INTRODUCTION: Women`s marathon of Tokyo Olympic Games 2020 was held on the 7th of August in Sapporo, Japan. This region has recorded, exactly in this day its highest temperature in 21 years, at 6 a.m. at the start of the race, the sky was clear, the temperature was 25 °C and a relative humidity was 82%. The temperature rose during the competition to above 30°C with relative humidity above 80%, which means values of WBGT exceeding 30, already in the dangerous category. With this type of warning, the acclimatization process becomes essential in an athletes sports preparation, not only to improve sport performance but also to preserve physical integrity. METHODS: In this study, two female athletes underwent a period of 15 days of acclimation. The process was carried out in the climatic chamber (CC) located at the laboratory of Industrial Aerodynamics (LAI) of the University of Coimbra. In all sessions, the duration was 60 min, temperature was 34.2°C±0.4, relative humidity 77.2% ±5.4%, and air speed was lower than 2.1 m s-1 at the chest level. A treadmill (HP Cosmos) was used for all sessions, VO2 max tests were performed on the first day of each acclimatization period using a gas analyser (Metamax cortex), to define the work intensity of the following acclimatization sessions (60% of VO2 max). Oxygen consumption tests were performed at days 1, 7 and 15. Urine temperature and density, body mass, heart rate variability before and after effort were controlled (Polar V800+H10 chest band). In addition, heart rate and lactate concentration (spectrophotometer Dr. Lange) and fluid consumption, loss of plasma volume and percentage of loss of body mass throughout each session were also controlled. RESULTS: Due to the acclimatization process the following changes were observed: an increase of 3.3% in the athletes running velocity and a decrease in oxygen consumption around 2.1 ml/kg/min, for the same percentage of VO2max; plasma volume loss varied from 9.98% to 3.97%; a decrease of urine density (1013 to 1007.5) and heart rate (151 to 138 bpm), a 1073 ml/h increase in sweat rate; an increase of percentage of loss of body mass (1.12% to 2,72%); as regards Heart Rate Variability, a decrease in LF/HF ratio (3.46 ms2 to 1.38ms2), Mean RR increased (951 - 1203 ms) and LnRMSSD increased (3.69 ms to 4.44 ms). The core temperature after the exercise had not large variations. Its average was 38°C ±0.53°C, which is 2.9°C±0.38°C above the initial temperature, across all training sessions. Mean blood lactate was 1.23 ± 0.27 mmol/l. CONCLUSION: The acclimatization process showed a clear improvement in performance indicators and the decrease in sympatho-vagal activity prior to exercise may suggest an adaptation to thermal adverse conditions. Additionally, decreases in the plasma volume loss, heart rate, ventilation and oxygen consumption were verified for the same intensity which is essential to be able to maintain body thermoregulation and hight levels of performance in adverse weather conditions.
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