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The human thermoregulatory system and its response to thermal stress
Abstract
An overview of the physiology of human thermoregulation is presented, including a discussion of the principle of heat balance and the various heat exchange pathways together with physiological adaptations during thermal challenges. Thermoeffector responses (i.e., eccrine sweating, cutaneous vasodilation) during heat stress are examined, as well as the thermoregulatory mechanisms activated during passive heat/cold stress, exercise, and postexercise, such as shivering and nonshivering thermogenesis. Aspects related to nonthermal modulators of thermoeffector responses are explored and the effects of body composition, aerobic fitness, heat acclimation, sex, age, chronic disease (i.e., diabetes), hydration, and cardiovascular function on the body's capacity to dissipate heat are discussed.
... Eğer metabolik ısı kazanımı, ısı kaybının üstüne çıkarsa vücut kor sıcaklığında -Egzersiz Fizyolojisi ve Temel Kavramlarda kademeli bir artış görülmektedir. Bu koşullar altında vücut sürekli bir şekilde ısı depolar ve potansiyel olarak tehlikeli seviyede vücut kor sıcaklığına neden olur (Kenny & Flouris, 2014). ...
... Çünkü genellikle istemli kontrol altındaki iskelet kaslarının istemsiz bir tepkisidir. Titremenin ritmik kas kasılmasıyla oluştuğu göz önüne alındığında, metabolik etkileri hafif düzeyde egzersize benzer şekilde ve VO2max'ın %40'ına (veya dinlenme metabolizma hızının beş katına) kadar yükselebilir (Kenny & Flouris, 2014). Titreme sırasında kas kasılmalarını sürdürmek için gereken enerji karbonhidratların, lipidlerin ve proteinlerin oksidasyonu yoluyla sağlanır. ...
Vücut ısı dengesi
Termoregülasyon
Sıcak ortamda egzersiz
Soğuk ortamda egzersiz
Yüksek irtifada egzersiz
Su altında egzersiz
... [12] In the ScienceDirect system, at the request of "Metabolic Heat Production", a page is formed with a selection of information on heat production with abstracts of articles (https://www.sciencedirect.com/topics/engineering/metabolic-heat-production), where such works as [6] and a number of works to determine thermal comfort and the effect of clothing on human heat production. The review work [6] investigates thermoeffective reactions during heat stress, as well as the mechanisms of thermoregulation. ...
... where such works as [6] and a number of works to determine thermal comfort and the effect of clothing on human heat production. The review work [6] investigates thermoeffective reactions during heat stress, as well as the mechanisms of thermoregulation. Aspects related to non-thermal modulators of thermoeffective reactions are explored and the effects of body composition, aerobic fitness, heat acclimatization, gender, age, chronic diseases (eg diabetes), hydration and cardiovascular function on the body's ability to dissipate heat are discussed. ...
A classification of the tasks of human thermoregulation is proposed, which allows combining various scientific studies into one system, which takes into account environmental factors, biological feedback in the body and different properties of the constituent parts of the human thermoregulation system. The proposed multifactorial model of the human thermoregulation system is based on combining the classification of tasks represented by a group of main factors (temperature, time and the characteristic size of the system under consideration), with the classification of tasks in the studied sciences. The article discusses a number of the most significant scientific works for each area of research. Identified and systematized the most important problems for study in each of them from the point of view of heat and mass transfer and energy conversion at different levels of the description of biosystems in the human body.
... As a result, individuals may experience discomfort, fatigue and reduced capacity to perform various physical activities (Vanos et al., 2010), which may delay the body's regeneration process or even negatively impact patients' health. Furthermore, during short exposures to a hot environment, older people and patients with chronic disease or poor fitness more frequently experience increased levels of thermal strain, as a result of impairments in their sudomotor and vasomotor capacity to dissipate excessive heat (Stapleton et al., 2013;Kenny & Flouris, 2014). ...
