The critical level of water deficit causing a decrease in human exercise performance: a practical field study.

To analyse the critical level of water deficit which causes a decrease in aerobic and anaerobic exercise performance, a step test score (STS) and 10 s maximal anaerobic power (MAP) output during cycling exercise were measured in two experiments (Ex-1, n=7, and Ex-2, n=9), before and after baseball practice, using subjects who played regularly. The measurements in both Ex-1 and Ex-2 were repeated under four conditions of fluid ingestion (FI) (FI of 80%, 60%, 40%, and 20% of the total sweat loss) on hot summer days. The subjects were allowed free access to a sports beverage, maintained at 10-15 degrees C, within any given FI condition during the exercise. The [mean (SEM)] duration of the exercise and the environmental conditions (wet bulb globe temperature) were similar between Ex-1 [3.52 (0.14) h and 29.2 (0.6) degrees C, respectively] and Ex-2 [3.82 (0.12) h and 29.2 (0.4) degrees C, respectively]. In both Ex-1 and Ex-2, the loss of body mass (Delta m(b)) increased significantly as FI decreased. In Ex-1, the STS significantly decreased ( P<0.05) at values of delta m(b) in excess of 2.4 (0.2)% (40%FI). In Ex-2, the MAP remained unchanged at values of delta m(b) up to 2.5 (0.3)% (40%FI), while the MAP significantly decreased ( P<0.05) at values of delta m(b) of 3.9 (0.2)% (20%FI). These results suggest that there is a critical level of water deficit at which a decrease in aerobic and anaerobic performance occurs, and that aerobic performance may be more adversely influenced by dehydration than anaerobic power output during exercise-induced dehydration.