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Physiological cost and thermal envelope: A novel approach to cycle garment evaluation during a representative protocol

This study aimed to examine thermoregulation in different clothing assemblies during a representative cycling exercise protocol. Six men undertook cycling exercise simulating representative thermal exchange challenges while wearing low (LOW), intermediate (INT1 and INT2), or high (HI) amounts of clothing. Exercise was conducted at 14.5 °C, 46.8% relative humidity and included a “flat” [45 min at 35% peak power output (PPO), wind speed 8.3 m/s], “uphill” (30 min at 55% PPO, wind speed 3.6 m/s), and “downhill” (20 min at 50 W, wind speed 16.7 m/s) stage. Rectal temperature changed with the exercise stage and was independent of clothing assembly. In contrast, an “envelope” was evident for mean body temperature, resulting from differences in mean skin temperature between the LOW and HI conditions. The elevated mean body temperature in HI was associated with increased physiological “cost,” in the form of increased sweat production and heart rate. Physiological cost provides a better index of clothing performance than deep body temperature in the “thermoregulatory zone,” as a consequence sports clothing should attempt to optimize the balance between comfort and reduced physiological cost.

During cycling exercise metabolic heat production varies mainly as a function of power output, which is typically stochastic due to terrain and tactical influences (Jeukendrup, 2002;Vogt et al., 2007). Simultaneously, the avenues for biophysical heat exchange may be affected: alterations in cycling speed will influence convective cooling, as well as evaporative cooling, through effects on the boundary layer around the body. Consequently, multiple thermal exchange scenarios can arise in a single cycling event. For example, during downhill cycling, metabolic heat production may be low (Vogt et al., 2007), whereas convective and evaporative heat losses can increase due to elevated airflow (Saunders et al., 2005). In uphill cycling, metabolic heat production is typically increased (Vogt et al., 2007), whereas lower airflow will decrease heat loss by convection and evaporation (Saunders et al., 2005). This variety of thermal exchange scenarios can make the selection of appropriate clothing for a given cycling event problematic, particularly when opportunities for adding or removing clothing are limited, such as during competition.
Clothing affects biophysical heat exchange by increasing insulation and acting as a barrier to convective and evaporative heat exchange (Berglund & Gonzalez, 1977;Nagata, 1978;Gavin, 2003). However, excessive clothing will lead to increased sweating in an attempt to regulate body temperature. If the clothing acts as a barrier to evaporation this may cause skin wettedness, discomfort (Pascoe et al., 1994), and increased thermal stress (Brownlie et al., 1987); the cooling efficiency of sweating is also reduced when moisture is wicked away from the skin before it evaporates (Havenith et al., 2013). Thus, the ideal clothing for preventing heat loss is that which blocks air movement, but allows the evaporation of water vapor from the skin if sweating occurs (Gavin, 2003). In contrast, clothing for facilitating heat loss should have minimal insulation and pose little resistance to the evaporation of sweat from the skin or convective airflow (Gavin, 2003), while clothing that reflects solar radiation will reduce heat gain from the environment (Nielsen, 1990).
Although manikins can measure heat flux, it is often difficult to accurately replicate the friction, drag, and cling between layers and against skin that is encountered during exercise, as well as the subsequent impact on the microclimate between the skin and clothes (Pascoe et al., 1994). Manikins also lack the important associated thermoregulatory and perceptual responses. It has also previously been noted that research examining the influence of clothing on thermoregulation has generally failed to investigate high exercise intensities (Gavin, 2003), to mimic the airflow of outdoor conditions (Gavin, 2003), or to employ prolonged and ecologically valid sportsrelated exercise protocols (Davis & Bishop, 2013). This is particularly pertinent with regard to cycle clothing, which is often used during high-intensity exercise and in conditions with high airflow rates. To our knowledge, no previous studies have attempted to simulate a range of different thermal exchange scenarios in a single protocol to evaluate cycle garment performance in an ecologically relevant manner.
Finally, attempts to evaluate thermal strain during exercise have often focused on changes in deep body temperature as the primary index of physiologic stress (Moran et al., 1998). Although this may have historical precedent (Haldane, 1905) and is likely practical under conditions of un-compensable heat stress (Gonzalez et al., 1997), the equilibrated deep body temperature of an exercising human is independent of ambient conditions across a wide range of environments and is determined by the external work rate (Nielsen, 1938). Sustained exercise at work rates and environment combinations that do not allow thermal balance (i.e., outside the "thermoregulatory zone"; Tipton, 2006) result in an inexorable rise in deep body temperature until the attainment of critical values associated with exercise termination (González-Alonso et al., 1999). Thus, during prolonged exercise athletes typically select work rates that allow thermal compensability in their environment (Schlader et al., 2011), i.e., they remain within the thermoregulatory zone. In such situations, the efficacy of deep body temperature as a measure of physiologic strain is limited; this includes many ecologically valid exercise scenarios. It has long been known that even under thermally compensable conditions thermoeffector responses, such as heart rate, peripheral blood flow, and sweating are elevated as thermal stress is increased, in advance of un-compensable alterations in deep body temperature (Lind, 1963); this "physiological cost" can provide an index of the physiological strain.
Accordingly, the aim of the present study was to examine the thermoregulatory and perceptual responses of trained cyclists during a representative cycling exercise protocol simulating different thermal exchange scenarios, while wearing a range of different cycle clothing assemblies. We hypothesized that the physiological cost associated with cycle clothing when undertaking an ecologically valid cycle test within the thermoregulatory zone would provide a useful index of clothing performance.