... As a simplification, RMR is often identified with the value of 1 MET (Metabolic Equivalent), which is a physiological concept commonly used to express energy costs of different levels of physical activity (Byrne et al., 2005;Heydenreich et al., 2019). During physical exercise metabolic heat production rises, as contracting muscles require constant energy supply to perform their work (Kenny & Flouris, 2014). For various forms of physical activities approximated MET levels can be assigned by multiplying standard RMR values, as has been done in Appendix A of ISO 8996 standard (ISO 8996, 2004) or in the Compendium of Physical Activities by Ainsworth (Ainsworth et al., 2011). ...
The objective of this study was to assess biothermal conditions in the selected Polish health resorts for specific forms of climatic therapy. We calculated Universal Thermal Climate Index (UTCI) for ten-year period (2008- 2017) and then added adjustment terms, taking into account changes in metabolic rates during various physical activities from resting to vigorous exercise. The adjusted UTCI values increased with rising activity, implying that warmer parts of the year were unsuitable for intensive forms of climatotherapy. These results demonstrate that the UTCI adjustment procedure provides well-balanced assessments of bioclimatic conditions for the purpose of climatic treatment considering the level of activity
... Starting from Maxwell's equations and applying the appropriate 'electric' tissue properties (ε, σ, μ), the complete interactions of both the EMF and the AOR with living organisms are revealed [5][6][7]. The use of the bioheat equation [8,9] has been considered effective for the RF thermal protection on the basis of specific absorption rate (SAR), considering the advanced heat dissipation mechanisms of the human body [10][11][12]. Investigation on the interactions of the central (CNS) and the peripheral (PNS) nervous system with the low frequency EMF (ELF) set the protection system on the basis of electrostimulation [5]. The EMF limiting system ensures sufficient protection from the above health effects containing several quantities, that is (measurable, external field) Action Levels (ALs) and (non-measurable, internal field) Exposure Limit Values (ELVs) [3]. ...
Background:
The electromagnetic spectrum spans over an enormous range from 0 up to more than 1020 Hz in the deep ionizing region, significant exposures exist in specific occupational environments. Between the ionizing and the electromagnetic fields (EMF) part of the spectrum, the 'optical radiation' (OR) region has specific properties. Comparative and concise evaluation enables action prioritization.
Methods:
Following the transposition and implementation periods of the artificial optical radiation (AOR) and EMF European Directives, the Hellenic Ministry of Labour in collaboration with the Greek Atomic Energy Commission (EEAE) and the National Technical University of Athens, conducted thorough occupational exposure investigation in Greece. Using dedicated measuring equipment and procedures, the majority of EMF emitting installations in Greece and also AOR emitting installations including arc welding, lasers and PC monitors has been assessed.
Results:
Measurement results from occupational settings reveal that it is the non-coherent metal arc welding AOR that can pose even sub-second overexposures. Rare EMF overexposures are manageable and EMF concern is not justified. Maintenance procedures demand proper attention. Preliminary laser safety assessment reveals OHS gaps and potential eye and skin hazards. Blue light exposure from computer monitors is well below safety limits.
Conclusions:
This electromagnetic spectrum risk assessment conducted in Greece enables the justification of the real occupational hazards, in this sense: i) EMF exposure assessment has to be concentrated to maintenance procedures; ii) AOR measuring setups are challenging and standardized measurement procedures are missing, and iii) AOR overexposures from arc welding pose significant eye and skin hazards.
... Maintaining a flow of air, either inside (PPE-PAPR-in) the PPE or on its surface (PPE-PAPR-out), contributes to reducing the internal heat footprint, which in turn improves perspiration and the sensation experienced by those who use it. The use of this type of PPE could avoid the thermal discomfort effect and the negative consequence of thermal stress [30][31][32][33][34]. ...
