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

Abstract.

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°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)°C, respectively] and Ex-2 [3.82 (0.12) h and 29.2 (0.4)°C, respectively]. In both Ex-1 and Ex-2, the loss of body mass (Δm _b) increased significantly as FI decreased. In Ex-1, the STS significantly decreased (P<0.05) at values of Δm _b in excess of 2.4 (0.2)% (40%FI). In Ex-2, the MAP remained unchanged at values of Δm _b up to 2.5 (0.3)% (40%FI), while the MAP significantly decreased (P<0.05) at values of Δm _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.