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ISSN : 1598-7248 (Print)
ISSN : 2234-6473 (Online)
Industrial Engineering & Management Systems Vol.18 No.4 pp.600-608
DOI : https://doi.org/10.7232/iems.2019.18.4.600

Comparison of the Effect of Cold Fluid and Crushed Ice Ingestion on Thermoregulatory Responses during Physical Activity in a Simulated Hot-Humid Environment

Titis Wijayanto*, Valentina Kunti Bratadewi, Ghani Furqon Abdul Rahman, Moony Sinawang, Harendra Surya Prakasa
Laboratory of Ergonomics, Department Mechanical and Industrial Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia
Corresponding Author, E-mail: twijaya@ugm.ac.id
May 7, 2019 September 15, 2019 October 18, 2019

ABSTRACT


It has been reported that internal precooling via fluid ingestion is useful for hyperthermia prevention during exercise; however, its application in the occupational setting is unclear. This study aims to compare the effect of pre-cooling using cold fluid and crushed ice ingestion on thermoregulatory responses during simulated occupational activity in a hot and humid environment. Nine healthy males completed treadmill exercise for 30-min at an intensity of 65% of predetermined HRmax in a chamber set at 36.6±0.8°C of air temperature and 72±6% of relative humidity. Before the exercise, they preceded ingested 7.5 ml kg-1 body mass of cold drink (4°C), crushed ice (-2°C), or mineral water (28°C) for pre-cooling. Ingesting crushed ice before the activity significantly lowered the pre-activity tympanic temperature than the cold beverage. Also, crushed ice ingestion showed more significant effects in lowering heart rate and improving heat sink at the post-activity phase compared to cold beverage ingestion. These findings suggest that crushed ice ingestion may be more practical to minimize physiological and subjective thermal strain during occupational activity than ingesting cold fluid.



초록


    1. INTRODUCTION

    Physical works, such as firefighting, industrial activities, military drills and operations, and sports, causes body temperature to rise in proportion to metabolic heat production. When the core temperature increases rapidly and exceeds the permissible limit for work in a hot environment, the human body will suffer heat stress resulting in reduced working performance and performance impairment. An excessive increase in core temperature places the workers at risk for the development of heat-related illnesses that threaten workers' safety.

    Several studies have proposed strategies in preventing hyperthermia in a tropical environment using body cooling techniques (Hayashi and Tokura, 1996;Walker et al., 2014;Shirish et al., 2016). These techniques include precooling (Yeo et al., 2012;Tokizawa et al., 2014;Takeshima et al., 2017) and post-activity cooling (Barwood et al., 2009). One popular strategy for preventing hyperthermia during physical activity in a hot environment is precooling. The idea of this technique is that reducing the pre-activity core temperature may increase heat storage capacity. It then delays hyperthermia-induced fatigue and prevents the critically high core temperature, thus attenuating the physiological strain during heat stress (Yeo et al., 2012;Ross et al., 2013).

    There are several precooling methods, and they are divided into two methods, external body cooling, and internal body cooling methods. Ross et al. (2013) defined an external pre-cooling method as the application of cold medium (e.g., cold water, ice vest) to the skin before performing physical activity. This method includes exposing whole body cold water immersion (González-Alonso et al., 1999;Siegel and Laursen, 2012), partial cold water immersion (Goosey-Tolfrey et al., 2008, Racinais et al., 2009;DeGroot et al., 2013), and wearing cooling garment (Mokhtari Yazdi and Sheikhzadeh, 2014;Shirish et al., 2016). According to several works of literature, these cooling strategies are the most commonly used external cooling. The mechanism of this cooling method is that cooled body surface may afterward cool the cutaneous circulating blood and lessen the increase of core temperature during physical activity (Kay et al., 1999;Price et al., 2009). Furthermore, external cooling, for example, using a fan and water sprayed over the entire body, has been proved to reduce thermal strain in the heat (DeGroot et al., 2013;Tokizawa et al., 2014). However, external body cooling techniques such as cold-water immersion and wearing cooling garments have some logistical issues (Siegel and Laursen, 2012) and limited cooling power (Ross et al., 2013).

    The second method that can be used to cool the body before physical activity is an internal cooling method. Internal precooling is defined as taking a cold medium with cooling capability into the body through ingesting (e.g., cold beverage or ice slushy) or inhaling (e.g., cold air) to decrease core body temperature. The ingested cold medium (e.g., cold beverage) readily absorbs heat from the body to equilibrate with the surrounding tissues (Ross et al., 2013). This cooling method appears as a more suitable technique in reducing core temperature for occupational settings and gains considerable attention as a method to cool body temperature and improve performance in heat due to its practical logistic simplicity (Siegel and Laursen, 2012;Siegel et al., 2012;Ross et al., 2013).

