Football in the Heat: Performance, Health and Mitigation Strategies

Abstract

Introduction. The increasing exposure to hot environmental conditions in football poses a growing challenge. This is driven by both, an expanding match calendar that includes more matches during the hotter summer months, alongside climate change increasing the presence of hotter conditions. Physical activity generates metabolic heat, which needs to be dissipated to maintain a stable core temperature (Tcore), primarily via skin vasodilation and sweating. In hot and humid conditions, the effectiveness of these thermoregulatory mechanisms is reduced, resulting in elevated cardiovascular strain and accelerated fatigue. This results in an impaired performance and can lead to potential health complications, such as exertional heat illnesses and alterations of injury rates when exercising in the heat. Consequently, football federations have begun implementing heat mitigation strategies and policies, such as additional cooling breaks, at varying thresholds of heat stress. Nonetheless, understanding of the effects of heat on real-world football performance and the efficacy of heat mitigation strategies remains limited, with most evidence derived from laboratory settings. Aims. This thesis presents five original investigations addressing five key aims: The first three observational studies aimed at investigating the associations between environmental temperatures and i) match running performance, ii) match play characteristics, and iii) injury occurrence in real-world football matches. The fourth experimental study (iv) aimed to assess the effects of pre-cooling and cooling breaks on thermoregulatory, physiological and performance responses during real football matches. The fifth observational study’s aim (v) was to describe Tcore responses in football and identify associations with physiological and performance factors. Methods. Three large observational studies were performed to address aims i-iii), characterizing the effects of heat on football matches. Large multi-league data sets were collected from open-source websites or shared directly by league organisations, containing match information (location, date, time, etc.) and performance (running or technical-/tactical actions) or injury data per match for both teams. Retrospectively, temperature (T) and wet-bulb globe temperature (WBGT) were aligned to each of these matches using a weighted interpolation of historical environmental data from up to four weather stations close to the match venue. For the first study, mixed linear models were applied to assess associations between environmental temperatures and match running performance across 1610 matches from four professional football leagues. In the second study, mixed linear models were used to identify associations between environmental temperatures and match characteristics across 1585 matches from four professional football leagues. The third study used generalized linear mixed models to investigate associations between environmental temperatures and match injury occurrence across 2612 matches from two professional football leagues. To address aim iv) and v), a field-based data collection was conducted involving four football matches played in WBGT ranging from 24 to 33 °C. In three of those matches pre-cooling and 3-minute cooling breaks (CB; consisting of iced towels and 5 °C drinks) were compared to performing 3-minute drinking breaks (DB; a passive rest with a 17 °C drink). One match was played as a regular match without additional breaks. For Study 4, mixed linear models were performed to investigate the effects of cooling on Tcore, heart rate (HR), match running, hydration status, fluid balance and perceptual markers, such as the rating of fatigue (RoF), rating of perceived exertion (RPE) and thermal sensation (TS). For Study 5, the Tcore responses of the players were explored, and factors associated with developing high Tcore values identified via mixed linear models, controlling for the effects of different environmental conditions and heat mitigation strategies used. Results. The observational studies addressing the first three aims of this thesis, revealed performance changes and indicated a potential small relationship to injury occurrence when playing football in hotter conditions. Study 1 showed reductions in total (p < 0.001) and high-speed running (p < 0.001) distances when WBGT was higher. Study 2 showed that in higher WBGT fewer passes (p < 0.001), including passes into the final third (p = 0.002), take-ons (p < 0.001) and touches (p < 0.001) were performed, while the success rate of passes increased (p = 0.002). In Study 3, there was a trend for higher injury occurrence in higher WBGT in one of the observed leagues (p = 0.05), but additional data, particularly from matches played in hot conditions, is needed to confirm this relationship. In the experimental matches, players experienced substantial heat strain (peak Tcore = 39.2 ± 0.5; range: 37.9 – 40.1 °C) even under moderate conditions of 25 °C WBGT. Implementing additional 3-minute breaks effectively mitigated the continuous rise in Tcore. Compared to drinking only, cooling reduced sweat loss (p = 0.005), but also fluid intake (p = 0.002) and improved perceptual responses (p range: < 0.001 to 0.05) in moderate heat stress (25 °C WBGT). Under more severe heat stress (33 °C WBGT), cooling also reduced sweat loss (p = 0.02) and body mass loss (= 0.007) and further lowered Tcore (p < 0.001) and HR (p = 0.007), whilst preserving match running. When controlling for the variations in WBGT and heat mitigation strategies, higher Tcore peaks were associated with higher total (p = 0.004) and low-to-moderate-speed (p = 0.001) distance, heart rate (p = 0.008) and sweat loss (p = 0.002). Goalkeepers experienced similar Tcore peaks as outfield players, but typically reached those peaks at the end of the warm-up, due to their limited in-match movement. Conclusion. This thesis provides robust field-based evidence on associations with playing football under heat stress. Hotter environmental conditions were associated with an impaired running performance and decreased technical and tactical match play actions. Potential implications for an elevated injury risk in the heat need further investigations. Playing football in hot conditions also resulted in a substantial thermoregulatory and physiological strain, which was mitigated by implementing pre- cooling and cooling breaks, particularly under severe heat conditions. Even at moderate WBGT levels (25 °C WBGT), players exhibited high Tcore values, which was associated with covering more total and low-to-moderate-speed distance. Based on these findings an earlier implementation of additional cooling breaks might be beneficial to mitigate players heat strain. Football teams should consider the adjustments of playing style in the heat in tactical consideration and consider cooling to improve players performance and wellbeing. The findings underscore the need for ongoing monitoring and further research to develop and update evidence-based heat policies in football and protect players health and performance in a warming climate.