• football
• heat stress
• performance
• recovery
• health

Escalating environmental temperatures are expected to influence performance and planning of several future football events, for example, FIFA World Cups, regular league and continental cup matches and climate projections for Tokyo (Olympic Games postponed to August 2021) implies that athletes are facing high heat exposure in the next Olympic football tournament. Prompted by global warming, but also with current conditions and historic heat-events in mind, it is relevant to consider the consequences for football play and players health. Heat stress has detrimental effects on endurance performances and sudden or extreme exposure is a major health concern.1 2 Moreover, heat stress could, in some situations, create an unsportsmanlike competitive advantage for the home team3 and fair play may require special planning and precaution procedures.

Scientific football research with control and hot matches completed with standardised set-up1 as well as retrospective analyses of the 2014 FIFA World Cup2 reveal a characteristic change in game activity pattern including a marked reduction in high-intensity running. However, the overall fatigue development is not affected and peak sprinting speeds appear slightly higher when playing in the heat. The majority of sprint efforts in a game last 1–3 s4 and may benefit from a faster cross-bridge contraction velocity at higher muscle temperatures, without being impaired by central nervous system dysfunction for such short duration efforts. In contrast, impairments in aerobic capacity, central nervous system fatigue provoked by hyperthermia during prolonged motor activation,5 as well as adjusted pacing in the heat are likely to explain the lower total and high-intensity running distances observed when playing football in hot environmental conditions. While findings of altered running patterns fit well with the physiological framework explaining heat effects on performance, it is somewhat surprising that technical parameters seem to be preserved or even improved for percentage of successful passes.1 2

Muscle soreness, slow performance recovery, delayed replenishment of muscle glycogen stores, elevated plasma creatine kinase and high myoglobin levels following regular football matches reflect significant muscle damage,4 and rhabdomyolysis provoked by heat stress is an admissible concern. However, an experimental match in extreme heat (air temperature >40°C) with severe hyperthermia and dehydration levels up to 5% for individual players, did not aggravate performance recovery or measures of muscle damage compared with the matched game in thermoneutral settings.6

Both complex motor-performance and cognitively dominated tasks are negatively affected by the combined high-levels of hyperthermia and moderate dehydration (~2% body weight deficit) typically observed during football match-play in the heat.1 7 However, physical performance impairments may partially be prevented by heat acclimatisation8 and, provided that players are adequately adapted, the technical quality of the game seems to be preserved.1 2 8 International or league matches at venues with sudden heat-waves or large geographical temperature differences, where players would be exposed to high heat with limited time to adapt (eg, from weekend to mid-week match) should clearly account for this aspect.8 Importantly, there is considerable inter-individual variance in the fashion that players are affected by the heat1 and, although athletes in general adapt faster to heat than untrained individuals, 1 week of acclimatisation may not be adequate for all players.8

Recommendations

Healthy football as well as fair play in hot environmental conditions is possible! However, it requires extensive preparation for players, precaution from the medical staff and planning of future football events.

It is advised to use appropriate heat stress indices and analyses to foresee issues9 and for matches where one or both teams have limited time to acclimatise, consider scheduling of the match to time of day with minimal heat stress (eg, after sunset to avoid solar radiation). Likewise, additional and adequate duration of hydration/cooling breaks should be considered to reduce potential health impacts. Modifications of match procedures will eventually be a political decision, but facing a future with increased risk of sudden and extreme heat exposure, the concern for players health is certainly an issue that requires special attention and qualified debate.

For tournaments allowing adequate time to acclimatise, playing in the heat does not require additional recovery days between matches, but we emphasise the importance of in-match breaks and focus on rehydration after matches as well as training to prevent accumulative hypohydration.

Player’s physical performance will be affected under high heat scenarios, but technical skills seem to be preserved and even improved (see figure 1 for overview). Teams may accordingly benefit from a game strategy prioritising ball possession rather than transition-based tactics and reliance on high work rates. If properly prepared and hydration opportunities allowed, concerns for players health does not appear substantially different than for a range of other environmental conditions. Monitoring hydration, a controlled (submaximal) heat-tolerance test, and special awareness towards players with signs of heat illness may assist team medical doctors in early identification of potential heat vulnerable players.

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Figure 1

Overview of the main performance and physiological parameters affected by environmental heat-stress during football match play in red text the main performance and physiological factors affected by soccer match play. In the heat (and in blue text the reference values or comparison to match play in control condition without environmental heat stress). Arrows in bold indicate relative change compared with control match play, that is, a small reduction in total and large hampering of high-intensity running and a small increase sprint speed and successful passes as the main performance changes. The large sweat loss, high core and muscle temperatures are normally well-tolerated in acclimatised athletes, but are highlighted as health attention issues to be considered—especially during sudden exposure or the initial adaptation period.