Objective:
The objectives of this study were (a) to determine the physical impact of the personal protective equipment (PPE) used in COVID-19 care, specifically the impact on the hydration state of the temperature and the comfort of the healthcare workers who use it, and (b) to show the high-fidelity simulated environment as an appropriate place to test the experimental designs to be developed in real environments for COVID-19.
Background:
All healthcare staff use full PPE in the care of COVID-19 patients. There are problems, such as excessive sweating, which have not been quantified thus far.
Methods:
A descriptive pilot design was used in a simulated high-fidelity setting. There was paired activity, with mild-moderate physical activity, between 45 and 60 min continuously, with the COVID-19 PPE. Sixteen intensive care nurses were selected. The before-after differential of weight, thirst, weight use of the PPE, body temperature, thermal body image, general and facial warmth sensation, and perspiration sensation were measured.
Results:
All subjects lost weight in the form of sweat with both PPEs during the simulation scenario, with a mean of 200 g (0.28% of initial weight), and increased thirst sensation. Body thermal image increased by 0.54 °C in people using the full COVID-19 PPE.
Conclusions:
The use of PPE in the management of critically ill COVID-19 patients generates weight loss related to excessive sweating. The weight loss shown in this pilot test is far from the clinical limits of dehydration. The use of ventilated PPE, such as PAPR, reduce the body temperature and heat sensation experienced by the users of it; at the same time, it improves the comfort of those who wear it. The simulated environment is a suitable place to develop the piloting of applicable research methodologies in future studies in a real environment.
... Players spent over 70% of the time in the lowest Z1 and Z2. It has been shown in the past that low intensity exercise is related to a small metabolic heat production [32]. Therefore, it is suggested that the stored heat in the body was not high enough to get overheated and affect performance. ...
This study compared performance parameters of two wheelchair basketball games under
hot (30.3 ◦C, 52% relative humidity) and temperate (21.6 ◦C, 30% relative humidity) environmental conditions and described the characteristics of wheelchair basketball. Eight wheelchair basketball players from two teams were monitored during two games using an indoor position tracking system. Total distance, mean- and peak-speed, playing-time, number of sprints, sprints per minute, heart rate and rate of perceived exertion were recorded. Additionally, athletes with a lesion level above and below T6 were compared. No measured parameter differed between the games. Across quarters (Q) mean velocity (m/s) (Q1: 1.01; Q2: 1.10; Q3: 1.18; Q4: 1.06; p < 0.001) and sprints per minute (Q1: 16;
Q2: 14; Q3: 23; Q4: 14; p = 0.033) differed significantly, independent of the conditions. Descriptive statistics did not reveal differences between the groups with a lesion level below or above T6. In the present study, hot environmental conditions seemed not to have an impact on activity parameters of wheelchair basketball players. It was speculated that the game intensity and therefore metabolic heat production was too low; consequently, the athletes had a sufficient heat loss to prevent a decrease in performance during the play in hot conditions.
... Furthermore, it appears that performance impairment is dependent on the duration of the event, with longer ones involving greater degradation [31]. In real condition, this is particularly true where athletes are unable to compensate for the combination of endogenous (exercise-induced) and environmental heat stress [32][33][34][35][36]. ...
