Combined active and passive heat exposure induced heat acclimation in a soccer referee before 2014 FIFA world cup

Background: The 2014 FIFA World Cup was held in Brazil, where the climatic conditions presented a significant thermoregulatory and perceptual challenge to those unfamiliar with the heat and humidity. Case Presentation: This case report documents the adaptation induced by a novel mixed methods (isothermic and passive) heat acclimation (HA) regime for a northern European professional soccer match official prior to the tournament. The intervention involved 13 HA sessions over an 18 day period comprising five isothermic HA sessions whereby intermittent running was used to target and maintain tympanic temperature (Tytemp) at 38°C for 90 min, and seven passive HA sessions of 48°C water bathing for 30 min. The athlete performed a heat stress test (HST) (35 min running at four incremental intensities in 30°C) and a repeated high-intensity running test (as many 30 s self-paced efforts as possible, to a maximum of 20, with 30 s passive recovery) before and after the intervention. The mixed methods HA regime increased plasma volume (+7.1%), and sweat loss (+0.9 L·hr-1), reduced exercising Tytemp (-0.6°C), and mean body temperature (-0.5°C). High-intensity running performance improved after HA (+29%), as did the perception of thermal comfort during exercise (-0.3 units). Conclusion: This data evidences the effectiveness of a practical, mixed methods HA strategy, remotely implemented around training and competition, at inducing the heat acclimation phenotype in a high-level soccer match official. Key words: Interval training; Core temperature; Exercise; Performance; Plasma volume