Participants
Six male trained cyclists volunteered to participate and provided their written informed consent. Inclusion criteria were a satisfactory medical examination including a 12-lead electrocardiogram; a satisfactory health history questionnaire, with no contraindications for maximal exercise; and a peak power output (PPO) > 300 W during an initial incremental ramp exercise test. The mean [standard deviation (SD)] age, height, mass, PPO, and maximum rate of oxygen uptake (VO2max) of the participants were 29 (9) years, 1.79 (0.10) m, 74.06 (8.27) kg, 400 (51) W, and 59.1 (4.3) mL/kg/min, respectively. The study was approved by the University's Biosciences Research Ethics Committee.

Experimental design
Following a preliminary incremental exercise trial for the determination of PPO and VO2max, participants undertook four experimental trials wearing a different clothing assembly on each occasion. The order in which participants undertook the experimental trials was determined using a Latin square. Each participant undertook their exercise trials at the same time of day (± 1 h), with at least 48 h between tests.

Experimental procedure
The preliminary incremental exercise test took place under ambient laboratory conditions, with a fan available to provide cooling if requested by the participant. The participant commenced exercise at a power output of 60 W, which was increased by 30 W/min until the participant could no longer maintain the required power output. The final power achieved at the end of the exercise test was termed the PPO, with the highest VO2 obtained from a Douglas bag collection > 15 s termed the VO2max.
The experimental trials took place in an environmental chamber at an ambient temperature of 14.5 (0.2)°C and relative humidity of 46.8 (2.9)%. During each trial participants undertook an exercise protocol lasting ∼2 h, which was designed to simulate a range of representative thermal exchange scenarios. The protocol consisted of (a) 45-min moderate-intensity exercise (35% PPO) with a moderate air speed (8.3 m/s), simulating a "flat" cycling thermal exchange scenario; (b) 30-min high-intensity exercise (55% PPO) with a low air speed (3.6 m/s), simulating an "uphill" cycling thermal exchange scenario; (c) 5-min rest period with no wind, simulating a brief rest period; (d) 20-min low-intensity exercise (50 W) with a high air speed (16.7 m/s), simulating a "downhill" cycling thermal exchange scenario; (e) 5-min rest period with no wind; (f) 10-km self-paced time trial with a moderate air speed (8.3 m/s), to obtain an index of performance.
All exercise trials took place on the participant's own bicycle, mounted on a Computrainer (Racer Mate, Seattle, Washington, USA) ergometer calibrated in accordance with previously described methods (Davison et al., 2009). During the 10-km time trial, participants completed a virtual racecourse constructed using the Computrainer 3D software (Racer Mate). The distance completed was displayed throughout the time trials, but all other feedback was blinded to the participant. Wind was simulated by use of a 0.5 m industrial fan (Fläkt Woods, Colchester, Essex, UK) positioned at a height of 1.15 m (to center point of fan) and 0.50 m from the center point of the front wheel of the participant's bicycle. Air speed was measured at a height of 1.40 m and distance of 0.60 m from the fan using an anemometer (Meterman TMA10; Wavetek, San Diego, California, USA).

Preliminary incremental exercise trial
During the preliminary test, power output was controlled using the Computrainer control panel, and consecutive 60-s expired gas collections were obtained using the Douglas bag method. Expired gases were analyzed to determine gas fractions and volumes using a gas analyzer (Servomex PLC, Crowborough, East Sussex, UK) calibrated using gases of known concentration and dry gas meter (Harvard Apparatus Ltd., Edenbridge, Kent, UK), respectively.

