Effect of pre-cooling by cold-water immersion on continuous sweat conductivity kinetics during submaximal exercise in the heat.

Cold-water immersion (CWI) is widely used as a pre-cooling strategy to improve exercise tolerance in hot environments, yet its effects on the temporal organization of sweating remain poorly understood. This study investigated the effects of whole-body CWI on sweating dynamics during submaximal exercise in the heat and the associated changes in body temperature and body heat storage. Fifteen endurance-trained participants completed two randomized experimental trials consisting of 45 min of cycling in a hot environment (35 °C, 30 % relative humidity), either following whole-body CWI or without pre-cooling. Core temperature, skin temperature, whole-body sweat rate (WBSR), and body heat storage were assessed. Sweating dynamics were continuously monitored using a wearable microfluidic device providing real-time measurements of conductivity-derived sweat sodium chloride concentration equivalent ([NaCl] equivalent). CWI markedly reduced body heat storage (-10.84 ± 1.33 vs. 0.06 ± 0.26 W m-2, p < 0.001). The onset of detectable sweating occurred significantly later following CWI (1462 ± 291 vs. 558 ± 147 s, p < 0.001), and WBSR was significantly lower (0.84 ± 0.20 vs. 1.38 ± 0.32 L h-1, p < 0.001). Relative sweat [NaCl] equivalent drift indices did not differ between conditions, indicating similar changes in conductivity-derived sweat [NaCl] equivalent once detectable sweating had occurred. Overall body heat storage remained significantly lower following CWI compared with the control condition (1.33 ± 0.27 vs. 1.59 ± 0.24 W m-2, p < 0.001). These findings indicate that CWI delays the appearance of detectable sweating through a transient reduction in body heat storage at exercise onset. As body heat storage progressively increased during exercise, detectable sweating occurred with similar conductivity-derived sweat [NaCl] equivalent changes despite the delayed onset. Continuous sweat monitoring provided novel insight into the temporal characteristics of sweating responses following pre-cooling.