Our conditions were sufficiently stressful to simulate typical American football preseason training in hot, humid conditions (Figure 2). Subjects were required to exercise intensely (heart rate [HR] of 85–95% of age-predicted maximum) and to reach a criterion minimum core temperature (38.0° C). Each data collection session consisted of a warm-up, followed by 60 minutes of intense exercise in the heat with 5-minute rest breaks every 20 minutes, followed by 30 minutes of recovery in the shade. During the 5-minute rest breaks and continuously during 30 minutes of recovery, participants wore the randomly assigned experimental superficial cooling garment or a T-shirt (see Instruments for more details). We collected data during 6 time-matched periods (Figure 1) corresponding to the beginning and end of each 5-minute break provided and every 5 minutes during 30 minutes of recovery. Meals, snacks, and hydration status were controlled for each trial by asking participants to maintain a consistent diet before each data collection session.
We purposefully sampled 16 male participants from a pool of college students meeting the inclusion criteria, and 10 were included in data analysis (age, 22.6 ± 1.6 years; height, 176.0 ± 6.9 cm; mass, 76.5 ± 7.8 kg; body fat, 15.6 ± 5.4%). Six participants were excluded from data analysis because either they did not put forth enough effort to reach the T gi >38.0° C threshold or the temperature capsule did not reach the duodenum in time for data collection. We recruited participants who were acclimatized to the environmental conditions by exercising outdoors in the tropical climate in which data collection occurred. Participants were screened by the investigators and included in the investigation if the following criteria were met (1): had no chronic health problems (2); exercised outdoors a minimum of 30 minutes per a day, 3 days a week (3); had no history of heat stroke in the past 3 years (4); had no history of cardiovascular, metabolic, or respiratory disease (5); were not febrile; and (6) were not currently taking any supplements or drugs that influence thermoregulation. All subjects read and signed the informed consent form before participation, and the University's The University of Hawaii at Manoa Institutional Review Board approved this investigation.
The cooling vest (Cool58 Phase Change Vest; Polar Products, Inc., Akron, OH, USA) is marketed by the manufacturer as an “evaporative, phase-change” cooling vest that stores water in special polymers built throughout the vest with a thin layer of fabric between the water storage compartments and the body (Figure 3). These vests are charged by soaking them in an ice water for at least 10 minutes before its use. Following manufacturer’s instructions, the vest was submerged and soaked in ice water maintained at 2.8 ± 1.0° C (measured using submerged thermometer), which was maintained by adding more ice as necessary. When removed the vest from the ice slush, it was allowed to drip for 1–2 minutes, then placed it directly on the skin. The vest was fitted to the participant and placed directly on the skin with self-adhesive straps to hold the material close to the skin (Figure 3).
Perceptual sensations were measured using 3 scales: thermal, thirst, and RPE, and an ESQ. The thermal sensations scale (33) was used to assess participants' perception of heat or cold and included a 9-point rating scale with 0 indicating unbearably cold and 8 indicating unbearably hot. The thirst sensations scale (16) was used to determine participants' level of thirst and consisted of a 9-point rating scale with 1 indicating not thirsty at all and 9 indicating very, very thirsty. The RPE scale (7) was used to assess participants' perceived level of physical effort or exercise intensity and consisted of a 15-point scale (6–20) with 7 indicating very, very light and 19 indicating very, very hard. Finally, the ESQ (31) was used to identify symptoms of heat illness. The ESQ consisted of 14 questions including, “I felt lightheaded, I had a headache, I felt dizzy,” etc.
Data Collection Procedures
Following screening and enrollment, participants were familiarized with the data collection procedures by reporting to the Human Performance Laboratory where investigators recorded participant physical characteristics: body mass, height, age, body mass index, body surface area, and body composition (skinfolds). Each participant’s baseline euhydrated condition was determined 3 days before data collection and verified before starting the experiment. Heart rate training range was predicted using the HR maximum score and multiplying it by the percent of HR maximum (85–95%). Before returning to the laboratory the following day for data collection, participants were provided with instructions for preparing for the experimental protocol (1): perform only activities of daily living during the 24 hours before each experiment (2); consume a consistent diet, including hydration behaviors, before each data collection session to reduce the effects of different nutrient intake on exercise performance; and (3) bring a dry change of clothes to each data collection session.
