Glycerol-induced hyperhydration and rehydration: Effect on tennis skill performance
The purpose of this study was to evaluate the effectiveness of glycerol as a hydrating and rehydrating agent on advance level tennis play. Eleven male subjects (age 26.5 ± 2.1 yrs; weight 77.4 ± 3.1 kg) completed two counter-balanced, double blind trials. Each trial was divided into three phases: (1) hyperhydration in which subjects were given a bolus of glycerol (G) (1.0 g · kg -1 + 4.0 ml water · kg -1 ) or a placebo (P) (4.0 ml water · kg-1 ) followed by 18.0 ml water · kg-1 over 150 min, (2) 120 min of exercise-induced dehydration (EID), (3) rehydration in which subjects were given a second bolus of glycerol (0.5 g · kg -1 + 2.0 ml water · kg-1 ) or P (2.0 ml water · kg-1 ) with each condition followed by 9.0 ml water · kg-1 over 90 min. Following each phase, subjects performed 5 and 10 m sprint tests, a repeated-effort agility test, and tennis skill tests. During hyperhydration, urine volume (UV) was significantly less (G: 595 ± 83, P: 1559 ± 138 ml; p = 0.001) and fluid retention significantly greater (G: 65 ± 5, P: 9 ± 7%; p = 0.001) during the G trial. Additionally, glycerol ingestion significantly increased plasma volume (PV) (G: 7.32 ± 1.00, P: 1.05 ± 0.67%; p = 0.001). Following EID, body weight was reduced in both groups but was not significantly different between groups (G: -2.71 ± 0.08, P: -2.67 ± 0.09%). During rehydration, PV restoration was significantly greater during the G trial (G: 1.55 ± 0.77, P: -4.36 ± 1.28%; p = 0.001). Throughout the entire trial, UV was significantly less (G: 952.7 ± 121.2, P: 1813.6 ± 148.5 ml; p = 0.001) and fluid retention significantly greater (G: 71.4 ± 3.4, P: 45.8 ± 4.0%; p = 0.001) in the G trial. In both the 5 and 10 m sprint tests significantly slower sprint times were observed after EID compared to post hyperhydration. Following rehydration, sprint times were significantly faster than post EID times but were not significantly different from post hyperhydration times. No significant difference existed between the two groups for the repeated effort agility tests and tennis skill tests. The results of this study indicate that a fluid ingestion regimen including glycerol appears to improve hydration status prior to exercise and may also offer a more rapid restoration of PV after exercise than does water alone. However, while glycerol improved hydration status, tennis skill related performance was not enhanced following EID or rehydration.