, respectively) via radiation, convection and conduction (Fig. 23.1).

Figure 23.1

Factors that influence heat gain and heat loss during exercise

Humans possess the innate ability to sweat, allowing for heat loss through evaporative mechanisms. Evaporation is the primary avenue of heat loss when exercising in the heat, where the evaporation of sweat (i.e. transformation of a liquid into a gas) serves to dissipate thermal energy. As such, dripping of sweat does not allow for heat dissipation, and warm and humid environments (e.g. tropical) that limit evaporation are more challenging than hot and dry environments (e.g. desert). Maintaining homeostasis requires reaching an equilibrium between metabolic heat production and heat gain or loss (Fig. 23.1). The ability of humans to turn energy into mechanical output is estimated at 25% or less, the remaining 75% of energy consumed generates heat. This heat generated by muscle contractions induces a large initial increase in muscle temperature,4 thereafter driving an ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.