    Many studies have reported that internal cooling via cold fluid (Kay and Marino, 2000;Burdon et al., 2010) or crushed ice (Ihsan et al., 2010;Yeo et al., 2012) is effective for preventing hyperthermia and improving exercise performance. Some of those studies examined the effect of beverage temperature ingested during physical activity on core temperature and exercise performance (Lee and Shirreffs, 2007;Lee et al., 2008). A study by Lee et al. (2008) reported that cold water (4ºC) ingestion before exercise reduced the rectal temperature by 0.5ºC and improved time to exhaustion during exercise. However, a more aggressive internal body cooling process was observed when ingesting ice slurry mixture (Siegel et al., 2012). Moreover, crushed ice or ice slurry ingestion appears to be a practical and effective method for lowering core temperature (Ihsan et al., 2010;Yeo et al., 2012;Onitsuka et al., 2015). Siegel et al. (2010) also reported that ice slurry ingestion (-1ºC) before exercise in a warm environment significantly reduced core temperature as compared to ingesting cold water (4ºC). Furthermore, it has been shown that thermal strain reduction was achieved with ice slurry ingestion before physical work in heat (Pryor et al., 2015).

    Most of the studies cited above focus on exercise or physical activity in a warm, humid environment (Riera et al., 2014;Takeshima et al., 2017) or in a hot, dry environment (Morris et al., 2016;Gerrett et al., 2017;Naito and Ogaki, 2017). There are relatively few studies focus on ice slurry or cold fluid ingestion for the occupational setting in a hot and humid environment. Based on this consideration, the purpose of the present study was to investigate the effects of precooling using crushed ice or cold fluid ingestion during simulated occupational activity in a hot and humid environment on physiological responses as compared with cold fluid ingestion. We posit that crushed ice ingestion would significantly reduce and lower core temperature. Lowering the core temperature would be beneficial in improving body heat storage capacity during occupational activity in the hot and humid environment compared to the cold fluid ingestion.

    2. METHODS

    2.1 Participants

    Participants of the present study were nine healthy males with a mean ± SD age of 22.1 years (±0.9), the body mass of 70.8 ± 4.2 kg, a height of 1.69 ± 0.35 m, body surface area of 1.8 ± 0.1 m2, and maximal heart rate (HRmax) of 188 ±2 bpm. We recruited active male students who exercised regularly and were accustomed to performing an exercise in the heat. The aforementioned is essential to make sure that they did not have any potential injuries or body cardiac disease. None of them were professional athletes, and they were not examined for their heat acclimation status.

    The experimental protocol was designed to meet the Helsinki Declaration and was approved for the ethical clearance by the local committee of ethics. Participants have been explained the detail of the experiment protocol and the potential risks before participation. Written informed consent was then obtained after explanation. Dur ing the entire period of the experiment, participants were instructed to maintain their habitual diet to diminish the effect of food intake variation on the variable measured. They were asked to abstain from eating, drinking alcoholic and caffeinated beverages, smoking, and exercise for at least two hours before the experiment.

    2.2 Experiment Procedures

    2.2.1 Preliminary Measurement

    On the first day of the experiment period, participants performed a submaximal exercise test before the main test session to estimate VO2peak. In this session, they performed a graded exercise test on a treadmill in a chamber set at air temperature (Ta) of 28 ± 0.5°C and relative humidity (RH) of 50 ± 3% r.h., according to the Bruce treadmill protocol (Bruce, 1974). VO2peak protocol started with a 3-min walking at 2.74 km h-1 on a 10% slope. The speed and slopes of the treadmill were then gradually increased every 3 min until the participants could not continue the exercise due to volitional exhaustion. Heart rate (HR) was recorded every second throughout the session using an HR monitor (RS800, Polar Electro, Finland). Participants reported Rated Perceived Exertion (RPE) during the last 15 s of each stage. Criteria for the achievement of VO2peak were HR ≥ 95% of agepredicted HRmax (220 - age in years), and rating of perceived exertion (RPE) was higher than 18; all the participants met these two criteria. VO2peak was not directly measured but estimated from the ASCM formula for Bruce protocol. The results of this preliminary measurement were used to estimate the treadmill speed, and slope equaled to 65% predetermined HRmax for the main experiment sessions.

