The time taken to reach volitional exhaustion also is impaired in hot compared with temperate conditions.³ Although numerous investigators have provided valuable data to help explain the reduced ability to sustain exercise in hot conditions, the reasons still are not understood fully. The impairment often is attributed to the development of hyperthermia because, in laboratory-based investigations, exercise is consistently terminated voluntarily at core temperatures of approximately 40°C, regardless of the initial temperature, acclimation status, or hydration levels of participants.^4,5 Researchers have proposed that exercise termination at such temperatures prevents the body from reaching temperatures that would lead to the onset of potentially fatal heat illness.^5,6 Although consistency in the core temperatures is observed at voluntary exercise termination in laboratory tests, core temperatures in excess of 40°C have been recorded after athletic competition.⁷ The ability to exceed temperatures of approximately 40°C during exercise demonstrates that the mechanisms that limit exercise in a hot environment can be overridden and that temperatures in excess of 40°C can be tolerated during exercise in sufficiently motivated athletes.

Cooling the body before exercise has regularly been shown to increase the time taken to reach volitional exhaustion and to improve exercise performance in hot conditions.^8,9 The reason for the improvement often is attributed to a reduction in core temperature at any comparative time point during the exercise bout, but improvements have occurred in the absence of core- temperature reductions after decreases in skin temperature.^8,10 Cooling the neck region via a practical neck CC can enhance exercise performance in hot conditions without altering the physiologic response to the exercise bout.² The head, neck, and face are regions of high allesthesial thermosensitivity,¹¹ and cooling the neck has been shown to more effectively alleviate heat strain than cooling the same surface area of the trunk.¹² Investigators have proposed that the neck might be an optimal site to cool because of its close proximity to the thermoregulation center,^12,13 which is located at the base of the brain and receives afferent signals regarding the thermal state of the body from many deep and peripheral thermoreceptors.¹⁴ Researchers have suggested that self-paced exercise in a hot environment is regulated by integrated feedback from many physiologic and perceptual systems (commonly referred to as the central governor theory).^15,16 Cooling the neck region might enhance exercise performance by providing a false signal of the body's thermal state, allowing the selection of a faster, “unnatural” pace. If cooling the neck region provides a false signal that allows performance to be improved, it also would prolong exercise in a hot environment by masking the extent of the thermal strain experienced, allowing the participant to tolerate a higher core temperature and level of thermal strain by overriding the thermal signals governing the termination of exercise. This has obvious potential performance-enhancing implications; however, it also might pose a threat to the health of the participant. If the extent of the thermal strain experienced is masked heavily, the athlete might experience potentially dangerous body temperatures that put him or her at risk for heat illness, so caution might be required when using cooling during exercise in a hot environment. Therefore, the aim of our study was to investigate whether the enhanced ability to exercise in a hot environment observed when cooling the neck region occurs as a result of dampening the perceived level of thermal strain experienced and the subsequent overriding of inhibitory signals.

Exercise performance is impaired in hot compared with moderate environmental conditions,¹ and this impairment is attributed to a centrally regulated decrease in the pace selected during such self-paced exercise performance tests.^15,16 The mechanisms governing this decrease are unknown, but researchers have proposed that feedback from a variety of peripherally and centrally located receptors (eg, thermoreceptors, chemoreceptors, baroreceptors) provides information regarding the state of the body and results in the selection of a pacing strategy that enables the task to be completed within homeostatic limits.^15,16,23,24 This central governor theory was developed in opposition to the critical core temperature hypothesis, which proposes that exercise in the heat is limited because individuals can experience dangerously high core temperatures^4,5; however, the models have very similar themes and, therefore, are not as contrasting as previously suggested. The central governor theory can only be applied to self-paced performance tests, whereas the critical core temperature theory can only be applied to fixed-intensity tests; this fundamental methodologic difference explains much of the conflict. Performance can be enhanced by approximately 6% in a hot environment with the application of a CC without any alterations in the physiologic response to the exercise bout.² It seemed prudent to suggest that the application of the CC might have provided a false signal regarding the body's thermal strain that allowed a faster pace to be adopted; however, the concept of the false signal was difficult to fully elucidate using a fixed- or known-endpoint performance test model.

