Are All Heat Loads Created Equal? ROBERT D. MEADE and GLEN P. KENNY Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Montpetit Hall, Ottawa, CANADA ABSTRACT
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MEADE, R. D., and G. P. KENNY. Are All Heat Loads Created Equal? Med. Sci. Sports Exerc., Vol. 49, No. 9, pp. 1796–1804, 2017. Purpose: We evaluated physiological responses during exercise at a fixed evaporative requirement for heat balance (Ereq) but varying combinations of metabolic and environmental heat load. Methods: Nine healthy, physically active males (age: 46 T 8 yr) performed four experimental sessions consisting of 75 min of semirecumbent cycling at various ambient temperatures. Whole-body dry heat loss (direct calorimetry) was monitored continuously as was heat production (indirect calorimetry), which was adjusted to achieve an Ereq of 400 W. The resultant metabolic heat productions and ambient temperatures for the sessions were as follows: (i) 440 W and 30-C (440 [30]), (ii) 388 W and 35-C (388 [35]), (iii) 317 W and 40-C (317 [40]), and (iv) 258 W and 45-C (258 [45]). Whole-body evaporative heat loss was determined via direct calorimetry. Esophageal (Tes) and mean skin (Tsk) temperatures as well as HR were monitored continuously. Mean body temperature (Tb) was calculated from Tes and Tsk. Physiological strain index (PSI) was determined from Tes and HR. Results: Endexercise evaporative heat loss and Tb were similar between conditions (both P Q 0.48). Tes was greater in 440 [30] (37.67-C T 0.04-C) and 388 [35] (37.58-C T 0.07-C) relative to both 317 [40] (37.35-C T 0.06-C) and 258 [45] (37.20-C T 0.07-C; all P e 0.05). Further, Tsk was different between each condition (440 [30], 33.85-C T 0.16-C; 388 [35], 34.53-C T 0.08-C; 317 [40], 35.67-C T 0.07-C; and 258 [45], 36.54-C T 0.08-C; all P G 0.01). In 440 [30], HR was elevated by about 13 and 18 bpm relative to 317 [40] and 258 [45], respectively (both P G 0.01). Finally, PSI was greater in both 440 [30] and 388 [35] compared with 317 [40] and 258 [45] (all P e 0.04). Conclusions: Exercise at a fixed Ereq resulted in similar evaporative heat loss and Tb. However, the Tes, Tsk, HR, and PSI responses varied depending on the relative contribution of metabolic and environmental heat load. Key Words: SWEATING, CORE TEMPERATURE, HEAT BALANCE, THERMOREGULATION, HEAT STRAIN, EXERCISE
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uring exercise and/or exposure to hot conditions, body heat storage and thereby the change in mean body temperature are determined by the difference between the rate of metabolic heat production (i.e., heat produced as a by-product of metabolism) and the rate of heat loss via dry (i.e., conduction, convection, and radiation) and evaporative mechanisms (24). However, dry heat loss is driven by the temperature gradient between the skin and the environment and can therefore be negative (i.e., net dry heat gain) when ambient temperature exceeds that of the skin (~34-C) (24). For simplicity, human heat balance is often described in terms of the evaporative requirement for heat balance (Ereq), which is calculated as the rate of metabolic heat production minus dry heat loss (7–10,12,14,20,39). In fact, Ereq explains approximately 71%–90% of the variation in evaporative heat loss and thereby whole-body sweat rate (assuming all produced sweat is evaporated) during compensable exercise (10,14).
Recent work has compared thermoregulatory responses between groups of varying body morphology, sex, and/or fitness in context of the biophysical determinants of human heat balance (7–10,12,15,17,18,20,39). From these studies, it is clear that the core temperature response during exercise in compensable conditions is strongly related to metabolic heat production (i.e., exercise intensity), whereas wholebody sweat rate is determined by Ereq (7–10,12,20,39). It is important to note that most of these studies compared physiological responses between exercise bouts in which exercise intensity (and therefore metabolic heat production) was modified but ambient conditions (and therefore environmental heat loads) were similar. Thus, the observed relationship between core temperature and heat production implies a similar relationship also exists between core temperature and Ereq (after adjusting for between-group differences in body mass and/or surface area). It is possible, however, that even at a fixed Ereq, the body temperature (i.e., core and skin) responses during exercise are influenced by the relative magnitude of environmental and metabolic heat loads secondary to alteration in the perfusion of various tissue beds (i.e., core, muscle and skin) and thereby compartmental heat distribution (the primary avenue of heat transfer within the body is convection via blood) (42). If the environmental contribution to Ereq is large (i.e., high ambient temperatures), local and reflex mechanisms induce an increase in skin blood flow, which can reach approximately 8 LIminj1, resulting in elevated skin temperature that acts to
Address for correspondence: Glen P. Kenny, Ph.D., School of Human Kinetics, University of Ottawa, 125 University, Room 367 Montpetit Hall Ottawa, Ontario, Canada K1N 6N5; E-mail: [email protected]. Submitted for publication December 2016. Accepted for publication April 2017. 0195-9131/17/4909-1796/0 MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ Copyright Ó 2017 by the American College of Sports Medicine DOI: 10.1249/MSS.0000000000001309
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METHODS Ethical approval. The current experimental protocol was approved by the University of Ottawa Health Sciences and Science Research Ethic Board and conforms to the Declaration of Helsinki. Written informed consent was obtained from all volunteers before their participation in the study. Participants. Nine habitually active (Q3 dIwkj1 of structured physical activity, Q30 min in duration) males volunteered for one screening visit and four experimental sessions. Participants were nonsmoking, with no history of metabolic or cardiovascular disease. The physical characteristics (mean T SD) of the participants were as follows: age, 46 T 8 yr; height, 1.76 T 0.07 m; body mass, 82.4 T 10.1 kg; body surface area, 1.99 T 0.15 m2; body fat percentage, 21.6% T ˙ O2peak), 45.1 T 5.7%; and peak oxygen consumption (V j1 j1 8.5 mLIkg Imin . Males were chosen given the potential modulation in the control of body temperature associated with fluctuations in female sex hormone levels (i.e., estrogen and progesterone) (5,6) that occur throughout the menstrual cycle. Furthermore, females exhibit a reduced capacity to dissipate heat independent of sex-related differences in body morphology and aerobic capacity in comparison with males (15–18). Experimental design. All participants completed one screening and four experimental sessions. Participants arrived at the laboratory on the day of each session after consuming a small breakfast (i.e., dry toast and juice) that did not include tea or coffee. Before each session, participants were also asked to avoid alcohol consumption and to not perform any exercise for 24 h before experimentation.
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Furthermore, to ensure participants reported to the laboratory well hydrated, they were instructed to drink 500 mL of water the night before as well as the morning of each session. Body height, mass, density, and fat percentage as well ˙ O2peak were determined during the screening session. as V Body height was measured using an eye-level stadiometer (model 2391; Detecto, Webb City, MO), whereas body mass was measured using a high-performance digital weighing terminal (model CBU150X; Mettler Toledo Inc., Mississauga, ON, Canada). Body surface area was subsequently calculated from the measurements of body height and mass (13). Body density was measured using the hydrostatic weighing technique and subsequently used to calculate body fat percentage (38). A maximal incremental semirecumbent cycling protocol ˙ O2peak. While being moniwas used for the assessment of V tored by an automated indirect calorimetry system (MCD Medgraphics Ultima Series; MGC Diagnostics, Saint Paul, MN), the participants cycled for 1 min at a starting external work rate of 100 W. The workload was increased 20 WIminj1 every min thereafter until the participant could not maintain a pedaling cadence of at least 50 rpm or reached volitional fa˙ O2peak was taken as the highest rate of oxygen contigue. V sumption averaged for 30 s. Upon arrival to the laboratory on the day of the experimental sessions, participants provided a urine sample for the measurement of urine-specific gravity after which a measurement of body mass was obtained. Thereafter, the participants entered the calorimeter chamber regulated to the ambient temperature for that day"s trial (see next section). Participants were then instrumented with skin temperature sensors after which they remained seated in a semirecumbent position. Following 30 min of baseline measurements, the participants performed a 75-min semirecumbent cycle protocol at a fixed Ereq of 400 W. Importantly, this Ereq was chosen to ensure heat balance was achieved by all participants and that body temperature (i.e., core, skin, and mean body) and cardiovascular (i.e., HR) responses were stable at end exercise. In total, each participant completed four counterbalanced sessions at ambient temperatures of 30-C, 35-C, 40-C, and 45-C wherein dry heat exchange and metabolic heat production were monitored every minute and the latter was adjusted to achieve the target Ereq. Thus, each trial was performed at a fixed Ereq but varying environmental (achieved by adjusting ambient temperature) and metabolic (achieved by adjusting external work) heat loads. As a result, the respective metabolic heat productions (mean T SD) and ambient temperatures for each trial were as follows: 1) 440 T ˙ O2peak) and 30-C (440 W 19 W (equivalent to 43% T 8% V ˙ O2peak) and [30]), 2) 388 T 12 W (equivalent to 40% T 7% V 35-C (388 W [30]), 3) 317 T 16 W (equivalent to 31% T 7% ˙ O2peak) and 40-C (317 [40]), and 4) 258 T 22 W (equivalent V ˙ O2peak) and 45-C (258 [45]). Throughout to 25% T 6% V exercise, participants wore only athletic shorts and provided subjective ratings of thermal sensation and perceived exertion at 15-min intervals. After the protocol, a measurement of nude body mass was taken.
