Mean neck skin temperature is shown in Figure 3. There was a significant main effect for trial (F2,12 = 145.6, P < 0.001), time (F18,108 = 33.7, P < 0.001), and trial × time interaction (F36,216 = 12.6, P < 0.001). Tneck was significantly colder during the 90-min TTpre in CC compared with NC (P < 0.001), CCreplaced compared with NC (P < 0.001), and CCreplaced compared with CC (P = 0.003). At the commencement of the time trial (t = 75 min), there was a significant main effect for trial (F2,12 = 73.2, P < 0.001), and neck temperature was significantly lower in CCreplaced compared with NC and CC trials (P < 0.001 for both); however, there was no significant difference between NC and CC (P = 0.210). Neck temperature remained significantly lower until the 55th min in CC (P < 0.05) and for the duration of CCreplaced (P < 0.001) compared with NC.
HR and rectal temperature.
HR and Trectal increased significantly during the trial (main effect of time on HR, F18,108 = 203.4, P < 0.001; main effect of time on Trectal, F18,108 = 206.4, P < 0.001); however, there were no significant differences between trials for Trectal (F2,12 = 1.3, P = 0.320) or HR (F2,12 = 0.8, P = 0.454). There were no significant differences between trials in the changes observed between 0 and 90 min for Trectal (F2,12 = 0.3, P = 0.727) or HR (F2,12 = 0.79, P = 0.478). Further analysis revealed that there were no significant differences between trials for Trectal or HR at the commencement of the time trial phase (t = 75 min) (F2,12 = 0.7, P = 0.496 and F2,12 = 0.6, P = 0.583, respectively). Trectal and HR data observed at 0, 75, and 90 min are shown in Table 1.
All perceptual data (TS, TSneck, RPE, FS) significantly changed over time (main effect of time on TS, F18,108 = 32.4, P < 0.001; main effect of time on TSneck, F18,108 = 47.8, P < 0.001; main effect of time on RPE, F17,102 = 50.0, P < 0.001; main effect of time on FS, F18,108 = 10.3, P < 0.001). There were no significant main trial or interaction effects for RPE or FS (F2,12 = 0.7, P = 0.540 and F2,12 = 0.03, P = 0.971, respectively).There was no main trial effect (F2,12 = 3.4, P = 0.1) for TS, but there was a significant interaction (trial × time, F36,216 = 1.6, P = 0.002). TSneck was significantly different between trials (main effect of trial, F2,12 = 22.8, P < 0.001) and was lower in CCreplaced compared with NC (P = 0.003) and CC (P = 0.004) and in CC compared with NC (P = 0.006) (Fig. 4).
At the beginning of the time trial (75 min), there was no significant difference between trials for RPE (F2,12 = 2.3, P = 0.143), but there was a difference between trials for TS (F1.1,6.4= 7.7, P = 0.029). Participants reported feeling significantly cooler in CCreplaced compared with CC (P = 0.001), but there were no differences between NC and CC (P = 0.599) or between NC and CCreplaced (P = 0.334). There was a significant main effect for TSneck (F2,12 = 18.5, P < 0.001). Pairwise comparisons revealed that participants reported a significantly lower TSneck in CCreplaced compared with CC (P = 0.006) and NC (P = 0.013), but there was no difference between CC and NC (P = 0.127).
Body fluid balance.
There were no significant differences in the volume of water voluntarily consumed (F2,12 = 2.2, P = 0.152) or in the volume of sweat lost (F2,12 = 2.0, P = 0.183) between trials. Participants consumed 0.85 ± 0.50, 0.90 ± 0.50, and 0.68 ± 0.24 L of water and lost 2.01 ± 0.39, 2.18 ± 0.31, and 1.88 ± 0.19 L of sweat during the NC, CC, and CCreplaced trials. There was a trend for participants to drink less and lose less body mass in CCreplaced trials; however, because of the large individual differences, there were no significant differences observed. The mean plasma volume changes observed in the NC, CC, and CCreplaced were −2.4% ± 8.27%, 0.4% ± 6.4%, and −5.8% ± 8.2% (F2,12 = 0.6, P = 0.583).
Whole-blood lactate and glucose concentrations increased over time (main effect of time on lactate, F1.9,11.3 = 105.4, P < 0.001; main effect of time on glucose, F1.8,11.1 = 4.9, P = 0.005), but there were no significant main effect differences between trials for whole blood lactate (F2,12 = 1.7, P = 0.216) or glucose (F2,12 = 0.1, P = 0.866). There was no significant increase in cortisol levels over time (F4,24 = 2.0, P = 0.139) or significant differences between trials (F2,12 = 1.5, P = 0.269). Plasma concentrations of cortisol, serotonin, and dopamine are displayed alongside whole-blood lactate and glucose concentrations in Table 2. Concentrations of serotonin and dopamine increased over time (main effect of time on serotonin, F4,24 = 5.7, P < 0.03; main effect of time on dopamine, F4,24 = 20.6, P < 0.001); however, there was no difference between trials for either neurotransmitter (main effect of trial on serotonin, F2,12 = 0.8, P = 0.462; main effect of trial on dopamine, F2,12 = 0.5, P = 0.790). Baseline concentrations of neuroendocrinological showed some variability between trial days (cortisol = 20% ± 6%, serotonin = 33% ± 15%, dopamine = 18% ± 8%). There were no significant differences between trials for the magnitude of change during the 90 min for cortisol (F2,12 = 3.4, P = 0.075), serotonin (F2,12 = 3.9, P = 0.055), or dopamine (F2,12 = 0.4, P = 0.716) (Table 2).
