Rehydration Strategies For Prolonged Post-immersion Exercise Performance When Working In The Heat

Abstract

Dehydration caused by cold-water immersion can reduce exercise capacity, especially in the heat where cardiovascular and thermoregulatory demands are increased. PURPOSE: The purpose of this study is to determine if post-immersion rehydration strategies maintain performance during sustained exercise in the heat. METHODS: Subjects (n = 5; 23 ± 2y; VO2max: 50.5 ± 6.2 ml/kg·min-1; BMI: 25.5 ± 2.2; BF: 10.7 ± 4.0%) completed one non-immersed control trial (CON) and two cold-water immersion (18.0 ± 0.2 °C) trials. Upon egress, subjects were either partially (HALF) or fully (FULL) rehydrated with water during a weighted ruck march (20.4 kg; 5.6 kph) at 50% VO2max in a warm environment (29.6 ± 0.1 °C; 50.1 ± 0.3% RH). CON trial received no water during the ruck march. Subjects then completed a 15-min intermittent exercise protocol (iEPT) with target speeds at 0, 25, 50 75, and 100% of their maximum sprint speed. The final 2-mins was a variable run in which they were instructed to cover as much distance as possible. Distance (km), heart rate (HR), core temperature (TC), rating of perceived exertion (RPE), and thermal comfort (TC) were recorded during the iEPT. Blood lactate (BL) was measured at the beginning and end of the iEPT. Data were analyzed with repeated measures one-way ANOVA. RESULTS: Initial hydration level and nude weight were not different between trials. During immersion, subjects lost 2.2 ± 0.5 and 2.3 ± 0.6% of body mass for HALF and FULL, respectively, and were rehydrated with 0.74 ± 0.3 L (HALF) and 1.6 ± 0.5 L (FULL) of water during the ruck. During the iEPT, distance covered during the targeted run (AVG: 1.90 ± 0.2 km; p = 0.11), and the variable run (AVG: 0.33 ± 0.08 km; p = 0.20) were not different between trials. HR during the iEPT was not different (p = 0.53) between trials, but TC was lower for FULL compared to CON (p = 0.047). BL was increased at the end of iEPT (p < 0.01) but was not different (p = 0.55) between trials. RPE (p = 0.38), TC (p = 0.11) were not different between trials. CONCLUSIONS: Both partial and full rehydration after cold-water immersion maintains performance when compared to a non-immersed control trial. Providing fluids during prolonged post-immersion exercise also maintains thermoregulation and comfort when working in the heat.