Exercise, largely through metabolic heat production, increases body temperature, and the rise in body temperature is accelerated when exercise is performed in warm to hot conditions (8,17). An accelerated rise in core temperature appears to limit performance because of a variety of mechanisms. For example, the time to reach the proposed critical body temperature often associated with fatigue in endurance activities will be shortened (12,19). Further, the cardiovascular strain, impairments in central nervous system functioning, and alterations in metabolism (e.g., greater glucose use leading to depletion) that are associated with hyperthermia and fatigue will also occur sooner (9,10,12,18). Consequently, any attempt to delay the rise in body temperature during exercise may improve performance (8,12,19).
One approach to improving exercise tolerance in warm conditions is to begin exercise with a lower or cooled body temperature. In theory, beginning exercise with a cooler body temperature increases the heat storage capacity, thus delaying fatigue associated with hyperthermia (19). The lowering of body temperature before exercise, called precooling, can be accomplished via a number of methods, including but not limited to, cold water immersion, cold air exposure, and cooling vests (19).
Both cold water immersion and cold air exposure have been effectively used to lower body temperature (3,15,16). Associated with these reductions in body temperature have been consistent improvements in endurance performance (3,15,16,23). Although effective, cold water immersion and cold air exposure are not without their limitations, and these consist of the need for specialized equipment, poor transportability to a field setting, athlete discomfort, and cost (19).
Cooling vests, with fewer of the above-mentioned limitations, are arguably the most practical cooling method. Additionally, a cooling vest can be worn during a competitor's warm-up or precompetition routine. Considering a warm-up provides a number of potential benefits, such as better oxygen delivery, accelerated rate of oxidative reactions, and an elevated V̇O2 at the start of exercise, all of which may aid endurance performance; this feature may be advantageous (2). Conversely, if the warm-up increases the degree of thermoregulatory strain a performer may be predisposed to the development of hyperthermia-related fatigue. Thus, the use of a cooling vest during the warm-up may allow for the benefits while limiting the degree of thermal strain.
The use of cooling vests during the warm-up has been shown to significantly limit the increase in body temperature generally associated with warming up (1,13,22) and enhance endurance performance (1,22). Arngrimsson et al. (1) examined the effects of wearing a cooling vest during a warm-up period on the time required to complete a 5-km time trial (TT). Body temperature after the warm-up was significantly cooler after the wearing of the vest in comparison to when no vest was worn. Beginning the 5-km TT with a cooler body temperature translated into a 1.1% (or 13 seconds; 1,134 vs. 1,147 seconds) improvement in running performance. Likewise Webster et al. (22) noticed a significant improvement in running time at 95% V̇O2max (to exhaustion) with the use of a cooling vest during the warm-up (164.6 seconds) than when no vest was worn during the warm-up (115.3 seconds). Together, these findings appear to support the use of a cooling vest during warm-up in preparation for high-intensity, short-duration endurance performance. Whether the benefits of this strategy would be sufficient and persistent enough to aid longer duration (e.g., 10 km) endurance exercise is unknown because there is a lack of research examining the effectiveness of precooling with cooling vests on continuous running performance of a longer duration.
Therefore, the purpose of this study was to investigate whether wearing a cooling vest during a warm-up would result in a faster 10-km TT among experienced runners when compared to 10-km performance without precooling.
Experimental Approach to the Problem
A repeated measures, within subjects design was used. Subjects attended a total of 4 sessions. The first session was to familiarize each subject with performing a 10-km TT on a treadmill. During the second session, subjects performed a V̇O2 peak test. Sessions 3 and 4 (the experimental sessions) involved completing a 10-km TT either with or without prior cooling, with the order of these 2 sessions randomized. During these final 2 sessions subjects either wore a cooling vest (V) or just a t-shirt (control [C]) during the 30-minute warm-up that preceded the 10-km TT. The cooling vest was removed before the TT during V. Dependent variables included time to complete the 10-km TT, 2-km split times, core temperature, heart rate (HR), ratings of perceived exertion, and thermal sensation (ThS). All sessions were separated by at least 6 days (5).
