Introduction
Sporting events and training camps often take place in hot environments which present major challenges to thermoregulation, hence more risks of exertional heat illness (1). The ability to reduce heat strain through various avenues of heat dissipation is, therefore, important not only for performance but also for safety. Many intrinsic and external factors influence thermoregulation, including clothing (9). Clothing acts as a barrier to heat transfer by increasing thermal insulation (i.e., insulation of the clothing and the surface air layer trapped within the clothing) and evaporative resistance (i.e., resistance of evaporative heat transfer) and can impair thermoregulation (18). Concerted efforts have been made to develop synthetic fibers for keeping exercising individuals cooler, drier, and more comfortable in the heat. However, it has been a common observation that there is either no difference (13,30) or an opposite effect (14,19) on thermoregulation, when comparing synthetic with natural fibers. Typically, there is also no difference in clothing comfort or thermal sensation (TS) between fiber thermal properties (13,14,19,30). It should be pointed out that most previous studies examining fiber thermal properties and thermoregulatory responses have incorporated lower exercise intensities mimicking work-related protocols (7).
In recent years, a variety of synthetic fibers and a mixture of synthetic and natural fibers have been engineered, aiming for enhanced sweat absorption and moisture transfer (7). The proposed advantage in a blend of fibers is the superior vapor absorption and transportation compared with cotton alone (6). Cotton fiber has been shown to have superior properties compared with synthetic fibers for vapor absorption (7), with soybean fiber being proposed to have superior ability to transfer moisture through the fiber, thereby, reducing the thermal insulation (6). As such, increased moisture permeability allows enhanced evaporative heat transfer and drier clothing, thus, potentially reducing the body heat storage and leading to better thermal comfort of the exercising human. Recent studies (6,26) have shown significantly lower skin temperature, microenvironment temperature (i.e., the environment underneath the clothing), TS, and enhanced comfort with a blend of synthetic (i.e., blend of polyester fibers) and natural fibers (i.e., soybean fiber) compared with a traditional cotton fiber in a moderate environment (i.e., 18–20° C). It remains to be determined whether improvements in thermoregulation and comfort with these modern fibers are only evident in thermoneutral environments.
A recent study has suggested that clothing fabric construction could also effectively reduce the thermal insulation (31). Clothing fabric construction refers to the method used to produce a fabric such as weaving or knitting. Fabric density (the number of stitches in a specific direction in a knit fabric) is also a characteristic of fabric construction. A denser knit, with more stitches per inch, may reduce convective air flow and, therefore, have a potential negative impact on clothing ventilation. Although novel clothing design may not influence the actual clothing insulation, such engineered design could enhance greater ventilation and convective air flow which, in turn, may alter the microenvironment (7). This is due to enhanced air flow moving stagnant air out of the microenvironment, allowing the external ambient air to circulate and, thereby, reducing the thermal insulation and evaporative resistance (16). It can be expected that convective heat loss during exercise could be improved, especially when winds exist. Because the majority of the existing studies were conducted on thermal manikins (7,15,35) with limited results based on exercising human volunteers (11,13,21,30), the applicability of the advanced clothing fabric construction on thermoregulation in hot environments remains to be evaluated.
Accordingly, this study aimed to quantify the effect of fabric construction and a blend of fibers, compared with a standard cotton fiber on thermoregulation, microenvironment temperature, and clothing comfort during moderate-intensity exercise in a hot environment. We wished to determine whether the fiber content and fabric construction would effectively reduce heat strain, sensations of effort, and enhance clothing comfort compared with natural cotton fibers when exercising in the heat.
Methods
Experimental Approach to the Problem
This study was designed to examine clothing fiber content and fabric construction on thermoregulation along with various markers of comfort during exercise in a hot environment. Direct measurement of the microenvironment temperature was also examined underneath the clothing. The testing environment was set at 32° C (30–35% relative humidity) with the wind speed mimicking treadmill speed at 11 km·hr−1. The environmental condition was chosen to represent a typical exercise bout on a summer day. The exercise bout represented a moderate exercise intensity consisting of a 45-minute treadmill run at 60% of maximal oxygen uptake (V̇o2peak).
Participants underwent 3 trials in a counterbalanced order, once in each clothing ensemble. Trials were conducted at approximately the same time of day to minimize circadian rhythms with a minimum of 48 hours separating each trial. Participants were asked to refrain from heavy exercise for 24 hours and to refrain from caffeine, tea, and alcohol for 24 hours before each trial.
