Whole body sweat rate prediction: outdoor running and cycling exercise

Our aim was to develop and validate separate whole body sweat rate prediction equations for moderate to high-intensity outdoor cycling and running, using simple measured or estimated activity and environmental inputs. Across two collection sites in Australia, 182 outdoor running trials and 158 outdoor cycling trials were completed at a wet-bulb globe temperature ranging from ∼15°C to ∼29°C, with ∼60-min whole body sweat rates measured in each trial. Data were randomly separated into model development (running: 120; cycling: 100 trials) and validation groups (running: 62; cycling: 58 trials), enabling proprietary prediction models to be developed and then validated. Running and cycling models were also developed and tested when locally measured environmental conditions were substituted with participants' subjective ratings for black globe temperature, wind speed, and humidity. The mean absolute error for predicted sweating rate was 0.03 and 0.02 L·h-1 for running and cycling models, respectively. The 95% confidence intervals for running (+0.44 and -0.38 L·h-1) and cycling (+0.45 and -0.42 L·h-1) were within acceptable limits for an equivalent change in total body mass over 3 h of ±2%. The individual variance in observed sweating described by the predictive models was 77% and 60% for running and cycling, respectively. Substituting measured environmental variables with subjective assessments of climatic characteristics reduced the variation in observed sweating described by the running model by up to ∼25%, but only by ∼2% for the cycling model. These prediction models are publicly accessible (https://sweatratecalculator.com) and can guide individualized hydration management in advance of outdoor running and cycling.NEW & NOTEWORTHY We report the development and validation of new proprietary whole body sweat rate prediction models for outdoor running and outdoor cycling using simple activity and environmental inputs. Separate sweat rate models were also developed and tested for situations where all four environmental parameters are not available, and some must be subsequently estimated by the user via a simple rating scale. All models are freely accessible through an online calculator: https://sweatratecalculator.com. These models, via the online calculator, will enable individualized hydration management for training or recreational cycling or running in an outdoor environment.

The effect of 8-day oral taurine supplementation on thermoregulation during low-intensity exercise at fixed heat production in hot conditions of incremental humidity

PURPOSE

To determine the effect of taurine supplementation on sweating and core temperature responses, including the transition from compensable to uncompensable heat stress, during prolonged low-intensity exercise of a fixed-heat production (~ 200W/m2) in hot conditions (37.5 °C), at both fixed and incremental vapour-pressure.

METHODS

Fifteen females (n = 3) and males (n = 12; 27 ± 5 years, 78 ± 9 kg, V ˙ O2max 50.3 ± 7.8 mL/kg/min), completed a treadmill walking protocol (~ 200W/m2 heat production [Ḣprod]) in the heat (37.5 ± 0.1 °C) at fixed-(16-mmHg) and ramped-humidity (∆1.5-mmHg/5-min) following 1 week of oral taurine supplementation (50 mg/kg/bm) or placebo, in a double-blind, randomised, cross-over design. Participants were assessed for whole-body sweat loss (WBSL), local sweat rate (LSR), sweat gland activation (SGA), core temperature (Tcore), breakpoint of compensability (Pcrit) and calorimetric heat transfer components. Plasma volume and plasma taurine concentrations were established through pre- and post-trial blood samples.

RESULTS

Taurine supplementation increased WBSL by 26.6% and 5.1% (p = 0.035), LSR by 15.5% and 7.8% (p = 0.013), SGA (1 × 1 cm) by 32.2% and 29.9% (p < 0.001) and SGA (3 × 3 cm) by 22.1% and 17.1% (p = 0.015) during the fixed- and ramped-humidity exercise periods, respectively. Evaporative heat loss was enhanced by 27% (p = 0.010), heat-storage reduced by 72% (p = 0.024) and Pcrit was greater in taurine vs placebo (25.0-mmHg vs 21.7-mmHg; p = 0.002).

CONCLUSION

Taurine supplementation increased sweating responses during fixed Ḣprod in hot conditions, prior to substantial heat strain and before the breakpoint of compensability, demonstrating improved thermoregulatory capacity. The enhanced evaporative cooling and reduced heat-storage delayed the subsequent upward inflection in Tcore-represented by a greater Pcrit-and offers a potential dietary supplementation strategy to support thermoregulation.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

Sex differences in adaptation to intermittent post-exercise sauna bathing in trained middle-distance runners

Background

The purpose of this study was to investigate the effect of sex on the efficacy of intermittent post-exercise sauna bathing to induce heat acclimation and improve markers of temperate exercise performance in trained athletes.

