Fatigue occurs earlier when working at corresponding exercise intensities in hot compared with cool conditions (30). This has been attributed to either "central fatigue," as a consequence of achieving a critical deep-body temperature (9), or to anticipatory changes in pacing strategy attributable to uncompensible rates of heat storage towards the start of exercise (30). These mechanisms may be activated when work intensity is high and exercise duration is relatively short (< 40 min), and they are exacerbated by high environmental heat load. If premature fatigue does not occur directly as a consequence of thermal load during exercise of longer duration (> 90 min), alterations in central neurotransmitter substances such as serotonin (7) and interleukin-6 (IL-6 (25)) increase the sensation of fatigue and are known to contribute to the premature cessation of exercise in normothermic conditions. This mechanism may also be influential in the heat (25).
Many of the theories associated with premature fatigue in the heat acknowledge the existence of a reduction in the "drive" or motivation to exercise (3,9,20). This psychological component is widely acknowledged (18,20) but rarely investigated, and it has not been quantified. One means of examining this component is to alter the psychological capability in dealing with hyperthermia and the perceptions associated with exercising in the heat. Research in sport psychology has demonstrated that the administration of tailored psychological skills training (PST) can suppress the sensations of fatigue arising from high-intensity exercise in trained individuals in normothermic environments (22,27).
PST, comprising goal setting, arousal regulation, mental imagery, and positive self-talk, has been reported to produce improvements in performance of between 2% (elite swimmers, 100-m swim time (11)) and 7.5% (triathletes' time to complete gymnasimum triathlon (27)). It is thought that PST provides motivation, optimizes arousal state according to task, facilitates skilled movement, and minimizes the development of negative cognitive states (27). In contrast, exercise in the heat diminishes motivation (18), alters arousal (21), impairs neuromuscular recruitment (knee extension (20), and causes negative mood states (17).
Recently, it has been demonstrated that the benefit of PST extends to stressful environmental, as well as competitive, situations. Barwood et al. (1) report that PST can help with the conscious suppression of the strong respiratory drive that arises from immersion in cold water. This suggests that PST can be tailored to help counteract the negative consequences of the thermal environment. The current study examined whether this was the case for exercise performance in the heat and, in so doing, attempted to quantify the psychological component associated with exercising in the heat. It was hypothesized that a 4-d PST package would significantly increase the distance covered during 90 min of exercise in the heat, by enabling subjects to better tolerate higher deep-body temperatures and associated fatigue sensations.
METHODS
The study received ethical approval from a local ethics committee, and all subjects gave their written informed consent to participate. After a medical examination, 18 healthy male volunteers acted as subjects. They were not heat acclimated, but they were accustomed to treadmill running. They refrained from strenuous exercise for 72 h before experimentation, were nonsmokers, and abstained from alcohol and caffeine consumption for 24 h before experimentation.
The study employed a test-retest design in which each subject undertook three runs (R1, R2, R3) in the heat (30°C, 40% RH). After R2, subjects were matched according to the difference in performance between R1 and R2 into two independent groups (matched pairs had run within 270 m of each other). The matched subjects were then randomly allocated to a control group (CG), which continued normal daily activity between R2 and R3, and a psychological skills group (PSG), in which subjects completed a psychological intervention between R2 and R3.
Preliminary Assessment
V˙O2max (mL·kg−1·min−1) was determined 2 wk before R1, using a continuous incremental running test with online breath-by-breath analysis (Oxycon-delta, Jaeger, Germany). Subjects ran on a level treadmill (Powerjog, GX200, London, UK), with workloads increased by 1 km·h−1 each minute from 6 to 13 km·h−1, after which the incline was increased by 1-2% each minute, depending on ratings of perceived exertion (2) and heart rate (Cardiosport, UK). Exercise continued until volitional exhaustion. V˙O2max or peak V˙O2 was classified according the British Association of Sport and Exercise Science (4) criteria.
Main Study
Each subject ran at the same time of day, and a minimum of 4 d elapsed between runs. On arrival at the laboratory the subjects voided, and body mass was measured (naked and clothed mass; Ohaus digital weighing scales, I-10, Canada). Subjects wore only shorts, socks, and running shoes; identical clothing was worn for each run. After changing into his running kit, each subject was instrumented (see below). Before exercise, and after the subject had stood upright for a minimum of 20 min to minimize postural alterations in plasma volume (10), 20 mL of venous blood was drawn. The subject then entered the climate chamber and sat on a chair situated on a treadmill (Powerjog, London, UK). At this stage, each subject was reminded that he should try to cover as much distance (maximum effort) as possible during their exercise bout until volitional exhaustion, or until 90 min had elapsed. The subject provided a resting sample of expired air, collected in a Douglas bag, before beginning running.