    2.2.2 Main Experiment Sessions

    On arrival at the laboratory, participants had their semi-nude body mass weighed. They were asked to clean their ear canal to make sure there was no visible hair and cerumen before a tympanic temperature probe (CE Thermo, Nipro Corporation, JAPAN) was inserted. The tympanic temperature probe was then gently inserted into the ear canal and positioned towards the tympanic membrane after it was cleaned. The ear was then tightly sealed and covered using surgical tapes. Then, skin thermistors (LT- 8A; Gram Corporation, Saitama, Japan) were attached to eight body regions (forehead, chest, back, lower arm, hand, thigh, calf, instep). An HR monitor (RS800, Polar Electro, Finland) were also attached to each participant’s chest. The participants then wore a cotton t-shirt and NOMEX IIIA coverall before starting the experiment.

    After the instrumentation, the participants entered a chamber controlled at 28.7 ± 0.4°C and 50.8 ± 3.5% r.h. After 10 min of resting in the chamber, they ingested cold fluid (CF), crushed ice (CI), and water (CO) 30-min before performing treadmill activity. The ingested beverages were 7.5 mL.kg-1 body mass of isotonic beverage (Pocari sweat, Otsuka, Japan) either at a temperature of 4ºC for CF or -2ºC for CI. The cold fluid was prepared by refrigerating the beverage to bring the temperature to 4ºC. The crushed ice was made by grinding the frozen isotonic beverage using a commercially available food blender. It was then stored in an insulated cup to maintain the temperature stable at -2ºC. For the control condition (CON), the participant ingested the same amount of water (28ºC) before the main experiment. The three experiment sessions were conducted at the same time in the afternoon to control the circadian variation of core temperature. The two experiments were separated by 2 – 4 days.

    5 min before the commencement of the physical activity, participants then moved to the preconditioned chamber set at 36.6 ± 0.8ºC and 72 ± 6% r.h. to perform a 30-min treadmill exercise and a 30-min recovery period (a total of 60-min trial). The treadmill speed was started at 4 km.hr-1 and gradually increased up to the predetermined speed (equals to 65% predetermined HRmax) within 2 min and was maintained until the end of the exercise. The test was terminated if their tympanic temperature exceeded 38.5ºC, their heart rate reached more than 90% of their age-predicted HRmax, or if any volunteer felt unable to continue the exercise.

    2.3 Measurement and Calculation

    Physiological responses measured in this study included tympanic temperature (Tty), skin temperatures and heart rate (HR). Tty was monitored every second using an infrared tympanic temperature sensor (CE Thermo, Nipro Corporation, JAPAN; resolution of 0.01°C, the accuracy of ± 0.1°C). Skin temperatures (Tsk) on eight body regions (forehead, chest, back, lower arm, hand, thigh, calf, instep) were recorded every 2-s using a thermorecorder (LT-8A; Gram Corporation, Saitama, Japan).

    Mean skin temperature ( T ¯ s k ) was estimated from a modified Hardy and DuBois’ as shown in Equation (1)

    T ¯ s k = 0.07 ( T f o r e h e a d ) + 0.35 ( T c h e s t + T b a c k 2 ) + 0.14 ( T l o w e r a r m ) + 0.05 ( T h a n d ) + 0.19 ( T t h i g h ) + 0.13 ( T c a l f ) + 0.7 ( T i n s t e p )
    (1)

    Mean body temperature ( T ¯ b ) was then calculated from Tty and T ¯ s k as follow

    T ¯ b = 0.8 ( T t y ) + 0.3 ( T ¯ s k )
    (2)

    Body heat storage (ΔS) was estimated every 5-min as the product of body mass (bm) per body surface area (AD), human body specific heat (cbody=0.965 W/kg/ºC), and the increase in mean body temperature ( Δ T ¯ b ) from the baseline or before the cold fluid or crushed ice ingestion.

    Δ S = b m c b o d y Δ T ¯ b A D 1
    (3)

    HR was recorded every second throughout the experiment using an HR monitor (RS800, Polar Electro, Finland). Subjective responses included thermal sensation (TS; 9-point thermal sensation scale ranging from – 4 = “very cold” to +4 = “very hot”), thermal comfort (TC; 7- point thermal comfort scale ranging from -3 = “very uncomfortable” to +3 = “very comfortable), and rating of perceived exertion (RPE; 15-point Borg’s RPE scale ranging from 6 = “no exertion” to 20 = “maximal exertion/ exhaustion”) were measured at 10-min interval throughout the experiment session.