Subjects
Approval was obtained from the local institutional review board for use of human subjects. Written informed consent was obtained from all participants before participation. Eight male athletes competing in National Collegiate Athletic Association (NCAA) Division II athletics for cross-country and soccer were recruited. A minimum V̇o2peak of 55 ml·kg−1·min−1 was required for participation. Mean ± SD characteristics of the participants were as follows: age, 22.5 ± 1.3 years; height, 180.6 ± 6.8 cm; body mass, 75.6 ± 7.4 kg; body fat percentage (24), 7.9 ± 3.1%; and V̇o2peak, 62.3 ± 3.7 ml·kg−1·min−1. Participants were accustomed to exercise in the heat on a regular basis; however, participants were not examined for heat acclimatization status nor did they complete a heat acclimatization period.
Procedures
Participants completed a maximal exertion running trial on a motor-driven treadmill (3620 PC Treadmill; VacuMed, Ventura, CA, USA) to determine V̇o2peak. An incremental running protocol was executed until participants achieved volitional exhaustion or could no longer maintain the required speed. Metabolic data were continuously monitored using a calibrated system (Max II metabolic system; AEI technology, Pittsburgh, PA, USA), along with heart rate (HR) (Polar Inc., Port Washington, NY, USA). Rating of perceived exertion (RPE) was collected during the last 15 seconds of each stage using the OMNI RPE pictorial 0–10 scale (33). Criteria for achieving V̇o2peak were (a) RPE ≥ 9; (b) respiratory exchange ratio ≥1.1; (c) plateau of V̇o2 with increased workload; and (d) >85% of age-predicted maximum HR (i.e., 220—Age) (17). Two or more of these 4 criteria were met by all participants. V̇o2peak data were subsequently used to determine the individual workload for all trials.
Participants were asked to empty their bladders into a clear container for the examination of urine specific gravity (Usg) (A300CL; Atage Co., Tokyo, Japan). If participants arrived at the laboratory dehydrated as examined by Usg of >1.020, they were not allowed to perform the trial and rescheduled for another day. Nude body mass was determined (Detecto-Medic; Detecto Scales, Inc., Brooklyn, NY, USA) before and immediately after each trial, which was performed privately with the use of a curtain to separate investigators and participants. Participants were not allowed to consume fluids during the trial. A Polar HR monitor was fitted around the chest. Participants self-inserted a rectal thermocouple (Physitemp Instruments, Inc., Clifton, NJ, USA) 8 cm beyond their anal sphincter. Microenvironment sensors (DS1923, iButton; Maxim Innovation Corp., Sunnyvale, CA, USA) were placed underneath the clothing and worn throughout the trial. Sensors were positioned at 3 areas: chest, back, and lateral side of the body by the fourth and fifth ribs. Sensor sites were prepared in accordance with Deren et al (8). Briefly, skin sites were shaved, removing excess hair, and wiped with an alcohol swab before placement of the sensors. Sensors were placed on the skin with the actual sensor facing away from the skin and secured with surgical tape (Blenderm 3M; St. Paul, MN, USA). These sensors have been previously validated against thermistors (29,34). Participants were instructed to sit quietly for 15 minutes while they were instrumented with chest, forearm, thigh, and calf skin thermocouples (Physitemp Instruments, Inc.) (25). Additionally, 2 skin thermocouples were placed on the upper arm and lower back. These thermocouples were applied to have additional measurements underneath the clothing.
Participants wore 3 short-sleeved t-shirts during the trial with each shirt composed of a different fiber (Figure 1). Investigators provided each participant with a short-sleeved t-shirt when arriving to the laboratory. Thus, each participant had their own shirt assigned to them. The fibers consisted of 100% cotton (Fruit of the Loom, Bowling Green, KY, USA), a blend of natural fibers (blend) (50% cotton and 50% soybean fiber; Nature's Cave, Sudbury, ON, Canada), and a synthetic fiber (synthetic) (100% polyester fiber; Under Armour, Baltimore, MD, USA) (Figure 1). The construction of all 3 fabrics was knitted fabrics (Figure 2). The fabric construction of the synthetic shirt was claimed by the manufacturer to facilitate ventilation through the clothing (Figure 2). The knit size of the meshed synthetic fabric in height and width was 0.29 and 0.16 mm, respectively. Table 1 provides the fabric density and weights of all 3 fabrics. The fabric density was quantified through a fabric count for each of the fabrics evaluated in the study (Table 1) (ASTM D3887-96 Standard Specification for Tolerances for Knitted Fabrics). Participants wore athletic shorts, underwear, as well as socks and running shoes during the trials. Investigators provided each participant the same athletic shorts, underwear, and socks for each trial with all materials the same for each participant.