Main Outcome Measures
Our main outcome measures were quantified during rest breaks and period. We assessed thermoregulatory and cardiovascular responses: core body temperature (T gi), chest skin temperature (T chest), and HR as the main outcome measures. For 3 days before the study, we measured participants' hydration status to determine the baseline euhydrated condition. In addition, before and after each data collection session, we determined hydration status consisting of sweat rate, volume of fluid consumed (F vol), body mass loss (BML), urine volume (U vol), urine color (U col) (2), and urine specific gravity (U sg). We also assessed perceptual sensations consisting of the thermal sensations scale (33), thirst sensations scale (16), and the Borg rating of perceived exertion (RPE) scale (7). An environmental symptoms questionnaire (ESQ) (31) was used to identify symptoms of heat illness. Environmental conditions were recorded at 15-minute intervals throughout data collection with WBGTo in the direct sun and in the shade.
Participants arrived at the laboratory 2 hours before beginning the exercise to swallow the ingestible temperature capsule (VitalSense; Philips Respironics, Inc., Bend, OR, USA) and allow for it to pass into the duodenum. Participants also provided a urine sample at this time for hydration status testing using a digital clinical refractometer (Pen-Urine SG; Atago USA, Inc., Bellevue, WA, USA). After voiding, participants were weighed on a digital platform scale (model PS6600 ST; Befour, Inc., Saukville, WI, USA) wearing only shorts, then fitted with a HR monitor (model FT1; Polar-Electro, Inc., Lake Success, NY, USA). Because of the limitation of the expense of the dermal patches, chest skin temperature was selected as a representative sample of skin exposed to the cooling vest. The skin on the left chest at the midclavicular line and midway between the clavicle and the nipple was rubbed with an alcohol pad (shaved, if necessary) and the dermal patch (VitalSense; Philips Respironics, Inc.) was then attached to the chest. Participants wore their own T-shirt, shorts, socks, and running shoes and were asked to wear the same or similar clothes for each data collection session.
Warm-up and Anaerobic Exercise
Once prepared, participants then walked to the outdoor research area in the shade adjacent to the synthetic turf practice field. After resting seated for at least 5 minutes, investigators recorded baseline measures of T gi, T chest, HR, thirst sensations, thermal sensations, RPE, and ESQ. At 15-minute intervals throughout data collection, investigators monitored environmental conditions using the thermal environment monitor (model 36; QUESTemp, Inc., Oconomowoc, WI, USA) attached to a tripod set at 36 in above the synthetic turf and allowed to equilibrate for at least 5 minutes. After recording the data, the monitor was moved and equilibrated in the shade. In addition, turf surface temperature and humidity in the sun and ground temperature and humidity in the shade were measured by placing the wand of a temperature-humidity monitor (model 11-661-21; Thermo-Fisher Scientific, Inc., Pittsburgh, PA, USA) on the ground for at least 10 minutes.
Participants were lead through a standardized exercise protocol for 60 minutes, which is designed to achieve the following goals: (a) maintain an average HR of 85–95% of age-predicted maximum, using the equation (191.5−0.007 × age2) (10,19) and (b) reach a threshold T gi criteria of ≥38.0° C during the exercise session. Participants who reached or exceeded 40° C were allowed to continue if desired and monitored for signs or symptoms of EHI. One participant exceeded the 40° C safety limit and was required to rest until his T gi decreased to a safer level. No participant exhibited signs or symptoms of EHI during the experiment. The exercise protocol consisted of a supervised training program with a 12-minute warm-up of dynamic stretching exercises immediately followed by 60 minutes of a variety of standard football speed and agility drills (Table 1). The exercises were designed to mimic the first half of an National Football League-style football practice with noncontact training and conditioning activities. To simulate the intensity and duration of an actual football practice, exercise intensity was monitored by the participants and recorded by the investigators at the end of each 20-minute interval using the HR monitors. Participants were asked to put forth a maximum effort for the duration of each interval and to maintain an average of 85–95% of HR maximum during the drills.