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modulate dry heat exchange (11,23,37). By contrast, in a situation with greater metabolic heat production (i.e., high exercise intensity), skin blood flow similarly increases, albeit not to the same degree as when environmental heat load is high (23). In this case, a large proportion of available blood flow is diverted to the active musculature (the primary site of endogenous heat production), to support the increased metabolic demand. This may result in comparatively greater increases in core temperature and cardiovascular strain as the capacity for increases in muscle blood flow (up to ~20–25 LIminj1 during cycling [3,4,22,34]) is larger than that of the skin (3,4,11,22,23,34,37). To the best of our knowledge, however, a direct evaluation of the relationship between Ereq and body temperature and cardiovascular responses to exercise is lacking. The purpose of the current study was to evaluate wholebody sweat rate as well as physiological (i.e., thermal and cardiovascular) and perceived (i.e., thermal sensation and RPE) strain during exercise at a fixed Ereq, but varying contributions of metabolic and environmental heat load. It was hypothesized that although evaporative heat loss and whole-body sweat rate would be consistent at a fixed Ereq, greater increases in core temperature and HR would be observed during conditions with greater exercise intensity and thereby metabolic heat production (and lower environmental heat loads).
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Measurements. The modified Snellen whole-body direct airflow calorimeter was used to measure the rate of evaporative heat loss and dry (radiation + convection + conduction) heat exchange (36). Calorimeter inflow and outflow values for absolute humidity and air temperature were collected at 8-s intervals. Absolute humidity was measured using high-precision dew point hygrometry (RH Systems model 373H, Albuquerque, NM), whereas air temperature was measured using high-precision resistance temperature detectors (Black Stack model 1560; Fluke Calibration, American Fork, UT). Air mass flow through the calorimeter was measured by differential thermometry over a known heat source placed in the effluent air stream. The data for absolute humidity, air temperature, and air mass flow were displayed and recorded on a personal computer with LabVIEW software (Version 7.0; National Instruments, Austin, TX). The rate of evaporative heat loss was calculated using the calorimeter outflow–inflow difference in absolute humidity, multiplied by the air mass flow and the latent heat of vaporization of sweat (2426 JIgj1 sweat). The rate of dry heat exchange was calculated using the calorimeter outflow–inflow difference in air temperature, multiplied by the air mass flow and specific heat capacity of air (1005 JIkg airj1I-Cj1). The rate of body heat storage was calculated as metabolic heat production minus total (evaporative + dry) heat loss. It should be noted that a relatively high mass flow of air is circulated through the calorimeter, ensuring the evaporation of the entirety of sweat produced from the skin (36,40). Therefore, whole-body sweat rate in grams per minute was calculated as evaporative heat loss (in W) multiplied by 60 s and divided by 2426 JIgj1 sweat. The rate of metabolic energy expenditure was simultaneously measured using indirect calorimetry. Expired gas samples were drawn from a 6-L fluted mixing box located within the calorimeter and subsequently analyzed for oxygen (O2) and carbon dioxide (CO2) concentrations using electrochemical gas analyzers (AMETEK model S-3A/1 and CD 3A; Applied Electrochemistry, Pittsburgh, PA). Thereafter, expired air was recycled back into the calorimeter chamber to account for respiratory dry and evaporative heat loss. Gas mixtures of 17% O2 and 4% CO2 (balance nitrogen) were used to calibrate the gas analyzers before each experimental session. A 3-L syringe was also used to calibrate the turbine ventilometer. Ereq, calculated as metabolic heat production minus dry heat loss, was monitored at 1-min intervals throughout the trial. Esophageal temperature was measured using a pediatric thermocouple temperature probe (Mon-a-therm; Mallinckrodt Medical, St. Louis, MO) inserted 40 cm past the entrance of the nostril. Skin temperature was measured at four locations (i.e., upper back, chest, thigh, and calf ) over the right side of the body using 0.3-mm-diameter T-type (copper/constantan) thermocouples (Concept Engineering, Old Saybrook, CT). Mean skin temperature was subsequently calculated using the following weightings based on those suggested by Ramanathan (35): 30% upper back, 30% chest, 20% thigh, and 20% leg. All temperature data were collected using a
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data acquisition module (HP Agilent model 3497A; Agilent Technologies Canada Inc., Mississauga, ON, Canada) at a sampling rate of 15 s and simultaneously displayed and recorded in spreadsheet format on a personal computer with LabVIEW software (Version 7.0, National Instruments). HR was recorded continuously and stored at a sampling rate of 15 s using a Polar coded WearLink transmitter, Polar RS400 interface, and Polar Trainer 5 software (Polar Electro, Oy, Finland). Thermal sensation was determined using the ASHRAE eight-point scale ranging from 0 (neutral) to 7 (very, very hot), whereas RPE was measured using the Borg 14-point scale ranging from 6 (no exertion) to 20 (maximal exertion). Urine-specific gravity was assessed from the urine samples acquired before each trial and compared with current guidelines to ensure adequate hydration (equivalent to a urinespecific gravity of G1.020 [2]). Data analysis. Values for the calorimetric (i.e., metabolic heat production, dry, and evaporative heat loss as well as Ereq, whole-body sweat rate, and body heat storage) and thermometric (i.e., esophageal, skin, and mean body temperatures) variables as well as HR were presented as an average of the final 15 min of baseline, and the 75-min exercise bout. Ereq was calculated as follows: Ereq ¼ ðM jW ÞjHD ;
where M j W is the rate of metabolic heat production and HD is the rate of dry heat loss. Furthermore, the physiological strain index (PSI) was calculated for end exercise using the following equation adapted from Moran et al. (33): 5
ðTes t jTes 0 Þ ðHRt jHR0 Þ þ 5 ; ð39:5jTes 0 Þ ð180jHR0 Þ
where Tes t and Tes 0 are esophageal temperature measured at end exercise and baseline, respectively. Similarly, HRt is HR measured at end exercise, whereas HR0 is baseline HR. Values for thermal sensation and RPE were presented as an average of two measurements taken 15 min before as well as at the end of baseline and exercise. From these measures, an integrated perceptual strain index (PeSI) was determined based on the index proposed by Tikuisis et al. (43). PeSI gives equal weightings to RPE and thermal sensation and was calculated during baseline and exercise as follows: 5
TS RPEj6 þ5 ; 7 14
where TS is thermal sensation. Statistical analysis. To compare whole-body heat exchange during exercise between conditions, metabolic heat production, dry heat loss, Ereq, evaporative heat loss, wholebody sweat rate, and body heat storage were evaluated with a one-way repeated-measures ANOVA with the factor of condition (four levels: 440 [30], 388 [35], 317 [40], and 258 [45]). To determine the relative influence of exercise performed at each of the four different combinations of metabolic and environmental heat loads yielding a similar
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TABLE 1. Whole-body heat exchange during 75-min of exercise at a fixed evaporative requirement for heat balance (Ereq) but various metabolic heat loads and ambient temperatures. 440 [30] Metabolic heat production (W) Dry heat loss (W) Ereq (W) Evaporative heat loss (W) Whole-body sweat rate (gIminj1) Body heat storage (W)
440 T 47 T 393 T 371 T 9.18 T 27 T
4* 5* 4 5 0.12 5
388 [35] 388 T j20 T 408 T 382 T 9.44 T 23 T
3* 4* 2 7 0.17 6
317 [40] 317 T j83 T 400 T 383 T 9.47 T 17 T
4* 4* 2 5 0.11 3
258 [45] 258 T j141 T 400 T 382 T 9.45 T 18 T
5* 6* 2 6 0.14 4
Values are expressed as means T 95% confidence interval. Values indicate an average of measurements taken during the final 15 min of the 75-min bout of continuous semirecumbent cycling. Ereq, evaporative requirement for heat loss (calculated as metabolic heat production minus dry heat exchange). *Significant difference vs all other conditions, P e 0.05.