Methods

Twenty-six trained runners (16 female; mean ± SD, age 19 ± 1 years, V̇O_2max F: 52.6 ± 6.9 mL⋅kg⁻¹⋅min⁻¹, M: 64.6 ± 2.4 mL⋅kg⁻¹⋅min⁻¹) performed a running heat tolerance test (30 min, 9 km⋅h⁻¹/2% gradient, 40 °C/40%RH; HTT) and temperate (18 °C) exercise tests (maximal aerobic capacity [V̇O_2max] and lactate profile) pre and post 3 weeks of normal exercise training plus 29 ± 1 min post-exercise sauna bathing (101–108 °C) 3 ± 1 times per week.

Results

Females and males exhibited similar reductions (interactions p > 0.05) in peak rectal temperature (− 0.3 °C; p < 0.001), skin temperature (− 0.9 °C; p < 0.001) and heart rate (− 9 beats·min⁻¹; p = 0.001) during the HTT at post- vs pre-intervention. Only females exhibited an increase in active sweat glands on the forearm (measured via modified iodine technique; F: + 57%, p < 0.001; M: + 1%, p = 0.47). Conversely, only males increased forearm blood flow (measured via venous occlusion plethysmography; F: + 31%, p = 0.61; M: + 123%; p < 0.001). Females and males showed similar (interactions p > 0.05) improvements in V̇O_2max (+ 5%; p = 0.02) and running speed at 4 mmol·L⁻¹ blood lactate concentration (+ 0.4 km·h⁻¹; p = 0.001).

Conclusions

Three weeks of post-exercise sauna bathing effectively induces heat acclimation in females and males, though possibly amid different thermoeffector adaptations. Post-exercise sauna bathing is also an effective ergogenic aid for both sexes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40798-021-00342-6.

Sweat rate and sweat composition following active or passive heat re-acclimation: A pilot study

The purpose of this study was to investigate local sweat rate (LSR) and sweat composition before and after active or passive heat re-acclimation (HRA). Fifteen participants completed four standardized heat stress tests (HST): before and after ten days of controlled hyperthermia (CH) heat acclimation (HA), and before and after five days of HRA. Each HST consisted of 35 min of cycling at 1.5W·kg⁻¹ body mass (33°C and 65% relative humidity), followed by a graded exercise test. For HRA, participants were re-exposed to either CH (CH-CH, n = 6), hot water immersion (water temperature ~40°C for 40 min; CH-HWI, n = 5) or control (CH-CON, n = 4). LSR, sweat sodium, chloride, lactate and potassium concentrations were determined on the arm and back. LSR increased following HA (arm +18%; back +41%, P ≤  0.03) and HRA (CH-CH: arm +31%; back +45%; CH-HWI: arm +65%; back +49%; CH-CON arm +11%; back +11%, P ≤ 0.021). Sweat sodium, chloride and lactate decreased following HA (arm 25–34; back 21–27%, P < 0.001) and HRA (CH-CH: arm 26–54%; back 20–43%; CH-HWI: arm 9–49%; back 13–29%; CH-CON: arm 1–3%, back 2–5%, P < 0.001). LSR increases on both skin sites were larger in CH-CH and CH-HWI than CH-CON (P ≤ 0.010), but CH-CH and CH-HWI were not different (P ≥ 0.148). Sweat sodium and chloride conservation was larger in CH-CH than CH-HWI and CH-CON on the arm and back, whilst CH-HWI and CH-CON were not different (P ≥ 0.265). These results suggest that active HRA leads to similar increases in LSR, but more conservation of sweat sodium and chloride than passive HRA.

