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1 Human Performance Laboratory, School of Sport and Exercise Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 2 Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Hassall Road, Alsager, Cheshire ST7 2HL, UK
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Abstract |
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6°C lower than CON, while other skin sites were similar or slightly warmer in the FC condition. Rectal temperature increased by 1.5°C with the same time course in both conditions. A relative bradycardia was observed during FC, with heart rate approximately 5 beats min–1 lower than CON (P < 0.05). Mean plasma lactate was lower during FC (FC, 5.0 ± 0.3 mmol l–1; CON, 5.9 ± 0.3 mmol l–1; P < 0.05) but no differences were observed for plasma glucose, which remained constant during exercise. Levels of PRL were maintained at 175 ± 17 mIU l–1 during exercise for FC, while values for CON increased to a peak of 373 ± 22 mIU l–1 so that towards the end of the exercise, for the same rectal temperature, PRL was significantly lower in the FC condition (P < 0.05). Global and breathing RPE were reduced but only towards the end of the 40 min of exercise during FC, whilst subjective thermal comfort was significantly lower during FC (P < 0.05). We confirm the substantial effect that FC has on the secretion of PRL during hyperthermic exercise but show that it makes a relatively small contribution to the perception of effort when compared to the effect of a cool total skin area as occurs with exercise in a thermoneutral environment.
(Received 29 June 2006;
accepted after revision 12 September 2006; first published online 14 September 2006)
Corresponding author T. Mündel: School of Sport and Exercise Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Email: t.mundel{at}bham.ac.uk
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Introduction |
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The increases in PRL secretion during exercise in the heat, with peak levels observed at exhaustion, have consistently been proposed to reflect an intolerable thermal strain and decreased desire to continue exercising, resulting in fatigue (Pitsiladis et al. 2002; Bridge et al. 2003b; Low et al. 2005). This reflex response to increasing body temperature may arise in the limbic area, hypothalamus or brainstem and involve serotonergic (5-HT) neurones that radiate from the raphe nuclei to higher centres concerned with motivation and motor drive (Hori & Harada, 1976). Central 5-HT and dopaminergic (DA) pathways play an important role in the regulation of body temperature (Cox et al. 1980) but 5-HT and DA activity cannot be assessed directly in humans; instead, many studies have used the release of PRL as an indirect measure, since PRL release from the anterior pituitary is stimulated by 5-HT and inhibited by DA activity in the hypothalamus (Bridge et al. 2003b). A study using functional brain imaging identified a strong correlation between 5-HT activity in the hypothalamus and the PRL response to a 5-HT2c receptor agonist, m-chlorophenylpiperazine (Anderson et al. 2002).
An increase in core temperature, as a result of both passive heating and exercise, is known to be a major stimulus of PRL secretion (Brisson et al. 1991; Low et al. 2005). A strong correlation has also been demonstrated between RPE and the increase in core temperature caused by cycling in a hot environment (Nybo & Nielsen, 2001). However, core temperature may not be the complete explanation because, for the same rectal temperature, exercise in a cool environment is associated with a lower RPE and PRL response compared to exercise in the heat (Pitsiladis & Maughan, 1999; Bridge et al. 2003a), suggesting that skin temperature affects both the perception of effort, as indicated by RPE, and the secretion of PRL.
Selective cooling of the face (FC) is particularly effective in mediating the PRL response to skin cooling during hyperthermic exercise (Brisson et al. 1989). Whilst there has been speculation that FC may directly cool the brain, thereby modifying hypothalamic output (Cabanac & Caputa, 1979a,b), this has been countered by more recent evidence that brain temperature is unaffected, so that any consequences of head cooling seem likely to be mediated by stimulation of skin afferents (Nelson & Nunneley, 1998; Nybo et al. 2002). Relatively recently, Armada-da-Silva et al. (2004) demonstrated that FC during a short (14 min) bout of cycling (63% of maximal aerobic power, Wmax) attenuated the hyperthermia-induced increase in RPE.