RESULTS Urine specific gravity. Participants were adequately hydrated (urine specific gravity G1.020 [2]) before each trial according to measures of urine specific gravity (mean T SD; 440 [30], 1.014 T 0.005; 388 [35], 1.013 T 0.007; 317 [40], 1.010 T 0.007; and 258 [45], 1.010 T 0.006), with no differences observed between conditions (main effect of condition, P = 0.22). Calorimetry. By design, both metabolic heat production (main effect of condition, P G 0.01) and dry heat loss (main effect of condition, P G 0.01) and thereby metabolic and environmental heat loads, respectively, were different, whereas Ereq (main effect of condition, P = 0.15) was similar between 440 [30], 388 [35], 317 [40], and 258 [45] at the end of the
75-min exercise bout (Table 1). In parallel to Ereq, no betweencondition differences in evaporative heat loss (main effect of condition, P = 0.67), whole-body sweat rate (main effect of condition, P = 0.66), or body heat storage (main effect of condition, P = 0.46) were noted (Table 1). Body temperature responses. No differences in baseline esophageal temperature were observed between conditions (all P Q 0.16; Table 2). Esophageal temperature (interaction of time and condition, P G 0.01) increased from baseline values during exercise in all conditions (all P e 0.05) and was greater at end exercise in 440 [30] and 388 [35] relative to both 317 [40] and 258 [45] (all P e 0.05; Fig. 1). In contrast to esophageal temperature, differences in baseline mean skin temperature (interaction of time and condition, P G 0.01) were noted between all conditions (all P G 0.01; Table 2) such that the mean skin temperature was elevated proportionally with increasing ambient temperature. With the exception of 440 [30] (P = 0.03), mean skin temperature did not increase from baseline values to the end of the 75-min exercise bout (all P Q 0.06). However, as with baseline, mean skin temperature was different between all conditions at end exercise (all P e 0.01; Fig. 1). In all conditions, mean body temperature (main effect of time, P = 0.01) increased from baseline (Table 2) to end exercise (440 [30], 37.29-C T 0.03-C; 388 [35], 37.28-C T 0.08-C; 317 [40], 37.19-C T 0.06-C; and 258 [45], 37.14-C T 0.08-C; all P e 0.02). However, no between-condition differences were observed (main effect of condition, P = 0.65). HR and PSI. No baseline differences in HR were observed between conditions (all P Q 0.13; Table 2). In all conditions, HR (interaction of time and condition, P G 0.01) increased during exercise (all P G 0.01) where a greater end exercise response was observed in 440 [30] in comparison with
TABLE 2. Baseline physiological and perceived strain responses. 440 [30] Esophageal temperature (-C) Mean skin temperature (-C) Mean body temperature (-C) HR (bpm) Thermal sensation
37.05 32.84 36.63 67 1.1
T 0.09 T 0.16* T 0.09 T2 T 0.2
388 [35] 37.03 34.01 36.72 64 2.1
T T T T T
0.05 0.14* 0.05 2 0.3
317 [40] 37.04 35.43 36.88 68 2.6
T T T T T
0.04 0.08* 0.04 2 0.2†
258 [45] 36.94 36.48 36.88 69 3.3
T 0.05 T 0.13* T 0.05 T1 T 0.3†‡
Values are expressed as means T 95% confidence interval. Values for body temperatures (i.e., esophageal, mean skin, and mean body) and HR indicate an average of measurements recorded during the final 15 min of the 30-min baseline periods. Values for thermal sensation taken as an average of two measurements taken at the 15- and 30-min points of baseline. P e 0.05. *Significant difference vs all other conditions. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35].
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fixed Ereq on the physiological and perceived strain, we compared esophageal temperature, mean skin temperature, mean body temperature, HR, and thermal sensation using a two-way repeated-measures ANOVA with the factors of time (two levels: baseline and exercise) and condition (four levels). Similarly, PSI, RPE, and PeSI as well as pretrial body mass and urine specific gravity were compared using a one-way repeated-measures ANOVA with the factor of condition (four levels). When a main effect was observed, post hoc comparisons were conducted using two-tailed paired samples t-tests adjusted for multiple comparisons using the Holm–Bonferroni procedure. The level of significance for all analyses was set at P e 0.05. All statistical analyses were completed using the software package SPSS 23.0 for Windows (IBM, Armonk, NY). Values are presented as mean T 95% confidence interval unless otherwise indicated.
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FIGURE 1—Esophageal (black bars) and mean skin (gray bars) temperature responses at end exercise. Values indicate an average of the final 15 min of the 75-min exercise bout and are presented as means T 95% confidence intervals. *Significant difference vs all other conditions. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35]. P e 0.05.
both 317 [40] and 258 [45] (both P G 0.01; Fig. 2). Furthermore, PSI (main effect of condition, P G 0.01) was greater in 440 [30] and 388 [35] relative to both 317 [40] and 258 [45] at the end of the 75-min exercise bout (all P e 0.04; Fig. 2). Perceived strain. Baseline thermal sensation (main effect of condition, P = 0.02; Table 2) was greater in 317 [40] in comparison with 440 [30] and in 258 [45] relative to both 440 [30] and 388 [35] (all P e 0.01). However, during exercise, no differences between conditions were observed (all P Q 0.08; Fig. 3). During exercise, RPE (main effect of condition, P G 0.01) was higher in both 440 [30] and 388 [35] in comparison with 258 [45] (both P e 0.05; Fig. 3). Finally, no differences in PeSI were observed between
conditions during exercise (440 [30], 3.9 T 0.3; 388 [35], 4.2 T 0.3; 317 [40], 4.0 T 0.3; 258 [45], 4.2 T 0.3; main effect of condition, P = 0.14).
DISCUSSION The current study marks the first comparison of wholebody heat loss (via direct calorimetry) and body temperature (via thermometry) responses during exercise at a fixed Ereq but varying metabolic and environmental heat loads. Although comparable evaporative heat loss, whole-body sweat rate, and mean body temperature responses were observed between conditions, core and skin temperature responses
FIGURE 2—HR (black bars) and PSI (gray bars) at end exercise. Values indicate an average of the final 15 min of the 75-min exercise bout and are presented as means T 95% confidence intervals. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35]. P e 0.05.
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were not uniform. Specifically, higher metabolic heat loads (and therefore lower ambient temperatures) during exercise were associated with greater elevations in core temperature (as determined by changes in esophageal temperature), whereas higher environmental heat loads resulted in comparatively higher mean skin temperatures. Furthermore, we observed greater HR and physiological strain (as estimated by PSI) in conditions with higher rates of metabolic heat production. However, differences in the contribution of metabolic and environmental heat load had minimal influence on indices of perceived strain (i.e., thermal sensation, RPE, and PeSI) during exercise. Thus, it is important that future work assessing core and skin temperature responses consider any differences in metabolic and environmental heat load on factors influencing compartmental heat distribution (e.g., regional tissue blood flow). Recent work has shown that during exercise in compensable conditions, 1) whole-body evaporative heat loss and thereby heat storage is dependent primarily on Ereq, and 2) the increase in core temperature is determined by metabolic heat production (i.e., exercise intensity) (7–10,12,20,39). Because these observations have typically been made during exercise bouts of varying intensity (based on metabolic heat production) performed in similar environments, it follows that a relationship between metabolic heat production and core temperature implies a relationship between Ereq and core temperature. However, despite similar mean body temperature and heat storage responses between conditions, we demonstrate varying core and skin temperature responses at a fixed Ereq. Specifically, core temperature was greater in conditions with higher rates of metabolic heat production and therefore lower ambient temperatures, whereas the mean skin temperature response followed the reverse pattern (Fig. 1). Given that the primary avenue of heat transfer in the body is convection via the blood, these responses may be
PHYSIOLOGICAL RESPONSES TO EXERCISE AT A FIXED Ereq
attributable to differences in regional (i.e., muscle, skin) tissue perfusion (42). To support the level of metabolism required for muscular contraction, increases in exercise intensity, such as that occurring in conditions with higher rates of metabolic heat production, are associated with proportional increases in blood flow to the active musculature, which can reach levels as high as approximately 20–25 LIminj1 during cycling (3,4,22,34). However, as ambient temperature increases, blood flow must also be diverted to the skin (up to approximately 4–6 LIminj1 during exercise [23]) to support dry heat exchange with the environment (11,23,37). Therefore, although previous as well as the current findings support the idea that Ereq is the primary determinant of evaporative heat loss and thereby the change in mean body temperature during exercise (10,14,24), the core and skin temperature responses at a given Ereq are influenced by the relative magnitude of environmental and metabolic heat load, likely secondary to differences in tissue perfusion (i.e., muscle and skin blood flows). During exercise, elevations in cardiac output, and thereby HR, act to increase perfusion of the active muscles as well as the skin to support muscular contraction and heat dissipation, respectively. The findings of the current study, however, suggest that increases in metabolic or environmental heat load do not confer similar changes in cardiovascular and physiological strain (as evaluated by changes in HR and PSI, respectively). Specifically, in the current study, conditions with greater metabolic heat loads (i.e., 440 [30] and 388 [35]) tended to induce greater increases in HR and PSI in comparison with conditions with greater environmental heat loads (i.e., 317 [40] and 258 [45]) (Fig. 2). This likely stems from differences in blood flow demands induced by increases in exercise intensity and ambient heat load as well as, in more extreme situations, the perfusion capacity of
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FIGURE 3—Thermal sensation (black bars) and RPE (gray bars) at the end exercise. Values are presented as means T 95% confidence interval. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35]. P e 0.05.
