Physiological responses of tropic-dwelling Malaysians during exercise in the heat 熱帯地住民マレーシア人の暑熱下運動時の生理反応に関する研究

博士論文

The objectives of this study are firstly to determine if tropical and temperate residents differ in their thermal responses during exercise in the heat and secondly if tropical residents show heat acclimation (HA) after exercise in the heat and thirdly evaluate the effects of exercise in and exposure to cold following HA. In the first study, eight Japanese-male students (JS) and eight Malaysian-male students (MS) were tested for thermal responses during exercise in the heat. Subjects spent an initial 10 min rest, 60 min of cycling at 40% VO2max and then 40 min recovery (Ta：32℃, rh：72%). The percent dehydration (％D), heat-activated sweat glands density (HASG), rectal temperatures (Tre) and mean skin temperatures (Tsk) were measured during the test. In the next study, sixteen Malaysian natives exercised for fourteen days (60 min day-1) in the heat for HA. Following this they were divided into two groups for deacclimation (DA). One group exercised in the cold (EXG, n=8) for 60 min-day-1 (morning) and was exposed to the cold for another 60 min (afternoon) for fourteen days. The other group was exposed to the cold (EPG, n=8) for 60 min each in the morning and afternoon (Ta：18.0℃, rh：58％) over the same period. All returned to exercise in the heat for reacclimation (RA) for 10 days. Subjects were tested before (day 1) and after HA (day 16), on the 4th (day 21) and 14th, (day 32) day exercise or exposure to cold and 3rd (day 36) and 10th(day 44) day return to exercise in the heat. For the tests subjects cycled on bicycle ergometer for 60 min at 60％ of VO2max in heat. Tre, Tsk, heart rate (HR) and blood parameters were measured during the test. In the first study the extent of ％D was significantly higher in the JS compared to MS. The HASG was significantly more in JS compared to MS at rest and recovery. Tre, was higher in MS during the test. Tsk was significantly higher starting at the 5th min of exercise until the end of the recovery period in MS compared to JS. In the second study, after the HA the ％D during test significantly increased from day 1 to day 16. Heart rate (HR), Tre, plasma lactate concentration. Hb and Hct at the 40th min of exercise were significantly lower after HA and serum osmolality were significantly higher. Tre and HR were used to determine the decay/gain of HA which was calculated using the formula described by Pandolf et al. (1977). During DA, EXG showed decay in Tre of 24％ and 35％ and HR of 29％ and 35％ on days 21 and 32, respectively. For EPG the corresponding decay was of 2％ and 9％ for Tre and 17％ and 17％ for HR. The percentage decay of HA in Tre at day 21 showed a significant difference between groups. After 10 days of RA, EXG showed gains of 11％ in Tre and 12％ in HR, while EPG showed gains of 47％ in Tre and 38％ in HR. In conclusion, tropical subjects when compare to temperate natives have lower sweat rates associated with of higher rectal temperatures and mean skin temperatures during rest, exercise and recovery period, indicating better adaptation to exercise in the heat. The differences are probably due to setting of the core temperature at a higher level and enhancement of dry heat loss, which occurred during passive heat exposure. Tropical subjects, although exposed to prolonged passive heat exposure, were not fully heat acclimatized as shown by the further adaptations in heart rate and rectal temperatures that occurred during the 14 days of exercise in the heat. Both exercise in the cold and exposure to cold caused a decay in HA. The decay tends to be greater in exercise in the cold than in mere exposure to cold. This may be due to the absence of heat stress stimuli and deadaptation of responses during exercise in the cold. On returning to exercise in the heat acclimation was faster than in the first 14 days, and tend to be quicker in those merely exposed to the cold.

CONTENTS 1. General Introduction 2.0 CHAPTER A : Effects of Exercise in the Heat on Thermoregulation of Japanese and Malaysian Males 3.0 CHAPTER B : Effects of Short-term Exercise in hte Heat on Thermoregulation, Blood Parameters, Sweat Secretion and Sweat Composition of Tropic-dweling Subjects 4. CHAPTER C: Decay of heat acclimation during exercise in cold and exposure to cold environment REFERENCES Abstract