Abbreviations: ANOVA: Analysis of variance; ATP: Adenosine triphosphate; BSA (m²): Body surface area; CH: Controlled hyperthermia; CH-CH: Heat re-acclimation by controlled hyperthermia; CH-CON: Control group (no heat re-acclimation); CH-HWI: Heat re-acclimation by hot water immersion; CV (%): Coefficient of variation; dt (min): Duration of a stimulus; F: Female; GEE: Generalized estimating equations; HA: Heat acclimation; HRA : Heat re-acclimation; HST: Heat stress test; LSR (mg·cm⁻²·min⁻¹) : Local sweat rate; LOD (mmol·L⁻¹): Limit of detection; M: Male; mx (mg): Mass of x; RH (%): Relative humidity; RT: Recreationally trained; SA (cm²): Surface area; t (min): Time; T: Trained; T_sk (°C): Skin temperature; T_re (°C): Rectal temperature; USG : Urine specific gravity; VO_2peak (mL·kg⁻¹·min⁻¹): Peak oxygen uptake; WBSL (L): Whole-body sweat loss; WBSR (L·h⁻¹): Whole-body sweat rate

Effect of regular precooling on adaptation to training in the heat

PURPOSE

This study investigated whether regular precooling would help to maintain day-to-day training intensity and improve 20-km cycling time trial (TT) performed in the heat. Twenty males cycled for 10 day × 60 min at perceived exertion equivalent to 15 in the heat (35 °C, 50% relative humidity), preceded by no cooling (CON, n = 10) or 30-min water immersion at 22 °C (PRECOOL, n = 10).

METHODS

19 participants (n = 9 and 10 for CON and PRECOOL, respectively) completed heat stress tests (25-min at 60% [Formula: see text] and 20-km TT) before and after heat acclimation.

RESULTS

Changes in mean power output (∆MPO, P = 0.024) and heart rate (∆HR, P = 0.029) during heat acclimation were lower for CON (∆MPO - 2.6 ± 8.1%, ∆HR - 7 ± 7 bpm), compared with PRECOOL (∆MPO + 2.9 ± 6.6%, ∆HR - 1 ± 8 bpm). HR during constant-paced cycling was decreased from the pre-acclimation test in both groups (P < 0.001). Only PRECOOL demonstrated lower rectal temperature (T_re) during constant-paced cycling (P = 0.002) and lower T_re threshold for sweating (P = 0.042). However, skin perfusion and total sweat output did not change in either CON or PRECOOL (all P > 0.05). MPO (P = 0.016) and finish time (P = 0.013) for the 20-km TT were improved in PRECOOL but did not change in CON (P = 0.052 for MPO, P = 0.140 for finish time).

CONCLUSION

Precooling maintains day-to-day training intensity and does not appear to attenuate adaptation to training in the heat.

Regional influence of nitric oxide on cutaneous vasodilatation and sweating during exercise-heat stress in young men

NEW FINDINGS

What is the central question of this study? Do regional differences exist in nitric oxide synthase (NOS)-dependent cutaneous vasodilatation and sweating during exercise-heat stress in young men. What is the main finding and its importance? Exercise-induced increases in cutaneous vasodilatation and sweating were greater on the chest and upper back compared to the forearm, although the NOS contribution to cutaneous vasodilatation was similar across all regions. Conversely, there was a greater NOS-dependent rate of change in sweating on the chest compared to the forearm, with a similar trend on the back.

ABSTRACT

While it is established that nitric oxide synthase (NOS) is an important modulator of forearm cutaneous vasodilatation and sweating during an exercise-heat stress in young men, it remains unclear if regional differences exist in this response. In 15 habitually active young men (24 ± 4 (SD) years), cutaneous vascular conductance (CVC) and local sweat rate (LSR) were assessed at three body regions. On each of the dorsal forearm, chest and upper-back (trapezius), sites were continuously perfused with either (1) lactated Ringer solution (control) or (2) 10 Mm N^ω -nitro-l-arginine (l-NNA, NOS inhibitor), via microdialysis. Participants rested in the heat (35°C) for ∼75 min, followed by 60 min of semi-recumbent cycling performed at a fixed rate of heat production of 200 W m^-2 (equivalent to ∼42% V ̇ O 2 peak ). During exercise, the chest and upper-back regions showed higher CVC and LSR responses relative to the forearm (all P < 0.05). Within each region, l-NNA attenuated CVC and LSR relative to control (all P < 0.05). However, the NOS contribution was not different across regions for the rate of change and plateau for CVC or for the LSR plateau (all P > 0.05). Conversely, there was a greater NOS contribution to the rate of change for LSR at the chest relative to the forearm (P < 0.05) with a similar trend for the back. In habitually active young men, NOS-dependent cutaneous vasodilatation was similar across regions while the NOS contribution to LSR was greater on the chest relative to the forearm. These findings advance our understanding of the mechanisms influencing regional variations in cutaneous vasodilatation and sweating during an exercise-heat stress.