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the muscle and skin vascular beds. During cycling, active muscle blood flow can increase from approximately 3 LIminj1 at rest to 20–25 LIminj1 at maximal exertion, with values of approximately 7 and 14 LIminj1 observed at 20 and 50% ˙ O2peak, respectively (exercise intensities comparable to the V current study) (3,4,34). By contrast, skin blood flow increases from about 0.3 LIminj1 at rest to a maximal of 8 LIminj1 during passively induced heat stress (11,23,37). However, as noted earlier, peak elevations during exercise are limited to 4– 6 LIminj1, or 50%–75% of the maximal response observed during whole-body passive heating (23). As can be seen, in situations of maximal blood flow requirements, the capacity of the skin to increase perfusion is relatively modest compared with that of the active musculature. Thus, even at a fixed Ereq, greater levels of metabolic heat production and therefore exercise intensity will result in greater levels of physiological strain relative to increases in environmental heat load. Perspectives. Our findings clearly indicate that the relative influence of exercise intensity and environmental conditions must be considered when comparing past and future research regarding the thermal and cardiovascular responses to exercise. Furthermore, the current study has important implications for evaluating different populations groups, even when the aforementioned factors are tightly controlled. For instance, aging is associated with altered control of muscle (19,22) and skin (23,24) blood flow, with reductions in both observed during exercise in older adults relative to their younger counterparts (19,23,24). Thus, it is possible that differences in core and skin temperature responses may be observed between young and older adults matched for biophysical factors during exercise in a given environment, even if whole-body heat storage is similar between groups. Indeed, we have demonstrated age-related differences in the relationship between increases in core temperature (via thermometry) and the change in body heat storage (determined with direct calorimetry) during exercise (27,41). Not only do the current findings have important implications for the assessment of hyperthermia and physiological strain during exercise, they also carry significant effect to practical situations such as protecting workers performing physically demanding tasks in hot conditions. For instance, the most recommended heat exposure guidelines for use by industry, the threshold limit values of the American Conference of Governmental and Industrial Hygienists, prescribe work-to-rest ratios that consider estimated work intensity (i.e., metabolic heat production) and environmental conditions with the main goal of maintaining core temperature within predefined limits (1,21,30). However, we show that these factors can influence the measured increase in core temperature (despite similar changes in mean body temperature). A recent analysis of the threshold limit values suggested that the prescribed work-to-rest ratios also influenced compartmental heat distribution, with the imposition of extended recovery periods resulting in an attenuated increase in core temperature (again, despite no differences in mean body temperature) (32). Altogether, the current study as well as
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previous work (32,42) highlights the fact that marked differences in compartmental heat exchange can occur during exercise secondary to differences in work intensity, work-to-rest allocation, and/or environmental conditions. Research is clearly required to elucidate the precise relationships between these factors as well as age not only to better define human thermoregulation but also to protect at-risk individuals during physical activity and/or exposure to hot environments. Considerations. It is important to consider the current study in the context of Lind’s (28,29) seminal work on the prescriptive zone—that is, the range of ambient conditions over which core temperature is influenced primarily by alterations in metabolic rate, with little or no influence of environmental heat load. Our findings appear to be in line with this concept at first glance. However, because the nature of the prescriptive zone is dependent on metabolic rate (and therefore heat load) (28), the conditions used fell outside of the critical ambient temperature range (except the 440 [30] condition). In combination with the observed similarities in whole-body heat loss and heat storage between conditions (Table 1), our findings support the proposed influence of differences in the relative magnitude of metabolic and environmental heat load on body heat distribution and the core and skin temperature responses. These findings should be considered in future work assessing the thermoregulatory responses to exercise between different environmental and/or exercise conditions as well as when comparing responses of different populations, given that factors such training status, age, and sex may modulate regional blood flow (e.g., muscle, skin) (23,26) and, therefore, body heat distribution (42) during exercise. To ensure an Ereq of 400 W, a heat load at which heat balance is easily achievable for most individuals, the exercise intensities used in the current study were relatively low ˙ O2peak). However, the corresponding absolute (25%–43% V intensities would be considered a moderate to heavy effort by the American Conference of Governmental and Industrial Hygienists (1), intensities in which a large portion work (~1/3 of the work shift) occurs in the mining (25) and electrical utilities (31) industries. Regardless, for a better understanding of whole-body hyperthermia and heat distribution in humans, future work is still required to determine the relative influence of metabolic and environmental heat load on the physiological responses to high to maximal intensity exercise, especially given the large differences in perfusion capacity of the active musculature and the skin (3,4,11,34,37).
CONCLUSION The current study demonstrates different core and skin temperature responses during exercise at a fixed Ereq but various metabolic and environmental heat loads. Specifically, although the core temperature response increased with the level of metabolic heat production, the mean skin temperature response increased with ambient temperature (and therefore environmental heat load). Furthermore, the HR and
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the PSI responses to exercise were greater at elevated exercise intensities. Altogether, this study demonstrates that the physiological responses to exercise vary depending on the relative contribution of metabolically and environmentally derived heat to total heat load, likely because of differences in compartmental heat storage as well as absolute tissue blood flow demands. Future work should be conducted to determine the relationship between these as well as previously described factors on the physiological responses observed during physical activity in the heat. This research was supported by the Ontario Ministry of Labour and in part by the Electrical Power Research Institute and a Natural
Sciences and Engineering Research Council Discovery Grant (RGPIN-06313-2014) and Discovery Grants Program—Accelerator Supplements (RGPAS-462252-2014) (funds held by G. P. Kenny). G. P. Kenny is supported by a University of Ottawa Research Chair Award. R. D. Meade is supported by a Natural Sciences and Engineering Research Council of Canada Alexander Graham Bell graduate scholarship (CGS-D). The authors greatly appreciate all of the volunteers for taking their time to participate in this study. They sincerely thank Mr. Dallon Lamarche and Ms. Pegah Akbari for their contribution to data collection as well as Mr. Michael Sabino of Can-Trol Environmental Systems Limited (Markham, ON, Canada) for his support. The results of the current report are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The authors declare no conflict of interest. The results of the present study do not constitute endorsement by the American College of Sports Medicine or the funding agencies mentioned.
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16. Gagnon D, Kenny GP. Sex modulates whole-body sudomotor thermosensitivity during exercise. J Physiol. 2011;589(Pt 24):6205–17. 17. Gagnon D, Kenny GP. Does sex have an independent effect on thermoeffector responses during exercise in the heat? J Physiol. 2012;590(23):5963–73. 18. Gagnon D, Kenny GP. Sex differences in thermoeffector responses during exercise at fixed requirements for heat loss. J Appl Physiol (1985). 2012;113(5):746–57. 19. Hearon CM Jr, Dinenno FA. Regulation of skeletal muscle blood flow during exercise in ageing humans. J Physiol. 2016;594(8): 2261–73. 20. Jay O, Cramer MN. A new approach for comparing thermoregulatory responses of subjects with different body sizes. Temperature (Austin). 2015;2(1):42–3. 21. Jay O, Kenny GP. Heat exposure in the Canadian workplace. Am J Ind Med. 2010;53(8):842–53. 22. Joyner MJ, Casey DP. Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs. Physiol Rev. 2015;95(2):549–601. 23. Kenney WL, Johnson JM. Control of skin blood flow during exercise. Med Sci Sports Exerc. 1992;24(3):303–12. 24. Kenny GP, Jay O. Thermometry, calorimetry, and mean body temperature during heat stress. Compr Physiol. 2013;3(4):1689–719. 25. Kenny GP, Vierula M, Mate´ J, Beaulieu F, Hardcastle SG, Reardon F. A field evaluation of the physiological demands of miners in Canada"s deep mechanized mines. J Occup Environ Hyg. 2012;9(8):491–501. 26. Koch DW, Newcomer SC, Proctor DN. Blood flow to exercising limbs varies with age, gender, and training status. Can J Appl Physiol. 2005;30(5):554–75. 27. Larose J, Boulay P, Sigal RJ, Wright HE, Kenny GP. Age-related decrements in heat dissipation during physical activity occur as early as the age of 40. PLoS One. 2013;8(12):e83148. 28. Lind AR. A physiological criterion for setting thermal environmental limits for everyday work. J Appl Physiol. 1963;18:51–6. 29. Lind AR. Effect of individual variation on upper limit of prescriptive zone of climates. J Appl Physiol. 1970;28(1):57–62. 30. Malchaire JB. The TLV work-rest regimens for occupational exposure to heat: a review of their development. Ann Occup Hyg. 1979;22(1):55–62. 31. Meade RD, Lauzon M, Poirier MP, Flouris AD, Kenny GP. The physical demands of electrical utilities work in North America. J Occup Environ Hyg. 2016;13(1):60–70. 32. Meade RD, Poirier MP, Flouris AD, Hardcastle SG, Kenny GP. Do the threshold limit values for work in hot conditions adequately protect workers? Med Sci Sports Exerc. 2016;48(6):1187–96. 33. Moran DS, Shitzer A, Pandolf KB. A physiological strain index to evaluate heat stress. Am J Physiol. 1998;275(1 Pt 2): R129–34.
