Sebastien Racinais, Magni Mohr, Martin Buchheit, Sven Christian Voss, Nadia Gaoua, Justin Grantham, Lars Nybo
Research and Education Centre, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar. Department of Sport and Health Sciences, College of Environmental Sciences, University of Exeter, Exeter, UK. Sport Science Department, Physiology Unit, ASPIRE, Academy for Sports Excellence, Doha, Qatar. Qatar Science and Technology Park, Doha, Qatar. Department of Exercise and Sport Sciences, Section of Human Physiology, University of Copenhagen, Copenhagen, Denmark.
Objectives: To identify the relationship between field performance in a hot environment and individual heat acclimatization responses in football players.
Methods: Nineteen semiprofessional football players completed a match in 21°C followed by 6 days of acclimatization in dry heat (38–43°C, 12–30% relative hazard) and a match in ∼43°C. A heat-response test (30 min walk+30 min seated; 44°C) was performed at the beginning and end of the acclimatization period.
Results: The acclimatization period increased sweat rate by 34% during a standard heat-exposure test and reduced sweat sodium concentration by 18% (both p≤0.005). Plasma volume changes showed large inter-individual differences (−10 to +20%). Match-running performance was impaired in hot ambient condition and demonstrated marked inter-individual differences (total distance −6.0±5.8%, high-intensity running −16.4±21.5%, both p≤0.002). Only hematological markers investigated during the heat-response test correlated with the ability of the player to cope with heat stress in a competitive situation; that is, changes in hematocrit between the heat-response tests were correlated to changes in total running during the game, r=−0.75; 90%CI −0.88 to −0.51.
Conclusions: Heat acclimatization responses and in turn, match-running performance in the heat, are highly individual. The players displaying the largest hematological adaptations were able to maintain the same activity when playing in the heat when in temperate conditions. As such, team doctors might use acclimatization indicators obtained from a heat-response test to predict the ability of individual players to cope with heat in competitive situations and individualize their preparation accordingly.
Several international sport events take place in the summer months and often in countries with hot environmental temperatures. Elite football players participating in events such as the FIFA World Cup in Qatar will be partly acclimatized as a result of physical training in temperate environmental conditions. However, these adaptations are different, or incomplete, and do not replace acclimatization per se. Even if heat acclimatization increases work capacity in both temperate and hot ambient conditions, endurance performance and its associated physiological responses remain impaired in the heat compared with temperate conditions. Acclimatization is dependent on the heat exposure characteristics. Natural acclimatization is more complex than artificial acclimation using a laboratory protocol, as it includes factors such as learning pacing strategies. Physiological responses to heat acclimatization also show some inter-individual variability. For example, the commonly observed lowering in core temperature has been reported to occur in some, but not all individuals following an acclimatization protocol. Therefore, identifying the specific acclimatization responses of athletes is an important factor to optimize performance in a hot environment. In addition, whereas most of the literature has described the responses of groups of soldier, worker or voluntary participants in laboratory situations, data on competitive athlete in real training conditions are needed. Therefore, a protocol was designed to characterize the individual acclimatization responses of North European football players who participated in 1-week acclimatization camp in Qatar. The aim of the study was to identify the relationship between field-running performance in a hot environment and individual heat acclimatization responses to a standardized heat-response test in football players. Given that evidence form the laboratory suggest that heat acclimatization facilitates performance, we hypothesized that individual physiological/adaptation responses would be associated with different game performance responses when playing in a hot environment.