After a verbal countdown, the subject commenced running on a treadmill set at 1% incline to reflect the metabolic demands of outdoor running (16). At all times, subjects had control of the treadmill speed, but they received no feedback on distance covered, current speed, or time; every effort was maintained by the experimental team to eliminate any temporal cues during data collection. At the end of each run, subjects were not informed of the distance they had covered; this information was provided during the debriefing period after the completion of R3. No warm-up was prescribed before exercise, but subjects were given the opportunity to stretch.
Treadmill running continued, with the speed adjusted by the subject using the treadmill control panel, for the full 90-min period, unless any of the withdrawal criteria were reached (deep-body temperature > 39.5°C; decision of subject (exhaustion) or independent medical officer (did not occur in the present study)). In R1, tap water was available ad libitum. The volume and timing of fluid consumption was noted throughout R1, and this profile was replicated in R2 and R3.
After completion of each run, the subject's treadmill speed was gradually slowed to a comfortable walking pace. A further 20-mL sample of venous blood was drawn while the subject stood, 4 min after the cessation of exercise. On exiting the thermal chamber, the subjects were reweighed (clothed and naked body mass) and instructed to sit in a tepid shower until deep-body temperature returned to normal resting level.
In the 4 d preceding R3, PSG subjects completed four 1-h PST sessions aimed at increasing distance covered in the final run. These included the following sessions:
Goal-setting.
Goals optimize motivation and feelings of control, mobilize effort, and focus concentration on a specific target (8). Outcome goals (e.g., improvement in distance covered or time to exhaustion) and process goals (e.g., controlled increase in RPE during R3) benefit endurance sport activities by up to 7.5% in competitive situations (27). Process goals provide positive feedback and reinforcement of behavior, and they mobilize effort toward achieving the outcome goal. In the absence of quantitative feedback during R1-R2, each PSG subject set an outcome goal to increase distance run by 5-10%, and a series of subgoals (process goals) by which to achieve the outcome. The process goals encouraged the subjects to mentally separate R3 into sections (three to six sections; 30- to 15-min periods) according to their sensation of exertion (RPE); PSG subjects estimated the timing of these sections and did not have any external temporal feedback. The subgoals were realistic according to the exertion experienced that is, to increase speed for the early sections of R3, but to only maintain, rather than reduce, intensity for the later sections.
Arousal regulation.
Arousal regulation prevents negative overarousal (somatic and cognitive anxiety), enabling performers to focus on relevant cues (for process goals) for task performance. PSG subjects identified their sources of overarousal (e.g., prerun feelings of anxiety; while running, feelings of fatigue leading to a reduction in work intensity). Subjects then practiced two relaxation strategies, progressive muscular relaxation (PMR (14)) and centering (13), to combat negative physical sensations experienced before and during R3. PMR involves tensing and relaxing the major muscle groups while lying down and breathing slowly. The repeated tensing and subsequent relaxation of the muscles helps to produce a feeling of being relaxed but alert. The PSG subjects were instructed to deploy PMR before their arrival for R3.
Centering requires the subject to change their center of consciousness from their head to their center of gravity (a point superior to the umbilicus) on the end of their exhalation. Initially, centering was practiced at rest, but then it was deployed during R3, when the subject became tempted to reduce work intensity. Centering provides a method for quick relaxation while maintaining focus on the outcome and process goals.
Mental imagery.
PSG subjects were instructed to identify the positive and negative images they had during R1 and R2. Negative images were used to direct attention and target critical times in R3 that require particular effort to overcome. Each subject visualized himself overcoming the fatigue at these points, using a competitive imagery sequence. PSG subjects visualized themselves in a competitive situation with their counterpart from the CG. When tempted to reduce their work intensity, PSG subjects deployed this imagery sequence with the CG subject appearing in advance of them in the race (motivational imagery). The opposite image (PSG subject in advance of the CG subject) was deployed on maintaining or increasing the speed of the treadmill (positive imagery reinforcing feelings of control). Mental imagery is an effective part of PST interventions, both independently and as part of a package of skills (19).
Positive self-talk.
