Athletes and workers must often perform in the heat after prior exposure to heat stress with some period of recovery. In addition, exercise scientists commonly use prior exercise-heat stress to achieve dehydration and to expose volunteers to the same stress while maintaining fluid intake for the control (euhydration). It is generally assumed that prior exercise-heat stress does not itself impact performance. However, it is uncertain (16) whether euhydrated performance after heat stress and recovery actually reflects a true baseline for comparison with dehydration.
Emerging data support the possibility that prior heat stress, despite recovery, might negatively impact subsequent tasks performed in the heat. During exercise-heat stress, Nybo et al. (19) demonstrated that cerebral blood flow velocity is reduced, whereas Wilson et al. (23) also demonstrated that whole-body heating increases cerebral vascular resistance to an orthostatic challenge. When Carter et al. (3) examined both measures before and after exercise-heat stress, they observed that decreases in cerebral blood flow velocity and increases in cerebral vascular resistance persisted into recovery despite a return to normothermia. It is therefore possible that prior heat stress can have a residual but pernicious impact on human physiology, which could affect exercise performance (17,22). At present, we are unaware of any investigation that has studied the impact of prior heat stress on aerobic time trial performance in the heat. Given this gap in the literature, further investigation is warranted.
One problem in measuring performance lies within the variability of the test itself and the associated confounders. Part of this variability rests with the fact that aerobic exercise performance in the heat itself can be impaired by numerous factors related to heat strain and independent of dehydration (7,16,18,20). On the basis of an extensive review of the dehydration literature (5) in environments of >30°C and dehydration of 2% to 7% of body mass, aerobic exercise performance can decline anywhere from 7% to 60% with the magnitude of the effect appearing to increase with exercise duration. Longer, open-ended exercise tests to exhaustion increase performance variability (13) and may introduce the potential for glycogen depletion to separately impact performance outcomes (8). If exercise performance is impaired by prior heat stress, the type of performance test used to test this hypothesis must be given careful consideration.
The purpose of the present study was to determine the effects of prior heat stress, despite recovery of body temperature, on aerobic time trial performance in the heat. We hypothesized that prior heat stress would degrade performance. The findings of this investigation have important implications not only for research methodology but for athletes, laborers, and military personnel who may work, train, or compete in the heat with intermittent opportunities for rest and rehydration. Care was taken to carefully control for hydration status, time of day, and prior heat exposure (e.g., by using nonacclimatized subjects, using two groups, and comparing performance to temperate conditions).
Two volunteer groups of nine men (total N = 18), who were not heat acclimated, volunteered to participate in this investigation. Volunteers in both groups were tested in a euhydrated state, with one group being exposed to 50°C for 3 h before testing (EUHPH), whereas the control group (EUH) was not exposed. Physical characteristics were as follows (mean ± SD): EUH, age = 23.2 ± 6.2 yr, mass = 77.1 ± 9.6 kg, body surface area (BSA) = 1.9 ± 0.1 m2, body mass index (BMI) = 24.5 ± 2.2 kg·m−2, and V˙O2peak = 45 ± 5 mL·kg−1·min−1; EUHPH, age = 23.2 ± 4.5 yr, mass = 88.3 ± 6.7 kg, BSA = 2.1 ± 0.1 m2, BMI = 27.1 ± 2.1 kg·m−2, and V˙O2peak = 44 ± 7 mL·kg−1·min−1. Appropriate institutional review boards approved this study. Before participation, each volunteer attended briefings informing them of the purpose of the experiment and the possible risks and completed a written informed consent document. Investigators adhered to policies for protection of human subjects as prescribed in Army Regulations 70-25 and US Army Medical Research and Materiel Command Regulation 70-25. The research was conducted in adherence with the provisions of 45 Code of Federal Regulations Part46.
Preliminary procedures and familiarization.
