The effect of altered body temperature on exercise in the heat: thermoregulatory and metabolic function

There is a paucity of information concerning the effect of a pre-manipulation of body temperature on exercise in a hot environment. Hence, the thermal impact of water immersion pre-treatment on physical endurance and physiological function during exercise in the heat (35°C, RH 50%) was investigated The efficacy of a novel method of whole-body pre-cooling, involving up to 60 min of water immersion at 29-24°C, for increasing exercise endurance in the heat was investigated. Compared to a neutral condition, pre-cooling increased the distance run (300 m ) by five male and three female subjects during a 30-min time trial. Moreover, pre-cooling attenuated the exercise-induced rise in core and skin temperature, whilst cardiac frequency was reduced and psychophysical status improved. The influence that pre-treatment immersion has on muscle metabolism and thermoregulation during exercise in the heat was investigated. Nine male subjects cycled for 35 min in the heat at 60 % of peak oxygen uptake on three occasions, with each trial preceeded by a different pre-treatment involving whole-body immersion in cool, neutral or hot water. Despite marked differences in pre-exercise core and muscle temperature, a similar pattern of muscle metabolism was observed during exercise in the heat in all conditions. Accordingly, muscle high-energy phosphate metabolism, lactate accumulation and triglyceride utilisation were not different, whilst muscle glycogen utilisation and plasma ammonia accumulation were increased at higher core and muscle temperatures. In contrast, the pre-treatments evoked marked differences in thermoregulatory function during exercise in the heat, with greater conductive heat loss and a reduced sweat rate and cardiac frequency for the precooled, compared with the neutral and pre-heated conditions. Following pre-heating, the circulatory strain imposed by exercise and acute heat stress would have been augmented by the 32 and 40 % greater loss of blood and plasma volume, compared to the pre-cooled condition. In this circumstance, cardiovascular instability, as indicated by the marked increase in cardiac frequency when compared with the other conditions, could be a primary factor limiting exercise endurance in a hot environment. These data indicate that following thermal pre-treatment, the changes to muscle metabolism are unlikely to influence endurance during exercise in the heat. Whilst muscle glycogen utilisation and plasma ammonia accumulation were increased by exercise hyperthermia, their role in fatigue during exercise in the heat is unclear. A more likely possibility is that, following water-immersion pre-cooling or preheating, altered thermoregulatory and cardiovascular function, through marked reductions in core and muscle temperatures, might be more important for determining endurance during exercise in the heat.