The work on face cooling discussed above suggests that the findings of Pitsiladis & Maughan (1999) and Bridge et al. (2003a) might be replicated by simply cooling the head rather than the total skin area as was the case when their subjects exercised in the cool environment, and to date, there have been no studies that have examined the effect of FC on both the RPE and PRL responses concurrently. Accordingly, the hypothesis of the present study was that FC during hyperthermic exercise would reduce both the RPE and the PRL response, suggesting a common mechanism linking these responses.
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Methods |
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All exercise tests were carried out on an electrically braked cycle ergometer (Lode Excalibur, Groningen, The Netherlands) operating in the pedal rate-independent mode. The protocol consisted of four visits. Visit 1 was an incremental exercise test to determine maximal oxygen consumption 
, Wmax and maximal heart rate (HRmax). Visits 2–4 involved exercising at 65%
Wmax for 40 min in a heated chamber maintained at 33°C. Visit 2 served to familiarize the subjects with the protocol and equipment, thereby minimizing practice effects. In one of the remaining two experimental rides, subjects received face cooling (FC) throughout the ride, the other ride acting as a control (CON). The study was carried out in a randomized, cross-over design with subjects blind to the purpose of the study.
Subjects
Ten healthy, non-heat-acclimated males volunteered their written informed consent to participate in the study. The study was performed according to the Declaration of Helsinki and was approved by the School of Sport and Exercise Sciences Local Ethics Subcommittee. The subjects' physical characteristics were (means ±
S.D.): age, 23 ± 2 years; body mass, 77 ± 8 kg; height, 180 ± 6 cm; 
, 56 ± 7 ml kg–1 min–1; Wmax, 297 ± 57 W; and HRmax, 193 ± 12 beats min–1. All subjects were familiar with cycle ergometry at this intensity and had completed the School's Health Questionnaire to rule out any obvious contra-indications for exercise. All trials were conducted during winter (November–February), during which time the subjects' acclimation to heat would be at a natural nadir.
Experimental design
Visit 1. Subjects performed an incremental exercise test to volitional fatigue at a self-selected cadence. The seat position, handlebar height and orientation were adjusted for each subject, and the same settings were used for all subsequent rides. The initial workload was 95 W, and this was increased by 35 W every 3 min until fatigue. Heart rate (HR) was monitored continuously (Polar Accurex Plus, Polar Electro Oy, Finland) as were O2 and CO2 (Oxycon Pro, Jaeger, Germany). The test was considered maximal if one of the following criteria was met: (1) final HR was within 10% of predicted maximum; (2) a clear plateau in O2 uptake was seen; or (3) respiratory exchange ratio was equal to, or above, 1.10. Maximal aerobic power was determined using the equation of Kuipers et al. (1985).
Visits 2–4. Subjects were advised to consume a diet high in carbohydrates in the 24 h period prior to each visit. To minimize differences in muscle glycogen content between visits, subjects were asked to record their diet in the 24 h period before the second visit and instructed to follow the same diet before each subsequent visit. On the morning of the test, subjects consumed their usual breakfast plus a bolus of 500 ml water to ensure adequate hydration and arrived at the laboratory at noon after a minimum 4 h fast and having abstained from exercise, alcohol, caffeine and tobacco for the previous 24 h.
On arrival, a cannula (20 gauge, Venflon, Becton Dickinson, Plymouth, UK) was inserted into an antecubital vein and kept patent with saline (Baxter, Norfolk, UK) during the test. Subjects were then asked to insert a rectal thermistor 10 cm beyond the anal sphincter and to empty their bladder before being weighed nude. Skin thermistors were attached to the forehead, dorsum of the hand, calf and lower back with surgical tape (TransporeTM, 3M) whilst the subject remained seated for 40 min. A resting blood sample was taken, after which a further oral bolus of water (8 ml (kg body weight)–1) was given. Subjects were then transferred to the heated chamber (33 ± 0°C, 27 ± 1% relative humidity) and exercised at a constant work rate of 65%
Wmax for 40 min. Subjects wore shorts and T-shirt. A fan set at 0.5 m s–1 was used to circulate the air in the chamber and was in the same position relative to the subjects for all trials. In the event of a subject needing to urinate during the test, they stopped pedalling and passed urine into a container whilst remaining in the heated chamber. Immediately following exercise, the subject was weighed nude to record weight loss, which was corrected for any urinary, respiratory (Snellen, 1966) and metabolic losses (Mitchell et al. 1972), and quantity of blood withdrawn.