Two weeks of preliminary testing preceded the experimental trials. Body mass was determined for each volunteer using an electronic scale (Model WSI-600; Mettler Toledo, Toledo, OH). Peak power (W) and V˙O2peak were measured using an incremental protocol on an electronically braked cycle ergometer (Lode Excalibur Sport; Lode, Groningen, The Netherlands) and a computer-based metabolic system with continuous gas exchange measurements (Parvo Medics, Inc, Sandy, UT). The cycle ergometer was used in the hyperbolic mode (pedal rate independent) for V˙O2peak testing while volunteers maintained a constant cadence of 60 ± 5 rpm to exhaustion. Briefly, volunteers began exercise at 40 W, and the workload increased by 20 W every minute until the subject reached volitional exhaustion. During V˙O2peak testing, heart rate (via monitor, Polar a3, Polar Accurex II, Polar Instruments; Polar Electro Inc., Woodbury, NY) and oxygen uptake, carbon dioxide, and minute ventilation were measured continuously. During preliminary testing, volunteers performed four familiarization sessions on the cycle ergometer to reduce training and learning effects (10). Each familiarization session took place in a 22°C, 20-30% relative humidity (RH) environment and consisted of 30 min of steady-state cycling (50% V˙O2peak) followed by a short rest break, then a 15-min maximal performance time trial. Gas exchange data measured during the first and the second steady-state familiarization rides were used to confirm the workload needed to elicit 50% V˙O2peak.
For five consecutive days during the second week of familiarization, volunteers consumed 2 L of sports drink after 6:00 p.m. On each subsequent morning, volunteers provided a first morning urine sample and had a blood sample (<5 mL) taken to measure plasma osmolality. In addition, nude body mass was measured before breakfast and after voiding. The 5-d measures of body mass, plasma osmolality, and urine specific gravity were averaged to establish a reliable baseline to determine euhydration. On the day of testing, volunteers with a combination of any two urine specific gravity <1.02, nude body mass within 1% of the 5-d average, or plasma osmolality <290 mOsmol·kg−1 H2O were considered euhydrated (22).
EXPERIMENTAL DESIGN AND TESTING
Previous heat exposure.
On the morning of each trial, nude body mass, urine specific gravity, and plasma osmolality were measured for comparison against the preceding week's 5-d average. Volunteers consumed a standardized breakfast of 540 kcal (16 g fat, 94 g carbohydrate, and 8 g protein) and 250 mL of water. In EUHPH, volunteers were instrumented for measurements of heart rate (Polar a3; Polar Electro Inc) and core temperature before entering the environmental chamber set at 50°C, ∼20% RH, 1.6 m·s−1 air speed. In the EUH group, volunteers rested in a comfortable environment (∼22°C).
Rectal temperatures were obtained from a telemetric temperature sensor (VitalSense Jonah™ Ingestible Capsule; Minimitter Inc, Bend, OR) representative of true rectal core body temperatures as they were inserted 8-10 cm (length of gloved index finger) beyond the anal sphincter. Previous pilot testing (Fig. 1) of this approach used simultaneously against a conventional rectal probe yielded excellent agreement (6), with smaller differences (≤0.05°C) than those reported between rectal probe and intestinal (conventional) pill temperatures (1). In all cases thus far in our laboratory (n = 62), pill expulsion has not occurred until defecation, probably due to the aid of cephalad contractions acting beyond the anal canal (>3-4 cm) (11). Thus, the two techniques appear equivalent for practical and experimental purposes.
The EUHPH group proceeded to walk on a treadmill at 3mph at 3.5% grade for 30 min, followed by 30 min of seated rest. This work/rest cycle continued for the duration of the 3-h dehydration protocol. The purpose of light walking exercise was to increase core temperature and to initiate sweating. Throughout the heat exposure, if core temperature reached 39.5°C, walking was discontinued and volunteers sat in the chamber for the remaining duration of the exercise cycle. Walking was resumed when core temperature fell below 38.5°C. Sweat loss volume was determined from changes in body mass measured every 30 min. Under the conditions tested, sweat volume and body mass losses were considered equivalent so that volunteers drank 1 mL of 0.05% NaCl and water solution to replace every 1 g of mass that was lost. A 90-min break followed heat exposure, where volunteers showered and relaxed. The purpose of this break was to allow core temperature to return to preheat exposure levels (4). After the break, nude body mass was again measured, and this value was compared with the preheat exposure value. If body mass did not equal preheat exposure values, additional fluid was provided.
Experimental performance testing.
Just before testing, group EUH was instrumented for measures of heart rate and core temperature as described for EUHPH, and these measures were continuously monitored and recorded during exercise. Upon entering the test environment set to 40°C, 20% RH, both EUH and EUHPH volunteers began the experiment by cycling for 30 min at 50% V˙O2peak. A 10-min break followed, after which a self-paced 15-min cycling performance time trial was completed. The 30-min exercise preload followed by a shorter time trial in this study was selected because 1) it has ecological validity for comparison to the similar exercise duration and energy system requirements (9) of the Army 2-mile run, and 2) it is a reliable performance test modality (13). During the steady-state ride, a pedal rate of 60 ± 5 rpm was used because in novice cyclists, endurance has been shown to be maximized at lower pedal cadences (15). The lode linear factor was individualized to elicit a 50% V˙O2peak workload for each volunteer at this cadence, which allowed ample room for self-paced improvement up to maximal sustainable workloads estimated at approximately 100 rpm from V˙O2peak testing. During the 15-min time trial, volunteers were blinded to all test parameters except time. Exercise heart rates and core temperatures were measured by automated means to limit distractions.