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1. ACGIH. Heat stress and strain. In: Documentation of the Threshold Limit Values for Physical Agents. Cincinnati, OH: ACGIH; 2013. pp. 206–15. 2. American College of Sports Medicine, Sawka MN, Burke LM, et al. American College of Sports Medicine Position Stand: exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377–90. 3. Calbet JA, Gonzalez-Alonso J, Helge JW, et al. Cardiac output and leg and arm blood flow during incremental exercise to exhaustion on the cycle ergometer. J Appl Physiol (1985). 2007;103(3): 969–78. 4. Calbet JA, Gonza´lez-Alonso J, Helge JW, et al. Central and peripheral hemodynamics in exercising humans: leg vs arm exercise. Scand J Med Sci Sports. 2015;25(4 Suppl):144–57. 5. Charkoudian N, Stachenfeld N. Sex hormone effects on autonomic mechanisms of thermoregulation in humans. Auton Neurosci. 2016;196:75–80. 6. Charkoudian N, Stachenfeld NS. Reproductive hormone influences on thermoregulation in women. Compr Physiol. 2014;4(2): 793–804. 7. Cramer MN, Bain AR, Jay O. Local sweating on the forehead, but not forearm, is influenced by aerobic fitness independently of heat balance requirements during exercise. Exp Physiol. 2012; 97(5):572–82. 8. Cramer MN, Jay O. Selecting the correct exercise intensity for unbiased comparisons of thermoregulatory responses between groups of different mass and surface area. J Appl Physiol (1985). 2014;116(9):1123–32. 9. Cramer MN, Jay O. Explained variance in the thermoregulatory responses to exercise: the independent roles of biophysical and fitness/fatness-related factors. J Appl Physiol (1985). 2015; 119(9):982–9. 10. Cramer MN, Jay O. Biophysical aspects of human thermoregulation during heat stress. Auton Neurosci. 2016;196:3–13. 11. Crandall CG, Gonza´lez-Alonso J. Cardiovascular function in the heat-stressed human. Acta Physiol (Oxf). 2010;199(4):407–23. 12. Dervis S, Coombs GB, Chaseling GK, Filingeri D, Smoljanic J, Jay O. A comparison of thermoregulatory responses to exercise between mass-matched groups with large differences in body fat. J Appl Physiol (1985). 2016;120(6):615–23. 13. Dubois D, Dubois EF. A formula to estimate the approximate surface area if height and weight are known. Arch Intern Med. 1916;17:863–71. 14. Gagnon D, Jay O, Kenny GP. The evaporative requirement for heat balance determines whole-body sweat rate during exercise under conditions permitting full evaporation. J Physiol. 2013; 591(11):2925–35. 15. Gagnon D, Jay O, Lemire B, Kenny GP. Sex-related differences in evaporative heat loss: the importance of metabolic heat production. Eur J Appl Physiol. 2008;104(5):821–9.
39. Smoljani( J, Morris NB, Dervis S, Jay O. Running economy, not aerobic fitness, independently alters thermoregulatory responses during treadmill running. J Appl Physiol (1985). 2014;117(12):1451–9. 40. Snellen JW, Chang KS, Smith W. Technical description and performance characteristics of a human whole-body calorimeter. Med Biol Eng Comput. 1983;21(1):9–20. 41. Stapleton JM, Larose J, Simpson C, Flouris AD, Sigal RJ, Kenny GP. Do older adults experience greater thermal strain during heat waves? Appl Physiol Nutr Metab. 2014;39(3):292–8. 42. Taylor NA, Tipton MJ, Kenny GP. Considerations for the measurement of core, skin and mean body temperatures. J Therm Biol. 2014;46:72–101. 43. Tikuisis P, McLellan TM, Selkirk G. Perceptual versus physiological heat strain during exercise-heat stress. Med Sci Sports Exerc. 2002;34(9):1454–61.
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34. Mortensen SP, Dawson EA, Yoshiga CC, et al. Limitations to systemic and locomotor limb muscle oxygen delivery and uptake during maximal exercise in humans. J Physiol. 2005;566(Pt 1): 273–85. 35. Ramanathan NL. A new weighting system for mean surface temperature of the human body. J Appl Physiol. 1964;19(3):531–3. 36. Reardon FD, Leppik KE, Wegmann R, Webb P, Ducharme MB, Kenny GP. The Snellen human calorimeter revisited, reengineered and upgraded: design and performance characteristics. Med Biol Eng Comput. 2006;44(8):721–8. 37. Rowell LB, Brengelmann GL, Murray JA. Cardiovascular responses to sustained high skin temperature in resting man. J Appl Physiol. 1969;27(5):673–80. 38. Siri WE. The gross composition of the body. Adv Biol Med Phys. 1956;4:239–80.
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MEADE, R. D., and G. P. KENNY. Are All Heat Loads Created Equal? Med. Sci. Sports Exerc., Vol. 49, No. 9, pp. 1796–1804, 2017. Purpose: We evaluated physiological responses during exercise at a fixed evaporative requirement for heat balance (Ereq) but varying combinations of metabolic and environmental heat load. Methods: Nine healthy, physically active males (age: 46 T 8 yr) performed four experimental sessions consisting of 75 min of semirecumbent cycling at various ambient temperatures. Whole-body dry heat loss (direct calorimetry) was monitored continuously as was heat production (indirect calorimetry), which was adjusted to achieve an Ereq of 400 W. The resultant metabolic heat productions and ambient temperatures for the sessions were as follows: (i) 440 W and 30-C (440 [30]), (ii) 388 W and 35-C (388 [35]), (iii) 317 W and 40-C (317 [40]), and (iv) 258 W and 45-C (258 [45]). Whole-body evaporative heat loss was determined via direct calorimetry. Esophageal (Tes) and mean skin (Tsk) temperatures as well as HR were monitored continuously. Mean body temperature (Tb) was calculated from Tes and Tsk. Physiological strain index (PSI) was determined from Tes and HR. Results: Endexercise evaporative heat loss and Tb were similar between conditions (both P Q 0.48). Tes was greater in 440 [30] (37.67-C T 0.04-C) and 388 [35] (37.58-C T 0.07-C) relative to both 317 [40] (37.35-C T 0.06-C) and 258 [45] (37.20-C T 0.07-C; all P e 0.05). Further, Tsk was different between each condition (440 [30], 33.85-C T 0.16-C; 388 [35], 34.53-C T 0.08-C; 317 [40], 35.67-C T 0.07-C; and 258 [45], 36.54-C T 0.08-C; all P G 0.01). In 440 [30], HR was elevated by about 13 and 18 bpm relative to 317 [40] and 258 [45], respectively (both P G 0.01). Finally, PSI was greater in both 440 [30] and 388 [35] compared with 317 [40] and 258 [45] (all P e 0.04). Conclusions: Exercise at a fixed Ereq resulted in similar evaporative heat loss and Tb. However, the Tes, Tsk, HR, and PSI responses varied depending on the relative contribution of metabolic and environmental heat load. Key Words: SWEATING, CORE TEMPERATURE, HEAT BALANCE, THERMOREGULATION, HEAT STRAIN, EXERCISE
D
uring exercise and/or exposure to hot conditions, body heat storage and thereby the change in mean body temperature are determined by the difference between the rate of metabolic heat production (i.e., heat produced as a by-product of metabolism) and the rate of heat loss via dry (i.e., conduction, convection, and radiation) and evaporative mechanisms (24). However, dry heat loss is driven by the temperature gradient between the skin and the environment and can therefore be negative (i.e., net dry heat gain) when ambient temperature exceeds that of the skin (~34-C) (24). For simplicity, human heat balance is often described in terms of the evaporative requirement for heat balance (Ereq), which is calculated as the rate of metabolic heat production minus dry heat loss (7–10,12,14,20,39). In fact, Ereq explains approximately 71%–90% of the variation in evaporative heat loss and thereby whole-body sweat rate (assuming all produced sweat is evaporated) during compensable exercise (10,14).
Recent work has compared thermoregulatory responses between groups of varying body morphology, sex, and/or fitness in context of the biophysical determinants of human heat balance (7–10,12,15,17,18,20,39). From these studies, it is clear that the core temperature response during exercise in compensable conditions is strongly related to metabolic heat production (i.e., exercise intensity), whereas wholebody sweat rate is determined by Ereq (7–10,12,20,39). It is important to note that most of these studies compared physiological responses between exercise bouts in which exercise intensity (and therefore metabolic heat production) was modified but ambient conditions (and therefore environmental heat loads) were similar. Thus, the observed relationship between core temperature and heat production implies a similar relationship also exists between core temperature and Ereq (after adjusting for between-group differences in body mass and/or surface area). It is possible, however, that even at a fixed Ereq, the body temperature (i.e., core and skin) responses during exercise are influenced by the relative magnitude of environmental and metabolic heat loads secondary to alteration in the perfusion of various tissue beds (i.e., core, muscle and skin) and thereby compartmental heat distribution (the primary avenue of heat transfer within the body is convection via blood) (42). If the environmental contribution to Ereq is large (i.e., high ambient temperatures), local and reflex mechanisms induce an increase in skin blood flow, which can reach approximately 8 LIminj1, resulting in elevated skin temperature that acts to
Address for correspondence: Glen P. Kenny, Ph.D., School of Human Kinetics, University of Ottawa, 125 University, Room 367 Montpetit Hall Ottawa, Ontario, Canada K1N 6N5; E-mail: [email protected]. Submitted for publication December 2016. Accepted for publication April 2017. 0195-9131/17/4909-1796/0 MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ Copyright Ó 2017 by the American College of Sports Medicine DOI: 10.1249/MSS.0000000000001309
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METHODS Ethical approval. The current experimental protocol was approved by the University of Ottawa Health Sciences and Science Research Ethic Board and conforms to the Declaration of Helsinki. Written informed consent was obtained from all volunteers before their participation in the study. Participants. Nine habitually active (Q3 dIwkj1 of structured physical activity, Q30 min in duration) males volunteered for one screening visit and four experimental sessions. Participants were nonsmoking, with no history of metabolic or cardiovascular disease. The physical characteristics (mean T SD) of the participants were as follows: age, 46 T 8 yr; height, 1.76 T 0.07 m; body mass, 82.4 T 10.1 kg; body surface area, 1.99 T 0.15 m2; body fat percentage, 21.6% T ˙ O2peak), 45.1 T 5.7%; and peak oxygen consumption (V j1 j1 8.5 mLIkg Imin . Males were chosen given the potential modulation in the control of body temperature associated with fluctuations in female sex hormone levels (i.e., estrogen and progesterone) (5,6) that occur throughout the menstrual cycle. Furthermore, females exhibit a reduced capacity to dissipate heat independent of sex-related differences in body morphology and aerobic capacity in comparison with males (15–18). Experimental design. All participants completed one screening and four experimental sessions. Participants arrived at the laboratory on the day of each session after consuming a small breakfast (i.e., dry toast and juice) that did not include tea or coffee. Before each session, participants were also asked to avoid alcohol consumption and to not perform any exercise for 24 h before experimentation.
