Introduction: The effect of heat stress and hydration status on performance during aerobic exercise has been studied extensively but their interaction effect on pacing and performance is still unclear. Therefore, the main goal of this study is to investigate the combined effect of hypohydration and heat stress on pacing pattern and exercise performance during a 40-km cycling time trial. Methods: 13 male cyclists performed 40-km cycling time trials in 25°C or 35°C in euhydrated (EU25, EU35) and hypohydrated (HYPO25, HYPO35) conditions. Relative humidity was 55±3% whereas air velocity was maintained at 7 m/s. Hypohydration was induced before starting the time trial by 50 min exercise in 30°C. Power output, heart rate, gastrointestinal temperature, skin temperature, thermal sensation and comfort, thirst sensation, and RPE were measured during the time trial. The significance of effects of experimental condition on the dependent variables was determined using two-way ANOVAs (or three-way ANOVAs to determine significance of effects over time). The practical meaningfulness of the performance effect was determined according to Batterham and Hopkins (2006). Results: Body mass at the start of the time trial was 1.1% (95% confidence interval (CI): 0.94-1.23%) lower in the HYPO trials than in the EU trials (F=267, P<0.001). At the finish, hypohydration levels were 2.1% (1.8 - 2.3%) of initial body mass in EU25, 2.7% (2.5 - 2.9%) in EU35, 3.1% (2.9 - 3.3%) in HYPO25 and 3.8% (3.6- 4.0%) in HYPO35. Heat stress increased finish time by 4:19 min (1:57 - 6:40 min; F=14.9, P=0.002) whereas a trend was observed for hypohydration (mean difference: 1:18 min ( -0.15 - 2.8 min); F=4.09, P=0.06). The chances that the effects are beneficial/trivial/harmful on the finish time of a 40-km cycling time trial in real-life competition are 0/1/99% for the heat stress intervention and 0/19/81% for the manipulation of hydration status. No interaction effect was found between heat stress and hydration status on finish time (F=0.10, P=0.75) and pacing pattern (F=0.72, P=0.70). Heat storage, skin temperature, thermal sensation/comfort and RPE were higher in the hot trials, whereas RPE and thirst sensation were higher in the hypohydration trials. Discussion: The negative effect of hypohydration on exercise performance during a 40-km cycling time trial is similar in hot and moderate conditions. Probably, the cooling provided by the high air velocity prevented hypohydration to become a performance limiting factor.
© Copyright 2014 19th Annual Congress of the European College of Sport Science (ECSS), Amsterdam, 2. - 5. July 2014. Veröffentlicht von VU University Amsterdam. Alle Rechte vorbehalten.