Self-talk was used to control negative statements that occurred before and during R3 by restructuring negative words into positive phrases. PSG subjects identified the negative cognitions they experienced in R1 and R2 (e.g., "my legs are stiff, I feel tired" or "the heat is overwhelming, I'll have to slow down") and constructed counteractive positive statements directed towards motivating themselves ("this is a challenge I'm going to meet, I have the mental tools to cope") and augmenting feelings of control over their running ("head up, shoulders back, and keep my stride length").
Positive self-talk is thought to minimize the development of negative cognitions that comprise the negative emotional state of cognitive anxiety (12); this has a negative performance effect. It also focuses attention toward task-relevant cues (process goals), enabling the performer to respond with positive behavior (increase running speed) on the appearance of a cue (27).
The PST interventions were overseen by an accredited sport psychologist. The CG were not aware of the content of PST or its delivery to the PSG. After PST, all PSG subjects completed R3, following the same protocol as described above.
Measurements
Skinfold thicknesses at eight different sites were measured on each subject by an accredited anthropometrist. Fluid intake and before and after naked and clothed body weights (OHAUS I-10 digital scales, Canada) were used to calculate sweat production and evaporation.
Aural temperature (Tau) was measured with a thermistor (Grant Instruments, Cambridge, UK) inserted into the right auditory meatus, close to the tympanic membrane and insulated. To ensure that Tau had equilibrated, the Tau of the left ear was measured using an infrared (IR) thermometer (Thermotek Plus, Ear Thermometer, model 718, SAAT Ltd, Israel). Runs did not commence until Tau was within 0.1°C of the value from the IR probe. Skin temperature (Tsk) was measured using thermistors (Grant Instruments, Cambridge, UK) attached to the skin by a single piece of tegaderm tape at four sites: chest, arm, thigh, and calf (23). Tau and Tsk were recorded each minute on a data logger (1250/1000 series, Grant Instruments, Cambridge, UK). Heart rate was displayed continuously throughout each run and recorded (1-min average) every 15 min during exercise, or after a substantial change in treadmill speed (HME Lifepulse, UK).
Speed (km·h−1) and distance covered (km) were recorded every 15 min. To ensure that subjects did not deduce elapsed time from the measurement intervals, additional random measurements were taken between the 15-min time points. Every 15 min, or after a substantial change in treadmill running speed, fractional concentration of oxygen and carbon dioxide and volume of expired air were measured and recorded (Servomex 1400, UK; Harvard dry gas meter, Harvard Instruments). Each change in treadmill speed and/or 15-min bout of exercise completed was accompanied with a perceptual measure of rating of perceived exertion (RPE (2)) and thermal comfort (TC). Subjects indicated their TC on a four-point scale ranging from 1 (comfortable) to 4 (unbearably uncomfortable).
After R3, PSG subjects completed a psychological skills-use questionnaire to assess their reactions to PST and the experimental outcomes (distance covered in R3). After R3, the comments of the subjects of both groups regarding their use of any psychological skills were noted.
Calculations
Mean skin temperature was calculated according to the formula (24):
Mean body temperature was calculated according to the formula (5):
Blood Sampling
Some of each sample (~18 mL) was transferred into untreated vacutainers and placed into a refrigerator (4°C) to clot. It was then prepared for later analysis via centrifugation (3000g for 10 min). The resultant supernatant was aliquotted into Eppendorf tubes and stored at −80°C for later analysis.
Blood lactate [BLa] and glucose concentrations [Bg] were determined using whole-blood samples. In determining [BLa] (mM), a minimum 7-μL aliquot of whole blood was used in a lactate-oxidase enzyme assay (Analox Instruments Ltd, Champion Lactate Analyser P-Lm5, London). [Bg] (mM) was measured using an Accu-Chek Active blood glucose monitor (Roche Diagnostics Ltd, England).
Prolactin and IL-6 concentration were determined using commercially available enzyme-linked immunoabsorbance assay (ELISA) kits (prolactin: DRG Instruments, Germany; IL-6: Diaclone, Besancon Cedex, France). Circulating prolactin levels were used as an index of central serotonergic activity (3). After assay preparation, serum concentrations were determined using a microplate absorbance reader (Versamax Microplate Reader, Sunnyvale, CA). Measures in whole or part blood were made in duplicate.
Statistical Analyses
Data were tested for normality (Kolmogorov-Smirnov test). The following variables were statistically compared using a repeated-measures analysis of variance with Tukey's post hoc analysis: distance covered (km), average speed (AVGspeed; km·h−1), percentage of R1 distance achieved in R2 and R3 (%R1; normalized between group comparison of kilometers), peak (Tsk peak), and mean Tmsk, Tau, Tb, and V˙O2 after 80 min of running. Groups were compared for differences in V˙O2max, using an independent-samples t-test. For all statistical tests, the α level was 0.05. Data are presented as means [SD] where appropriate.