Face cooling
The cycle ergometer was equipped with tri-bars, and subjects were asked to assume a racing position for both CON and FC rides. A thermistor placed at the centre of the forehead allowed constant measurement of facial temperature (Tfor), and the face was sprayed with a mist of cold water (4°C) to maintain Tfor < 28°C. Assuming the racing position on the ergometer prevented cooling of other body parts, and this was confirmed by skin temperature data from these regions.
Blood collection and analyses
Venous blood samples (8 ml) were collected into prechilled EDTA-containing tubes at rest, at 10 min intervals throughout the ride and 10 min post-exercise. One millilitre of blood was separated and analysed for haemoglobin and haematocrit. The remainder was centrifuged at 2300g for 10 min at 4°C, and the plasma separated and stored at –70°C until analysis. Haemoglobin concentration was measured using a Coulter®AC•T diffTM Analyser (Beckman Coulter Inc., Miami, FL, USA) and haematocrit was measured in triplicate by centrifugation. Changes in plasma volume were calculated from haemoglobin concentrations and haematocrit values using the equations of Dill & Costill (1974). Plasma glucose and lactate concentrations were determined enzymatically (Sigma Diagnostics, Dorset, UK) on a semi-automatic analyser (Cobas Bio, Basel, Switzerland). Prolactin was measured using a radioimmunoassay (Skybio Ltd, Bedford, UK). Average inter- and intra-assay coefficients of variation were 5.9 and 2.7%, respectively. All hormone analyses from a single subject were carried out in the same assay batch.
Gas, temperature and HR measurement during exercise
During the exercise trials, standard Douglas Bags (Cranleigh, Crowburough, UK) were used to collect expired air for 2 min at the 20 and 40 min time points and were measured with a Servomex analyser and dry gas meter (Harvard, Kent, UK) to determine expiratory minute ventilation 
, and rates of O2 uptake 
and CO2 elimination 
. Ambient temperature was measured during each ride, and relative humidity calculated from the wet and dry bulb thermometer differential. The rectal and skin thermistors were connected to a Squirrel data logger (Grant Instruments, Cambridge, UK) and values recorded every 5 min. Heart rate was recorded continuously throughout by telemetry (Polar Accurex Plus).
Perceptual measurement
Measurements were taken every 10 min and a minimum of 1 min after spraying, in the case of the FC trials. Global ratings of perceived exertion (RPE) were recorded using the 15-point Borg scale (Borg, 1982). Separate RPE for breathing (RPEbre) and legs (RPEleg) were recorded using the CR-10 scale to assess the extent that central (cardiopulmonary) and local (muscular) signals may contribute to global RPE (Noble et al. 1983). In addition, subjective thermal comfort was assessed using a 10-point scale (Frank et al. 1999).
Data and statistical analyses
All statistical analyses were carried out with the SPSS package, version 12.0 (SPSS Inc., Chicago, IL, USA). Data were tested for approximation to a normal distribution. Data were analysed using a two-way (time x trial) analysis of variance (ANOVA) for repeated measures. Values from ANOVA were assessed for sphericity and if necessary corrected using the Huynh-Feldt method. Following a significant F test, pairwise comparisons were identified using Tukey's honestly significant difference (HSD) post hoc procedure. Student's paired t tests were used for differences in weight loss and plasma volume changes. Otherwise, where data were not normally distributed (PRL), Friedman and Wilcoxon tests were used. A 5% significance level was adopted throughout. Data are reported as means ± S.E.M. and analyses performed on n = 10, unless stated otherwise.