To remove any bias, subjects were informed the study was investigating whether a warm-up in a cooling vest (a) aided performance because of cooling or (b) decreased performance because of greater energy expenditure during the warm-up because of the added weight of the vest (1).
Eight male endurance runners with racing experience ranging from 5 km to marathon distance volunteered for the study (mean ± SD: age, 33.7 ± 7.4 years; height, 179.6 ± 9.6 cm; V̇O2peak, 61.5 ± 5.8 ml·kg−1·min−1; Weekly mileage, 54 ± 18.6 km·wk−1; Running experience, 12.1 ± 9.8 years). Inclusion criteria consisted of completion of a recent 10-km race or TT with a pace of <7 minutes per mile (<4:18 km·h−1). Testing occurred during the winter and early spring; thus, these locally recruited participants were not accustomed to exercising in warm conditions. One participant withdrew because of reasons not related to the study, and as a result, complete data were collected for 7 participants. The study was approved by the university's Institutional Review Board and written informed consent, Physical Activity Readiness Questionnaire (PAR-Q) and health history were obtained before testing.
V̇O2 Peak Testing Protocol
To obtain V̇O2max, the subjects ran on the treadmill at a constant, self-selected speed ranging from 9.7 to 12.07 km·h−1 (6-7.5 mph) with the grade increasing by 2% every 2 minutes until the point of voluntary exhaustion (1). The subjects performed a cool-down after the test until the HR recovered to below 120 b·min−1 (11). During the test, expired gases were collected and analyzed with a metabolic cart (PARVO Medics TrueMax 2400, Sandy, UT, USA). Before each test, gas calibration was regularly performed against a mixture of known gas concentrations and flow rate calibration was made with a volume syringe (Hans Rudolph, Kansas City, MO, USA).
10-km Time Trial Protocol
The warm-up was designed to duplicate a typical warm-up routine used before endurance competition. The warm-up was 30 minutes and consisted of 2, 10-minute running phases, which were separated by 10 minutes of stretching. The pace during the second 10-minute running phase was 1.6 km·h−1 faster than the first 10-minute phase (1). The speed of each of the 2 10 minutes of running stages was established during familiarization. However, during V, each of the 10-minute stages was performed 0.8 km·h−1 slower (in comparison to C) to compensate for the extra metabolic work as a result of the weight of the vest (1). After the warm-up, and before beginning the 10-km TT during V, the vest was removed.
Running speed at the beginning of the 2 experimental TTs was set according to an average speed obtained during the 10-km TT of the familiarization session. Once each TT was underway, the speed was controlled by the subjects. To assist the subjects in pacing, distance ran was continuously displayed. Total time required to complete each 10-km TT and 2-km splits was recorded and used in the analysis. The subjects were not provided with their performance times until the end of the last session.
Subjects were given the same instructions before each TT. These directions included “to give your best effort,” “pace yourself,” and “freely change the speed of the treadmill as you feel necessary.”
During the warm-up and the 10-km TT of the familiarization session, the subjects were allowed to drink water ad libitum. The amount of water consumed and time of consumption were recorded so that the fluid consumption pattern could be reproduced during both experimental sessions.
Heart rate (Polar S810, Finland) and core temperature were recorded every minute during the warm-up and TT. Rating of perceived exertion (RPE) according to Borg's 20 point scale and ThS measurements were taken at rest, every 5 minutes during the warm-up, immediately before the simulated run and every 5 minutes during each TT with the final readings at the end of the 10-km TT. Thermal sensation was measured on a 5-point category scale (1 = “neutral” and 5 = “as hot as possible” (1).