PHYSIOLOGICAL RESPONSES TO EXERCISE AT A FIXED Ereq
Furthermore, to ensure participants reported to the laboratory well hydrated, they were instructed to drink 500 mL of water the night before as well as the morning of each session. Body height, mass, density, and fat percentage as well ˙ O2peak were determined during the screening session. as V Body height was measured using an eye-level stadiometer (model 2391; Detecto, Webb City, MO), whereas body mass was measured using a high-performance digital weighing terminal (model CBU150X; Mettler Toledo Inc., Mississauga, ON, Canada). Body surface area was subsequently calculated from the measurements of body height and mass (13). Body density was measured using the hydrostatic weighing technique and subsequently used to calculate body fat percentage (38). A maximal incremental semirecumbent cycling protocol ˙ O2peak. While being moniwas used for the assessment of V tored by an automated indirect calorimetry system (MCD Medgraphics Ultima Series; MGC Diagnostics, Saint Paul, MN), the participants cycled for 1 min at a starting external work rate of 100 W. The workload was increased 20 WIminj1 every min thereafter until the participant could not maintain a pedaling cadence of at least 50 rpm or reached volitional fa˙ O2peak was taken as the highest rate of oxygen contigue. V sumption averaged for 30 s. Upon arrival to the laboratory on the day of the experimental sessions, participants provided a urine sample for the measurement of urine-specific gravity after which a measurement of body mass was obtained. Thereafter, the participants entered the calorimeter chamber regulated to the ambient temperature for that day"s trial (see next section). Participants were then instrumented with skin temperature sensors after which they remained seated in a semirecumbent position. Following 30 min of baseline measurements, the participants performed a 75-min semirecumbent cycle protocol at a fixed Ereq of 400 W. Importantly, this Ereq was chosen to ensure heat balance was achieved by all participants and that body temperature (i.e., core, skin, and mean body) and cardiovascular (i.e., HR) responses were stable at end exercise. In total, each participant completed four counterbalanced sessions at ambient temperatures of 30-C, 35-C, 40-C, and 45-C wherein dry heat exchange and metabolic heat production were monitored every minute and the latter was adjusted to achieve the target Ereq. Thus, each trial was performed at a fixed Ereq but varying environmental (achieved by adjusting ambient temperature) and metabolic (achieved by adjusting external work) heat loads. As a result, the respective metabolic heat productions (mean T SD) and ambient temperatures for each trial were as follows: 1) 440 T ˙ O2peak) and 30-C (440 W 19 W (equivalent to 43% T 8% V ˙ O2peak) and [30]), 2) 388 T 12 W (equivalent to 40% T 7% V 35-C (388 W [30]), 3) 317 T 16 W (equivalent to 31% T 7% ˙ O2peak) and 40-C (317 [40]), and 4) 258 T 22 W (equivalent V ˙ O2peak) and 45-C (258 [45]). Throughout to 25% T 6% V exercise, participants wore only athletic shorts and provided subjective ratings of thermal sensation and perceived exertion at 15-min intervals. After the protocol, a measurement of nude body mass was taken.
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modulate dry heat exchange (11,23,37). By contrast, in a situation with greater metabolic heat production (i.e., high exercise intensity), skin blood flow similarly increases, albeit not to the same degree as when environmental heat load is high (23). In this case, a large proportion of available blood flow is diverted to the active musculature (the primary site of endogenous heat production), to support the increased metabolic demand. This may result in comparatively greater increases in core temperature and cardiovascular strain as the capacity for increases in muscle blood flow (up to ~20–25 LIminj1 during cycling [3,4,22,34]) is larger than that of the skin (3,4,11,22,23,34,37). To the best of our knowledge, however, a direct evaluation of the relationship between Ereq and body temperature and cardiovascular responses to exercise is lacking. The purpose of the current study was to evaluate wholebody sweat rate as well as physiological (i.e., thermal and cardiovascular) and perceived (i.e., thermal sensation and RPE) strain during exercise at a fixed Ereq, but varying contributions of metabolic and environmental heat load. It was hypothesized that although evaporative heat loss and whole-body sweat rate would be consistent at a fixed Ereq, greater increases in core temperature and HR would be observed during conditions with greater exercise intensity and thereby metabolic heat production (and lower environmental heat loads).
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Measurements. The modified Snellen whole-body direct airflow calorimeter was used to measure the rate of evaporative heat loss and dry (radiation + convection + conduction) heat exchange (36). Calorimeter inflow and outflow values for absolute humidity and air temperature were collected at 8-s intervals. Absolute humidity was measured using high-precision dew point hygrometry (RH Systems model 373H, Albuquerque, NM), whereas air temperature was measured using high-precision resistance temperature detectors (Black Stack model 1560; Fluke Calibration, American Fork, UT). Air mass flow through the calorimeter was measured by differential thermometry over a known heat source placed in the effluent air stream. The data for absolute humidity, air temperature, and air mass flow were displayed and recorded on a personal computer with LabVIEW software (Version 7.0; National Instruments, Austin, TX). The rate of evaporative heat loss was calculated using the calorimeter outflow–inflow difference in absolute humidity, multiplied by the air mass flow and the latent heat of vaporization of sweat (2426 JIgj1 sweat). The rate of dry heat exchange was calculated using the calorimeter outflow–inflow difference in air temperature, multiplied by the air mass flow and specific heat capacity of air (1005 JIkg airj1I-Cj1). The rate of body heat storage was calculated as metabolic heat production minus total (evaporative + dry) heat loss. It should be noted that a relatively high mass flow of air is circulated through the calorimeter, ensuring the evaporation of the entirety of sweat produced from the skin (36,40). Therefore, whole-body sweat rate in grams per minute was calculated as evaporative heat loss (in W) multiplied by 60 s and divided by 2426 JIgj1 sweat. The rate of metabolic energy expenditure was simultaneously measured using indirect calorimetry. Expired gas samples were drawn from a 6-L fluted mixing box located within the calorimeter and subsequently analyzed for oxygen (O2) and carbon dioxide (CO2) concentrations using electrochemical gas analyzers (AMETEK model S-3A/1 and CD 3A; Applied Electrochemistry, Pittsburgh, PA). Thereafter, expired air was recycled back into the calorimeter chamber to account for respiratory dry and evaporative heat loss. Gas mixtures of 17% O2 and 4% CO2 (balance nitrogen) were used to calibrate the gas analyzers before each experimental session. A 3-L syringe was also used to calibrate the turbine ventilometer. Ereq, calculated as metabolic heat production minus dry heat loss, was monitored at 1-min intervals throughout the trial. Esophageal temperature was measured using a pediatric thermocouple temperature probe (Mon-a-therm; Mallinckrodt Medical, St. Louis, MO) inserted 40 cm past the entrance of the nostril. Skin temperature was measured at four locations (i.e., upper back, chest, thigh, and calf ) over the right side of the body using 0.3-mm-diameter T-type (copper/constantan) thermocouples (Concept Engineering, Old Saybrook, CT). Mean skin temperature was subsequently calculated using the following weightings based on those suggested by Ramanathan (35): 30% upper back, 30% chest, 20% thigh, and 20% leg. All temperature data were collected using a
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data acquisition module (HP Agilent model 3497A; Agilent Technologies Canada Inc., Mississauga, ON, Canada) at a sampling rate of 15 s and simultaneously displayed and recorded in spreadsheet format on a personal computer with LabVIEW software (Version 7.0, National Instruments). HR was recorded continuously and stored at a sampling rate of 15 s using a Polar coded WearLink transmitter, Polar RS400 interface, and Polar Trainer 5 software (Polar Electro, Oy, Finland). Thermal sensation was determined using the ASHRAE eight-point scale ranging from 0 (neutral) to 7 (very, very hot), whereas RPE was measured using the Borg 14-point scale ranging from 6 (no exertion) to 20 (maximal exertion). Urine-specific gravity was assessed from the urine samples acquired before each trial and compared with current guidelines to ensure adequate hydration (equivalent to a urinespecific gravity of G1.020 [2]). Data analysis. Values for the calorimetric (i.e., metabolic heat production, dry, and evaporative heat loss as well as Ereq, whole-body sweat rate, and body heat storage) and thermometric (i.e., esophageal, skin, and mean body temperatures) variables as well as HR were presented as an average of the final 15 min of baseline, and the 75-min exercise bout. Ereq was calculated as follows: Ereq ¼ ðM jW ÞjHD ;
where M j W is the rate of metabolic heat production and HD is the rate of dry heat loss. Furthermore, the physiological strain index (PSI) was calculated for end exercise using the following equation adapted from Moran et al. (33): 5
ðTes t jTes 0 Þ ðHRt jHR0 Þ þ 5 ; ð39:5jTes 0 Þ ð180jHR0 Þ
where Tes t and Tes 0 are esophageal temperature measured at end exercise and baseline, respectively. Similarly, HRt is HR measured at end exercise, whereas HR0 is baseline HR. Values for thermal sensation and RPE were presented as an average of two measurements taken 15 min before as well as at the end of baseline and exercise. From these measures, an integrated perceptual strain index (PeSI) was determined based on the index proposed by Tikuisis et al. (43). PeSI gives equal weightings to RPE and thermal sensation and was calculated during baseline and exercise as follows: 5
TS RPEj6 þ5 ; 7 14
where TS is thermal sensation. Statistical analysis. To compare whole-body heat exchange during exercise between conditions, metabolic heat production, dry heat loss, Ereq, evaporative heat loss, wholebody sweat rate, and body heat storage were evaluated with a one-way repeated-measures ANOVA with the factor of condition (four levels: 440 [30], 388 [35], 317 [40], and 258 [45]). To determine the relative influence of exercise performed at each of the four different combinations of metabolic and environmental heat loads yielding a similar
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TABLE 1. Whole-body heat exchange during 75-min of exercise at a fixed evaporative requirement for heat balance (Ereq) but various metabolic heat loads and ambient temperatures. 440 [30] Metabolic heat production (W) Dry heat loss (W) Ereq (W) Evaporative heat loss (W) Whole-body sweat rate (gIminj1) Body heat storage (W)
440 T 47 T 393 T 371 T 9.18 T 27 T
4* 5* 4 5 0.12 5
388 [35] 388 T j20 T 408 T 382 T 9.44 T 23 T
3* 4* 2 7 0.17 6
317 [40] 317 T j83 T 400 T 383 T 9.47 T 17 T
4* 4* 2 5 0.11 3
258 [45] 258 T j141 T 400 T 382 T 9.45 T 18 T
5* 6* 2 6 0.14 4
Values are expressed as means T 95% confidence interval. Values indicate an average of measurements taken during the final 15 min of the 75-min bout of continuous semirecumbent cycling. Ereq, evaporative requirement for heat loss (calculated as metabolic heat production minus dry heat exchange). *Significant difference vs all other conditions, P e 0.05.
