Human tolerance to heat strain during exercise: influence of hydration

Abstract

This study determined whether 1) exhaustion from heat strain occurs at the same body temperatures during exercise in the heat when subjects are euhydrated as when they are hypohydrated, 2) aerobic fitness influences the body temperature at which exhaustion from heat strain occurs, and 3) curves could be developed to estimate exhaustion rates at a given level of physiological strain. Seventeen heat-acclimated men [maximal oxygen uptake (VO2max) from 45 to 65 ml.kg-1.min-1] attempted two heat stress tests (HSTs): one when euhydrated and one when hypohydrated by 8% of total body water. The HSTs consisted of 180 min of rest and treadmill walking (45% VO2max) in a hot-dry (ambient temperature 49 degrees C, relative humidity 20%) environment. The required evaporative cooling (Ereq) exceeded the maximal evaporative cooling capacity of the environment (Emax); thus thermal equilibrium could not be achieved and 27 of 34 HSTs ended by exhaustion from heat strain. Our findings concerning exhaustion from heat strain are 1) hypohydration reduced the core temperature that could be tolerated; 2) aerobic fitness, per se, did not influence the magnitude of heat strain that could be tolerated; 3) curves can be developed to estimate exhaustion rates for a given level of physiological strain; and 4) exhaustion was rarely associated with a core temperature up to 38 degrees C, and it always occurred before a temperature of 40 degrees C was achieved. These findings are applicable to heat-acclimated individuals performing moderate-intensity exercise under conditions where Ereq approximates or exceeds Emax and who have high skin temperatures.

Effect of a Cooling Vest on Core Temperature in Athletes With and Without Spinal Cord Injury

“…Although ACSM and Sawka report considerable attrition from exercise in AB persons when T c >40°C, Saboisky observed this at the lower temperature of 38.8°C. 9,37,38 Meanwhile in SCI athletes, Webborn noted volitional termination of exercise at T c >39°C. 25 Although 83% of the TP group reached this T c max >39°C in either condition (NV, V), none demonstrated symptoms consistent with heat-related illness and none terminated exercise prematurely.…”

Avaliação Da Perda Hídrica E Hábitos De Hidratação De Atletas Universitários De Futsal Competitivo

“…Os efeitos fisiológicos da desidratação podem ocorrer em qualquer grau em que esta se encontre. Observando as médias e os valores má-ximos dos percentuais de desidratação encontrados nesta pesquisa, sabe-se que as possíveis consequências fisiológicas implicadas são: início do comprometimento da termorregulação e o aumento da temperatura corporal central 16,23 , diminuição da capacidade aeróbica máxima e sensação intensa de sede 2,5,16 , o que pode prejudicar o desempenho cognitivo de indivíduos treinados e destreinados 24 . A TS dos atletas tanto do treino A como a do B foi superior a encontrada por outros autores ao estudar um time de futebol amador 15 (8,8 mL/min) e semelhante à obtida no estudo com atletas de judô 14 utilizando a mesma metodologia para se obter a taxa.…”

“…A pesagem pós-treino se deu ao final das sessões, após os atletas terem retirado o excesso de suor do corpo e trocado a vestimenta usada no treino por uma vestimenta seca, sendo esta semelhante à utilizada na pesagem feita inicialmente, conforme a orientação fornecida 13 . A partir das variações de peso verificadas nos treinos, foram empregadas fórmulas para calcular a perda hídrica relativa (PHR), a taxa de sudorese (TS) e o percentual de desidratação dos atletas, sendo este último avaliado segundo as classificações de Sawka et al 16 . As fórmulas utilizadas foram:…”

Hormonal Contraception, Body Water Balance and Thermoregulation

“…A reduction in the set point for temperature regulation secondary to blood volume expansion has profound effects on performance of physical activity in the heat because core temperature is maintained at a lower level and strain on the cardiovascular system is reduced. Conversely, dehydration (plasma volume loss) elevates exercise core temperature (81, 83) and decreases exercise tolerance (82,83).Estrogen may alter the threshold for thermoregulation during exercise in the heat. Core temperature responses to passive heating and exercise in heat are reduced during the follicular phase of the menstrual cycle, the cycle phase characterized by rising estrogen levels (47, 48, 53,73).…”

“…A reduction in the set point for temperature regulation secondary to blood volume expansion has profound effects on performance of physical activity in the heat because core temperature is maintained at a lower level and strain on the cardiovascular system is reduced. Conversely, dehydration (plasma volume loss) elevates exercise core temperature (81, 83) and decreases exercise tolerance (82,83).…”