The main finding of the current study is that replacing the cold collar at regular intervals improves time trial performance by 6.9% in the heat but offers no cumulative or additional benefit to that provided without replacing the cold collar. In the CC trial, performance was improved by ∼7.3% compared with the NC control trial, which confirms previous research that demonstrated that cooling the neck region via a practical CC improved the 15-min time trial performance in the heat (35).
There was no cumulative benefit of the sustained cooling in the present study, which is in contrast to previous literature that has suggested that the beneficial effects of a cooling intervention are dependent on a sufficient level of cooling provided and/or thermal strain experienced (23,25). For example, Nunneley et al. (23) reported that cooling the head had no effect on core temperature in trials conducted at 20°C and 30°C; however, it did reduce core temperature in the 40°C trials when the thermal strain was greatest. In an exercise setting, Palmer et al. (25) reported that sustained cooling of the head region during a bout of rest and subsequent exercise improved 15-min treadmill performance in a hot environment (33°C; 55% RH) by ∼2.5% compared with cooling at rest alone. No data comparing cooling at rest to no cooling were provided, but the data provided regarding cooling at rest versus cooling at rest and during exercise offered further tentative support for the notion of a cumulative benefit of sustained cooling. Such a benefit was not observed in the current study.
Unlike the present study, Palmer et al. (25) reported a reduction in rectal temperature with the sustained head cooling. Because of the inverse relationship observed between the ability to exercise and the levels of hyperthermia (12), the improvement in performance observed could be, in part, due to this reduction. In previous experimental studies investigating the practical CC used in this study (34,35), the collar was shown to have no effect on the physiological or hormonal responses to the exercise bout. The current study replicated these findings, and it was established that replacing the collar at 30-min intervals also had no effect on the rectal temperature or HR response to the 90-min preloaded time trial and that the hormonal response also remained unaffected. Reductions in core temperature caused by cooling interventions seem dependent on the cooling of peripheral blood, which circulates to the core (17); however, the neck only forms <10% of the body’s surface area and therefore has limited potential to reduce core temperature. Brain temperature is more important than body temperature in the regulation of exercise (6), and it has been proposed that cooling the neck may decrease brain temperature because the brain is supplied by the carotid arteries within the neck (40). Although mathematical modeling articles have proposed that superficial cooling of the brain may be possible (40), clinical (30) and experimental (8,24) data have failed to identify a reduction in brain temperature after superior cooling. As a result, it seems unlikely that brain temperature was affected in the current study; however, this remains to be confirmed.
Previously, it was proposed that peripheral hormonal concentrations are important markers of exercise stress (4); however, more recently, it has been suggested that central, rather than peripheral, levels are key in regulating exercise performance in hot environments (20,27–29). Brisson et al. (4) suggested that the effects of a cooling intervention on the peripheral hormonal concentrations were magnitude dependent, but the data from the current study and that reported previously (35) show that cooling the neck has no effect on such concentrations even when the cooling is sustained and pronounced. Although there were no differences between trials, concentrations of dopamine and serotonin were both elevated at the end of the trial (90 min) compared with all other time points, replicating previous findings using a CC (35). The release of dopamine and serotonin is dependent on the intensity of the stress (in this case exercise) to which it is released in response to, and so significant differences were only observed after the most intense part of the trial—the 15-min time trial. The measurement of peripheral concentrations has been questioned because of the limited crossover with central concentrations (23,30–32), and the current study also highlights the issue of intraindividual variation and the importance of establishing participant-specific biological variance if such variables are to be measured. As with other similar investigations (27–29), this was not established in the current study; however, data suggest that such measures should be taken.