RESULTS Urine specific gravity. Participants were adequately hydrated (urine specific gravity G1.020 [2]) before each trial according to measures of urine specific gravity (mean T SD; 440 [30], 1.014 T 0.005; 388 [35], 1.013 T 0.007; 317 [40], 1.010 T 0.007; and 258 [45], 1.010 T 0.006), with no differences observed between conditions (main effect of condition, P = 0.22). Calorimetry. By design, both metabolic heat production (main effect of condition, P G 0.01) and dry heat loss (main effect of condition, P G 0.01) and thereby metabolic and environmental heat loads, respectively, were different, whereas Ereq (main effect of condition, P = 0.15) was similar between 440 [30], 388 [35], 317 [40], and 258 [45] at the end of the
75-min exercise bout (Table 1). In parallel to Ereq, no betweencondition differences in evaporative heat loss (main effect of condition, P = 0.67), whole-body sweat rate (main effect of condition, P = 0.66), or body heat storage (main effect of condition, P = 0.46) were noted (Table 1). Body temperature responses. No differences in baseline esophageal temperature were observed between conditions (all P Q 0.16; Table 2). Esophageal temperature (interaction of time and condition, P G 0.01) increased from baseline values during exercise in all conditions (all P e 0.05) and was greater at end exercise in 440 [30] and 388 [35] relative to both 317 [40] and 258 [45] (all P e 0.05; Fig. 1). In contrast to esophageal temperature, differences in baseline mean skin temperature (interaction of time and condition, P G 0.01) were noted between all conditions (all P G 0.01; Table 2) such that the mean skin temperature was elevated proportionally with increasing ambient temperature. With the exception of 440 [30] (P = 0.03), mean skin temperature did not increase from baseline values to the end of the 75-min exercise bout (all P Q 0.06). However, as with baseline, mean skin temperature was different between all conditions at end exercise (all P e 0.01; Fig. 1). In all conditions, mean body temperature (main effect of time, P = 0.01) increased from baseline (Table 2) to end exercise (440 [30], 37.29-C T 0.03-C; 388 [35], 37.28-C T 0.08-C; 317 [40], 37.19-C T 0.06-C; and 258 [45], 37.14-C T 0.08-C; all P e 0.02). However, no between-condition differences were observed (main effect of condition, P = 0.65). HR and PSI. No baseline differences in HR were observed between conditions (all P Q 0.13; Table 2). In all conditions, HR (interaction of time and condition, P G 0.01) increased during exercise (all P G 0.01) where a greater end exercise response was observed in 440 [30] in comparison with
TABLE 2. Baseline physiological and perceived strain responses. 440 [30] Esophageal temperature (-C) Mean skin temperature (-C) Mean body temperature (-C) HR (bpm) Thermal sensation
37.05 32.84 36.63 67 1.1
T 0.09 T 0.16* T 0.09 T2 T 0.2
388 [35] 37.03 34.01 36.72 64 2.1
T T T T T
0.05 0.14* 0.05 2 0.3
317 [40] 37.04 35.43 36.88 68 2.6
T T T T T
0.04 0.08* 0.04 2 0.2†
258 [45] 36.94 36.48 36.88 69 3.3
T 0.05 T 0.13* T 0.05 T1 T 0.3†‡
Values are expressed as means T 95% confidence interval. Values for body temperatures (i.e., esophageal, mean skin, and mean body) and HR indicate an average of measurements recorded during the final 15 min of the 30-min baseline periods. Values for thermal sensation taken as an average of two measurements taken at the 15- and 30-min points of baseline. P e 0.05. *Significant difference vs all other conditions. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35].
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fixed Ereq on the physiological and perceived strain, we compared esophageal temperature, mean skin temperature, mean body temperature, HR, and thermal sensation using a two-way repeated-measures ANOVA with the factors of time (two levels: baseline and exercise) and condition (four levels). Similarly, PSI, RPE, and PeSI as well as pretrial body mass and urine specific gravity were compared using a one-way repeated-measures ANOVA with the factor of condition (four levels). When a main effect was observed, post hoc comparisons were conducted using two-tailed paired samples t-tests adjusted for multiple comparisons using the Holm–Bonferroni procedure. The level of significance for all analyses was set at P e 0.05. All statistical analyses were completed using the software package SPSS 23.0 for Windows (IBM, Armonk, NY). Values are presented as mean T 95% confidence interval unless otherwise indicated.
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FIGURE 1—Esophageal (black bars) and mean skin (gray bars) temperature responses at end exercise. Values indicate an average of the final 15 min of the 75-min exercise bout and are presented as means T 95% confidence intervals. *Significant difference vs all other conditions. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35]. P e 0.05.
both 317 [40] and 258 [45] (both P G 0.01; Fig. 2). Furthermore, PSI (main effect of condition, P G 0.01) was greater in 440 [30] and 388 [35] relative to both 317 [40] and 258 [45] at the end of the 75-min exercise bout (all P e 0.04; Fig. 2). Perceived strain. Baseline thermal sensation (main effect of condition, P = 0.02; Table 2) was greater in 317 [40] in comparison with 440 [30] and in 258 [45] relative to both 440 [30] and 388 [35] (all P e 0.01). However, during exercise, no differences between conditions were observed (all P Q 0.08; Fig. 3). During exercise, RPE (main effect of condition, P G 0.01) was higher in both 440 [30] and 388 [35] in comparison with 258 [45] (both P e 0.05; Fig. 3). Finally, no differences in PeSI were observed between
conditions during exercise (440 [30], 3.9 T 0.3; 388 [35], 4.2 T 0.3; 317 [40], 4.0 T 0.3; 258 [45], 4.2 T 0.3; main effect of condition, P = 0.14).
DISCUSSION The current study marks the first comparison of wholebody heat loss (via direct calorimetry) and body temperature (via thermometry) responses during exercise at a fixed Ereq but varying metabolic and environmental heat loads. Although comparable evaporative heat loss, whole-body sweat rate, and mean body temperature responses were observed between conditions, core and skin temperature responses
FIGURE 2—HR (black bars) and PSI (gray bars) at end exercise. Values indicate an average of the final 15 min of the 75-min exercise bout and are presented as means T 95% confidence intervals. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35]. P e 0.05.
