Does a Prolonged Work Day in the Heat Impair Heat Loss on the Next Day in Young Men?.

PURPOSE: Heat strain is known to be exacerbated on the second of consecutive work days. We therefore evaluated whether prolonged work in the heat would impair whole-body heat loss capacity on the next day. METHODS: To evaluate this possibility, we assessed changes in whole-body heat exchange and heat storage in eight young (26 years (SD 4)) males during heat stress tests performed on the same day prior to (Day 1), and on the day following (Day 2), a prolonged work simulation. Each heat stress test involved three, 30-min bouts of semi-recumbent cycling at fixed rates of metabolic heat production (200 (Ex1), 250 (Ex2) and 300 W.m-2 (Ex3)), each separated by 15-min recovery, in hot-dry conditions (40[degrees]C, 20% relative humidity). The work simulation (7.5 h) involved three moderate intensity intermittent work bouts (2 h), each separated by 30-min rest breaks, under similarly hot-dry conditions (38[degrees]C, 34% relative humidity). Total heat loss (evaporative +/- dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was quantified as the temporal summation of heat production and loss. RESULTS: Total heat loss did not differ between Day 1 and 2 (P=0.66), and averaged (mean+/-95% CI) 185+/-7 W (Ex1), 233+/-7 W (Ex2) and 261+/-5 W (Ex3) across test days. Consequently, the change in body heat storage was also similar between Day 1 and 2 (P=0.32), averaging 133+/-15 kJ (Ex1), 99+/-16 kJ (Ex2) and 184+/-15 kJ (Ex3) across test days. CONCLUSION: When assessed in controlled laboratory conditions in young men, prolonged work in the heat does not appear to impair whole-body heat loss nor exacerbate heat storage on the following day. (C) 2017 American College of Sports Medicine

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