Improved neural control of body temperature following heat acclimation in humans

KEY POINTS

With the advent of more frequent extreme heat events, adaptability to hot environments will be crucial for the survival of many species, including humans. However, the mechanisms that mediate human heat adaptation have remained elusive. We tested the hypothesis that heat acclimation improves the neural control of body temperature. Skin sympathetic nerve activity, comprising the efferent neural signal that activates heat loss thermoeffectors, was measured in healthy adults exposed to passive heat stress before and after a 7 day heat acclimation protocol. Heat acclimation reduced the activation threshold for skin sympathetic nerve activity, leading to an earlier activation of cutaneous vasodilatation and sweat production. These findings demonstrate that heat acclimation improves the neural control of body temperature in humans.

ABSTRACT

Heat acclimation improves autonomic temperature regulation in humans. However, the mechanisms that mediate human heat adaptation remain poorly understood. The present study tested the hypothesis that heat acclimation improves the neural control of body temperature. Body temperatures, skin sympathetic nerve activity, cutaneous vasodilatation, and sweat production were measured in 14 healthy adults (nine men and five women, aged 27 ± 5 years) during passive heat stress performed before and after a 7 day heat acclimation protocol. Heat acclimation increased whole-body sweat rate [+0.54 L h^-1 (0.32, 0.75), P < 0.01] and reduced resting core temperature [-0.29°C (-0.40, -0.18), P < 0.01]. During passive heat stress, the change in mean body temperature required to activate skin sympathetic nerve activity was reduced [-0.21°C (-0.34, -0.08), P < 0.01] following heat acclimation. The earlier activation of skin sympathetic nerve activity resulted in lower activation thresholds for cutaneous vasodilatation [-0.18°C (-0.35, -0.01), P = 0.04] and local sweat rate [-0.13°C (-0.24, -0.01), P = 0.03]. These results demonstrate that heat acclimation leads to an earlier activation of the neural efferent outflow that activates the heat loss thermoeffectors of cutaneous vasodilatation and sweating.

Regional contributions of nitric oxide synthase to cholinergic cutaneous vasodilatation and sweating in young men

NEW FINDINGS

What is the central question of this study? We evaluated whether regional variations exist in NO-dependent cutaneous vasodilatation and sweating during cholinergic stimulation. What is the main finding and its importance? Peak cutaneous vasodilatation and sweating were greater on the torso than the forearm. Furthermore, we found that NO was an important modulator of cholinergic cutaneous vasodilatation, but not sweating, across body regions, with a greater contribution of NO to cutaneous vasodilatation in the limb compared with the torso. These findings advance our understanding of the mechanisms influencing regional variations in cutaneous vasodilator and sweating responses to pharmacological stimulation.

ABSTRACT

Regional variations in cutaneous vasodilatation and sweating exist across the body. Nitric oxide (NO) is an important modulator of these heat loss responses in the forearm. However, whether regional differences in NO-dependent cutaneous vasodilatation and sweating exist remain uncertain. In 14 habitually active young men (23 ± 4 years of age), cutaneous vascular conductance (CVC_%max ) and local sweat rates were assessed at six skin sites. On each of the dorsal forearm, chest and upper back (trapezius), sites were continuously perfused with either lactated Ringer solution (control) or 10 mm N^ω -nitro-l-arginine (l-NNA; an NO synthase inhibitor) dissolved in Ringer solution, via microdialysis. At all sites, cutaneous vasodilatation and sweating were induced by co-administration of the cholinergic agonist methacholine (1, 10, 100, 1000 and 2000 mm; 25 min per dose) followed by 50 mm sodium nitroprusside (20-25 min) to induce maximal vasodilatation. The l-NNA attenuated CVC_%max relative to the control conditions for all regions (all P < 0.05), and NO-dependent vasodilatation was greater at the forearm compared with the back and chest (both P < 0.05). Furthermore, maximal vasodilatation was higher at the back and chest relative to the forearm (both P < 0.05). Conversely, l-NNA had negligible effects on sweating across the body (all P > 0.05). Peak local sweat rate was higher at the back relative to the forearm (P < 0.05), with a similar trend observed for the chest. In habitually active young men, NO-dependent cholinergic cutaneous vasodilatation varied across the body, and the contribution to cholinergic sweating was negligible. These findings advance our understanding of the mechanisms influencing regional variations in cutaneous vasodilatation and sweating during pharmacological stimulation.