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Figure 1.: Three short-sleeved t-shirts. A) Blend, (B) cotton, and (C) synthetic.
Figure 2.: Fabric construction of the 3 short-sleeved t-shirts. A) Blend, (B) cotton, and (C) synthetic.
Table 1.: Fabric density and weights of all 3 fabrics.
Each trial consisted of a 45-minute constant-speed running at 60% of V̇o2peak on a motor-driven treadmill (3620 PC Treadmill; VacuMed). Trials were terminated when rectal temperature (Tre) of 39.2° C was reached or if participants felt unable to continue. Participants were not allowed to consume water throughout the trial. Tre and skin temperature (Tsk) were recorded (TH-8 Thermometer; Physitemp) every 5 minutes, along with HR. Microenvironment temperature was recorded every minute with data from the microenvironment downloaded and analyzed after the trial. Weighted mean Tsk was calculated as (25). Weighted mean body temperature (Tb) was calculated as at rest and during exercise (5). The physiological strain index (PSI) was calculated as , where t is the time interval for measuring Tre/HR, and 0 is the initial Tre/HR (22).
Participants were asked to rate their TS, RPE, sweating sensation, skin wetness, and clothing comfort every 5 minutes during the trial. Each scale was verbally anchored for each participant before the study, and participants were instructed how to use and interpret the scale. Session RPE (S-RPE) and session TS (S-TS) were recorded 20 minutes after the exercise period. Participants were asked to rate how they found the overall session according to the OMNI scale based on how physically demanding the exercise trial was.
Statistical Analyses
Data (mean ± SD) were analyzed using SPSS (version 19; SPSS, Inc., Chicago, IL, USA). A 1-way repeated-measures analysis of variance was performed to compare clothing fabrics for the heat strain indices, microenvironment, RPE, comfort ratings, S-RPE, and S-TS. When appropriate, a Fisher's least significant difference was used to determine the statistical significance. An alpha level less than 0.05 was considered significant.
Results
No participant stopped because of signs or symptoms of heat ailments or requested to stop during the trial. Usg was <1.020 before all trials. There was no main effect of clothing on Tre (p = 0.43), Tsk (p = 0.83), and Tb (p = 0.43) (Figure 3). No main effect for additional thermocouple sites for clothing with upper arm temperature (synthetic 35.5 ± 0.4° C, cotton 36.1 ± 0.5° C, blend 35.5 ± 0.5° C; p = 0.83) or lower back temperature (synthetic 34.9 ± 0.4° C, cotton 35.0 ± 0.4° C, blend 34.6 ± 0.4° C; p = 0.21).
Figure 3.: Mean ± SD changes in rectal, skin, and body temperatures.
There was a significant main effect of clothing on the microenvironment temperature at the chest (p = 0.02) (Figure 4). Post hoc tests revealed a significantly cooler microenvironment temperature at the chest in synthetic vs. cotton (p = 0.02). There was no main effect of clothing on microenvironment temperature at the lateral side (p = 0.89) or back area (p = 0.46).
Figure 4.: Mean ± SD of microenvironment temperature. *p ≤ 0.05, synthetic vs. cotton.
No main effect was found for HR (synthetic 153 ± 28 b·min−1, cotton 154 ± 28 b·min−1, blend 154 ± 26 b·min−1; p = 0.94), PSI (synthetic 6.5 ± 2.5, cotton 6.6 ± 2.6, blend 6.5 ± 2.6; p = 0.89), body mass loss (synthetic 1.03 ± 0.23 kg, cotton 0.99 ± 0.17 kg, blend 1.02 ± 0.25 kg; p = 0.91), or RPE (synthetic 4.9 ± 2.2, cotton 4.8 ± 2.1, blend 4.9 ± 2.2; p = 0.89). No main effect was found for TS (p = 0.50), sweat sensation (p = 0.18), skin wetness (p = 0.12), or clothing comfort (p = 0.41) (Figure 5). There was a significant main effect of clothing on S-RPE (p = 0.03) and S-TS (p = 0.05) (Figure 6). Post hoc measures show that synthetic produced significantly lower S-RPE (p = 0.03) and S-TS (p = 0.04) than cotton but not for blend (S-RPE, p = 0.8; S-TS, p = 0.08). There was no difference between cotton and blend for S-RPE (p = 0.19) or S-TS (p = 0.35).
