In-play cooling strategies for sport in hot and humid conditions*

Over 40%, and rising, of the global human population currently reside in the climatically ‘torrid’ geographical zone (hot and humid environmental conditions).¹ High ambient temperatures and humidity during sporting events place an added burden onto the physical stress. This load inherently increases the risk of heat-related illness, which remains one of the leading causes of sudden death in exercising individuals.² For example, heat illness was the third most common specific cause of death (1.9 annual incidences) between 1990 and 2010 in high school and college American football players.³ Tennis is a popular global summer sport that features 4 pinnacle Grand Slam tournaments per year at the professional level. The United States and Australian tournaments often coincide with hot environmental conditions that have resulted in athletes aborting play due to heat-related health issues.⁴ This Discovery article highlights the study by Schranner et al.⁴ who examined the effectiveness of in-play cooling interventions during simulated tennis match-play performed in the most extreme environmental conditions observed during the United States Open in recent years (36°C and 50% relative humidity).

The study first sought to examine the effectiveness of the currently recommended in-play cooling strategy for professional tennis: ice wrapped in a wet towel around the neck, and cold towels simultaneously placed on the head and thighs during each mandatory rest period (following every ‘odd’ game and completion of set).⁴ Ice towels facilitate cooling through the transfer of heat from skin in direct contact with a cold towel (conductive heat loss).⁴ Due to the limited utility of conductive heat loss via covering a small surface area, the authors secondly sought to test the efficacy of a cooling strategy that promotes powerful evaporative heat loss. The strategy involved wetting uncovered skin on the neck, face, arms, and thighs with a damp sponge of cool/tepid water while being exposed to air flow generated by a fan at 6.4 m/s (FAN_wet) during each mandatory rest period.⁴ The authors hypothesized that the FAN_wet trial would best mitigate thermal strain.

Both the ice towel and FAN_wet interventions led to similar reductions in the rise of rectal temperature (∼0.5°C) over the course of the match in comparison to a control match (no cooling intervention) (Fig. 1).⁴ The cooling effect was evident from the seventh game of the first set during the FAN_wet intervention, and onwards from the first game of the second set during the ice towel intervention.⁴ Both cooling interventions positively influenced perceptual (rating of thermal sensation and perceived effort) and cardiovascular (heart rate) strain measures.⁴ Although the FAN_wet intervention did not further enhance rectal temperature reductions versus the existing policy of applying ice and cold towels, the practical utility of the FAN_wet trial remains attractive to lower levels of competition where the preparation and replenishment of ice might be difficult for players and officials.⁴ However, the effectiveness of the skin wetting component of the FAN_wet intervention for reducing the rise in core temperature may be somewhat stunted in humid conditions since maximal skin wettedness may already be elicited physiologically due to the associated reductions in sweating efficiency.⁵ Therefore in conditions similar to the United States Open (warm/humid), it could be hypothesized that additional airflow alone may elicit a similarly beneficial cooling response by improving sweating efficiency in the face of negligible dry heat gain.

Figure 1.

Change in rectal temperature over the course of a simulated tennis match. Rectal temperature was recorded following every ‘odd’ numbered game during each set (Games 1,3,5,7) and at the completion of every set (Game 8). First reading constitutes rectal temperature at rest. Only one participant finished the entirety of the CON trial. CON Control; ICE Ice towel; FAN_wet Fan with skin wetting. Adapted from Schranner et al.⁴

A follow-up study by the same research group using a similar methodology illustrated the importance of environmental conditions for the effectiveness of different cooling strategies during the mandatory rest periods in tennis.⁵ Simulating the very hot, but conversely to the United States Open, very dry conditions of the Australian Open Grand Slam tennis tournament (45°C, <10% relative humidity), the FAN_wet trial was more effective (∼0.6°C) at reducing the total rise in rectal temperature during a simulated match vs. an airflow alone intervention (no additional skin wetting).⁵ This relates to the greater rate of evaporation (due to increased skin wettedness) that can be sustained following additional skin wetting during dry environmental conditions in comparison to physiological sweating alone.⁵ Collectively, the data from these studies support the notion that anticipated environmental conditions during sporting events should inform recommended cooling strategies by extreme heat policy makers.

A number of cooling techniques aimed at mitigating the rise in core and skin temperature have been examined in research across a range of sports, including the use of ice vests, ice slurry drinks, menthol application, and cold water immersion. However, some of these methods are not accessible for lower levels of sport participation given financial and infrastructure constraints and many of these methods are only commonly implemented prior to competition. Therefore, pragmatic and effective brief in-play cooling strategies that potentially reduce the risk of heat illness remain attractive for a range of sports, not just tennis. Additional ‘cooling breaks’ may be as simple as ceasing activity for a period of time and represent a feasible strategy for field-based continuous sports. The Fédération Internationale de Football Association (FIFA) implemented additional 3-min ‘cooling periods’ during matches that occurred in ≥ 32°C wet bulb globe temperature (WBGT) conditions during the 2014 World Cup in Brazil. Players ceased competition and consumed extra fluid during the brief breaks at the 30-min mark of each half. The A-League (Australia) and Major League Soccer (United States) competitions operate during the hotter months of the year. Similar additional in-play cooling breaks are included in the extreme heat policy for both competitions, although, at more conservative environmental thresholds than that advocated by FIFA, at 26°C WBGT and ∼28°C WBGT respectively. However, there remains limited evidence as to whether these additional brief in-play ‘cooling breaks’ positively change the rise in core temperature and the subsequent risk of heat illness during field-based sports. Illustrated by the Schranner et al.⁴ and Lynch et al.⁵ research, sporting organisations also need to consider the environmental conditions that comprise a WBGT reading since hot/dry and warm/humid conditions may require different cooling policies.

Brief in-play cooling strategies are recommended by many sporting organizations, but in some cases, lack evidence-based research to support their inclusion. There is a need to develop cooling strategies that are pragmatic for not only professionals, but community-based amateur competitions that are vital for public health. The Schranner et al.⁴ study, and subsequently Lynch et al.,⁵ demonstrate the ability of simple and brief in-play cooling methods to reduce the rise in core temperature over the course of a match, and thus potentially reduce the risk of heat illness.