Core temperature (Tc) was measured using an ingestible core body temperature sensor (CorTemp, HQ Inc., Palmetto, FL, USA). After the manufacturer's guidelines, each subject was asked to swallow a temperature transducer 4 hours before each experimental session to allow enough time for the temperature transducer to enter the small intestine.
Before each of the experimental sessions, urine samples were collected and analyzed with a portable refractometer (Brix Refractometer, Manix Instruments, Tempe, AZ, USA) to determine preTT hydration status according to the following criteria: urine specific gravity (USG) of 1.020 or less (4). If a subject was dehydrated (USG > 1.020) he or she would have been asked to return to the laboratory on a different day; however, none of the participants reported to the laboratory in a dehydrated state. Also, nude body weight was recorded pre and postTT (Digital Scale Model T500E, A&A Scales, Prospect Park, NJ, USA). Before the postTT assessment of body weight, subjects were required to towel off. From these values, sweat losses during each TT were estimated by subtracting pre- from post-TT body weight and adding the amount of fluids consumed during the TT session (6).
In preparation for all sessions, subjects were instructed to avoid alcohol 48 hours before testing and avoid consuming caffeine 12 hours before testing. They were encouraged to drink water and noncaffeinated beverages liberally (1). Additionally, subjects were asked to consume 400-600 ml of water or a sport drink 2-3 hours before the test and 200-300 ml of water or a sport drink immediately before the test to ensure euhydration. Subjects were informed not to engage in unaccustomed exercise on either the day of, or the day before, to testing. The air temperature (24-26°C) and relative humidity (29-33%) were kept constant.
StaCool™ Under Vest (StaCool Industries Inc., Brooksville, FL, USA) made of airprene breathable material with a zipper on the front was used in this study. The vest had 4 pockets containing thermopacks: 2 anterior pockets located vertically on the right and left sides of the torso and 2 posterior pockets similarly positioned.
Independent t-tests were used to compare the 10-km TT times and the changes in body weight during the 2 TTs. A 2-way (condition × measurement time) analysis of variance (ANOVA) with repeated measures was used to compare HR, Tc, RPE, and ThS during warm-up and then during the TT. HR, Tc, RPE, and ThS were analyzed at 0, 10, 20, and at the end of the warm-up and at 0, 10, 20, 30, and at the end of the TT. Alpha level was established a priori at p ≤ 0.05. Additionally, effect sizes (ESs) using Cohen's d were calculated (7). Interpretations of the ESs were based on the following: <0.20 = Trivial, 0.21-0.50 = Small, 0.51-0.80 = Moderate, and >0.8 = Large.
10-km Time Trial Running Performance
Ten-kilometer run times were 2,533 ± 144 seconds (42.2 minutes) for C and 2543 ± 149 seconds (42.4 minutes) for V conditions and were not significantly different (p = 0.746) (ES = 0.07) (Table 1). Table 1 also displays the 2-km split times, which did not differ between the conditions (p = 0.761). Individual comparisons in 10-km TT for the 2 conditions are displayed in Figure 1.
Physiological Responses during the Warm-Up and 10-km Time Trial
The HR during both conditions significantly changed during the warm-up (effect of time: F(1,6) = 546.5, p < 0.001), but no interaction effect was evident (p = 0.103) (Figure 1). As illustrated in Figure 2, Tc showed no significant difference between C and V during the warm-up (p = 0.451).
A significant time effect was found for HR data during the 10 km TTs (effect of time: F(1,6) = 72.09, p = 0.003) (Figure 1). A repeated-measures ANOVA also reported a significant interaction effect (Effect of condition × time: F(1,6) = 24.96, p = 0.012) (Figure 2). No effect of condition was observed (p = 0.213). Peak HR during the TTs reached 184 (± 22.5) b·min−1 (or 101.6 [± 3.0] % of HRmax) in C and 181 (± 20) b·min−1 (or 100.1 [± 4.2] % of HRmax) in V, respectively (p = 0.512) (ES = 0.14).
