Thermoregulation in boys and men exercising at the same heat production per unit body mass

PurposeChild–adult thermoregulatory comparisons may be biased by differences in metabolic heat production (H˙p$$\dot{H}_{\text{p}}$$). We compared thermoregulatory responses of boys and men exercising at two intensities prescribed to elicit either a fixed H˙p$$\dot{H}_{\text{p}}$$ per unit body mass (BM) or a fixed absolute H˙p$$\dot{H}_{\text{p}}$$.MethodsTen boys (10–12 years) and 10 men (19–25 years) performed 4 × 20-min cycling at a fixed H˙p$$\dot{H}_{\text{p}}$$ per BM (W kg−1) at 35 °C and 35 % relative humidity (MENREL). Men also cycled (MENABS) at the same absolute H˙p$$\dot{H}_{\text{p}}$$ (in W) as the boys.ResultsH˙p$$\dot{H}_{\text{p}}$$ was lower in boys compared with MENREL, but similar to MENABS (mean ± SD, 233.6 ± 38.4, 396.5 ± 72.3, 233.6 ± 34.1 W, respectively, P < 0.001). Conversely, H˙p$$\dot{H}_{\text{p}}$$ per unit BM was similar between boys and MENREL, and lower in MENABS (5.7 ± 1.0, 5.6 ± 0.8 and 3.3 ± 0.3 W kg−1, respectively; P < 0.001). The change in rectal temperature was similar between boys and MENREL (0.6 ± 0.2 vs. 0.7 ± 0.2 °C, P = 0.92) but was lower in MENABS (0.3 ± 0.2 °C, P = 0.004). Sweat volume was lower in boys compared to MENABS (500 ± 173 vs. 710 ± 150 mL; P = 0.041), despite the same evaporative heat balance requirement (Ereq) (199.1 ± 34.2 vs. 201.0 ± 32.7 W, P = 0.87).ConclusionBoys and men demonstrated similar thermoregulatory responses to 80 min of exercise in the heat performed at a fixed H˙p$$\dot{H}_{\text{p}}$$ per unit BM. Sweat volume was lower in boys compared to men, despite similarities in absolute H˙p$$\dot{H}_{\text{p}}$$ and Ereq.