“Heat stress and sport performance” was the title of an excellent conference that took place at INSEP (National Institute of Sport, Expertise and Performance), Paris, on June 22nd and 23rd. The Scientific Committee of the conference, including my colleagues (and friends) Christophe Hausswirth, Yann Le Meur, Rob Duffield and Aaron Coutts managed to bring together sport physiologists, medical doctors, coaches and other elite sport professionals, including athletes, to present, discuss and debate the latest developments concerning heat exposure during training and competition in hot environments.

Below you can find the written abstract of my own invited lecture, entitled “Case studies about elite performance in the heat”.

High ambient temperature and humidity are associated with reduced capacity for endurance exercise. For instance, mathematical models and experimental studies indicate that metabolic heat production, heat storage and heat dissipation mechanisms are stressed during distance running in warm, humid environments. Heat stress could thus have caused or contributed to various widely reported counter performances of elite athletes at major international competitions, including for example marathon runners, triathletes and tennis players.

Various strategies have been developed to limit the negative impact of heat stress on performance. Some of these strategies consist of precooling interventions to reduce an athlete’s core temperature before and/or during competition. Heat acclimatization and optimal hydration practices are also important components of athletes' preparation for performance under thermal stress conditions. The aim of this paper is to report on the use of such strategies in elite sports.

Several studies report on the use and effects of popular athlete precooling strategies, which include the external application and/or ingestion of cold modalities including air, water and/or ice, separately or in combination, immediately prior to exercise (Quod et al. 2006, Ross et al. 2013). A recent meta-analysis indicates that precooling can effectively enhance well trained athletes' endurance performance in hot environments, and that cold drinks, cooling packs and cooling vests can be regarded as best-practice methods (Wegmann et al. 2012). Interestingly, the published research supporting the use of such strategies arrived well after their application in the sports arena. For instance, such strategies were widely researched and applied by Australian Institute of Sport physiologists in the lead-up to Atlanta 1996 and especially Athens 2004 Olympic and Paralympic Games.

Repeated active and/or passive (e.g. sauna) heat exposures are also popular strategies to prepare athletes to cope with heat stress. Heat acclimation through repeated exposures that elevate core temperature, skin temperature and sweat rate, and acclimatization through passive environmental heat exposure or training-induced heat stress induce positive physiological outcomes (i.e. increased plasma volume, enhanced skin perfusion, increased sweat rate and decreased sweat electrolyte content). Such thermoregulatory adaptations, which are obviously beneficial for endurance performance in the heat, might also contribute to improve endurance performance in thermoneutral environmental conditions (Lorenzo et al. 2010). Heat adaptation for competition in hot environments is a generalized strategy for endurance athletes, but heat adaptation for competition in thermoneutral conditions is gaining popularity (e.g. the 2012 Men's Under 23 Triathlon World Champion successfully used a heat stress training strategy in combination with simulated altitude training in the lead-up to winning his world title in a sea level thermoneutral environment).

Hyperhydration practices prior to prolonged exercise in the heat, as well as optimal hydration strategies during training and/or competition may confer athletes some advantages in terms of maintaining fluid balance and exercise performance. In a series of studies designed to assess the consequences of hydration status and heat stress on performance in different sports, we investigated the effects of a reduction in body mass attributable to unreplaced sweat losses on simulated cycling hill-climbing performance in the heat (30° C), and we found that dehydration-induced hyperthermia outweighed the theoretical benefit of a reduction in body mass on the power-to-mass ratio and energy cost during cycling (i.e. “functional dehydration”). We also investigated fatigue patterns in elite male soccer players in a hot environment (31° C) using activity profile assessments, and intermittent performance tests before and after a match, in combination with physiological measurements. Very high muscle temperatures were recorded (39.6–41.9° C) and a significant dehydration occurred during the game, which correlated with the fatigue index of a post-match repeated sprint test. We found similar results on professional tennis players competing in temperatures between 25.4 and 32.0° C. Players experienced moderate thermoregulatory strain (mean core temperature 38.5-38.9° C) and hypohydration, influencing several match notation variables and predisposing players to premature fatigue, performance decrements, and heat illness. Examples of countermeasures taken by elite athletes to prevent the occurrence of such negative outcomes include the use glycerol hyperhydration, ice slushy ingestion and/or paracetamol ingestion in endurance sports like cycling, triathlon and swimming, and intermittent sports like tennis and football.

We also assessed the effects of an alcohol and menthol based liquid solution which purportedly provides a high rate of evaporative cooling by wearing sweatbands soaked in it during indoor rowing in a warm environment (25.0°C, 65.0%). No substantial benefits were observed in heart rate, blood lactate, RPE and mean power during a submaximal row, nor in performance time, mean power, mean stroke rate, pacing strategy and total sweat loss during a 2000 m time trial. These results did not conclusively exclude the possibility that over a longer exercise duration and/or higher ambient temperatures, this product could provide measureable benefits that might aid training and competitive performance by increasing heat removal rate or reducing perceptions of thermal strain (Mujika et al. 2010). Anecdotally, the solution was successfully used in severely stressful thermal conditions in rowing and middle-distance triathlon competition.

References

Lorenzo S, Halliwill JR, Sawka MN, Minson CT. Heat acclimation improves exercise performance. J Appl Physiol. 2010;109:1140-1147.
Mujika I, González de Txabarri R, Pyne D. Effects of a new evaporative cooling solution during rowing in a warm environment. Int J Sports Physiol Perform. 2010;5:412–416.
Quod MJ, Martin DT, Laursen PB. Cooling athletes before competition in the heat. Comparison of techniques and practical considerations. Sports Med. 2006;36:671–682.
Ross M, Abbiss C, Laursen P, Martin D, Burke L. Precooling methods and their effects on athletic performance: a systematic review and practical applications. Sports Med. 2013;43:207-225.
Wegmann M, Faude O, Poppendieck W, Hecksteden A, Fröhlich M, Meyer T. Pre-cooling and sports performance: a meta-analytical review. Sports Med. 2012;42:545-564.

Conference book.

Eiffel Tower (Photo: Inigo Mujika)

First slide from the presentation.