| The present research was developed to investigate the separate and interactive influences of hydration status, fluid replacement, short-term aerobic training, heat acclimation, and aerobic fitness during exercise in the heat. The research focused on an uncompensable heat stress environment, where the capacity for evaporative heat loss is less than the amount required to maintain thermal steady state. This was produced by either light (3.5 km . h -1, 0% grade, no wind; Studies 1--3) or heavy (4.8 km . h-1, 4% grade, no wind; Study 1) exercise in the heat (40°C, 30% relative humidity) while wearing a multilayered clothing ensemble with a high insulative capacity and a low water vapour permeability. Study 1 investigated the influences of hydration status (euhydration versus hypohydration of 2.5% body mass), fluid replacement, and exercise intensity. Study 2 investigated the influences of short-term aerobic training and hydration status on unfit (LF) individuals. Study 3 investigated the influences of heat acclimation and hydration status on individuals of both LF and high fitness (HF). Hypohydration resulted in significant increases in cardiovascular and thermal strain and a decrease in exercise tolerance regardless of other factors. Fluid replacement decreased heart rate (fc) and increased tolerance time (TT) during fight exercise. In LF, two weeks of aerobic training or heat acclimation increased sweat rate (SR), but did not influence fc, rectal temperature (Tre), or TT. In HF, two weeks of heat acclimation increased SR, decreased T&d1;sk and Tre, but did not influence f c or TT. Neither training nor heat acclimation in LF was able to increase TT to a level similar to that in HF before undergoing heat acclimation. A greater DeltaTre over the course of the exercise was present in HF, through a combination of both a lower initial, and a higher endpoint, Tre. The higher endpoint Tre in HF was consistent regardless of hydration, training, or acclimation status. It is concluded that even mild hypohydration can significantly impair physiological responses and tolerance to exercise in an uncompensable heat stress environment, and that high aerobic fitness and habitual activity imparts a degree of protection that cannot be replicated by either short-term training or heat acclimation. |