Experimental trials
Deep body temperature was measured using a thermistor (Grant Instruments, Cambridge, UK) self-inserted in the rectum, 15 cm beyond the anal sphincter (Squirrel Data Logger; Grant Instruments). Skin temperature (Tsk) was measured using thermistors (DS18B20 T-3, MSR Electronics GmbH, Henggart, Switzerland) attached to the skin by a single piece of Tegaderm™ (3M, St. Paul, MN, USA) tape at six sites on the right side of the participant at chest, back, bicep, forearm, thigh, and calf. Deep body temperature and skin temperatures were recorded every 60 s throughout experimental trials using an MSR data logger (MSR12; MSR Electronics GmbH). Ambient conditions were measured using a Wet Bulb Globe Temperature weather station (Grant Instruments). Mean body temperature was determined according to Colin et al. (1971) and mean skin temperature using Ramanathan (1964). Throughout the experimental trial participants wore a heart rate monitor (Polar RS800; Polar Electro Oy, Kempele, Finland), which recorded heart rate every 15 s. Fluid intake and pre and post naked (following towel drying) and clothed body weights (OHAUS I-10 digital scales, OHAUS Corporation, Parsippany, NJ, USA) were used to estimate total sweat production. Local sweat rate was determined using a ventilated capsule positioned on the forehead under the cycling helmet (Q-Sweat quantitative sweat measurement system, model 1.0; WR Medical Electronics, Co., Maplewood, Minnesota, USA). To provide an index of peripheral blood flow, a laser Doppler probe (Moor Instruments, Axminster, UK) was lightly taped to the right index finger, with data corrected against a biological zero taken at the start of each trial by inflating a blood pressure cuff to 250 mmHg to momentarily occlude the brachial artery. Five minutes before the end of each exercise stage of the experimental trial, a 60-s expired gas collection was obtained and analyzed to determine the rate of oxygen uptake (VO2). Rating of perceived exertion (RPE; Borg, 1982) and perceptual measures of thermal comfort and thermal sensation (Zhang, 2003) were obtained at 5-min intervals throughout the trials.

Data analyses
Data are presented as mean (SD) unless otherwise stated. Where data are available at 60-s intervals (i.e., deep body temperature, mean body temperature, mean skin temperature, heart rate) they were averaged for each participant over the final 5 min of each of the exercise stages of the experimental protocol, i.e., final 5 min of the 45-min flat stage, final 5 min of the 30-min uphill stage, final 5 min of the 20-min downhill stage for comparisons. Where data are reported at 5-min intervals (i.e., RPE, thermal comfort, thermal sensation), the final data point recorded for each exercise stage was used for analysis. Between-group differences were determined by Friedman's test, with significant differences examined post hoc using the Wilcoxon signed-rank test. The alpha value was set a priori at 0.05.

Thermal measures
There were no significant differences between clothing conditions in rectal temperature at any of the comparison points throughout the exercise trials ( Fig. 1(a)). In general, rectal temperature rose toward a plateau throughout the first 45 min of exercise, with a further increase occurring during the subsequent 30-min uphill exercise stage. During the 20-min simulated downhill cycling stage, a reduction in rectal temperature was observed. In contrast, significant differences in mean skin temperature were evident between all of the clothing assemblies at comparison point 1 and comparison point 3. At the second comparison point, mean skin temperature was not different between INT1 vs INT2 and LOW vs INT2, but was different between all other conditions ( Fig. 1(b)). Significant differences in mean body temperature were evident at comparison point 1 and comparison point 3 between all conditions except INT1 vs INT2. At comparison point 2, differences in mean body temperature were evident between all conditions except INT1 vs INT2 and LOW vs INT2 (Fig. 1(c)). Across the experimental protocol a clear thermal envelope was evident for mean skin temperature and mean body temperature, the upper and lower boundaries of which were defined by the HI and LOW clothing conditions, respectively.
Site-specific chest and back temperature data are presented together in Fig. 1(d), with significant differences between conditions denoted accordingly. Chest temperature was different between all conditions at each comparison point, with the exception of H1 vs INT2 at the third comparison point. INT1 chest temperature was lower than INT2 at each comparison point, although both were positioned within the boundaries of the thermal envelope defined by HI and LOW. Back temperatures were similar between INT1 and INT2 across the exercise protocol, and a clear thermal envelope was defined by the HI and LOW conditions. However, back temperature in INT1 and INT2 approached the upper end of the thermal envelope at the second comparison point.