    2.4 Data Presentation and Statistical Analysis

    Time-course measured data were calculated 5 min average for statistical data analysis. Data were tested to check its normality assumption, and all data met the normality assumption before the main statistical test. The data were then evaluated using a two-way (time x treatment) within-subject ANOVA. In the case there was no significant interaction between time and treatment in twoway ANOVA, the paired t-test with Bonferroni correction at the same point of time was used to determine the significant differences among the three conditions. Statistical analysis was performed using JASP Version 0.8.5.1 for Mac OS (JASP-Team, 2018). Statistical significances were established at P<0.05. Data are presented in the mean ± standard error of the mean (SE).

    3. RESULTS

    3.1 Body Temperature

    Figure 1 displays Tty during 10 min fluid or ice ingestion, 20 min baseline, 30 min physical activity, and 30 min recovery period. The ingestion of CI caused the Tty to fall significantly from 10 min after the ingestion to 10 min before starting the physical activity. Meanwhile, CF ingestion slightly decreased Tty from 15 min after the ingestion to 10 min before starting the activity. Significant differences in Tty between CON and CI were observed 15 min after ingestion until 5-min before starting the physical activity (p < 0.05). No significant difference was observed in Tty between CON and CF. The comparison between CI and CF showed significantly lower Tty in CI from 15 min after the ingestion to 5 min before starting the physical activity (p < 0.05).

    The Tty increased progressively for all the three conditions during the physical activity. Statistical analysis revealed significantly higher Tty during physical activity in the CON condition started from 15 min of physical activity until the end of the recovery period in comparison with CI (p < 0.05). No significant differences in Tty during physical activity and during recovery were found in the comparison between CF and CI trials.

    T ¯ s k at the end of physical activity was higher in the CON condition (36.3 ± 0.1ºC) compared to CI condition (36.1 ± 0.2ºC) or CF condition (36.2 ± 0.1ºC). However, statistical analysis revealed no significant differences in T ¯ s k observed among the three experimental conditions at any of the sampled time (p > 0.05).

    3.2 Heart Rate

    Figure 2 shows HR during 10 min ingestion, 20 min baseline, 30 min physical activity, and 30 min recovery period. The ingestion of the CI caused the HR to fall significantly compared to CF ingestion. Statistical analysis showed that the HR was lower after CI ingestion from 15 min after the ingestion to 5 min before the physical activity as compared to the CON condition (p < 0.05). In comparison with the CF condition, HR started to fall significantly from 20 min after the ingestion to 5 min before starting the physical activity (p < 0.05). HR then gradually increased throughout the physical activity period for all the three trials.

    There were no significant differences in HR during physical activity between CI and CF trials. However, HR in the CON trial was significantly higher than the CI trial from 10 min to 25 min during the physical activity compared to the CI trial (p < 0.05). After the cessation of physical activity, the HR decreased progressively for all three trials but remained lower in the CI trial from the first 15 to 20 min of recovery period compared to the CON trial (p < 0.05).

    3.3 Body Heat Storage

    As shown in Figure 3, the amount of heat stored in the body (ΔS) was decreasing after CI or CF ingestion during the pre-activity phase. The differences between these two trials in the pre-activity period were not significantly different (p < 0.05). However, ΔS after CI and CF ingestion were lower from 15 min after the ingestion to 5 min before starting the physical activity in comparison with the CON trial (p < 0.05). The ΔS then started to increase as the commencement of the physical activity for the three trials. There were no significant differences in heat storage among the three trials until 25 min of physical activity. However, ΔS was significantly higher from 25 min of physical activity until the end of recovery in the CI trials than those observed in the CF and CON trials (p < 0.05).

    3.4 Subjective Responses

    Participants reported lower thermal sensation (TS) after CI ingestion before physical activity compared to the CON trial (p < 0.05) and CF trial (p < 0.05). TS was significantly increased as the commencement of the physical activity, but CI and CF trials remained lower until 10 min of recovery period than the CON trial (p < 0.05). As displayed in Figure 4, the participant reported the lowest TS in CI trial at the end of the recovery period (p < 0.05). There were no significant differences in TC among the three trials during the pre-activity period. Thermal comfort started to decrease during physical activity. It was significantly lower in the CON trial throughout the physical activity and recovery period as compared with CF and CI trials (p < 0.05).