Pediatric Thermoregulation: Considerations in the Face of Global Climate Change

Predicted global climate change, including rising average temperatures, increasing airborne pollution, and ultraviolet radiation exposure, presents multiple environmental stressors contributing to increased morbidity and mortality. Extreme temperatures and more frequent and severe heat events will increase the risk of heat-related illness and associated complications in vulnerable populations, including infants and children. Historically, children have been viewed to possess inferior thermoregulatory capabilities, owing to lower sweat rates and higher core temperature responses compared to adults. Accumulating evidence counters this notion, with limited child–adult differences in thermoregulation evident during mild and moderate heat exposure, with increased risk of heat illness only at environmental extremes. In the context of predicted global climate change, extreme environmental temperatures will be encountered more frequently, placing children at increased risk. Thermoregulatory and overall physiological strain in high temperatures may be further exacerbated by exposure to/presence of physiological and environmental stressors including pollution, ultraviolet radiation, obesity, diabetes, associated comorbidities, and polypharmacy that are more commonly occurring at younger ages. The aim of this review is to revisit fundamental differences in child–adult thermoregulation in the face of these multifaceted climate challenges, address emerging concerns, and emphasize risk reduction strategies for the health and performance of children in the heat.

Physiology of sweat gland function: The roles of sweating and sweat composition in human health

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

Nine-, but Not Four-Days Heat Acclimation Improves Self-Paced Endurance Performance in Females

Although emerging as a cost and time efficient way to prepare for competition in the heat, recent evidence indicates that "short-term" heat acclimation (<7 days) may not be sufficient for females to adapt to repeated heat stress. Furthermore, self-paced performance following either short-term, or longer (>7 days) heat acclimation has not been examined in a female cohort. Therefore, the aim of this study was to investigate self-paced endurance performance in hot conditions following 4- and 9-days of a high-intensity isothermic heat acclimation protocol in a female cohort. Eight female endurance athletes (mean ± SD, age 27 ± 5 years, mass 61 ± 5 kg, VO2peak 47 ± 6 ml⋅kg⋅min-1) performed 15-min self-paced cycling time trials in hot conditions (35°C, 30%RH) before (HTT1), and after 4-days (HTT2), and 9-days (HTT3) isothermic heat acclimation (HA, with power output manipulated to increase and maintain rectal temperature (T rec) at ∼38.5°C for 90-min cycling in 40°C, 30%RH) with permissive dehydration. There were no significant changes in distance cycled (p = 0.47), mean power output (p = 0.55) or cycling speed (p = 0.44) following 4-days HA (i.e., from HTT1 to HTT2). Distance cycled (+3.2%, p = 0.01; +1.8%, p = 0.04), mean power output (+8.1%, p = 0.01; +4.8%, p = 0.05) and cycling speed (+3.0%, p = 0.01; +1.6%, p = 0.05) were significantly greater in HTT3 than in HTT1 and HTT2, respectively. There was an increase in the number of active sweat glands per cm2 in HTT3 as compared to HTT1 (+32%; p = 0.02) and HTT2 (+22%; p < 0.01), whereas thermal sensation immediately before HTT3 decreased ("Slightly Warm," p = 0.03) compared to ratings taken before HTT1 ("Warm") in 35°C, 30%RH. Four-days HA was insufficient to improve performance in the heat in females as observed following 9-days HA.

Upper body sweat mapping provides evidence of relative sweat redistribution towards the periphery following hot-dry heat acclimation