RESULTS
The average [SD] conditions for the runs were 30.06°C [0.21] and 41.40% RH [5.01]. The physical characteristics (mean [SD]) of the respective groups were, for CG (N = 8): age 28 [5] yr, height 1.73 [0.04] m, mass 72.83 [6.74] kg, body fat 15.74 [3.31]%, V˙O2max 63.49 [6.17] mL·kg·min−1; and for PSG (N = 10): age 23 [3] yr, height 1.77 [0.05] m, mass 69.31 [6.06] kg, body fat 14.38 [1.96]%, V˙O2max 67.06 [4.49] mL·kg·min−1. There were no differences between groups in V˙O2max (P = 0.174). Groups were uneven because two subjects did not show comparable variability between R1 and R2, and so they were added to the PSG.
The distances covered in the CG did not differ between runs. In the PSG, there were no differences in the distance run between R1 and R2, but they ran significantly farther in R3 (8%; 1.15 km). Statistical analysis suggests that this difference did not become significant until the final 15 min of exercise were complete (P = 0.016; β = 0.857; Fig. 1B), although the improvement in distance covered was consistent from the 30th minute of exercise onward (Fig. 1B). Distances run in R1-R3, and relative differences between each run in the CG and PSG, are displayed in Table 1 and Figure 1.
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TABLE 1:
Distance (absolute and differences) achieved during in both groups.
FIGURE 1:
Average distance run in R 1, R 2, and R 3 in the CG (A; N = 8) and PSG (B; N = 10). Absolute differences between R 2 and R 3 (km) are displayed next to each column.
There were no differences in peak or average RPE or TC within or between groups (P > 0.05). These variables peaked or reached a plateau between the 75th and 90th minutes of exercise in all subjects in all runs. We found the following RPE values at 90 min for CG: R1, 16.2 [2.1]; R2 16.5 [0.5]; and R3, 16.8 [0.6]. In the PSG, we found the following RPE values at 90 min: R1, 16.6 [2.2]; R2, 16.3 [2.2]; and R3, 16.9 [2.5]. RPE and TC data are displayed in Figure 2.
FIGURE 2:
Average RPE (solid lines, Y 1) and TC (dashed lines, Y 2) votes during R 1-R 3 in the CG (A; N = 8) and PSG (B; N = 10).
There were no significant differences in the absolute or change (Fig. 3) in Tau, Tmsk, or Tb between runs in either group (P > 0.05). Start Tau were CG: R1: 36.88 [0.39], R2: 36.92 [0.34]°C, and R3: 36.87 [0.36]°C; PSG: R1: 36.76 [0.28], R2: 36.58 [0.46]°C, and R3: 36.78 [0.37]°C. Peak Tau in the CG were R1: 38.46 [0.74], R2: 38.51 [0.74]°C, and R3: 38.53 [0.45]°C and were achieved in minute 79 [15] in R1, minute 85 [9] in R2, and minute 79 [17] in R3, respectively. In the PSG peak, Tau were R1: 38.70 [0.39], R2: 38.74 [0.49]°C, and R3: 38.85 [0.53]°C and were achieved in minute 79 [12] in R1, minute 82 [15] in R2, and minute 81 [10] in R3. The peak Tb achieved during each run was CG: R1: 37.43 [0.57], R2: 37.48 [0.48]°C, and R3: 37.65 [0.45]°C, where Tsk averaged at R1: 33.83 [1.13], R2: 34.20 [0.73]°C, and R3: 33.54 [1.40]°C. In the PSG, Tb peaked at R1: 37.80 [0.37], R2: 37.82 [0.53]°C, and R3: 38.03 [0.54]°C, where Tsk averaged at R1: 34.17 [1.00], R2: 34.69 [0.81]°C, and R3: 34.72 [0.73]°C.
FIGURE 3:
Average ΔTau in R 1-R 3 in the CG (A; N = 8) and PSG (B; N = 10).
