Exercise time to fatigue and the critical limiting temperature: effect of hydration

The data support the model that anticipation of impending thermal limits reduces efferent command to working skeletal muscle ensuring cellular preservation.The purpose of this study was to investigate the effect of active pre-warming combined with three regimens of fluid ingestion: (1) fluid replacement equal to sweat rate (FF), (2) fluid replacement equal to half the sweat rate (HF), and (3) no fluid replacement (NF). Eight males cycled to voluntary fatigue at 70% of peak power output (PPO) in 31.3±0.4°C, 63.3±1.2% relative humidity in a randomised fashion in either of FF, HF or NF conditions. For each trial the time to fatigue test was preceded by 2×20 min active pre-warming periods where subjects also cycled at 70% PPO. Subjects commenced each exercise period with identical rectal temperatures (Tre). The rate of increase in Tre for each condition during the first 20 min of active pre-warming was not different. However, the rate of increase in Tre was significantly reduced in the second active pre-warming period for all fluid conditions but no differences between conditions were noted. During the fatigue test, the rate of increase in Tre for FF was 0.29°C h'1 and 0.58°C h'1 for HF but were not significantly different. The rate of increase in Tre for the NF trial was 0.92°C h'1 and was significantly higher compared to the FF trial. Overall mean skin temperatures and mean body temperatures were higher for NF compared to FF and HF. The rate of heat storage during the fatigue test was similar for FF (80.1±11.7 W m'2) and HF (73.0±13.7 W m'2) conditions but increased to 155.8±31.2 W m'2 (P<0.05) in the NF trial. The results indicate that fluid ingestion equal to sweat rate has no added benefit over fluid ingestion equal to half the sweat rate in determining time to fatigue over 40 min of sub-maximal exercise in warm humid conditions. Fluid restriction accelerates the rate of increase in Tre after 40 min of exercise, thereby reducing the time to fatigue.