Performance was assessed as the total amount of work completed (kJ) in 15 min. Although cycling is a body mass independent exercise modality, performance relative to body mass (kJ·kg−1) was also examined due to the larger body mass of group EUHPH. Briefly, although the relative fitness of both groups was well matched, the heavier group EUHPH had a higher absolute V˙O2peak. This corresponds to a larger linear factor setting at the same relative 50% V˙O2peak workload, which could bias the total work performed if all other factors (time, rpm) remained constant. To control for this factor when interpreting the total work performed, both absolute and relative work were examined. The percent change in performance relative to the best practice trial at 22°C was also calculated for comparison between EUH and EUHPH as another way of evaluating the effects of heat stress and residual heat stress on performance.
The primary outcome variable of interest in this experiment was time trial performance. Group performances were compared using an independent samples t-test, as were other variables of interest at single time points. A mixed model two-factor ANOVA with repeated measures on the second factor was used to make all group × time comparisons. F values were adjusted for sphericity where appropriate, and main or interaction effects were investigated by Newman-Keuls post hoc test. An analysis selecting conventional α (0.05) and β (0.20) parameters showed that eight subjects in each group would provide sufficient power to detect a 10% difference in time trial performance between groups. This estimate was made using the mean total work (∼175 kJ) and the coefficient of variation (CV = 5%) calculated from trials of negligible difference during 2 wk of time trial practice. The desire to detect a twofold change from the %CV was chosen based on the likelihood of experimental perturbations producing unique performance infidelity (12). Group sample sizes of nine allow detection of desired differences with a CV as high as 7%. Graphical data are presented with unidirectional error bars for clarity of presentation. All data are presented as means ± SD, except where indicated.
Volunteers in the EUH and the EUHPH groups completed all aspects of testing. Volunteers in both groups were well matched for relative V˙O2peak and body fat. In both the EUH and the EUHPH trials, volunteers were euhydrated before the start of testing because their body mass was within 1% of the 5-d average, urine specific gravity was below 1.020, and plasma osmolality was <290 mOsm·kg−1. In the EUHPH, trial body mass did not significantly change from preheat exposure to preexperimental testing because sweat losses were matched with fluid intake (3.5 ± 0.05 L; Table 1).
Total accumulated work was not different (P > 0.05) between the EUH (150.5 ± 28.3 kJ) and the EUHPH (160.3 ± 24.0 kJ) trials. Expressed relative to body mass to control for group differences in absolute V˙O2peak, differences were even smaller at 1.96 kJ·kg−1 (EUH) and 1.82 kJ·kg−1 (EUHPH). The percent change in time trial performance relative to the best 15-min practice time trial at in 22°C, 20% RH, during familiarization was also not different (P > 0.05) between the EUH (18.7% ± 9.2%; 95% confidence interval [CI] = 12.7-24.7) and the EUHPH (15.0% ± 7.8%; 95% CI = 9.9-20.1) trials (Fig. 2).
During heat exposure, core temperature for the EUHPH group increased from preheat (37.0°C ± 0.3°C) to postheat (38.25°C ± 0.5°C) exposure but returned to baseline (37.2°C ± 0.2°C, P > 0.05) before the start of experimental testing. Heart rates and core temperatures were not different (P > 0.05) during the steady-state or performance time trial rides between the EUH and the EUHPH trials. However, there was a significant main effect for time (P < 0.05), where both heart rate and core temperature increased during exercise (0, 15, and 30 min and end of performance time trial; Fig. 3A and B). Ratings of perceived exertion were also similar during steady state and not different (P < 0.05) between the EUH (18.2 ± 1.3) and the EUHPH (19.0 ± 1.5) trials at the end of the aerobic time trial.
This study quantified the effects of prior heat stress on subsequent aerobic time trial performance in the heat. This is the first study to have examined possible residual heat stress effects (despite recovery as indicated by body temperature) on subsequent aerobic time trial performance while carefully controlling for potential confounders (hydration and prior heat stress). Our findings indicate that prior heat exposure did not significantly alter subsequent aerobic time trial performance in the heat. This was evident by a lack of difference between EUH and EUHPH in total accumulated work, total work relative to the best 15-min time trial performance achieved in temperate conditions during familiarization, and no differences in heat rate, core temperature, or ratings of perceived exertion during exercise.