Figure 5.: Mean ± SD of thermal sensation, skin wetness, sweating sensation, and clothing comfort.
Figure 6.: Mean ± SD of session RPE (S-RPE) and session thermal sensation (S-TS). *p ≤ 0.05, synthetic vs. cotton.
Discussion
Results demonstrate that there was no difference in thermoregulation, RPE, or various markers of comfort between clothing fiber content and fabric construction during moderate-intensity exercise in a hot environment. An intriguing finding is that the microenvironment at the chest region was significantly different for synthetic compared with that for cotton fibers. Another novel finding is that S-RPE and S-TS were significantly lower with synthetic compared with those with cotton fibers.
Within the past decade, a dramatic increase has occurred with sport textiles marketed to athletes and exercise enthusiasts. Modern sport textiles, especially synthetic fibers, have been promoted for keeping wearers cooler compared with natural fibers (7). The current study is in agreement with previous research (10,20,30,37,38) showing no difference in thermoregulation for either clothing fiber content or fabric construction compared with traditional cotton fibers. Davis and Bishop (7) summarized that no study examining sport textile clothing has shown a significant difference in reducing Tre when exercising in a moderate to warm environment despite using various clothing fibers and modes of exercise. Early work with synthetic clothing has either shown no difference or an increased Tre with synthetic fibers compared with cotton fibers (13,14,18). However, recent studies using methodologies more applicable to sport or recreational activities with short-sleeved synthetic t-shirts have shown no detrimental effect on thermal balance compared with cotton (10,20,30,38).
Several recent studies have shown a significant reduction in Tsk with either a blend of synthetics (26) or natural fibers (i.e., cotton and soybean) compared with cotton fibers (6). The reduction in Tsk has been shown during high- (26) and low-intensity (6) exercise with modes of testing simulating a soccer match (26) and a work-related protocol (6). These studies, however, used moderate environmental temperatures of 18 and 20° C (6,26). Current results go against previous findings from Dai et al. (6) in which a lower Tsk, microenvironment temperature, and humidity were observed for an identical blend of natural fibers (cotton/soybean) in comparison with cotton fibers. Equivocal results could stem from differences in methodologies with the current study using a higher exercise intensity and warmer environment. Beyond this, Dai et al. (6) also used a long-sleeved t-shirt compared with a short-sleeved t-shirt incorporated in the current study. Bishop et al. (3) suggested that clothing showing lower Tsk in a cool to moderate environment might be overwhelmed with increased sweat rates in a hot environment and coupling with higher exercise intensity.
Clothing fabric construction with engineered fabrics allowing enhanced ventilation through clothing provides a potential avenue to reduce thermal insulation with evaporative and convective air flows (36). The current study showed no significant difference on thermal balance with the constructed synthetic fabric proposed to have enhanced ventilation compared with other fabrics. However, the microenvironment temperature was significantly reduced at the chest region for synthetic compared with that for cotton fabric. Several recent studies have shown enhanced ventilation through clothing based on the construction of the fabric (11,30). Sperlich et al. (30) showed a significant reduction in the microenvironment humidity at the chest and back regions for polyester compared with a cotton t-shirt. The polyester shirts were constructed to purportedly enhance air flow helping to dissipate moisture more effectively through various channels in the fabric. However, there was no significant difference in Tsk or core temperature during exercise (30). Gonzales et al. (11) also reported a significantly lower torso Tsk and perceived hotness with clothing constructed of varying knit sizes (large, medium, and small). The lower torso Tsk and perceived hotness was shown with large compared with small knit sizes (11). Potential differences in Tsk among the current study and those of Gonzales et al. (11) could stem from the knit sizes of the clothing. The knit size of the synthetic fabric of the current study was 0.16 mm in width, whereas Gonzales et al. (11) showed the greatest influence on skin and perceived hotness with larger knits of 3.5 mm in width. Equivocal results may also arise from durations of the exercise protocol. The current study used a 45-minute continuous exercise protocol at a moderate intensity with Gonzales et al. (11) having participant cycle for 15 minutes at 150 W on a cycle ergometer. Although Gonzales et al. (11) showed moderate to large effect size for the larger-knit jerseys on Tsk in comparison with small-knit jerseys, the potential meaningful benefits to performance are unclear. Furthermore, the influence the large knits would have on Tre is unknown because this was not examined by Gonzales et al. (11). Specifically, future work should be done to understand how the knit size of the fabric could potentially enhance clothing ventilation and the influence this would have on thermoregulation and performance.