Participants assigned to the control condition remained seated with their T-shirts on in the shade throughout the 5-minute rest breaks and recovery. Participants assigned to the cooling condition were asked to remove their T-shirt, put on the charged cooling vest (maintained at a temperature, 2.8 ± 1.0° C), and remain seated throughout the 5-minute rest break. To avoid the confounding effects of dehydration in both conditions, participants were permitted to drink cold water ad libitum from individual water bottles at the end of each rest break after data were collected (to avoid artificially affecting the ingested thermistor in the gastrointestinal tract). The amount of fluid consumed was recorded for data analysis (Table 2).
Upon completion of the exercise protocol, participants moved to the adjacent research area in the shade for 30 minutes of seated recovery. In the control condition, participants rested in a seated position wearing their T-shirt. In the cooling condition, participants rested wearing the charged cooling vest. The vest was not removed until T gi reached baseline levels. At 5-minute intervals during recovery, we assessed T gi, T chest, HR, and perceptual sensations. Following recovery, participants toweled off, changed into dry clothes, and were weighed. We asked participants to refrain from voiding until weighed, then we collected U vol and performed urinalysis consisting of U col and U sg (2). Participants were provided with snacks (pretzels, bananas) and were required to rehydrate with at least 500 ml of cold water before leaving the laboratory.
The treatment effects were evaluated using a randomized, crossover design. All data were statistically analyzed using separate 2-way repeated measures (condition × time) analysis of variance (ANOVA) for exercise and recovery and were reported as mean ± SD. The difference between pretest and posttest ([INCREMENT] = pre-post) were analyzed with a 2 × 8 ANOVA with repeated measures on the second factor. When statistical differences were found, the Tukey Least Significant Differences test for post hoc analysis was used, and post hoc t-tests with Bonferroni corrections were used to determine pairwise differences among periods. Estimates of effect size were reported as partial η2 value, and power was reported as 1 − β for each parameter. Pearson correlations were used to analyze the relationship between the thermoregulatory and cardiovascular responses and the perceptual responses for the end of exercise. Descriptive statistics were calculated for participant's physical characteristics, environmental conditions, and hydration status. Occasional missing data points were replaced with the series mean. For all analyses, the α level was set at 0.05 and statistical software (SPSS Statistics 20; IBM, Inc., Chicago, IL, USA) was used.
Thermoregulatory and Cardiovascular Responses
The T gi data from the intermittent cooling during exercise breaks resulted in a blunted or flattened thermoregulatory response in the cooling condition (Figure 4). For T gi during exercise, we found no significant differences between the conditions (F 1,18 = 0.183; p = 0.674; partial η2 = 0.010; 1 − β = 0.069); however, we did find a significant interaction effect (F 5,90 = 2.77; p = 0.022) with post hoc testing revealing a significant (p = 0.010; 95% confidence interval, 0.057–0.595) decrease in T gi during break 3 (pretest = 38.50 ± 0.13° C vs. posttest = 38.18 ± 0.13° C) in both the conditions. For T gi during recovery, we found no significant differences between the conditions (F 1,18 = 0.590; p = 0.452; partial η 2 = 0.032; 1 − β = 0.113), but we did find a significant main effect for test (F 5,90 = 6.681; p < 0.001) with T gi progressively decreasing during recovery in both the conditions. For [INCREMENT]T gi, we found no significant interaction or main effect in both the condition (F 1,18 = 2.243; p = 0.152; partial η2 = 0.111; 1 − β = 0.294). Cooling rate revealed no significant interaction (F 8,144 = 1.760; p = 0.090; partial η2 = 0.089; 1 − β = 0.741) or main effects.
The T chest data from the intermittent cooling during exercise resulted in dramatic decreases in the cooling condition (Figure 5). For T chest during rest breaks, we found a significant interaction effect (F 5,90 = 25.853; p < 0.001) with significantly (p < 0.001) decreased skin temperature in the cooling condition (T chest = 31.85 ± 0.43° C) compared with the control condition (T chest = 34.38 ± 0.43° C). For T chest during recovery, we found a significant interaction effect (F 5,90 = 12.671; p < 0.001) with significantly (p < 0.001) lower skin temperature in the cooling condition (T chest = 31.24 ± 0.47° C) compared with the control condition (T chest = 33.48 ± 0.47° C). For the difference in [INCREMENT]T chest, we found a significant interaction effect (F 5,90 = 17.189; p < 0.001) with [INCREMENT]T chest significantly (F 1,18 = 43.038; p < 0.001) larger in the cooling condition ([INCREMENT]T chest = −1.465 ± 0.14° C) than in the control condition ([INCREMENT]T chest = −0.21 ± 0.14° C).