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Results |
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All 10 subjects successfully completed both experimental trials. They were cycling at the same intensity in both trials (77 ± 1%
for CON and 78 ± 1%
during FC; n.s.) and cadence was maintained at 93 r.p.m. during both trials with only a 5 r.p.m. decrease by 40 min. Heart rate (Fig. 1) increased significantly (P < 0.001) from 5 min to the end of each trial, with a relative bradycardia during the FC trial, the rate being approximately 5 beats min–1 lower than during the CON trial (P < 0.05).
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Core temperature before exercise was the same for CON and FC, and there were no differences in rectal temperature between trials (n.s.; Fig. 2A). Rectal temperature increased significantly from 36.9 ± 0.1°C at the start of exercise to 38.5 ± 0.1 and 38.4 ± 0.1°C for CON and FC, respectively (P < 0.001). Forehead temperature remained constant during exercise in both trials at 26.7 ± 0.4°C for FC and 33.4 ± 0.5°C for CON, with a mean intertrial difference of 6.7 ± 0.5°C (P < 0.001; Fig. 2B). Mean skin temperature, for sites other than the forehead, tended to be higher by 0.4 ± 0.3°C for FC, but this was not significant (Fig. 2C).
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Plasma lactate (Fig. 3A) increased in the first 10 min of both trials and then remained relatively constant. After 30 and 40 min of exercise, lactate was significantly lower during FC (FC, 5.0 ± 0.3 mmol l–1; CON, 5.9 ± 0.3 mmol l–1; P < 0.05). No changes were observed over time for concentrations of plasma glucose, and there were no differences between trials (Fig. 3B). Plasma volume changes were not different between trials (CON, –10 ± 2%; FC, –6 ± 2%; n.s.), indicating that differences between the trials in blood metabolites or hormones were probably not a consequence of haemoconcentration.
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Global RPE (Borg scale) increased significantly during exercise (P < 0.01), and values for FC were significantly lower than for CON by 40 min, with values reaching 14.1 ± 0.8 and 15.5 ± 0.7, respectively (P < 0.05, Fig. 5A). Ratings of RPEbre (Fig. 5B) increased significantly (P < 0.001) during CON but remained stable for FC so that by 40 min values were significantly higher for CON (5.1 ± 0.7) compared to FC (3.8 ± 0.4; P < 0.05). Local/muscular signals (RPEleg) contributing to global RPE increased significantly (P < 0.001) during the course of each trial but did not differ between trials, reaching 6.1 ± 0.7 and 5.5 ± 0.7 for CON and FC, respectively (n.s.).
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Discussion |
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Cardiorespiratory and metabolic responses
Face cooling caused heart rate to be significantly lower than CON by 5 beats min–1. Similar observations were made by Brisson et al. (1989), who studied subjects cycling in a warm environment (27 ± 5°C) for 30 min, by which time their heart rate with face cooling was approximately 9 beats min–1 lower than without face cooling. This relative bradycardia may be due to one or both of the following reasons. In humans, it is possible to simulate the diving response seen in marine mammals by immersion of the face in cold water (Gooden, 1992). Perhaps the most important stimulus for the diving response is stimulation of the ophthalmic division of the trigeminal nerve, which serves the forehead, eyes and nose. When the face is placed in cold water, stimulation of the (thermosensitive) trigeminal nerve results in alterations of vagal tone and a classic deceleration of the heart (Heistad et al. 1968). This effect has also been demonstrated whilst performing exercise (Bjertinaes et al. 1984). Alternatively, vasoconstriction of the vessels beneath the cooled area may improve venous return and stroke volume, and consequently decrease heart rate (Booth et al. 1997). Therefore, with FC there appears to be a reduced cardiovascular strain during exercise in the heat.
