Thirteen healthy and fit men [age = 27 ± 8 (SD) yr, height = 177 ± 5 cm, mass = 75 ± 7 kg, body fat = 14 ± 5%, maximal O₂ consumption = 51 ± 4 ml · kg¹ · min¹] participated in an experiment designed to test their thermoregulatory response to a challenging cold exposure after 5 h of demanding mixed exercise during which only water was consumed. Subjects expended 7,314 ± 741 kJ on cycling, rowing, and treadmill-walking machines, performed 8,403 ± 1,401 kg · m of mechanical work during resistance exercises, and completed 120 inclined sit-ups. Subjects then assumed a seated position in a 10°C air environment while wearing shorts, T-shirt, rain hat, and neoprene gloves and boots. After 30 min the subjects were showered continuously with cold water (~920 ml/min at 10°C) on their backs accompanied by a 6 km/h wind for up to 4 h. Blood samples were taken from the nondominant arm every 30 min during the exposure and assayed for energy metabolites, hormones, indexes of hydration, and neurotransmitters. Counterbalanced control trials without prior exercise were also conducted. Blood insulin was higher during the control trial, whereas values of glycerol, nonesterified fatty acids, -hydroxybutyrate, lactate, cortisol, free triiodothyronine, and thyroxine were lower. Three subjects lasted the maximum duration of 4.5 h for control and fatigue trials, with final rectal temperatures of 36.43 ± 0.21 and 36.08 ± 0.49°C, respectively. Overall, the duration of 172 ± 68 (SD) min for the fatigue trial was not significantly different from that of the control trial (197 ± 72 min) and, therefore, was not affected by the preexposure exercise. Although duration was positively correlated to body fatness and shivering intensity, the latter was not correlated to any physical characteristic or the fitness level of the individual.

shivering; hypothermia; thermoregulation; heat debt; rectal temperature

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