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were not uniform. Specifically, higher metabolic heat loads (and therefore lower ambient temperatures) during exercise were associated with greater elevations in core temperature (as determined by changes in esophageal temperature), whereas higher environmental heat loads resulted in comparatively higher mean skin temperatures. Furthermore, we observed greater HR and physiological strain (as estimated by PSI) in conditions with higher rates of metabolic heat production. However, differences in the contribution of metabolic and environmental heat load had minimal influence on indices of perceived strain (i.e., thermal sensation, RPE, and PeSI) during exercise. Thus, it is important that future work assessing core and skin temperature responses consider any differences in metabolic and environmental heat load on factors influencing compartmental heat distribution (e.g., regional tissue blood flow). Recent work has shown that during exercise in compensable conditions, 1) whole-body evaporative heat loss and thereby heat storage is dependent primarily on Ereq, and 2) the increase in core temperature is determined by metabolic heat production (i.e., exercise intensity) (7–10,12,20,39). Because these observations have typically been made during exercise bouts of varying intensity (based on metabolic heat production) performed in similar environments, it follows that a relationship between metabolic heat production and core temperature implies a relationship between Ereq and core temperature. However, despite similar mean body temperature and heat storage responses between conditions, we demonstrate varying core and skin temperature responses at a fixed Ereq. Specifically, core temperature was greater in conditions with higher rates of metabolic heat production and therefore lower ambient temperatures, whereas the mean skin temperature response followed the reverse pattern (Fig. 1). Given that the primary avenue of heat transfer in the body is convection via the blood, these responses may be
PHYSIOLOGICAL RESPONSES TO EXERCISE AT A FIXED Ereq
attributable to differences in regional (i.e., muscle, skin) tissue perfusion (42). To support the level of metabolism required for muscular contraction, increases in exercise intensity, such as that occurring in conditions with higher rates of metabolic heat production, are associated with proportional increases in blood flow to the active musculature, which can reach levels as high as approximately 20–25 LIminj1 during cycling (3,4,22,34). However, as ambient temperature increases, blood flow must also be diverted to the skin (up to approximately 4–6 LIminj1 during exercise [23]) to support dry heat exchange with the environment (11,23,37). Therefore, although previous as well as the current findings support the idea that Ereq is the primary determinant of evaporative heat loss and thereby the change in mean body temperature during exercise (10,14,24), the core and skin temperature responses at a given Ereq are influenced by the relative magnitude of environmental and metabolic heat load, likely secondary to differences in tissue perfusion (i.e., muscle and skin blood flows). During exercise, elevations in cardiac output, and thereby HR, act to increase perfusion of the active muscles as well as the skin to support muscular contraction and heat dissipation, respectively. The findings of the current study, however, suggest that increases in metabolic or environmental heat load do not confer similar changes in cardiovascular and physiological strain (as evaluated by changes in HR and PSI, respectively). Specifically, in the current study, conditions with greater metabolic heat loads (i.e., 440 [30] and 388 [35]) tended to induce greater increases in HR and PSI in comparison with conditions with greater environmental heat loads (i.e., 317 [40] and 258 [45]) (Fig. 2). This likely stems from differences in blood flow demands induced by increases in exercise intensity and ambient heat load as well as, in more extreme situations, the perfusion capacity of
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FIGURE 3—Thermal sensation (black bars) and RPE (gray bars) at the end exercise. Values are presented as means T 95% confidence interval. †Significant difference vs 440 [30]. ‡Significant difference vs 388 [35]. P e 0.05.
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the muscle and skin vascular beds. During cycling, active muscle blood flow can increase from approximately 3 LIminj1 at rest to 20–25 LIminj1 at maximal exertion, with values of approximately 7 and 14 LIminj1 observed at 20 and 50% ˙ O2peak, respectively (exercise intensities comparable to the V current study) (3,4,34). By contrast, skin blood flow increases from about 0.3 LIminj1 at rest to a maximal of 8 LIminj1 during passively induced heat stress (11,23,37). However, as noted earlier, peak elevations during exercise are limited to 4– 6 LIminj1, or 50%–75% of the maximal response observed during whole-body passive heating (23). As can be seen, in situations of maximal blood flow requirements, the capacity of the skin to increase perfusion is relatively modest compared with that of the active musculature. Thus, even at a fixed Ereq, greater levels of metabolic heat production and therefore exercise intensity will result in greater levels of physiological strain relative to increases in environmental heat load. Perspectives. Our findings clearly indicate that the relative influence of exercise intensity and environmental conditions must be considered when comparing past and future research regarding the thermal and cardiovascular responses to exercise. Furthermore, the current study has important implications for evaluating different populations groups, even when the aforementioned factors are tightly controlled. For instance, aging is associated with altered control of muscle (19,22) and skin (23,24) blood flow, with reductions in both observed during exercise in older adults relative to their younger counterparts (19,23,24). Thus, it is possible that differences in core and skin temperature responses may be observed between young and older adults matched for biophysical factors during exercise in a given environment, even if whole-body heat storage is similar between groups. Indeed, we have demonstrated age-related differences in the relationship between increases in core temperature (via thermometry) and the change in body heat storage (determined with direct calorimetry) during exercise (27,41). Not only do the current findings have important implications for the assessment of hyperthermia and physiological strain during exercise, they also carry significant effect to practical situations such as protecting workers performing physically demanding tasks in hot conditions. For instance, the most recommended heat exposure guidelines for use by industry, the threshold limit values of the American Conference of Governmental and Industrial Hygienists, prescribe work-to-rest ratios that consider estimated work intensity (i.e., metabolic heat production) and environmental conditions with the main goal of maintaining core temperature within predefined limits (1,21,30). However, we show that these factors can influence the measured increase in core temperature (despite similar changes in mean body temperature). A recent analysis of the threshold limit values suggested that the prescribed work-to-rest ratios also influenced compartmental heat distribution, with the imposition of extended recovery periods resulting in an attenuated increase in core temperature (again, despite no differences in mean body temperature) (32). Altogether, the current study as well as
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previous work (32,42) highlights the fact that marked differences in compartmental heat exchange can occur during exercise secondary to differences in work intensity, work-to-rest allocation, and/or environmental conditions. Research is clearly required to elucidate the precise relationships between these factors as well as age not only to better define human thermoregulation but also to protect at-risk individuals during physical activity and/or exposure to hot environments. Considerations. It is important to consider the current study in the context of Lind’s (28,29) seminal work on the prescriptive zone—that is, the range of ambient conditions over which core temperature is influenced primarily by alterations in metabolic rate, with little or no influence of environmental heat load. Our findings appear to be in line with this concept at first glance. However, because the nature of the prescriptive zone is dependent on metabolic rate (and therefore heat load) (28), the conditions used fell outside of the critical ambient temperature range (except the 440 [30] condition). In combination with the observed similarities in whole-body heat loss and heat storage between conditions (Table 1), our findings support the proposed influence of differences in the relative magnitude of metabolic and environmental heat load on body heat distribution and the core and skin temperature responses. These findings should be considered in future work assessing the thermoregulatory responses to exercise between different environmental and/or exercise conditions as well as when comparing responses of different populations, given that factors such training status, age, and sex may modulate regional blood flow (e.g., muscle, skin) (23,26) and, therefore, body heat distribution (42) during exercise. To ensure an Ereq of 400 W, a heat load at which heat balance is easily achievable for most individuals, the exercise intensities used in the current study were relatively low ˙ O2peak). However, the corresponding absolute (25%–43% V intensities would be considered a moderate to heavy effort by the American Conference of Governmental and Industrial Hygienists (1), intensities in which a large portion work (~1/3 of the work shift) occurs in the mining (25) and electrical utilities (31) industries. Regardless, for a better understanding of whole-body hyperthermia and heat distribution in humans, future work is still required to determine the relative influence of metabolic and environmental heat load on the physiological responses to high to maximal intensity exercise, especially given the large differences in perfusion capacity of the active musculature and the skin (3,4,11,34,37).
CONCLUSION The current study demonstrates different core and skin temperature responses during exercise at a fixed Ereq but various metabolic and environmental heat loads. Specifically, although the core temperature response increased with the level of metabolic heat production, the mean skin temperature response increased with ambient temperature (and therefore environmental heat load). Furthermore, the HR and
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the PSI responses to exercise were greater at elevated exercise intensities. Altogether, this study demonstrates that the physiological responses to exercise vary depending on the relative contribution of metabolically and environmentally derived heat to total heat load, likely because of differences in compartmental heat storage as well as absolute tissue blood flow demands. Future work should be conducted to determine the relationship between these as well as previously described factors on the physiological responses observed during physical activity in the heat. This research was supported by the Ontario Ministry of Labour and in part by the Electrical Power Research Institute and a Natural
Sciences and Engineering Research Council Discovery Grant (RGPIN-06313-2014) and Discovery Grants Program—Accelerator Supplements (RGPAS-462252-2014) (funds held by G. P. Kenny). G. P. Kenny is supported by a University of Ottawa Research Chair Award. R. D. Meade is supported by a Natural Sciences and Engineering Research Council of Canada Alexander Graham Bell graduate scholarship (CGS-D). The authors greatly appreciate all of the volunteers for taking their time to participate in this study. They sincerely thank Mr. Dallon Lamarche and Ms. Pegah Akbari for their contribution to data collection as well as Mr. Michael Sabino of Can-Trol Environmental Systems Limited (Markham, ON, Canada) for his support. The results of the current report are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The authors declare no conflict of interest. The results of the present study do not constitute endorsement by the American College of Sports Medicine or the funding agencies mentioned.
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