V˙O2 data taken after 30, 60, and 80 min are displayed in Table 2. There were significant differences between groups in V˙O2 after 80 min (P = 0.038) in R1 only, with the CG working at a higher relative intensity (P = 0.006). Within groups, the CG did not differ in work intensity across runs at the 80th minute of exercise, whereas the PSG were working significantly harder in the 80th minute of R2 (P = 0.043) and R3 (P = 0.017) than R1. There were no differences within or between groups at either 30 or 60 min of exercise. HR data did not differ either within or between groups, averaging, in each run, PSG: R1 148 [11] bpm, R2 151 [12] bpm, 154 [12] bpm; and CG: R1 147 [12] bpm, R2 146 [10] bpm, and R3 147 [12] bpm.
TABLE 2:
Peak V˙O2 expressed as a percentage of V˙O2max after 30, 60, and 80 min of exercise in the heat in the CG and PSG.
Fluid balance and sweat data are presented in Table 3. No significant differences were identified in these data, although sweat evaporated by the PSG in R2 cp R3 was close to being accepted as significantly greater (P = 0.06).
TABLE 3:
Sweat produced (L), evaporated (L[%]), and fluid consumed in R 1-R 3 in the CG and PSG.
There were no consistent significant differences in the biochemical data (P > 0.05) The results for ΔIL-6, prolactin, ΔBg, and ΔBLa are presented in Table 4. Baseline values for [Bg] were CG: R1 5.04 [0.70] mM, R2 4.94 [0.98] mM, R3 4.93 [0.55] mM; and PSG: R1 4.87 [0.45] mM, R2 4.84 [0.30] mM, R3 4.91 [1.11] mM. [BLa] at baseline was CG: R1 1.80 [1.00] mM, R2 1.54 [0.67] mM, R3 1.93 [0.77] mM; and PSG: R1 1.67 [0.95] mM, R2 1.48 [0.56] mM, R3 1.64 [0.64] mM.
TABLE 4:
Postexercise and ΔIL-6, prolactin, blood glucose, and lactate levels in R 1-R 3 in the CG and PSG.
The rating of the usefulness of each psychological skill before and during R3 is presented in Table 5. Mental imagery and goal-setting were reported to be most useful in preparing to run in the heat, whereas mental imagery, positive self-talk, and goal-setting were most useful while running. Overall, mental imagery was rated as most effective, with arousal regulation least useful.
TABLE 5:
PSG subject ratings of psychological skill use before and during R 3 (0 = not at all useful to 10 = very useful; N = 10).
DISCUSSION
This study examined, for the first time, the impact of a psychological intervention on endurance exercise performance in the heat. The finding that the PSG ran, on average, 8% (1.15 km; Fig. 1B) farther after PST suggests that psychological factors can have a significant impact on running performance in the heat. This finding extends the efficacy of PST in environmental extremes from breath holding in cold water (1) to exercising in the heat.
The PSG improved their performance by running faster from the 15th minute onward (Fig. 1B). However, after PST, neither deep-body temperature nor RPE exceeded the values seen in previous runs (Fig. 2). The PSG suppressed the temptation to reduce their exercise intensity, and although RPE ratings were not significantly lower after PST, they did remain unchanged despite the significant increase in distance covered in R3. This supports the findings of Tikuisis et al. (28), who have suggested that the learned ability to suppress RPE and sensations of hyperthermia could be a means by which performance is improved in trained individuals.
Neither the CG nor PSG reached temperatures that are thought to cause the termination of exercise in trained participants (9). However, it should be noted that in the present work, Tau was measured, whereas others have reported rectal (Tre (28)) or esophageal temperature (Tes (9)). Tre is normally slightly higher than Tau (15); our subjects may have been a little closer, but still well below, the temperatures associated with the termination of exercise. Interestingly, the PSG subjects did not seem to be thermoregulating at the end of R1 (decline in Tau at minute 75), which may indicate inadequate pacing strategy or fatigue, whereas in both R2 (comparable rate of rise in Tau but lower Tau) and R3, Tau was approaching a plateau. The increased running speed from the 15th minute in R3 is reflected by the tendency for Tau to be higher throughout this run (compared with R2) and plateau at a higher level, thus demonstrating an improved tolerance of stored heat over a prolonged period. This raises the question of where the additional heat produced during this run went (assuming constant mechanical efficiency). Although it did not reach statistical significance (P = 0.06), it is worth noting that the average volume of sweat evaporated was numerically higher in R3 compared with R1 and R2 in the PSG. Therefore, the additional heat production may have been balanced by increased evaporative heat loss. An estimation of this possibility can be obtained by comparing the difference in the average rate of heat storage between R2 and R3 (1.5 kJ·min−1 based on the V˙O2 measured at 80 min, where ME = 25% and 1 L V˙O2 per minute = 20.2 kJ·min−1) with the difference in the average rate of evaporative heat loss between the two conditions (4.5 kJ·min−1, where latent heat of vaporization = 2.4 MJ·L−1, Table 2). Even ignoring the fact that Tb was higher in R3 of this group (which would account for some of the extra heat being produced), this analysis suggests that the additional sweat being evaporated in R3 compared with R2 was more than sufficient to balance the greater work-related heat production seen in R3, and by an amount that would account for the fact that not all of this additional sweat was evaporated.