Physiological measures
A significant effect of clothing condition on heart rate was evident at the second comparison point, with heart rate being higher in the HI condition than in either the LOW condition or INT2 (Fig. 2). Between-condition differences in heart rate at the other comparisons points were not statistically significant. Significant differences in forehead sweat rate were evident between LOW vs HI and HI vs INT1 at comparison point 1 (Fig. 3). Sweat rate was increased at the second comparison point, but was not different between conditions, suggesting that the peak local sweating rate had been achieved in all conditions. At the third comparison point, differences in sweat rate were evident between each condition with the exception of INT1 vs INT2 and LOW vs INT1. Total sweat production was 0.35 (0.06), 0.43 (0.05), 0.42 (0.06), and 0.71 (0.06) L/h for LOW, INT1, INT2, and HI, respectively, and was different between all clothing conditions with the exception of INT1 and INT2. In contrast, no significant differences were evident in the rate of oxygen uptake, or fingertip blood flow, at any of the comparison points. Mean power output sustained during the 10-km performance trial was 272 (15)

Perceptual measures
The RPE was not different between the clothing conditions during the first two comparison points, but was significantly higher in INT2 than LOW at the third comparison point. Thermal sensation data indicated that participants felt significantly warmer in HI than in LOW, INT1, or INT2 at the first comparison point. At the second comparison point, participants felt significantly hotter in HI than in LOW or INT2, and also in INT1 and INT2 than in LOW. At the third comparison point, participants felt significantly warmer in HI than in LOW, INT1, or INT2. Finally, there were no significant between-condition differences in thermal comfort at the first or third comparison point, although participants felt significantly less comfortable in HI than in LOW, INT1 or INT2 at the second comparison point.

