Preceding high-intensity interval exercise alters cooling responses during temperate-water immersion after exercise-induced hyperthermia.

Post-exercise cooling after exercise-induced hyperthermia is commonly evaluated by the rate of core temperature decline. However, cooling may also depend on the physiological state at the onset of cooling. This study examined whether preceding exercise condition influences responses to 20 °C temperate-water immersion after exercise-induced hyperthermia. Fifteen recreationally active adults completed randomized crossover trials involving low-intensity exercise with restricted evaporative heat loss and high-intensity interval exercise in a hot environment (40 °C, 20 % relative humidity). Exercise continued until core temperature reached 39.5 °C or volitional fatigue, followed by a 5-min preparation period and sternum-level immersion until core temperature declined to 37.5 °C. Heating rate and peak core temperature did not differ between conditions. Cooling time was longer after high-intensity interval exercise than after low-intensity exercise (22.93 ± 10.93 vs. 9.93 ± 4.92 min, p < 0.0001), and overall cooling rate was lower (0.10 ± 0.04 vs. 0.23 ± 0.08 °C·min-1, p < 0.0001). High-intensity interval exercise elicited greater metabolic and cardiovascular strain during exercise and was followed by lower cutaneous vascular conductance and skin temperature during immersion, whereas post-exercise body mass loss was similar between conditions. Although mean cooling rates exceeded the commonly cited minimum acceptable threshold in both conditions, cooling was markedly slower after high-intensity interval exercise. These findings indicate that the cooling response to temperate-water immersion is not determined by pre-immersion core temperature alone, but can vary according to the preceding exercise condition.