Exercise-Heat Strain during Hypohydration: Interaction with Heat Acclimation and Aerobic Fitness

摘要

During military operations in hot environments, the combined metabolic and environmental heat stress must be dissipated by the soldier to enable sustained physical exercise performance. The primary factors influencing a soldier's thermoregulatory responses during exercise-heat stress are acclimlation state, aerobic fitness and hydration level. During physical exercise in the heat, sweat output often exceeds water intake, resulting in hypohydration, which is defined as a body water deficit. Hypohydration causes a greater heat storage and reduces physical exercise performance relative to euhydration levels. The greater heat storage is attributed to a decreased sweating rate as well as a decreased cutaneous blood flow. The greater the level of hypohydration, the greater the magnitude of cardiovascular and thermoregulatory strain and reduced physical exercise performance. Rectal temperature and heart rate responses are elevated above euhydration levels by about 0.15 C and 4 b/min for each percent decrease in body weight during exercise-heat stress, respectively. Likewise, sweating rates are decreased by about 29/sq m/h for each percent decrease in body weight. When hypohydrated, heat acclimation decreases thermoregulatory and cardiovascular strain in a comfortable environment, but only decreases cardiovascular strain in hot environments during exercise.