Exercise during hot-water immersion in divers habituated to hot-dry and hot-wet conditions

Purpose: Diving in warm water increases thermal risk during exercise compared to thermoneutral waters. The purpose of this study was to evaluate exercise endurance in warm- and hot-water conditions in divers habituated to wet or dry heat. Methods: Nineteen male divers completed this study at the Navy Experimental Diving Unit. Subjects were assigned DRY or WET heat habituation groups. The DRY group (n=9) cycled at 125-150W for one hour in a non-immersed condition (34.4◦C, 50%RH), while the WET group (n=10) cycled at 50W for one hour while immersed in 34.4◦C water. Exercise time to exhaustion was tested on an underwater cycle ergometer in 35.8◦C (WARM) and 37.2◦C (HOT) water at 50W. Core temperature (T_c) was continuously recorded and for all dives. Results: Time to exhaustion was reduced in HOT compared to WARM water (p<0.01) in both DRY (92.7±41.6 minutes in 35.8◦C vs. 43.4±17.5 minutes in 37.2◦C) and WET (95.9±39.2 minutes in 35.8◦C vs. 53.4±27.5 minutes in 37.2◦C) groups, but did not differ between groups (p=0.62). Rate of T_c rise was greater with higher water temperature (p<0.01), but was not different between groups (p=0.68). Maximum T_c (p=0.94 and p=0.95) and T_c change from baseline (p=0.38 and p=0.34) was not different between water temperatures or habituation group, respectively. Conclusion: Endurance decreased with increased water temperature but was not different between WET and DRY. Divers became exhausted at a similar core temperature during WARM- and HOT-water exercise. Mechanisms and applications of heat acclimation for warm-water diving should be further explored.