    The rating of perceived exertion (RPE) for all three trials was depicted in Figure 5. There were no significant differences in RPE throughout the experimental protocol for all three trials. RPE in the CON trial was higher during physical activity than the other trials, although the differences were not statistically significant. During the recovery period, RPE decreased significantly. However, the participants still reported higher RPE during the first 10 to 20 min of the recovery period in the CON trial compared to CI and CF trials (p < 0.05).

    4. DISCUSSION

    This study investigated the compared effects of crushed ice ingestion with cold fluid ingestion before physical activity in a hot and humid environment on physiological and subjective responses. The main findings of the present study are as follow: (1) crushed ice ingestion before physical activity significantly reduced pre-activity tympanic temperature and heart rate as compared with cold fluid ingestion, (2) crushed ice ingestion also considerably increased body heat storage capacity during physical activity until the end of the recovery period compared to cold fluid ingestion.

    In the present study, we found that ingesting crushed ice before physical activity in hot and humid environments reduced the pre-activity tympanic temperature by 0.4ºC compared to cold fluid ingestion. The tympanic temperature also remained lower throughout the experimental protocol. The effect of crushed ice ingestion before physical activity in lowering pre-activity tympanic temperature mimics the findings of the earlier studies reporting that ice ingestion provided effective internal pre-cooling in lowering lower pre-exercise core temperature, rectal temperature (Siegel et al., 2010) or gastrointernal temperature (Ihsan et al., 2010) compared with cold water ingestion. A study by Siegel et al. (2010) found that ingesting ice slurry (-1ºC) before starting the exercise in the heat lowered the pre-exercise rectal temperature by 0.32ºC as compared ingesting cold water.

    This study did not find any significant differences in mean skin temperature among the three trials, suggesting that it may be difficult to explain whether the lower tympanic temperature in this study was due to heat loss through the skin. Hence, it is possible to explain that lowering core temperature via crushed ice ingestion before physical activity in a hot environment was likely caused by a more massive heat sink, which increased body heat storage capacity. This explanation was based on the thermodynamic characteristic of water/ice and the changing of its physical state from solid (ice) to liquid. Siegel et al. (2010) suggested that a large heat sink caused by the ingestion of crushed ice might be due to the unique characteristics of crushed when changing from solid phase to liquid, which requires more considerable thermal energy. Crushed ice absorbs a more significant amount of energy during its phase change from a solid to a liquid. Therefore, crushed ice ingestion can cool the internal body more effectively than cold fluid alone. These explain the higher increase in body heat storage capacity during physical activity after the ingestion of crushed ice as compared with cold fluid ingestion, which is in agreement with the previous study (Siegel et al., 2010).

    Furthermore, a higher capacity for heat storage after ingesting crushed ice as precooling would provide a more significant margin for metabolic heat production in lessening thermoregulatory strain during physical activity in the hot and humid environment compared with the control condition (Booth et al., 1997). In the present study, we observed a lower heart rate when crushed ice was ingested than the control condition. Earlier findings reported that ingesting low temperature of beverage attenuated the increase of heart rate compared with ingesting the same amount of warm or hot water (Lee and Shirreffs, 2007;Lee et al., 2008, 2013). Tan and Lee (2015) suggest that the lower heart rate after ingesting cold beverages in the form of crushed ice or cold fluid showed the occurrence of peripheral vasoconstriction without an increase in metabolic heat production. This would be beneficial in reducing physiological thermal strain during physical activity in a hot and humid environment.

    However, except for the difference in pre-activity heart rate, no significant difference in heart rate was found between crushed ice ingestion and cold fluid ingestion during physical activity. This finding mimics the results in several studies reporting no significant difference in heart rate between cold fluid and crushed ice (Naito and Ogaki, 2017) or ice slush beverage (Stanley et al., 2010) ingestion during exercise in the heat. The temperature difference between cold fluid and crushed ice in this study might not be enough to significantly influence the attenuation of heart rate during physical activity in a hot and humid environment.

    In this study, the thermal sensation was the only subjective responses showing a significant difference between crushed ice and cold fluid ingestion. Participants reported lower pre-activity thermal sensation after crushed ice ingestion as compared with cold fluid ingestion. Reduction thermal sensation during precooling with crushed ice ingestion is commonly reported in the previous studies reporting lower pre-exercise thermal sensation after ingesting ice than after ingesting cold fluid (Ihsan et al., 2010;Siegel et al., 2010;Onitsuka et al., 2015). It is suggested that lower thermal sensation after ingesting ice might be due to the reduction of core temperature and facial temperature after ingesting crushed ice (Onitsuka et al., 2015). In addition to the reduction of core temperature, there has been a piece of strong evidence that perceived a cold sensation of cold in the mouth activated oral temperature sensitive regions of the brain as recorded by functional magnetic resonance imaging (Guest et al., 2007). It suggests a possible sensory effect of crushed ingestion before physical activity.