The improvements in performance observed in the current investigation were not matched with significant alterations in TS, FS, or RPE. The RPE results are different from those reported previously in a study of similar design (35) and also differ from those reported elsewhere recently after the administration of a menthol mouth rinse (21) and a dopamine reuptake inhibitor (37). The administration of a dopamine reuptake inhibitor reduced the impairment observed in time trial performance in a hot environment from −30% to −19% with the same RPE. The findings from Watson et al. (37) and Mundel and Jones (21) suggest that tolerance to hyperthermic exercise can be improved at the cortical level after alterations in sensation but also that exercise is limited by other mechanisms in addition to perception. This is tentatively supported in the current study by the assessment of pleasure and displeasure using the FS, which showed no differences with the cooling interventions despite altered performance replicating previous findings (34). Thermal sensation data differed from previous literature (35). In the current study, participants were required to differentiate the levels of thermal comfort they experienced at the neck from the rest of their body, and this was an additional measurement adopted in the present study compared with the previous (35). Thermal sensation of the neck was significantly reduced via the application of the CC compared with the NC trial and by the replacement of the CC compared with the NC and CC trials. It has been proposed that the neck is an optimal site for cooling because of its proximity to the thermoregulatory center (31); however, the extent to which cooling the neck affects perceived thermal states compared with cooling elsewhere has not been investigated. It has been established that the face is a site of high alliesthesial thermosensitivity and that cooling the face region results in a two- to fivefold greater suppression in thermal discomfort than cooling areas of the trunk and limbs (9) and so the neck may have similar qualities. It seems likely that the lack of difference in thermal comfort reported in this study is explained by the addition of data collection regarding the thermal sensation specific to the neck region and previous data reporting a combined thermal comfort. It is well documented that improvements in thermal sensation offer a benefit to exercise performed in a hot environment, and the data from the current study support the literature stating that head and neck cooling may offer a performance benefit (9,31,34). Interestingly, replacement of the CC did not improve performance in comparison to simply wearing the CC from the start despite a loss of cooling being observed in the CC trial and a lowered perception of thermal sensation being reported in CCreplaced, which suggests that there is a limit to the extent of deception and the beneficial effect of altered perception.
The performance benefit observed in the previous experimental study investigating the modified CC (35) was attributed to an up-regulation in the pace selected because of a positive alteration in the level of perceived thermal comfort. In the present study, there were no significant differences in the pacing strategy adopted. Participants initially adopted a faster pace in the CCreplaced trial, but they were then unable to increase the pace selected as progressively as in the CC trial. These data suggest that the collar replacement may have provided a false signal, which resulted in the adoption of an initial pace in excess of what was sustainable, whereas the single application of the collar allowed for a progressive increase in pace during the performance test. The idea that the pacing strategy could be influenced by cooling the neck is due to the association between hyperthermia and the down-regulation of self-selected pace (19). It has been proposed that, during self-paced exercise, the intensity is regulated by a complex network of feedback and feed-forward systems regarding the physiological state of the body to allow for the completion of the task within homeostatic limits (19,32). Data from the current study and from a previous investigation using the same protocol (35) suggest that cooling the neck enhances preloaded time trial performance in a hot environment by masking the extent of the thermal strain; however, the present study suggests that there is a limit to the gain that can be achieved and to the extent to which the mechanisms that regulate exercise in the heat can be deceived. Interestingly, the gain was achieved despite relatively low final rectal temperatures (∼38.90°C–38.97°C). Wearing a CC can enable greater tolerance of higher Trectal (34), and so it is possible that the benefit to self-paced exercise could be greater still in more thermally challenging situations.
The critical core temperature and central governor theories are the two main theories proposed to explain the impairment in sporting performance observed in hot temperatures and both models propose that there are mechanisms in place to prevent the onset of a dangerously high internal temperature (12,19,32,36). Wearing a CC during hyperthermic exercise enables participants to tolerate higher levels of thermal and cardiovascular strain during exercise (34), and this has potential risks for user safety. Whereas other species have evolved mechanisms to safely tolerate high internal temperatures (e.g., selective brain cooling) (6), humans rely on the delicate balance between heat production and heat loss. It would be attractive to suggest that the data from the current study suggest a physiological, evolutional, or biochemical limit to the benefit of cooling due to the lack of a difference between CC and CCreplaced trials; however, the lack of differences is more likely due to the nature of the test. There is a limit to the magnitude of an improvement, which can be observed in the time trial performance in a homogenous population. The fixed duration of a time trial dramatically improves the reliability of the test in comparison to an open-ended capacity test (15), but it does so by reducing the potential for variance to occur. As a result, there is also a limited potential for performance differences, and therefore, it is possible that the replaced collar may have a cumulative performance benefit in longer duration or open-loop tests.
Cooling the neck can improve time trial performance in a hot environment, although maintaining the neck at a reduced temperature via the replacement of a practical CC offers no additional benefit. Cooling the neck region does not alter the physiological or hormonal response to running exercise performed in high ambient temperatures; however, it does improve the subjective rating of thermal comfort, and this improvement in thermal comfort may improve performance by masking the thermal strain of the body.
The authors thank all of the participants for their time and efforts and Dr. Philip Hennis, Dr. Hannah MacLeod, and Mr. Ian Varley for their assistance with data collection. The studies conducted in this article were conducted while the authors were at Nottingham Trent University, United Kingdom.
The authors have no relationships or affiliations with any companies or manufacturers to disclose.
The results presented in this article do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2011The American College of Sports Medicine
HYPERTHERMIA; THERMOREGULATION; TREADMILL; EXHAUSTION; FATIGUE; PACING