Purpose: Produce a detailed upper-body sweat map and evaluate changes in gross and regional sweating rates (RSR) and distribution following heat acclimation (HA). Methods: Six males (25 ± 4 yrs) completed six consecutive HA days (45°C, 20% rh) requiring 90 minutes intermittent exercise to maintain a 1.4°C rectal temperature (Tre) rise. Pre- and post-HA upper-body RSR were measured at 55% (Intensity-1; I1) and 75% VO2 max (Intensity-2; I2) using a modified absorbent technique. Results: From day one to six of HA, work rate increased (n.s.), heart rate, Tre, and skin temperature were similar, and gross sweat loss (GSL) increased (P < 0.001). During pre and post-HA experiments, relative workloads were similar (Pre-I1 54 ± 3, Post-I1 57 ± 5%VO2max; Pre-I2 73 ± 4, Post-I2 76 ± 7%VO2max). Post-HA GSL was significantly higher (Pre 449 ± 90 g.m-2 h-1, Post 546 g.m-2 h-1; P < 0.01). Highest RSR were observed on the central back both pre and post-HA at I1 (pre 854 ± 269 post 1178 ± 402g.m-2 h-1) and I2 (pre 1221 ± 351 post 1772 ± 396 g.m-2 h-1). Absolute RSR increased significantly in 12 (I1) to 14 (I2) of the 17 regions. Ratio data indicated significant post-HA relative RSR redistribution, with decreased relative contributions to whole-body sweating on the back, chest staying the same and arms increasing. Conclusions: Hot-dry HA significantly increased GSL in aerobically trained males at I2. Absolute RSR significantly increased in I1 and I2, with a preferential relative redistribution towards the periphery of the upper-body.

Menstrual cycle phase does not modulate whole body heat loss during exercise in hot, dry conditions

Menstrual cycle phase has long been thought to modulate thermoregulatory function. However, information pertaining to the effects of menstrual phase on time-dependent changes in whole body dry and evaporative heat exchange during exercise-induced heat stress and the specific heat load at which menstrual phase modulates whole body heat loss remained unavailable. We therefore used direct calorimetry to continuously assess whole body dry and evaporative exchange in 12 habitually active, non-endurance-trained, eumenorrheic women [21 ± 3 (SD) yr] within the early-follicular, late-follicular, and midluteal menstrual phases during three 30-min bouts of cycling at increasing fixed exercise intensities of 40% (Low), 55% (Moderate), and 70% (High) peak oxygen uptake, each followed by a 15-min recovery, in hot, dry conditions (40°C, 15% relative humidity). This model elicited equivalent rates of metabolic heat production among menstrual phases ( P = 0.80) of ~250 (Low), ~340 (Moderate), and ~430 W (High). However, dry and evaporative heat exchange and the resulting changes in net heat loss (dry ± evaporative heat exchange) were similar among phases (all P > 0.05), with net heat loss averaging 216 ± 43 (Low), 287 ± 63 (Moderate), and 331 ± 75 W (High) across phases. Accordingly, cumulative body heat storage (summation of heat production and loss) across all exercise bouts was similar among phases ( P = 0.55), averaging 464 ± 122 kJ. For some time, menstrual cycle phase has been thought to modulate heat dissipation; however, we show that menstrual cycle phase does not influence the contribution of whole body dry and evaporative heat exchange or the resulting changes in net heat loss or body heat storage, irrespective of the heat load. NEW & NOTEWORTHY Menstrual phase has long been thought to modulate thermoregulatory function in eumenorrheic women during exercise-induced heat stress. Contrary to that perception, we show that when assessed in young, non-endurance-trained women within the early-follicular, late-follicular, and midluteal phases during three incremental exercise-induced heat loads in hot, dry conditions, menstrual phase does not modify whole body dry and evaporative heat exchange or the resulting changes in body heat storage, regardless of the heat load employed.

Uncoupling psychological from physiological markers of heat acclimatization in a military context