The HR data from intermittent cooling during exercise resulted in almost identical responses for both conditions (Figure 6). For HR during rest breaks, we found no significant difference (F 1,18 = 0.308; p = 0.586; partial η2 =0.017; 1 − β = 0.082) between the cooling condition (HR = 147.6 ± 2.6 b·min−1) and the control condition (HR = 150.1 ± 3.5 b·min−1); however, we did find significant differences between pre test and post test (F 5,90 = 429.796; p < 0.001). For HR during recovery, we found no significant (F 1,18 = 1.552; p = 0.229; partial η2 = 0.079; 1 − β = 0.219) differences between the conditions; however, we did find significant differences between pre test and post test (F 5,90 = 21.919; p < 0.001).
The thermal responses (Figure 7) were significantly (F 1,18 = 43.038; p = 0.026) lower in the cooling condition (4.4 ± 0.2 points) compared with the control condition (5.0 ± 0.2 points). The thirst responses (Figure 8) approached significance (F 1,18 = 5.038; p = 0.051; partial η2 = 0.359; 1 − β = 0.517) with thirst response in the cooling condition being lower (4.5 ± 0.3 points) than in the control condition (5.3 ± 0.4 points). The RPE responses (Figure 9) were not significantly (F 1,18 = 2.291; p = 0.164; partial η2 = 0.203; 1 − β = 0.273) different between the conditions. No other significant differences were found. No symptoms of EHI using the ESQ were found for either group. No significant correlations (Table 3) were found between thermoregulatory, cardiovascular, and perceptual responses at the end of exercise in the control condition or the cooling condition.
Hydration status (Table 4) was not significantly (F 1,9 = 0.008; p = 0.993; partial η2 = 0.001; 1 − β = 0.051) different between the conditions. However, both conditions became significantly hypohydrated during the exercise session with a mean % BML of −2.2 ± 0.8% (F 1,9 = 182.637; p < 0.001). The urinalysis data revealed no significant differences between the conditions (U col F 1,9 = 2.548; p = 0.145; partial η2 = 0.221; 1 − β = 0.298; U sg F 1,9 = 1.514; p = 0.250; partial η 2 = 0.144; 1 − β = 0.197); however, both U col and U sg indicated significant increase after exercise. In both the conditions, U col was significantly (F 1,9 = 56.077; p < 0.001) increased by 33.3% after exercise (6.8 ± 0.2) compared with preexercise (4.5 ± 0.4). In both the groups, U sg significantly (F 1,9 = 17.612; p = 0.002) increased by 30% after exercise (1.022 ± 0.002) compared with preexercise (1.019 ± 0.002). No significant differences between the conditions were found for F vol (p = 0.250) or sweat rate (p = 0.961).
Environmental conditions measured in the sun and in the shade are demonstrated in Figure 2. Outdoor environmental conditions were significantly higher (t 20 = 4.99; p < 0.001) in the sun (WBGTo = 27.1 ± 0.8° C) than in the shade (WBGTo = 25.4 ± 0.8° C).
The most important findings of this investigation were that although we found significantly decreased thermoregulatory response in the superficial cooling condition (T gi = 38.2 ± 0.2° C vs. T gi = 38.5 ± 0.1° C for cooling and control conditions, respectively), these decreases were small and not clinically important. Furthermore, overall cooling rates were not significantly different between the cooling and control conditions, indicating that the superficial cooling vest was not effective in reducing the core body temperature increase during intense exercise in the heat. Finally, although we elected to evaluate intermittent cooling followed by continuous cooling during recovery, most applications of this type of “phase change” cooling vest would most likely be used during recovery from exercise only. We found no significant differences between cooling and control conditions during 30 minutes of recovery from intense exercise in the heat. This finding implies that recovery by sitting in the shade and rehydrating vs. using a superficial cooling vest are equally effective in reducing core body temperature after intense exercise in the heat.