Tc at the start of the 10-km TT was 37.7 ± .72° C in C and 37.3 ± .73° C in V (Figure 3). A paired sample t-test indicated this initial difference in Tc between the conditions approached significance (p = 0.067) (ES = 0.5). Tc during the 10-km TTs did not differ significantly between the conditions (p = 0.621) and showed neither a time effect (p = 0.084), nor a condition effect (p = 0.099) (Figure 3).
No significant differences in RPE were found between the conditions (p = 0.08) but throughout each TT RPE significantly increased over time (Effect of Time: F(1,6) = 144.9, p = 0.001) (Figure 4). The ThS data during the TTs displayed the identical trends as RPE (Figure 5).
Weight (Sweat) Loss during the Time Trials
The weight loss during the TTs equaled 1.99 ± 0.19 kg for C and 2.07 ± 0.35 kg for V, with no significant difference between conditions (p = 0.449).
Urine Specific Gravity
There was no significant difference in pre-TT USG values between the C (1.015 ± 0.006) and V (1.011 ± 0.071) conditions (p = 0.240).
This investigation examined whether wearing a cooling vest during a warm-up would result in a faster 10-km TT among endurance runners when compared to TT performance without prior cooling. Results indicate wearing a cooling vest during a 30-minute warm-up did not improve the time to complete the 10-km TT.
The absence of any improvement in 10-km TT performance contradicts previous research examining the influence of precooling with a cooling vest on running ability. For example, Arngrimsson et al. (1) observed a small (13 seconds, 1.1%) but significant improvement in 5-km running time as a result of wearing a cooling vest during the warm-up. Running time to exhaustion at 70% of maximum HR and running time to exhaustion at 95% V̇O2max (after 30 minutes of running at 70% V̇O2max) were both significantly lengthened as a result of pre-exercise cooling using a vest (21,22).
One explanation for the lack of improvement in TT performance may be the degree of cooling experienced by V. The use of the cooling vest during the warm-up produced a lower pre-exercise core temperature than in C (37.3-37.7°C). Although this degree of cooling is comparable to the 0.5°C reduction in core temperature observed after 60 minutes of precooling with a vest (13) and the 0.4°C reduction in body temperature after 38 minutes of precooling during a warm-up (1), it was not significant and perhaps inadequate to aid 10-km TT performance. Booth et al. (3) observed a 4% increase in distance ran during 30 minutes when precooling (via cold water immersion) reduced rectal temperature, in comparison to the control, by 0.7°C. A 20-minute application of a cooling vest resulted in an approximately 0.6°C reduction in tympanic temperature and a subsequent 2.2-minute improvement in time ran during a graded-exercise test (21). Collectively, these findings indicate the 0.4°C lowering of core temperature of this study, although large enough to show a moderate ES, was not physiologically sufficient to provide any benefit to performance.
The effectiveness of the cooling vest may have been limited considering the vest was worn during the warm-up. This practice is common in previous research and has proven effective at lowering pre-exercise core temperature (relative to no precooling) (1,13). However, a cooling vest worn without a warm-up was more effective at reducing temperature than when a vest was worn during warm-up (21). When used during a warm-up some of the cooling effects of the vest are offset by the metabolic heat production of the contractile activity, consequently limiting the degree of cooling. In this study, the small amount of cooling was inadequate to aid performance, perhaps suggesting cooling strategies should be employed at times other than warm-up.
The duration of the TT may be another reason for the lack of improvement in performance. Approximately 42 minutes was required to complete the 10-km TT. Studies examining the effectiveness of precooling during the warm-up on TT or actual running have observed performance and physiological benefits in distances between 4 and 6 km (1,13) where by similar or alternative precooling methods have reported benefits on running time to exhaustion tests lasting approximately 30 minutes (3,16,21). These findings, when combined with those of the present study, point to the possibility of an upper limit duration of exercise that can benefit from precooling and the 42-minute duration of the present study exceeded this limit. It may also be that the degree of cooling necessary to improve longer duration endurance performance may be larger than that necessary to improve shorter performance.