Heat acclimatization may help personnel who travel to areas with a hot climate (WBGT > 27 °C), making them operationally more efficient and performant through improvements in physiological and psychological parameters. Their work-related physical activities may aid active heat acclimatization. However, it is unknown whether adding physical training to improve adaptation is effective, particularly if there is sufficient time for full acclimatization, classically reached after 15 days. Thirty French soldiers (Training group, T) performed a progressive and moderate (from three to five 8-min running sets at 50-60% of their speed at VO_2max with 4-min periods of active recovery in between) aerobic training program upon arriving at their base in United Arab Emirates (~40 °C and 20% RH). A control group (30 soldiers; No Training, NT) continued to perform only their usual outdoor military activities (~5 h d^-1). A field heat stress test (HST: three 8-min running sets at 50% of the speed at VO_2max) was performed before (D0), during (D10), and after (D15) the heat acclimatization period to assess physiological and psychological changes. An 8-km trial in battledress was then performed at D17. Although physiological modifications were mostly similar (p < 0.001 for all) for both groups (rectal temperature at the end of the HST: -0.58 ± 0.51 vs -0.53 ± 0.40 °C, HR at the end of the HST: -21 ± 12 vs -19 ± 9 bpm, and sweat osmolality: -47 ± 30 vs -26 ± 32 mOsmol.l^-1 between D15 and D0 for T and NT groups, respectively), thermal discomfort (-31 ± 4 vs -11 ± 5 mm between D15 and D0, p = 0.001) and rates of perceived exertion (-3.0 ± 0.4 vs -1.4 ± 0.3 D15 and D0, p = 0.001) were much lower in the T than NT group during the HST. HST-induced modifications in facial temperature only decreased in the T group (-1.08 ± 0.28 between D15 and D0, p < 0.001). Moreover, there was a difference in perceived thermal discomfort during the 8-km trial (40 ± 20 vs 55 ± 22 mm for the T and NT groups, respectively, p = 0.010). Thus, a 15-day, low-volume training regimen during a mission in a hot and dry environment has a modest impact on physiological adaptation but strongly decreases the perceived strain of exertion and climate potentially via greater reductions in facial temperature, even during a classical operational physical task in a military context.

Age Modulates Physiological Responses during Fan Use under Extreme Heat and Humidity

PURPOSE

We examined the effect of electric fan use on cardiovascular and thermoregulatory responses of nine young (26 ± 3 yr) and nine aged (68 ± 4 yr) adults exposed to extreme heat and humidity.

METHODS

While resting at a temperature of 42°C, relative humidity increased from 30% to 70% in 2% increments every 5 min. On randomized days, the protocol was repeated without or with fan use. HR, core (Tcore) and mean skin (Tsk) temperatures were measured continuously. Whole-body sweat loss was measured from changes in nude body weight. Other measures of cardiovascular (cardiac output), thermoregulatory (local cutaneous and forearm vascular conductance, local sweat rate), and perceptual (thermal and thirst sensations) responses were also examined.

RESULTS

When averaged over the entire protocol, fan use resulted in a small reduction of HR (-2 bpm, 95% confidence interval [CI], -8 to 3), and slightly greater Tcore (+0.05°C; 95% CI, -0.13 to 0.23) and Tsk (+0.03°C; 95% CI, -0.36 to 0.42) in young adults. In contrast, fan use resulted in greater HR (+5 bpm; 95% CI, 0-10), Tcore (+0.20°C; 95% CI, 0.00-0.41), and Tsk (+0.47°C; 95% CI, 0.18-0.76) in aged adults. A greater whole-body sweat loss during fan use was observed in young (+0.2 kg; 95% CI, -0.2 to 0.6) but not aged (0.0 kg; 95% CI, -0.2 to 0.2) adults. Greater local sweat rate and cutaneous vascular conductance were observed with fan use in aged adults. Other measures of cardiovascular, thermoregulatory, and perceptual responses were unaffected by fan use in both groups.

CONCLUSIONS

During extreme heat and humidity, fan use elevates physiological strain in aged, but not young, adults.

Sweating as a heat loss thermoeffector

In humans, sweating is the most powerful autonomic thermoeffector. The evaporation of sweat provides by far the greatest potential for heat loss and it represents the only means of heat loss when air temperature exceeds skin temperature. Sweat production results from the integration of afferent neural information from peripheral and central thermoreceptors which leads to an increase in skin sympathetic nerve activity. At the neuroglandular junction, acetylcholine is released and binds to muscarinic receptors which stimulate the secretion of a primary fluid by the secretory coil of eccrine glands. The primary fluid subsequently travels through a duct where ions are reabsorbed. The end result is the expulsion of hypotonic sweat on to the skin surface. Sweating increases in proportion with the intensity of the thermal challenge in an attempt of the body to attain heat balance and maintain a stable internal body temperature. The control of sweating can be modified by biophysical factors, heat acclimation, dehydration, and nonthermal factors. The purpose of this article is to review the role of sweating as a heat loss thermoeffector in humans.

The physiological strain incurred during electrical utilities work over consecutive work shifts in hot environments: A case report

PURPOSE

In this article, we evaluated physiological strain in electrical utilities workers during consecutive work shifts in hot outdoor conditions.

