The Thermophysiology Of Uncompensable Heat Stress PDF Download

This study examined the efficacy of hyperhydration approaches during compensable and uncompensable exercise-heat stress and the impact of hyperhydration on physiologic response and tolerance to heat strain. The general approach was to determine if 1-h pre-exercise hyperhydration (29.1 ml-kg LBM-1 with or without glycerol 1.2 g-kg LBM-1) provided a physiologic advantage. During compensable heat stress (CHS) the evaporative heat loss required (Ereq = 293 W.m-2) to maintain steady-state core temperature was less than the maximal capacity (Emax = 462 W.m-2) of the climate for evaporative heat loss (Ereq/Emax = 63%). During uncompensable heat stress (UCHS) the Ereq (366 W.m-2) was greater than Emax.(88 W.m-2; Ereq/Emax = 416%) and core temperature continued to rise until exhaustion from the heat strain occurred. Eight heat-acclimated men completed 5 trials (euhydration, glycerol hyperhydration, and water hyperhydration both with and without rehydration (replace fluid lost during exercise) in CHS and 3 trials (control, glycerol hyperhydration and water hyperhydration) in UCHS. During exercise in the heat (35 deg C, 45% rh) there was no difference between hyperhydration methods for increasing total body water. During CHS, hyperhydration did not alter core temperature, skin temperature, whole body sweating rate, local sweating rate, sweating threshold temperature, sweating sensitivity, or heart rate responses compared to euhydration trial. Likewise, no difference was found between water and glycerol hyperhydration for these physiologic responses. During UCHS, hyperhydration did not alter thermal or cardiovascular (stroke volume, cardiac output, blood pressure, total peripheral resistance) responses or heat strain tolerance compared to the control trial.