The thermal strain associated with the present study was sufficient to cause subjects to reduce their work intensity in the later stages of some of their runs. The consistency of the RPE ratings further suggests that the protocol produced a relatively constant level of demand. Selkirk and McLellan (26) have suggested that the critical body temperature associated with the cessation of exercise is different for trained versus untrained individuals, being about 38.7°C in the untrained, but higher in trained individuals. Accordingly, the subjects in the present study (mean V˙O2max: 66 mL·kg−1·min−1; N = 18) should have had higher critical body temperatures than those measured during R1-R3. In the absence of any distinct temporal or pacing cues in the environment, all subjects, despite being instructed to work maximally, regulated their work intensity and remained below an average deep-body temperature of 38.7°C (Fig. 3). Thus, they seem to have been adjusting their pace to remain below their critical body temperature. Recently, it has been suggested that subjects consciously alter their pacing strategy in the heat according to their rate of heat storage to prevent hyperthermia impairing their performance (29,30).
This finding provides support for a conscious component in thermoregulation during exercise in the heat. It is possible that this component is altered by PST. However, although it is clear from the present study that the PSG group demonstrated an improvement in running performance that the CG did not show, it is not possible to determine exactly what it was about the PST that produced this improvement. In theory, it could have been just one skill contained in the intervention; goal-setting has previously shown a similar magnitude of performance improvement (8,27). It may even have been a placebo effect associated with greater contact with the experimenters. However, both the literature (8,19,27) and the subjective feedback reports of the PSG group (Table 5) suggest that it was the content and combination of skills employed within the PST that was effective. The least effective component of the PST seems to have been arousal regulation. The other aspects of PST were rated more highly, with multiple goal setting leaving PSG subjects sensing definite improvements in performance and feeling "more determined" an able to "push myself harder." This is a characteristic of trained athletes (20). Some of this motivation may have been derived from the imagery strategy that was used to stimulate effort by visualizing being in a race scenario. Similarly, Thelwell and Greenlees (27) found that images of experiencing and dealing with pain from high-intensity exercise helped their subjects get through an exercise bout.
The final aspect of PST, positive self-talk, may have helped control negative cognitions, such as thoughts of failure leading to anxiety, which can have a negative impact on exercise performance (6). Whether self-talk had an anxiolytic effect is not clear, but negative thoughts and cognitions may distract the individual from relevant environmental cues, which are important to task performance. Self-talk in the current study was designed to minimize these effects (6).
Although it is difficult to determine the precise mechanism that enabled the PSG to run farther in R3, it is possible to reach some tentative conclusions about what did not produce this improvement. The findings related to central serotonergic activity, as indicated by prolactin and IL-6 levels after exercise, suggest that these "central" mechanisms were less influential than originally thought, particularly if dopaminergic activity is preserved through this exercise duration. Both IL-6 and prolactin increased significantly postexercise, nearing levels that have previously been shown to induce feelings of heightened fatigue and negative mood disturbance (25). However, neither measure showed a consistent relationship with either RPE, or the distances run in R3, suggesting that they did not underpin either of these findings.
One further point is worthy of note. We have shown that there is a psychological component to performance in the heat, and this component is susceptible to manipulation. This finding may help explain both the wide variety of performance observed in the heat in people with similar physiological and acclimation status and the ability of some to drive themselves to the point of collapse (23). Assuming PST results in more generic than specific alterations, our findings also have significant implications for studies that adopt an "unblinded" experimental design. In the heat, this might include rehydration or cooling studies. If subjects believe that their performance might be improved by an intervention, then it seems that such a psychological influence can alter performance by 8%, on average.
In conclusion, PST can improve running performance in the heat by an average of 8%. The precise mechanisms underpinning these improvements are unclear, but their implications for unblinded experimental design are clear.
The authors would like to thank the subjects for their reliability and forbearance. Also thanks to Mrs. Julia Allen for her assistance in the laboratory and Dr. Anuj Goyle and Dr. Avijit Datta for providing Independent Medical cover.
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