Subjects were cycling at the same approximate aerobic intensity in both trials (77%
), and there were no differences in any of the respiratory parameters of ventilation, O2 uptake or CO2 elimination (Table 1), which suggests that the energy consumption, gross efficiency and relative rates of fuel oxidation were not altered by FC. Plasma lactate was lower with FC, whilst plasma glucose remained constant and unchanged. These results were unexpected, and previous studies investigating the effects of face cooling during exercise hyperthermia have not reported measures of blood metabolites (Brisson et al. 1989, 1991; Nybo et al. 2002), although Bridge et al. (2003a) observed lower levels of lactate during cycling at 20°C compared to 35°C (4.3 ± 0.9 and 5.2 ± 0.8 mmol l–1, respectively; P
= 0.059). In the present study, subjects were cycling at very similar intensities in the two trials, indicating that the difference in lactate did not result from differences in overall workload. A lower concentration of lactate during FC may reflect a small redistribution of blood flow from the face towards the working muscles, resulting in an improved lactate clearance; however, this is unlikely because in the present study, mean skin temperature for sites excluding the forehead was actually higher with FC (Fig. 2C), suggesting a redistribution of blood flow towards the cutaneous layer, to enhance heat dissipation, rather than towards the active muscles. Although not measured in this study, it is possible that FC reduced circulating catecholamines; Brisson et al. (1989) demonstrated that by 30 min of exercise with face cooling, concentrations of noradrenaline and adrenaline were beginning to plateau when compared with exercise without cooling.
Prolactin, RPE and thermal comfort
It has been demonstrated that changes in blood pH and osmolality are not responsible for stimulating the release of prolactin (Brisson et al. 1986) and, likewise, in the present study, it seems unlikely that there is a causal relationship between concentrations of plasma lactate or glucose and the release of prolactin. Lactate concentrations, although elevated above resting levels, remained relatively unchanged after 10 min for both trials, whilst prolactin levels continued to rise during exercise in the control trial. Moreover, plasma glucose concentrations changed very little during exercise and, by 40 min, were equal to, or above, resting levels, with a similar response reported between trials.
Brisson et al. (1989) observed significantly lower plasma prolactin as a result of face cooling during exercise in a warm environment (27 ± 0.5°C), and we have replicated this observation at a higher ambient temperature. Although previous studies have reported that the increase in prolactin mainly results from a rise in core temperature of > 1°C or above 38°C (Brisson et al. 1991; Radomski et al. 1998; Low et al. 2005), this is clearly not the case in the present study, since rectal temperatures increased with the same time course and to the same degree in both trials (1.5°C/38.5°C), and this clearly indicates the importance of the temperature of the facial skin in the secretion of PRL. These observations suggest that the reduced secretion of prolactin observed during exercise when the total skin area was relatively cool (Bridge et al. 2003a) can be replicated by cooling the face alone. Whilst there has been speculation that cooling the head/face leads to direct cooling of the brain, thereby modifying hypothalamic output (Cabanac & Caputa, 1979a,b), there is now good evidence showing that brain temperature remains unaffected (Nelson & Nunneley, 1998; Nybo et al. 2002) and therefore any action on prolactin release is more likely to be a direct consequence of skin afferent stimulation. In support of this, Mündel et al. (2006) have demonstrated that elevated skin temperature, independently of core temperature, leads to increased circulatory prolactin.
In the present study, global RPE followed the same time course in both trials until 30 min; thereafter, FC blunted the rise and, by 40 min, RPE with FC was significantly lower when compared with CON. Rectal temperature increased almost linearly and without any difference between trials; therefore, it is unlikely that a lower RPE by 40 min would be due to a lower rectal temperature. Although plasma lactate was significantly lower with FC at 30 and 40 min, this was not reflected in the scores of RPEleg, which is thought to report the extent that local metabolic (muscular) signals contribute to global RPE (Noble et al. 1983). Global RPE during physical exertion is thought to derive from multiple sources, including sensory signals from the working muscles and information pertaining to cardiovascular and respiratory load (Ekblom & Goldbarg, 1971). Face cooling significantly reduced ratings of RPEbre, and it is possible that the altered perception of breathing may, in part, have mediated the reduced global RPE seen towards the end of the exercise. It appears that in the present study, stimulation of touch- and temperature-sensitive receptors on the face during FC may have altered the perception of breathing, and sensory signals pertaining to ventilatory load have previously been shown to correlate with changes in global RPE (Robertson, 1982).
