Low-intensity exercise delays the shivering response to core cooling

Hypothermia can occur during aquatic exercise despite production of significant amounts of heat by the active muscles. Because the characteristics of human thermoregulatory responses to cold during exercise have not been fully elucidated, we investigated the effect of low-intensity exercise on the shivering response to core cooling in cool water. Eight healthy young men (24 ± 3 yr) were cooled through cool water immersion while resting (rest trial) and during loadless pedaling on a water cycle ergometer (exercise trial). Before the cooling, body temperature was elevated by hot water immersion to clearly detect a core temperature at which shivering initiates. Throughout the cooling period, mean skin temperature remained around the water temperature (25°C) in both trials, whereas esophageal temperature (T_es) did not differ between the trials (<i>P</i> > 0.05). The T_es at which oxygen uptake (V̇o₂) rapidly increased, an index of the core temperature threshold for shivering, was lower during exercise than rest (36.2 ± 0.4°C vs. 36.5 ± 0.4°C, <i>P</i> < 0.05). The sensitivity of the shivering response, as indicated by the slope of the T_es-V̇o₂ relation, did not differ between the trials (-441.3 ±177.4 ml·min^-1·°C^-1 vs. -411.8 ± 268.1 ml·min^-1·°C^-1, <i>P</i> > 0.05). The thermal sensation response to core cooling, assessed from the slope and intercept of the regression line relating T_es and thermal sensation, did not differ between the trials (<i>P</i> > 0.05). These results suggest that the core temperature threshold for shivering is delayed during low-intensity exercise in cool water compared with rest although shivering sensitivity is unaffected.