We found that the core body temperature of individuals exercising in the heat remained elevated, despite efforts of intermittent, superficial cooling (Figure 4). In agreement with previous investigations (13,25,33), superficial cooling vests were not effective in lowering T gi; however, the use of the cooling vest did attenuate the increase of T gi during intense exercise in the heat. Furthermore, the cooling rate of the current investigation was similar to that of the previous investigation of the current researchers (vest group = 0.03 ± 0.01° C·min−1, no-vest group = 0.03 ± 0.01° C·min−1) with a similar 30-minute recovery period (25). These cooling rates are far inferior to the plethora of evidence relating to cooling rates of hyperthermic individuals during cold water immersion, which are consistently reported to be 0.16–0.2° C·min−1 (0.29–0.36° F·min−1) or upward of 0.35° C·min−1 (0.63° F·min−1) when multiple elements that influence cooling rates are in place (9).
Similar to our findings, other studies found significant decreases in the rate of increase of T gi with intermittent cooling during exercise in the heat (5,28). Price et al. (28) evaluated the physiological effects of intermittent, superficial cooling on exercise in the heat during a simulated soccer match. A significant decrease in T gi was found between the first and second bouts of exercise in the pre plus midcooling condition. Similarly, Barr et al. (5) found that in firefighters, T gi and thermal sensations were significantly lower during the second bout of exercise when wearing a cooling vest in between 2 exercise bouts. Both studies (5,28) provided participants a 15-minute rest break between exercise bouts. The differences in time between the 15-minute rest breaks compared with the 5-minute rest breaks in the current investigation likely explains why T gi decreased significantly compared with the control condition. In addition, in the present study, during recovery, the overall decrease in T gi was larger in the control condition because the final temperatures were higher in the control condition, providing a larger temperature gradient for cooling. Although our experimental protocol was based on the National Athletic Trainer’s Association recommendations of 5-minute rest breaks every 20 minutes of exercise in Red Flag conditions (6), longer rest breaks may be required to achieve sufficient cooling to reduce elevated core body temperature and prevent dangerous hyperthermia from developing.
Superficial cooling applications vary considerably in concept, design, and application. Cadarette et al. (8) studied the effects of constant cooling vs. intermittent cooling vs. no cooling in military personnel wearing chemical protective clothing during continuous treadmill walking for 80 minutes. The protective uniform consisted of a liquid cooling garment with cold water circulating continuously, intermittently (2 minutes on and 2 minutes off), or not at all. These researchers found that continuous and intermittent cooling during exercise was effective in lowering T gi (8). These applications of superficial cooling may be effective in soccer games, firefighter training, and military convoys, where longer breaks or seated activities allow continuous cooling. However, when applying these findings to generalized athletic settings such as football practices and games, the rest periods are much shorter and the equipment required continuous cooling is impossible in these settings.
Heart rate responses (Figure 6) were not affected by the superficial cooling, indicating that the cardiovascular strain experienced by the participants was not attenuated with the cooling garment. Rest in the shade and rehydration was equally effective in reducing the cardiovascular strain. Armstrong et al. (4) reported that in football athletes, cardiovascular strain was increased in individuals wearing a full football uniform compared with control subjects. However, cessation of exercise reduced HR regardless of the equipment configuration. Clearly, during exercise in hot, humid environments, removing equipment, resting in the shade or air-conditioned environment, and rehydration are effective strategies in reducing thermoregulatory and cardiovascular strain. The use of superficial cooling garments may attenuate the increase in core body temperature during exercise in the heat, but these garments are not particularly useful in accelerating core body cooling or reducing cardiovascular strain during exercise in the heat. Proper precautions must be followed when athletes are intensely exercising in the heat. Using superficial cooling garments intermittently during exercise may provide a small measure of protection but seated rest and rehydration in the shade at 15- to 20-minute intervals is essential to avoid dehydration, hyperthermia, and EHI.