Other afferent signals that have been proposed to influence global RPE are psychophysical phenomena such as pain and discomfort (Robertson, 1982). Increased discomfort may arise from a change in subjective thermal comfort, and in the present study, ratings of thermal comfort increased significantly during exercise with both trials, suggesting that a raised core temperature has a considerable impact on how comfortable subjects are with the thermal environment. Face cooling significantly improved thermal comfort compared with CON and, since ambient and rectal temperatures were similar in both trials, it appears that skin temperature, or more appropriately temperature of the face, is important in determining thermal comfort. In a carefully controlled study by Cotter & Taylor (2005), an assessment of intersite cutaneous thermosensitivities was achieved by using whole-body thermal clamping and applying both cooling and warming stimuli to 10 skin sites in the physiological range of 30–40°C. The authors found that cooling the face resulted in a two- to fivefold more powerful suppression of sweating and thermal discomfort when compared with cooling any of the other skin sites (Cotter & Taylor, 2005). Further support is derived from observations that cooling the head improves thermal comfort and cognitive performance under hyperthermic conditions (Nunneley et al. 1971, 1982). Moreover, a recent study by Arens et al. (2006) concluded that in a warm environment: (1) the head feels warmer than the rest of the body; and (2) overall sensation and comfort follow the warmest local (head) sensation, which may explain why this part of the body is most sensitive/responsive to cooling.
Nevertheless, despite finding a significant effect of face cooling on RPE in the present study, the difference between FC and CON was not so marked as that found by Pitsiladis & Maughan (1999) in RPE between exercise in cool and hyperthermic conditions when rectal temperatures were the same (
40 min of exercise). In that study, there was a significant difference in RPE early in exercise, whereas with face cooling, significant differences were seen only towards the end of exercise. The difference probably reflects the quantity of cool skin in the two situations and suggests that RPE is sensitive to skin temperature all over the body, although the relatively small area of the face does indicate that the face must be a relatively important and sensitive area.
In the present study, maintaining forehead temperature below 28°C was chosen because previous observations made during exercise under relatively cool ambient conditions (10–11°C) have shown skin temperature to be maintained at approximately this temperature, with perceived exertion and prolactin responses being attenuated when compared with the same exercise in the heat (Galloway & Maughan, 1997; Pitsiladis & Maughan, 1999; Pitsiladis et al. 2002). However, to our knowledge, no published study has investigated the possibility of a ‘dose–response’ effect of FC during exercise in the heat, whereby the magnitude of effect is proportional to the temperature by which the face is cooled. Further, although there have been reports of FC during exercise in the heat improving time to exhaustion by 51 ± 12% (Marvin et al. 1999), additional investigation needs to be conducted to determine whether these effects would improve exercise performance. Nonetheless, this and previous studies (Nunneley et al. 1982; Armada-da-Silva et al. 2004; Cotter & Taylor, 2005; Arens et al. 2006) have demonstrated the large effect that cooling the face can have on perceptual/behavioural responses during heat stress, which may better maintain motivation during exercise in a hot environment.
Our original hypothesis was that: (1) face cooling would attenuate the prolactin release associated with hyperthermic exercise; and (2) overall ratings of perceived exertion would be reduced in a similar manner, suggesting a common mechanism linking the perceived exertion and prolactin responses. Our results demonstrate that: (1) face cooling during 40 min of cycling in the heat inhibited the prolactin release associated with an increase in core temperature, whilst the attenuation of perceived exertion was not as complete; and (2) the attenuation of perceived exertion with face cooling may be mediated in part by an effect on the perception of breathing and subjective thermal comfort. These results suggest that the temperature of only a small part of the skin (10%) can have a disproportionately large effect on the hormonal and perceptual responses to exercise-induced hyperthermia, and confirm the importance of the face/head in determining subjective thermal comfort.
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) 
Denotes significant difference between trials; *denotes significant difference from 5 min value within a trial.