In the present study, aerobic exercise in the heat after previous heat stress resulted in a 15% reduction in performance, and heat exposure alone resulted in an 18% decrement (P > 0.05). Given these findings, it appears that previous heat exposure on the same day does not contribute to a further performance decrement during subsequent aerobic exercise. Investigations of the impact of previous heat stress on subsequent physiological responses, aside from exercise performance, are limited. Using a protocol very similar to the one used in the present study (3-hexposure to 45°C, 2-h recovery period), Carter et al. (3) reported that after exposure to heat, a reduction in cerebral blood flow velocity and increased cerebral vascular resistance remained during a subsequent orthostatic challenge in 22°C. On the basis of the findings of Carter et al. (3), we hypothesized that these residual physiological changes in cerebral autoregulation might be accentuated further during exercise at 40°C (17) and would degrade performance to a greater extent than exposure to heat alone. However, our results showed that prior heat exposure did not alter subsequent aerobic time trial performance in the heat. As the methods of measuring cerebral blood flow velocity and cerebral vascular resistance used by Carter et al. (2) are extremely difficult to make during exercise due to movement artifact and were not the focus of this study, we can only speculate that either changes in cerebral autoregulation were not present or were insufficient to alter exercise performance. However, tenable support for null findings can be gleaned (4) from investigations showing that neither residual heat stress (rectal temperature +1.0°C) nor moderate dehydration (2.7%) had any effect on very high intensity exercise in a temperate environment, although both factors can influence cerebral autoregulation (3,19,23). It should also be made clear that the principal importance of these findings lies in their methodological and practical applications and less so in elucidating a possible mechanism(s) of action.
Neilsen et al. (16) noted that the effect of elevated temperature, body water loss, and exercise cannot be easily separated experimentally and that there appeared to be an interaction between these variables in the reduction of work capacity. Furthermore, Caldwell et al. (2) observed that differences in maximal work capacity may arise from the specific method of the treatment used to cause dehydration. On the basis of two extensive reviews (5,21), dehydration (2-7% body mass) reduced aerobic exercise performance from 7% to 60% in environments warmer than 30°C. Reasons for the wide range of performance decrements include factors that coexist with dehydration, such as hyperthermia and accelerated substrate depletion, or the independent effects of environmental heat stress on aerobic exercise performance (7,18,20). Indeed, we observed an approximately 17% decrement in performance at 40°C when compared with 22°C despite careful control over other fatigue factors (Fig. 2). In the present study, we were able to eliminate the confounding influence of dehydration and hyperthermia on subsequent aerobic time trial performance in the heat by matching sweat loss with fluid intake during the dehydration protocol and by allowing body core temperature to recover during the 90-min rest period. Furthermore, our choice of exercise performance task was minimally confounded by these factors, was highly reliable (CV = 5%), and yet was long enough to be considered aerobic in nature (9). This type of performance test also has exceptional applicability to actual performance tasks or competitions that have a predetermined end point.
The findings of the present study are important, as many research protocols use these very same methods when studying the effects of body water deficits on human physiology. These findings may also afford important insight for workplace settings, where individuals may work in the heat, rest, and then resume work in the heat. However, any application of these findings must consider adequate recovery of body core temperature and hydration state. In the present study, volunteers were given a 90-min break out of the heat, where they showered and any fluids lost were replaced to maintain euhydration. During this period, heart rate and core temperatures nearly recovered to preheat stress levels. It is likely that if dehydration of >2% was not corrected or if body core temperature was not allowed to recover, greater performance decrements would occur (14) during subsequent aerobic exercise.
The findings of this study demonstrate that prior heat stress does not have a residual effect on subsequent aerobic time trial performance in the heat, provided that after initial heat exposure, any level of dehydration is corrected and body core temperature is allowed to recover. The findings of this study are important not only as they pertain to research methodology but for athletes, laborers, and military personnel who may work, train, or compete in the heat.
The authors would like to thank the volunteers who donated their time and effort to participate in this study. In addition, the authors would like to thank Robert Carter for his editorial assistance. The views, opinions, and/or findings in this report are those of the authors and should not be construed as official Department of the Army position, policy, or decision unless so designated by other official designation. All experiments were carried out in accordance to state and federal guidelines. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
Funding: No outside funding was received for this study.
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