Discussion
The ultimate validation of any clothing assembly relies on testing with humans under the actual conditions of intended use (Pascoe et al., 1994). The present study is the first to employ a prolonged and ecologically valid sports-related exercise protocol to evaluate the performance of different cycle garment assemblies during exercise in a range of representative thermal exchange scenarios, which mimic the airflow of outdoor conditions. In all clothing assembly conditions, the rectal temperature response was independent of the clothing worn. Rectal temperature increased throughout the flat cycling exercise stage, with a further increase during the uphill exercise stage, prior to a reduction during the downhill exercise stage. This finding is consistent with other studies that have shown that the temperature at which deep body temperature is regulated during exercise is primarily related to the relative exercise intensity (Nielsen, 1938;Lind, 1963;Saltin & Hermansen, 1966;Gant et al., 2004). In contrast, mean skin temperature, and consequently mean body temperature, was influenced by the clothing assembly. In each instance the mean skin and mean body temperature was highest in the HI condition and lowest in LOW, with INT1 and INT2 positioned toward the lower end of the thermal envelope. Because work output was identical between each of the exercise conditions (except during the TT), these differences must result solely from alterations in heat loss capacity as a consequence of the different clothing assemblies. The boundaries of the thermal envelope defined by the LOW and HI condition should be a useful reference for clothing manufacturers by providing an indication of the realistic limits for heat loss and heat retention that are possible for each exercise stage via alterations in garment assembly selection, or design.
The site-specific temperature data from the chest and back are useful in illustrating the non-uniform nature of the torso skin temperature during cycling exercise in an environment where a representative air velocity is pro-vided. Although not examined statistically, an elevated back temperature relative to chest temperature was particularly evident at the higher air velocities in INT1, INT2, and LOW conditions (comparison points 1 and 3). In addition, it is interesting to note that the back temperature in INT1 and INT2 conditions approached the upper limit of the thermal envelope for back temperature (e.g., HI condition) at the second comparison point. We believe that these site-specific data are useful for two reasons: first, they demonstrate the importance of using a representative air velocity when examining the performance of sporting garments; failure to do so will lead to an inaccurate representation of the skin : clothing microclimate during exercise, and will diminish heat loss to the environment (Saunders et al., 2005). Second, the observation that back temperature in the INT1 and INT2 conditions approached the upper (HI) limit of the thermal envelope at the second comparison point implies that increased back ventilation may be a beneficial design feature for these assemblies in this thermal exchange scenario.
Sweat production was influenced by the clothing assembly worn, and differences in local sweat rates were evident at the first and third comparison points. However, local sweat rates were not different between conditions at the second comparison point, possibly indicating that the peak local sweating rate had been reached. Nevertheless, heart rate was elevated in HI relative to INT2 and LOW at the second comparison point. Presumably, these augmented effector responses with increased amounts of clothing occurred as a consequence of the elevated mean skin temperature and mean body temperature, in order to facilitate heat loss, and as such, demonstrate the increased physiological cost associated with regulating deep body temperature while wearing different garment assemblies. It is well established that sweat losses of ∼2% body weight can result in significant impairments in performance (Cheuvront et al., 2003), whereas an elevated heart rate during submaximal exercise at a fixed intensity is indicative of increased physiological strain. However, there was no significant difference in 10-km time trial (TT) performance between the clothing conditions in the present investigation, although, numerically, the trends for performance times were consistent with the differences in mean body and mean skin temperature. This indicates that the increased physiological cost was tolerated by the trained cyclists participating in this study, although the participant numbers used in the present study will have impact upon the ability to detect a statistically significant difference; it remains to be confirmed whether a statistically significant effect would have been evident with a larger number of participants, or if the increased physiological cost would have been tolerated by less welltrained participants, or during exercise in a warmer exercise environment. Nevertheless, it would appear logical to suggest that garments intended for use in sports performance should be designed to achieve the best thermal response at minimum physiological cost. In most prolonged exercise activities, during which participants must remain in the thermoregulatory zone (i.e., able to thermoregulate), it will be the physiological cost of thermoregulating, rather than body temperature per se that will be the most illuminating measure of clothing performance.
Differences in thermal sensation between clothing conditions were evident at each of the comparison points, indicating that the clothing assemblies were sufficiently different to induce significant perceptual differences. In contrast, there was no difference in thermal comfort between conditions at the first and third comparison points, although thermal comfort was significantly lower in the HI condition than in the other conditions at the end of the uphill exercise stage. The observation that participants generally remained comfortable and that there were no between-condition differences in thermal comfort during the first and third stage suggests that the range of clothing assemblies selected were realistic under these conditions. Taken together these findings emphasize the complex nature of thermal comfort perception, which may remain relatively constant (comparison points 1 and 3) in the presence of a variety of whole-body thermal sensations. Interestingly, the RPE did not differ between groups at the point at which a significant difference in thermal comfort was detected. This would imply that thermal comfort is not an important factor determining perception of exertion, or that there is a lag in the relationship between thermal comfort and RPE. It should be noted that RPE was higher in INT2 than LOW at the third comparison point, although we question the practical significance of this finding. Further research is required to understand the nature of the factors influencing thermal and effort perception under the non-uniform and dynamic exercise conditions employed in the present study, and frequently encountered outside of the laboratory environment.
In summary, the representative cycle exercise protocol employed in this study enabled cycle garment performance to be evaluated across a range of different thermal scenarios, in a single exercise test. Rectal temperature remained in the thermoregulatory zone and changed concomitant with the intensity of exercise and was independent of the clothing assembly. In contrast, a clear thermal envelope was evident for mean body temperature, which resulted from differences in mean skin temperature as a consequence of the various garment assemblies used. Thus, future studies examining clothing performance during exercise within the thermoregulatory zone should not use deep body temperature as the sole index of thermal performance. The elevated mean body tempera-ture with increased levels of clothing resulted in an elevated rate of sweat production and heart rate at points during the exercise trials. Thus, regulating deep body temperature within the thermoregulatory zone under different clothing conditions is associated with varying physiological cost. This physiological cost may have the potential to impair performance, although this was not apparent in the present study. We propose that the thermal envelope and physiological cost of thermoregulating within the thermoregulatory zone represent useful concepts for evaluating the performance of sporting apparel. Finally, laboratory studies examining the performance of sporting apparel should seek to accurately replicate the range of metabolic and thermal demands likely to be encountered by the end user.

Perspective
There have been calls for studies examining the influence of clothing on thermoregulation to employ prolonged, ecologically relevant, sports-related exercise protocols including high exercise intensities and mimicking the airflow of outdoor conditions (Gavin, 2003;Davis & Bishop, 2013). This study used a novel protocol to examine thermoregulatory and physiological responses during prolonged cycling exercise in representative thermal exchange challenges, with different cycle clothing assemblies; the concepts of physiological cost and thermal envelope have been introduced. During most prolonged exercise activities athletes remain in the thermoregulatory zone; we proposed the magnitude of thermoeffector responses necessary to maintain the regulated deep body temperature (i.e., physiological cost), rather than body temperature per se as an index of clothing performance. Differences in physiological cost were evident between clothing assemblies in the absence of change in deep body temperature. Thermal envelope refers to boundary thermal profiles elicited by clothing assemblies toward the upper and lower end of those that might be worn under given environmental conditions. This envelope in mean body temperature was demonstrated in this study and might represent a useful approach for providing an indication of the realistic limits for heat loss and heat retention possible via alterations in garment assembly selection or design.
Key words: Environmental physiology, clothing, temperature.