    Furthermore, Onitsuka et al. (2015) suggested that thermoreceptors in the mouth, esophagus, and the abdominal area might sense more cooling sensation when ingesting crushed ice, and thus reduced pre-exercise thermal sensation lower than when ingesting cold fluid. However, although thermal sensation was lower after ingesting crushed ice before exercise, thermal comfort between cold fluid ingestion and crushed ice ingestion before starting physical activity in the hot and humid environment was not significantly different. Participants reported that both ingesting crushed ice and cold fluid provided cooling sensations, and their ratings of thermal comfort were neutral for both conditions. After the commencement of the exercise, both thermal sensation and thermal comfort were not significantly different between the cold fluid and crushed ice.

    We did not find any significant differences in the perceived exertion between the two trials, except for the significant difference in comparison with the control trial. Similar to our findings, Naito and Ogaki (2017) also reported no significant difference in rating perceived exertion during cycling in an environment set at 35ºC and 30% r.h., between crushed ice and cold fluid ingestion. A similar response has been reported for perceived exertion in exercise performance (Siegel et al., 2010;Naito and Ogaki, 2017). Lee et al. (2008) suggested that beverage temperature influenced thermal sensation and perceived exertion during exercise; cold beverages lower perceived exertion than a warm beverage. Therefore, we speculated that cold fluid and crushed ice might already lower perceived exertion during exercise in a hot and humid environment when they were compared with ingesting warmer beverages, although the difference was only observed during the recovery period.

    It seems that ingesting either crushed ice or cold fluid as an internal precooling strategy would yield the same effects on thermal comfort and perceived exertion. Schlader et al. (2009) suggested that subjective responses and perceived exertion during exercise were more closely related to changes in skin temperature than core temperature. In this study, we did not find any significant difference in mean skin temperature between the three conditions this may explain why perceptual responses were similar between precooling using crushed ice and cold fluid. We considered that the temperature difference between crushed ice and cold fluid was not significantly large; therefore, their effects on perceived exertion and thermal comfort were not significantly different.

    5. CONCLUSION

    In conclusion, our present study has shown that crushed ice ingestion before performing physical activity in a hot and humid environment was able to reduce preactivity core temperature significantly. Ingesting crushed ice as precooling also results in higher heat storage capacity than ingesting cold fluid. From these results, it may be concluded that crushed ice ingestion before an occupational activity in a hot and humid environment may be a more preferred and practical approach to minimize physiological and subjective thermal strain as compared to ingesting cold fluid.

    ACKNOWLEDGMENT

    Part of this study is supported by the DTMI UGM research grant (No. 836/H1.17/TMI/LK/2018) from the Department of Mechanical and Industrial Engineering, UGM. Any opinions, findings, conclusions, and recommendations expressed in this paper are those of authors and do not necessarily reflect the views of the employers or granting organizations.

    Figure

    IEMS-18-4-600_F1.gif

    Tympanic temperature (mean ± SE) throughout the experimental protocol. a shows a significant difference between CON and CI trials at p<0.05; b shows a significant difference between CI and CF at p < 0.05.

    IEMS-18-4-600_F2.gif

    Heart rate (mean ± SE) throughout the experimental protocol. a shows significant a difference between CON and CI at p<0.05; b shows significant a difference between CF and CI at p<0.05.

    IEMS-18-4-600_F3.gif

    Heat body storage (mean ± SE) throughout the experimental protocol. a shows a significant difference between CON and CI trials at p<0.05. b shows a significant difference between CON and CF at p<0.05. c shows a significant difference between CI and CF at p < 0.05.

    IEMS-18-4-600_F4.gif

    Thermal sensation and thermal comfort (mean ± SE) throughout the experimental protocol. a shows a significant difference between CON and CI trials at p<0.05. b shows a significant difference between CON and CF at p<0.05. c shows a significant difference between CI and CF at p < 0.05.

    IEMS-18-4-600_F5.gif

    Thermal sensation, thermal comfort, and rating of perceived exertion (mean ± SE) throughout the experimental protocol. a shows a significant difference between CON and CI trials at p<0.05; b shows a significant difference between CON and CF at p<0.05.

    Table

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