Siegler, J. C., Hillman, A., Vince, R., Taylor, L., McNaughton, L. (June 02, 2010). The influence of heat and dehydration on cycling time-trial performance. Presentation 1683 at the 2010 Annual Meeting of the American College of Sports Medicine, Baltimore, Maryland; June 2-5.

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This study investigated the effects of prolonged submaximal exercise and time-trial performance in the heat in euhydrated or dehydrated states. Healthy male trained club cyclists (N = 5) completed 90 minutes of ergometer cycling at 95% lactate threshold followed by a 5-km time-trial in four different randomized conditions: heat dehydration, heat euhydration, ambient dehydration, and ambient euhydration. All trials were completed in an environmentally controlled heat chamber. Venous blood was obtained pre- and post-trial and subsequently analyzed for hematocrit, and hemoglobin, changes in plasma volume, whole blood electrolyte concentrations, and heat shock protein 70. Nude body weight was obtained pre- and post-trial, and core temperature, heart rate, and rating of perceived exertion were monitored continuously throughout the exercises.

Plasma volume declined in all conditions, with the dehydration conditions resulting in the greatest overall loss (~8 %) compared to ~3 % for the euhydration conditions. Significant interaction effects were observed for both whole blood sodium and chloride, but not potassium. Post-hoc analysis revealed both dehydration conditions caused significant increases in sodium and chloride, while the euhydration conditions caused significant declines in the concentrations of both substances. Heat shock protein 70 expression (lymphocytes) was not affected by any of the four conditions. There was a significant interaction effect for core temperature with all conditions exhibiting higher core temperatures post-90 minutes. Heart rate was significantly elevated in all conditions at 80 to 90 minutes into the exercises compared to the initial 10 minutes, but did not differ between conditions. In terms of 5-km time-trial performance, the hot dehydrated condition resulted in a significant increase in time-to-completion compared to all other conditions.

Implication. While significant levels of dehydration were apparent in the ambient dehydration condition (~3.2 %), only the hot dehydrated condition (~4.2%) resulted in a significant decline in 5-km time-trial performance.

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