Ambient temperature and exercise capacity : effects of substrate and fluid provision
This series of studies began with an examination of the effects of ambient temperature (4°C-31°C) on exercise capacity at approximately 70% of VO2max. In this first study, exercise capacity was greatest at 11°C (93.5(16.4) min) and shortest at 31°C (51.6(10.5) min) with intermediate exercise times at 4°C (81.4(27.2) min) and 21°C (81.2(16.3) min). In the trial at 31°C, Tre, Tsk, HR, RPE, and sweat rate were all significantly higher and total CHO oxidation was significantly lower than during any other trial. At 21°C Tre, Tsk, and sweat rate were all significantly higher, and total CHO oxidation lower, than during the 4°C and 11°C trials. These results suggest that heat stress and subsequent failure of thermoregulation or circulatory collapse rather than complete depletion of endogenous muscle glycogen were the possible limiting factors during exercise at 21 and 31°C. At 4°C and 11°C total CHO oxidation was similar suggesting that CHO depletion may have limited exercise capacity in these trials. The reduced exercise capacity at 4°C compared to 11°C can be explained by the observed increase in the rate of CHO oxidation during exercise in the 4°C trial. The second study of this series examined exercise capacity at approximately 60% of VO2max in a hot (30°C) environment with ingestion of either a dilute CHO drink (2%) aimed at fast fluid replacement or a concentrated CHO drink (15%) aimed at CHO provision. In this study we clearly observed that a 2% drink was most effective in delaying the onset of fatigue during prolonged exercise at 30°C but a 15% drink was also effective at delaying fatigue when compared with no fluid. The results of the studies performed in this series have implications for the optimisation of exercise performance in different environmental conditions and indicate that a drink with low CHO content and an emphasis on fluid replacement would be preferential in both cold and hot environments.