We examined the effect of mild dehydration (−1% of body mass) on an outdoor 5-km hill-climbing cycling ride in the heat. The data indicated that even a mild degree of dehydration could detrimentally affect cycling performance during hill cycling, probably due to greater thermal stress. We found that, by the end of the race, Core temperature (Tc) was greater in the dehydrated (DEH) than euhydrated (EUH) trial. We expected EUH athletes to have higher Tc due to greater exercise intensity, as supported by the literature.26,27 To the contrary, Tc was lower in the EUH than DEH trial, probably due to a greater sweating response.
In this regard, Walsh et al,28 who examined the performance of cyclists at 32°C, found that the time to exhaustion at 90% of maximal oxygen uptake was decreased by 31% when participants started cycling dehydrated by −1.8%. Below et al9 also showed that, when cyclists were dehydrated by less than −2%, their exercise performance was 6.5% lower. Similarly, Casa et al29 found that Tc was 0.22°C greater for every 1% of body mass lost during the DEH trial than the hydrated trial. Exercise intensity causes a rise in body Tc,26 especially in a hot environment. However, dehydration can play a modulating role in the extent of exercise-induced hyperthermia.5 Sweat sensitivity has been used as an index of thermoregulatory efficiency because it represents sweating as a response to a rise in Tc. Our data indicated that even −1% dehydration induces lower sweat sensitivity.
Armstrong et al20 examined the thermoregulatory responses during fixed 90-minute treadmill walking in the heat when participants were EUH or DEH with or without ad libitum water intake during exercise. Both sweating and sweat sensitivity were reduced in the hypohydrated trial (−3.9% body mass) when participants did not drink fluids during exercise, showing that dehydration modulates thermoregulatory responses. Armstrong et al20 also found that lower sweat sensitivity was associated with lower body mass and greater Osmp.
In our study, heart rate response during the 5-km cycling test did not differ between the EUH and DEH trials. Maybe the high intensity of the exercise induced near-maximal heart rate in both trials. Although we did not measure cardiac output during exercise, we speculate that both stroke volume and cardiac output might have been compromised as a response to dehydration. This response has been well documented with a greater degree of dehydration by others.1
The cycling cadence of our participants in each single kilometer of the performance trial was higher in the EUH than DEH condition, suggesting that dehydration hinders a cyclist's ability to maintain cycling cadence. We also found RPE was lower at the end of the EUH than DEH trial. We speculate that the combined effect of dehydration and hyperthermia influenced participants' perceptions of effort, possibly due to cardiovascular drift.1,30 All cyclists in our study had better performance during the EUH than DEH trial even though the lower body mass could have provided a potential performance benefit (ie, greater power-to-mass ratio) in the hill-climbing cycling course. In summary, our main finding was that mild dehydration decreased hill-climbing cycling performance via increased thermoregulatory strain and RPE.