METHODS

Four highly experienced electrical utilities workers were monitored during regularly scheduled work performed in hot conditions (∼34°C) on two consecutive days. Worker hydration (urine specific gravity) was assessed prior to and following work. The level of physical exertion was determined by video analysis. Body core temperature (T_core) and heart rate (HR; presented as a percentage of maximum, %HR_max) were monitored continuously. Responses were reported for each worker individually and as a group mean ± standard deviation.

RESULTS

According to current guidelines, all workers were dehydrated prior to work on both days (urine specific gravity: day 1, 1.025 ± 0.005; day 2, 1.029 ± 0.004) and remained dehydrated following work (urine specific gravity: day 1, 1.027 ± 0.015; day 2, 1.032 ± 0.004) except for one worker on day 1 (urine specific gravity of 1.005). On day 1, the proportion of the work shift spent at rest (as defined by the American Conference for Governmental and Industrial Hygienists, ACGIH) was 51 ± 15% (range: 30-64%). Time spent resting increased in all workers on the second day reaching 66 ± 5% (range: 60-71%) of the work shift. Work shift average T_core was 37.6 ± 0.1°C (range: 37.5-37.7°C) and 37.7 ± 0.2°C (range: 37.5-37.9°C) on days 1 and 2, respectively. Peak T_core surpassed the ACGIH recommended threshold limit of 38.0°C for work in the heat in three workers on day 1 (38.1 ± 0.2°C, range: 37.8-38.2°C) while all workers exceeded this threshold on day 2 (38.4 ± 0.2°C, range: 38.2-38.7°C). By contrast, work shift average (day 1, 67 ± 7%HR_max, range: 59-74%HR_max; day 2, 65 ± 4%HR_max, range: 60-70%HR_max) and peak (day 1, 90 ± 6%HR_max, range: 83-98%HR_max; day 2, 87 ± 10%HR_max, range: 73-97%HR_max) HR were similar between days.

CONCLUSION

This case report demonstrates elevations in thermal strain over consecutive work shifts despite decreases in work effort in electrical utilities workers during regular work in the heat.

Ten days of repeated local forearm heating does not affect cutaneous vascular function

The aim of the present study was to determine whether 10 days of repeated local heating could induce peripheral adaptations in the cutaneous vasculature and to investigate potential mechanisms of adaptation. We also assessed maximal forearm blood flow to determine whether repeated local heating affects maximal dilator capacity. Before and after 10 days of heat training consisting of 1-h exposures of the forearm to 42°C water or 32°C water (control) in the contralateral arm (randomized and counterbalanced), we assessed hyperemia to rapid local heating of the skin (n = 14 recreationally active young subjects). In addition, sequential doses of acetylcholine (ACh, 1 and 10 mM) were infused in a subset of subjects (n = 7) via microdialysis to study potential nonthermal microvascular adaptations following 10 days of repeated forearm heat training. Skin blood flow was assessed using laser-Doppler flowmetry, and cutaneous vascular conductance (CVC) was calculated as laser-Doppler red blood cell flux divided by mean arterial pressure. Maximal cutaneous vasodilation was achieved by heating the arm in a water-spray device for 45 min and assessed using venous occlusion plethysmography. Forearm vascular conductance (FVC) was calculated as forearm blood flow divided by mean arterial pressure. Repeated forearm heating did not increase plateau percent maximal CVC (CVC_max) responses to local heating (89 ± 3 vs. 89 ± 2% CVC_max, P = 0.19), 1 mM ACh (43 ± 9 vs. 53 ± 7% CVC_max, P = 0.76), or 10 mM ACh (61 ± 9 vs. 85 ± 7% CVC_max, P = 0.37, by 2-way repeated-measures ANOVA). There was a main effect of time at 10 mM ACh (P = 0.03). Maximal FVC remained unchanged (0.12 ± 0.02 vs. 0.14 ± 0.02 FVC, P = 0.30). No differences were observed in the control arm. Ten days of repeated forearm heating in recreationally active young adults did not improve the microvascular responsiveness to ACh or local heating.NEW & NOTEWORTHY We show for the first time that 10 days of repeated forearm heating is not sufficient to improve cutaneous vascular responsiveness in recreationally active young adults. In addition, this is the first study to investigate cutaneous cholinergic sensitivity and forearm blood flow following repeated local heat exposure. Our data add to the limited studies regarding repeated local heating of the cutaneous vasculature.