Original paper
The combined effects of hydration and exercise heat stress on choice reaction time

https://doi.org/10.1016/j.jsams.2006.03.006Get rights and content

Summary

The purpose of this investigation was to examine the combined effects of hydration and exercise heat stress on choice reaction time. On three separate occasions eight male subjects performed cycle exercise at ∼70% of peak power output in warm, humid conditions (31 °C, 63% relative humidity) for a maximum of 90 min or until exhaustion. Throughout the trials, subjects ingested either a volume of water equal to fluid loss (100  FR), a volume equivalent to ∼50% of fluid loss (50  FR), or no fluid (0  FR). A choice reaction time task was undertaken at rest, after 20 min of cycling, 40 min of cycling and at the conclusion of exercise. Mean reaction time for 100  FR was 342.2 ± 8.2 ms, 352.4 ± 7.5 ms for 50  FR and 345.6 ± 8.4 ms for 0  FR and were not significantly different. Choice reaction time was facilitated as the duration of exercise progressed with reaction time at 40 min and conclusion stages of exercise faster than at rest (P < 0.005). Choice reaction time and accuracy were affected by the number of choices, with choice reaction time increasing linearly with the number of choices (P < 0.005) and rate of incorrect responses increasing in the 4-choice task compared to the 1-choice and 2-choice task (P < 0.05). The results indicate that, in up to 90 min of exercise in warm, humid conditions, choice reaction time is not compromised by different levels of hydration.

Introduction

Previous studies have shown that heat stress alone can induce marked changes in repetitive psychometric tasks and mental performance.1, 2, 3 Although some conjecture exists, it is generally thought that, like physiological function, heat stress negatively affects mental performance.4, 5 One intervention strategy that can be used to offset the effects of heat stress is fluid ingestion,6, 7 as core temperature and heart rate responses are graded according to the level of hypohydration achieved during exercise. It has been previously shown that dehydration may influence the variations in mental performance, particularly during heat stress.8, 9 In the case of team sports, hypohydration has been suggested to affect the high degree of mental functioning for tactical changes, reading the play, anticipation and skill execution.10 However, there has been relatively little research conducted regarding the effects of level of hydration on mental performance, particularly during exercise.

Cian et al.11 showed that a reduction in performance of perceptual and decisional tasks occurred irrespective of whether hypohydration occurred due to exercise alone or due to a hot environment. However, this finding should be balanced against the suggestion that uncomfortable feelings emanating from exercise in the heat might result in performance decrements as a consequence of reduced attention demands on the exercise task and a shifting of attention to the feelings associated with thermal strain.12 It is also plausible that significant decrements in performance can be expected to occur when hypohydration is equivalent to 2% body mass loss, suggesting the existence of a critical limit or impairment threshold.9, 13 This hypothesis should be balanced with those studies that have shown significant decrements in physical performance with hypohydration equivalent to as little as 1% loss in body mass.14 It has also been suggested that the quantity of fluid ingested might influence cognitive performance, with better results reported for hyperhydrated compared with euhydrated subjects,11 although these improvements in mental performance have been shown following, but not during prolonged exercise.

It has also been reported that reaction time was significantly faster when subjects were hydrated, although there were no significant results for the other cognitive tasks administered.15 This seems to be a salient point as others have also shown a positive effect of exercise duration on cognitive processes. For example, when choice reaction time (CRT) and simple reaction time (SRT) tasks were performed during or immediately following exercise, an improvement in both CRT and SRT was observed.16, 17, 18 On this basis, it seems that reaction time is a useful variable that could be used to infer changes in mental processing. For instance, reaction time has been described as an important variable in a number of sporting situations where a performer must make various reactions to a ball or to manoeuvres of other performers.19

Although some studies suggest that decision-making processes are enhanced following long duration exercise,16, 17 we are unaware of any studies that have examined the combined effects of hydration level and heat stress on decision-making processes during exercise. For the purpose of this study, we chose a reaction time task given that an exercise bout in addition to the performance of a reaction time task has been described previously in terms of attentional resource allocation and defined in the literature as a dual task.17 Hence, the purpose of this investigation was to examine the extent to which the level of hydration and exercise heat stress affect choice reaction time during prolonged exercise.

Section snippets

Subjects

Eight males volunteered to participate in the study. The mean (±S.E.) age, height and mass were 24.5 ± 1 years, 179.9 ± 3.0 cm, and 77.6 ± 5.0 kg, respectively. All subjects were considered healthy, having completed a health history questionnaire and maximal cycle ergometer stress test. The participants could not be classified as elite cyclists, but each subject was actively involved in cycling either as a training method or as a regional competitive athlete. The range of values for peak oxygen uptake (

Results

Of the eight subjects who participated in the study, six completed 90 min of cycling in the 100  FR trial, five were able to sustain exercise for the same duration in the 50  FR trial, while only three subjects completed the 90 min in the 0  F trial. Only one subject successfully exercised for 90 min in each of the three experimental trials.

Discussion

The ingestion of fluid had no effect on CRT as the means for all trials were similar. This finding is in contrast with previous results15 which show significantly faster reaction time when subjects are hydrated. As the present investigation and previous work15 used similar exercise protocols and fluid ingestion regimes, it is difficult to reconcile the difference in the findings. However, one possible explanation is that in previous work15 subjects were hydrated using a commercial sport

Practical implications

  • It is unlikely that limited or no hydration during exercise in the heat will adversely affect mental functioning in competitive athletes.

  • Reaction time response is not altered by hydration during exercise but gets faster possibly due to the effect of exercise on arousal.

  • An athlete's capacity to make correct choices in relation to reaction time is unlikely to be affected by rising body temperature or hydration, but rather by the number of choices to be made.

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