The practice of physical activity in a variable climate during the same competition is becoming more and more common due to climate change and increasingly frequent climate disturbances. The main aim of this pilot study was to understand the impact of cold ambient temperature on performance factors during a professional cycling race. Six professional athletes (age = 27 ± 2.7 years; height = 180.86 ± 5.81 cm; weight = 74.09 ± 9.11 kg; % fat mass = 8.01 ± 2.47%; maximum aerobic power (MAP) = 473 ± 26.28 W, undertook ~20 h training each week at the time of the study) participated in the Tour de la Provence under cold environmental conditions (the ambient temperature was 15.6 ± 1.4 °C with a relative humidity of 41 ± 8.5% and the normalized ambient temperature (Tawc) was 7.77 ± 2.04 °C). Body core temperature (Tco) was measured with an ingestible capsule. Heart rate (HR), power, speed, cadence and the elevation gradient were read from the cyclists’ onboard performance monitors. The interaction (multivariate analysis of variance) of the Tawc and the elevation gradient has a significant impact (F(1.5) = 32.2; p < 0.001) on the variables (cadence, power, velocity, core temperature, heart rate) and on each individual. Thus, this pilot study shows that in cold environmental conditions, the athlete’s performance was limited by weather parameters (ambient temperature associated with air velocity) and race characteristics. The interaction of Tawc and elevation gradient significantly influences thermal (Tco), physiological (HR) and performance (power, speed and cadence) factors. Therefore, it is advisable to develop warm-up, hydration and clothing strategies for competitive cycling under cold ambient conditions and to acclimatize to the cold by training in the same conditions to those that may be encountered in competition.
... The human body generates heat as a part of homeostatic regulation for maintaining the core body temperature at 37 • C (98.6 • F). Muscular metabolism, which results from adenosine triphosphate's exothermic hydrolysis, varies for the activities being performed [2]. This heat transpires from deep tissues to the skin surface through blood flow. ...
This paper estimates metabolic rate and physiological response associated with different activity intensities through controlled climate chamber experiments. We investigate the effect of activity intensity, personal and environmental factors on the subjects' heart rate, oxygen consumption, energy expenditure, and metabolic rate (Mr). Forty-eight adult Indian subjects categorized based on age, gender, Body Mass Index (BMI), ethnicity and dietary practices are subjected to different temperature, relative humidity, and air velocity set-points. Cardio-Pulmonary Exercise Test (CPET) and Resting Metabolic Rate (RMR) Test are performed in a climate camber based on appropriate experimental protocols. We find substantial variation in the physiological responses concerning activity intensity. The difference in Mr for different age and BMI groups is statistically significant. Mr of stationary activities exhibits marginal variation with temperature change. However, Mr of non-stationary activities shows significant variation with temperature variations. The effect of relative humidity on Mr is significant for non-stationary activities. Ethnicity and dietary habits of the subjects depict good relationship with the mean Mr for non-stationary activities. There is a significant difference observed in the metabolic stabilisation period for different age group categories in non-stationary activities. We present the impact of these intrinsic variations of Mr on thermal comfort estimations.
... In this study, we found that increased fruit and vegetable intakes were associated with a reduction of inflammation in summer and/or spring but not in winter for most subgroups. In summer and spring, people tend to have increased outdoor physical activities [36]; increased outdoor physical activities plus higher temperatures lead to increased sweating and breathing [37][38][39], which can result in more water loss [40]. Approximately 1000 mL/day more water loss through sweat occurs in summer than in winter [41]. ...
Seasons can affect human inflammatory status and the occurrence of diseases, and foods may also have differential impacts on inflammation across seasons; however, few studies have investigated whether there are independent and joint impacts of seasons and red meat, fruit and vegetable intakes on inflammation in breast cancer survivors. We conducted a cross-sectional study by leveraging a large cohort, the Women’s Healthy Eating and Living (WHEL) study. The WHEL study comprised primarily early stage breast cancer survivors and collected blood samples, dietary intake, demographic, and health status information at baseline. We selected 2919 participants who provided baseline dietary information and had measurement of C-reactive protein (CRP), a general marker of inflammation. In our multivariable-adjusted analyses, we found that red meat intakes were positively associated, while fruit and vegetable intakes were inversely associated with CRP; blood collected in the winter season was associated with lower CRP when compared to summer; and increased smoking intensity and body mass index (BMI) as well as having cardio-metabolic conditions (such as heart disease or diabetes) were positively associated with CRP. Furthermore, we examined the joint associations of food intakes and the season of blood draw with CRP in different subgroups. We found that moderate intakes of red meat were associated with a reduction of CRP in winter but not in other seasons; increased intakes of fruit and vegetables were associated with reduced inflammation in most seasons except winter. These associations were observed in most subgroups except past smokers with pack-years ≥ 15, in whom we observed no benefit of red meat intakes in winter. Our study provides valuable evidence for considering seasonal impacts on inflammation and seasonal food impacts in different subgroups among breast cancer survivors. The results of our study are in line with one of the emphases of the current NIH 2020–2030 nutrition strategy plan—namely, pay attention to what, when, and who should eat.