The longer exercise duration of the present study meant a slower running pace in comparison to the pace observed by Arngrimsson et al. (1). As a result, metabolic heat production and the rate at which core temperature increased would have been reduced. Thus, thermal stress during the 10-km TT may not have been a limiting factor on performance, and if thermal stress did not constrain performance, then the modest precooling experienced by V would be of little benefit.
The highest core temperature achieved during C was 39.3° C, which is less than the proposed critical core temperature (ca. 40°C) often associated with limiting exercise performance (12,19). One reason for this may have been the moderate ambient temperature (24-26°C). In the studies by Arngrimsson et al. (1) and Ückert and Joch (21), both of whom observed an improvement in running performance as a result of using a cooling vest, the ambient temperature was between 30 and 32°C. Further, cycling performance in 30°C, but not in 25°C conditions, was enhanced with precooling (14). These findings support the idea that the effectiveness of precooling is influenced by environmental factors that increase the degree of heat strain during exercise. Accordingly, if the rise in core temperature during C was not adequate to stress physiological and perceptual responses, and thus limit performance, it is unlikely that any pre-exercise cooling would benefit performance.
Another indicator of the degree of thermal strain during exercise is HR. Studies examining the influence of precooling with a vest on HR during exercise suggest the rise in exercise HR is delayed with precooling (1,20,21). The delayed rise in HR was linked with a reduction in thermal strain because of a significantly lower body temperature. In this study, precooling failed to blunt the HR response indicating the reduction in core temperature elicited by V (in comparison to C) was not sufficient to alleviate any thermal strain during the TT. Additionally, the similarities in RPE, ThS, and sweat loss between the 2 conditions support this notion. The inability of the cooling vest to ease the degree of physiological and perceptual stress likely explains the lack of improvement in running performance.
A key determinant of endurance performance is the ability to maintain a constant pace (15). An initial pace that is too fast may impair performance during the later stages of an endurance performance. Pace, when self-selected, appears to be influenced by a number of factors, one of which is skin temperature (15). In this study, time to complete the first 2-km split was faster in V than in C. Further, in 2 of the 3 subjects who did not see an improvement as a result of wearing the vest the time to complete the first 2 km was considerably faster in V than in C (467 vs. 498 and 481 vs. 497 seconds). It is possible that the faster initial pace may have slowed performance during the later stages of the TT (e.g., because of metabolic fatigue), thus offsetting (or even outweighing) any of the benefits offered by the small amount of cooling. Although skin temperature was not assessed in this study, it is plausible the faster initial pace may have been because of a cooler skin temperature as a result of wearing the vest during the warm-up. Perhaps with more effective pacing at the onset of the TT a benefit from the small degree of precooling may have been realized.
In summary, although wearing a cooling vest during warm-up slightly lowered core temperature relative to that achieved with no precooling, this reduction was not sufficient to improve running performance. The small reduction in core temperature did not ease HR or perceptual responses during the TT, thus accounting for the lack of improvement. Additionally, the rise in core temperature, likely because of the moderate ambient air temperature, did not appear to be a performance-limiting factor. Consequently, the small amount of cooling was of little value. Future research should determine the degree of cooling necessary to enhance performance of different durations and under different environmental conditions. With respect to the cooling vest, research should be directed at evaluating the ideal conditions for its use (e.g., during warm-up, duration of application).
When endurance competitions are approximately 40 minutes in duration and are performed in environmental conditions that create a mild to moderate heat load, there does not appear to be a performance advantage to warming up with cooling vest. Although wearing a cooling vest does provide a small amount of cooling, the lack of improvement in endurance performance is likely a result of the primary source of fatigue under these environment conditions not being related to thermal strain. If a competitor elects to use a cooling vest care must be used to not select an initial pace that is too fast.
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