Hyperthermia is common among athletes and occurs in a variety of environments and athletic settings and when combined with dehydration and heat stress can lead to EHI. Using effective cooling modalities combined with rest and rehydration may be useful in preventing dangerous hyperthermia from developing. The effectiveness of a variety of cooling methods have been evaluated in individuals after exercise, including cold water immersion, shade, cold fans, vests, hoods, ice buckets, and ice towels (13). In athletes experiencing mild hyperthermia, cold water immersion and ice buckets have consistently resulted in a significantly greater decrease in core body temperature. Although these cooling strategies are recommended for the treatment of EHI (13), these cooling strategies are impractical for use in decreasing core body temperature during brief recovery periods between exercise bouts. Although this investigation was not about exercise performance, we found an extensive systematic literature review (29) reporting that aerobic exercise performance can be enhanced with the use of cooling modalities. In this review, cold water immersion, cooling vests, cooling collars, and hand cooling devices all provided subjects with some exercise benefits. Many of these modalities could be used during athletic events such as between tennis matches or during halftime of other outdoor events such as American football or soccer (29). Some specific cooling modalities may provide a benefit to performance when used before and in between exercise bouts; however, in cases of dangerous hyperthermia, cold water immersion is the modality with the highest cooling rate.
The important findings of this investigation were that thermal sensation was significantly lower in the cooling condition, indicating that superficial cooling was effective in reducing participants' sensation of heat. This finding is largely explained in that the cooling vest was effective in significantly lowering T chest. Although T gi was significantly different between the conditions, we found that at the end of break 1 and 2, the cooling condition produced a higher T gi than the control condition, indicating that rest in the shade alone is just as effective in reducing thermal strain as wearing the cooling vest. These findings are similar to previous investigations (8,28). When superficial blood is cooled, it is returned to the core of the body to reduce the temperature of the core; however, the cooling vest was not effective in blunting the elevation of T gi, which continued to increase in both conditions throughout exercise as a result of the exercise intensity and heat stress.
Participants' thermal perceptions during the cooling condition were very intense coldness initially; however, after 5 minutes of cooling, the participants' reported that the vest felt “like a wet T-shirt” and that it had lost much of its cooling capacity. Skin sensations are known to adapt over time, so it is not clear if the vest material decreased temperature or if the participants' cognitive perception of coldness had accommodated. According to Herrera et al. (22), ice massage, ice pack, and cold water immersion reduced sensory nerve conduction velocity. Thermal sensation sensitivity may have decreased after 5 minutes of cold application, potentially explaining why subjects commented on the change in temperature of the cooling vest. Coaches and athletic trainers should exercise caution when athletes are using superficial cooling vests during intense exercise in the heat because the vest imparts a sensation of coolness on the skin while the core body temperature is not reduced. This finding is important as athletes using intermittent cooling may continue to exercise intensely without pacing themselves and potentially develop dangerous hyperthermia.
Although both conditions resulted in hypohydration (2.2%), thirst sensations were slightly lower in the cooling condition (p = 0.051) than in the control condition. Thirst sensations may have been less in the cooling condition perhaps because as exercise progressed, participants in the cooling condition were less able to perceive hypohydration developing from the intense exercise and heat stress. According to Maresh et al. (26), thirst may be an underlying cue for increased RPE during exercise in a hot environment. In the current investigation, no correlations were found with thirst and RPE at the end of the exercise; however, as expected, RPE and thirst decreased following each rest break independent of superficial cooling. Furthermore, thirst did increase progressively at the end of each exercise bout throughout the investigation with the highest thirst rating at the end of the last exercise bout, regardless of the condition. As the exercise task progresses, exercise tolerance may be related to perceptions of thirst, potentially negatively impacting the perception of exercise intensity. Lower thirst sensation in the cooling condition is an important practical finding because subjects consumed less fluid and became dehydrated. Superficial cooling may make athletes feel cooler and therefore neglect to consume enough fluid to avoid dehydration and EHI.