It is of practical importance to emphasize that, because of the (commonly) limited ability of the environmental chamber to simulate air flow in all directions, the current study could only simulate air flow in one direction aiming directly at the front of participants. It remains plausible that the microenvironment for the synthetic fabric constructed to enhance ventilation could therefore be reduced in all regions with natural air flow in multiple directions. Sawka et al. (27) has summarized that hot skin impairs endurance performance. A reduced microenvironment in all regions could potentially affect perceptual and heat strain such as Tsk. Therefore, despite lack of global thermoregulatory effect, the observation that synthetic fabric construction reduced microenvironment temperature at the chest region where the only air flow was applied should not be underappreciated. Future research with sophisticated environmental chambers mimicking air flow in multiple directions or taken place in natural environments with a protocol resembling sport-specific exercise is warranted for better understanding the influence of clothing fabric construction on the microenvironment and Tsk in particular.
Synthetic fibers have superior properties for transporting water vapor through clothing where natural fibers have a limited ability to transport sweat (9). This ability of synthetic fibers to rapidly transport sweat helps the clothing remain drier (7). Studies have consistently shown less sweat retention and greater sweat efficiency with synthetic compared with cotton fibers (10,26). A drier fiber should reduce the clinginess of clothing and decrease contact with the skin and, therefore, enhance comfort (2,7,12). There was, however, no significant difference for TS, clothing comfort, sweating sensation, or skin wetness in the study. Also, no difference in RPE was shown throughout the trial. A few studies have shown greater comfort (26), lower TS (26,38), and perceived hotness (11) during moderate to high exercise intensities with temperatures ranging from 20 to 29° C and exercise durations of 15–47 minutes (11,26,38). However, most studies that have incorporated markers of comfort have typically failed to show any significant difference for perceptual responses among different clothing fibers (4,10,13,19,30,37).
To our knowledge, this is the first study describing global exertion and TS between clothing fibers. Importantly, results show a significantly lower rating of efforts for synthetic compared with cotton fibers. The same result was also observed for S-TS between synthetic and cotton fibers. It is unclear as to why differences in overall perception occurred among the 3 fibers despite no differences in perceived exertion or markers of comfort during the exercise. Rating of perceived exertion measures physical sensations induced by exercise intensity, whereas global exertion examined by S-RPE quantifies the total effort during an exercise session (28). This result could reflect, at least partly, an attenuation of the total loads over the testing session. Nonetheless, this novel finding suggests that sportswear made of synthetic fiber could sufficiently reduce negative sensations compared with natural cotton fabric during moderate exercise in a hot environment. It has been proposed that perception of exertion and comfort are key mediators in the central regulation of work rate, hence influence optimal exercise intensity and performance (23,32). Thus, synthetic fiber might present important advantages during the course of prolonged endurance events (e.g., marathon, cycling) occurring in hot environments. Future research incorporating time-trial performance or using protocols for longer duration in the heat is warranted to examine this hypothesis.
In conclusion, this study showed no difference with synthetic or a blend of natural fibers in reducing thermal stress or perceptual responses compared with cotton fibers when exercising in a hot environment. However, sportswear made of synthetic fabric constructed to allow for greater ventilation effectively reduced the microenvironment temperature at the chest, global exertion, and TS, and this effect might be positively exaggerated in natural environments with all-around air flow or during prolonged endurance sports in hot environments. Thus, these results provide novel perspectives in the understanding of sport textiles and for future designing of advanced sportswear.
Practical Applications
Within the context of the literature, no advantage is gained for reducing heat strain with synthetic clothing or sport textiles when exercising in warm to hot environments at moderate to high exercise intensities. However, strength and conditioning professionals should be aware that most laboratory studies are based on either thermal manikin or human volunteers performing low-to-moderate endurance exercises. Studies with high ecological validity on sport-specific situations are therefore lacking. Thus, the use of sport textiles should not be discouraged because the current and recent studies show no negative effects on thermoregulation or comfort during exercise. This study provides novel insights that synthetic fiber with fabric constructed to allow for greater ventilation and convective air flow could effectively reduce regional microenvironment temperature, global perceived exertion, and TS, which could have potential benefits for enhanced tolerance to exercise in the heat.
Acknowledgments
No sources of funding were used to assist in the preparation of this manuscript. The authors have no conflicts of interest that are directly relevant to the content of this article.
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