The time to volitional exhaustion also is impaired in a hot environment,³ and, when the intensity is fixed in such exercise tests, the point of voluntary exercise termination regularly occurs at core temperatures of approximately 40°C in laboratory investigations, regardless of the initial rectal temperature⁴ or acclimation status.⁵ Core temperatures in excess of 40°C have been reported after marathon races. This demonstrates that high internal temperatures can be tolerated and that the mechanisms limiting exercise can be overridden when sufficient incentive and a high level of motivation exist.⁷ Our data showed that cooling the neck region extended the time to reach volitional exhaustion by approximately 13.5%. Exercise was terminated at a higher rectal temperature when the neck region was cooled via the application of a neck CC. Participants voluntarily terminated exercise at a rectal temperature of 39.18°C ± 0.7°C in the NC trials but did not cease exercising until reaching an average temperature of approximately 0.4°C higher (39.61°C ± 0.45°C) in the CC trials (t₇ = −3.217, P = .02, d = 0.78). The HR at the termination of exercise was also higher in the CC trials; however, the subjective perceptual measurements revealed that, at the point of voluntary exercise termination, no difference existed in the level of thermal comfort or rating of perceived exertion. These data showed that participants reached voluntary exhaustion at similar perceived levels of thermal and physical stress and discomfort despite being under less thermal and cardiovascular strain in the NC trials. Similar findings were presented in 2 recent pharmacologic investigations in which cerebral concentrations of dopamine and noradrenaline were manipulated.^25,26 Roelands et al²⁵ and Watson et al²⁶ reported that higher levels of thermal and cardiovascular strain could be tolerated when cerebral dopamine concentrations were elevated and that the perceptual response to the level of strain was dampened. The excitability of cerebral dopaminergic neurons is temperature dependent,²⁷ so cerebral dopamine concentrations might be elevated by the application of the CC; however, this has not been investigated.

Authors⁹ of precooling studies often have reported improvements in subsequent exercise performance and time to volitional exhaustion and have attributed the improvements to a reduction in the rectal temperature at any given comparative point; however, improvements in a subsequent exercise bout have been observed without reductions in core temperature.¹⁰ This suggests that the benefit observed might not be dependent wholly on a reduction in core temperature and that the benefit might be due to an alternative cooling-induced alteration in the actual or perceived state of the body. Our data demonstrated that, during fixed-intensity exercise, cooling the neck region can dampen the perceived levels of TS and rating of perceived exertion, because participants' subjective ratings were the same despite higher core temperatures at commencement of and throughout exercise and higher HRs during the exercise bouts. Investigators^2,28 have reported that cooling the neck region has no effect on the core temperature or HR response to exercise, but, in both of these studies, the participants were exercising in cooler temperatures (30°C versus 32°C) and at a lower intensity (60% versus 70% O_2max) for most of the test. The effectiveness of cooling the head region in enhancing exercise capacity has been shown to depend on the level of thermal strain experienced,²⁹ and this might explain the differences observed. The elevation in core temperature despite the fixed work intensity suggests that cooling the neck region might alter the thermoregulatory drive and suppress the heat-loss mechanisms. However, the higher core temperatures observed during the exercise might have been due to unsystematic variation, as demonstrated by the higher mean starting core temperature. A suppressed heat-loss mechanism and a reduced perceived level of thermal strain both have potentially serious implications for the health and well-being of the participant. Subjective ratings help to regulate exercise intensity³⁰; therefore, any intervention that manipulates or dampens this feedback might be dangerous if it results in higher core temperatures.

Researchers³¹ have shown that brain, rather than core, temperature is the main determinant of exercise capacity, so another possible reason for the ergogenic effect of cooling the neck is a direct cooling of the arterial blood and a subsequent reduction in brain temperature. The preoptic and anterior regions of the hypothalamus, which make up the thermoregulatory center, are supplied primarily by the anterior cerebral and anterior communicating arteries.³² These arteries are supplied by the internal carotid artery; this relationship demonstrates the direct route that the arterial blood takes from the carotid arteries to the thermoregulatory center and explains why investigators have proposed that neck cooling might lower brain temperature. Using mathematical modeling, researchers have computed that a reduction in brain-surface temperature is possible to a depth of approximately 3 to 4 mm based upon typical cerebral blood flow and that reduced cerebral blood flow, as is observed during hyperthermic exercise,³³ would increase the depth of cooling achieved.³⁴ Although these researchers have computed that external cooling theoretically can reduce brain temperature, no one has established whether the cooling induced is practically significant or can occur in a human model. We did not measure cerebral blood flow or arterial temperature, so we acknowledge that a reduction in neck skin temperature does not necessarily mean a similar reduction in arterial blood temperature.

An inherent problem with cooling studies is the inability to blind the participants to the intervention and the resultant possibility of a placebo effect. In a recent review on the placebo effect in sports performance, Beedie and Foad³⁵ stated that both positive and negative placebo effects ranging from −1.9% to 50.7% have been reported on sport performance; however, most investigators have reported a positive effect of 1% to 5%. Although the magnitude of any cooling-induced placebo effect has not been established, the improvements in the time taken to reach volitional exhaustion observed in our study are far greater than the 1% to 5% reported. The CC also had no effect on the subjective perceptions of exertion or pleasure/displeasure. If the improvement observed was due to a placebo effect, it would have been mirrored by improvements in perceived levels of pleasure, but this was not the case, suggesting a real, rather than placebo, effect.