Cooling vest companies commonly advertise anecdotal evidence that superficial cooling can enhance performance because the subject feels cooler and is therefore less fatigued. We found that when participants wore the cooling vests, they informed investigators that it made them feel “reenergized,” “refreshed,” and “cold and ready to go.” However, when this evidence was transferred into a quantifiable measurement RPE, there were no significant differences between the conditions. This finding indicates that exercise caused an increase in RPE regardless of cooling and that rest, independent of cooling, decreased RPE. Indeed, in the cooling condition, we found no significant correlation between the RPE and thermal sensations, indicating that rest alone reduced RPE. This finding indicates that the cooling vest altered thermal perceptions of heat without affecting RPE or cardiovascular responses. Similar to the findings of Johnson et al. (24), the perceptual scales indicated that participants found it difficult to perceptually rate exercise conditions and exercise strain as hyperthermia developed. Coaches should be aware that intermittent superficial cooling during exercise does not appreciably decrease perceptions of exertion. Athletes exercising intensely in the heat, regardless of using a superficial cooling garment, must take regular rest and rehydration breaks to avoid dangerous hyperthermia.
A limitation of field research in general is that environmental conditions cannot be tightly controlled without an environmental chamber. However, we collected data at the same time each day for 11 days over a 3-week period in July-August in a tropical climate with minor variations in temperature (Figure 2). Furthermore, we assert that field research is more generalizable than laboratory research because it occurs in the actual conditions that American football players would be training and conditioning. Another limitation of this investigation was that the effects of intermittent, superficial cooling on exercise performance were not examined. Exercise performance was not the primary objective of our investigation, and many previous investigations, including a very detailed and thorough systematic literature review (29), have examined the effects of superficial cooling on exercise performance. One previous study (14) examined the effects of applying a cooling vest before exercise to determine a change in core body temperature; however, no significant results were found. In addition, limited evidence (15) exists to support the concept of self-pacing and the limitations of a critical internal temperature. In terms of performance, there is strong evidence in the literature that some specific cooling modalities may provide a benefit to performance when used before and in between exercise bouts (29). Therefore, future studies should be designed to determine the relationship between intermittent, superficial cooling, with how individuals perceive the skin coolness and modify behavior or exercise pacing.
Superficial cooling garments are a popular way for athletes to feel cooler while exercising in hot environments. These garments have been recently marketed to athletic trainers, athletes, and coaches as a means to rapidly reduce core body temperature and presumably for the prevention and treatment of EHI. The findings of this investigation were as follows: (a) although statistically significant, cooling did not sufficiently attenuate T gi to be clinically relevant; (b) intermittent cooling did decrease thirst, RPE, or HR; (c) although participants drank water ad libitum during rest breaks, they became mildly dehydrated with the cooling condition slightly more dehydrated (not significant) than the control condition; (d) in an athletic setting, intermittent, superficial cooling was effective in reducing thermal sensations and T chest; and (e) rest in the shade and rehydration was similarly effective to the superficial cooling garment in reducing core body temperature during and after exercise. In conclusion, because athletes may feel cooler but their core body temperature is still elevated, perceptual responses should be carefully considered when deciding to use cooling vests during extremely hot conditions. Athletes may feel cooler, but they may not be able to perceive the increase in core body temperature, making them potentially at further risk for EHI.
Coaches, athletic trainers, and strength and conditioning specialists should understand that cooling vests are not to be used for the treatment of EHI. However, these garments may be effective in reducing thermal sensations. Superficial cooling vests may be helpful in preventing heat illness or may be used in adjunct with full-body ice water immersion to treat EHI (9). It is important to remember that when athletes are exercising in the heat 5- to 10-minute rest and rehydration breaks must occur in the shade every 15–20 minutes, minimal equipment should be worn, and individuals at risk should not participate. Should dangerous hyperthermia occur (T gi >40° C), coaches should summon medical help and immediately begin the recommended “gold standard” treatment for rapidly cooling hyperthermic athletes using full-body ice water immersion (1,9).
The authors are grateful that Polar Products, Inc (Stow, OH) provided the cooling vests for this investigation. They also thank the participants for their time and efforts and the Kinesiology and Rehabilitation Science students who helped collect data for this investigation, especially Melissa Shrum who made an important contribution to our study. The authors have no conflict of interest and have no professional relationships with companies or manufacturers who will benefit from the results of the present study. There was no specific grant support for the study. The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
exertional heat illness; superficial cooling; heat strain