... It has long been established that muscular exercise induces an increase in CBT, which then triggers thermoregulatory processes to balance heat production and heat loss (Nielsen, 1938) as cited by Sawka et al. (2010). The increases in CBT found during exercise are a consequence of muscular metabolism to supply the energy required for muscular function (Kenny and Flouris, 2014). The heat produced during muscular metabolism is transferred from the muscles to the blood and the tissues surrounding the muscles, leading to an increase in CBT (Kenny et al., 2003). ...
Sleep inertia, the transitional state of reduced alertness and impaired cognitive performance upon waking, is a safety risk for on-call personnel who can be required to perform critical tasks soon after waking. Sleep inertia countermeasures have previously been investigated; however, none have successfully dissipated sleep inertia within the first 15 min following waking. During this time, on-call personnel could already be driving, providing advice, or performing other safety-critical tasks. Exercise has not yet been investigated as a sleep inertia countermeasure but has the potential to stimulate the key physiological mechanisms that occur upon waking, including changes in cerebral blood flow, the cortisol awakening response, and increases in core body temperature. Here, we examine these physiological processes and hypothesize how exercise can stimulate them, positioning exercise as an effective sleep inertia countermeasure. We then propose key considerations for research investigating the efficacy of exercise as a sleep inertia countermeasure, including the need to determine the intensity and duration of exercise required to reduce sleep inertia, as well as testing the effectiveness of exercise across a range of conditions in which the severity of sleep inertia may vary. Finally, practical considerations are identified, including the recommendation that qualitative field-based research be conducted with on-call personnel to determine the potential constraints in utilizing exercise as a sleep inertia countermeasure in real-world scenarios.
... Extreme heat is a "silent" killer because it's not a visible threat. Heat stress is created by the combined heat load from the environment (higher ambient temperatures increase the requirements for the body to dissipate heat, while increased ambient humidity restricts heat loss via sweat evaporation), clothing (restricts heat loss), and heat generated from metabolic processes (increases in metabolic rate augments the heat produced by the body and thus rate that heat must be dissipated) [16,17]. The resulting physiological, psychophysical, and performance effects characterize the heat strain response. ...
The projected rise in the frequency and intensity of extreme heat conditions associated with global climate change represent the greatest threat to human health of the twenty-first century. This threat is particularly great for heat-vulnerable populations such as the elderly and those employed in physically demanding occupations. As such, there is an immediate need to define appropriate heat action plans to protect human health. To increase our readiness and ability to protect people during extreme heat events, it is essential that we continue to develop our understanding of the physiological factors that contribute to increased heat vulnerability. This includes the inter- and intraindividual factors that modify physiological strain during extreme heat exposure or during work and/or physical activity in the heat. In this review, emphasis will be directed to the consequences of rising global temperatures on human health, including the cause-and-effect relationships between the thermal environment and the body’s physiological capacity to dissipate heat. Further, we examine how physiological adaptations, behavioral adjustments, and the implementation of heat management and monitoring strategies can mediate one’s susceptibility to heat exposure and work in the heat. Finally, we review current initiatives directed at creating communities and industries resilient to climate change and protecting the general public and workers against the projected rise in temperatures.
... Those exposed to higher risk include patients with cardiovascular and respiratory diseases, diabetics, patients with kidney disease, patients with mental disorders and the immobile. Their vulnerability can be further influenced by low socio-economic status, social isolation, poor access to health services and generally poor living conditions (Kenny and Flouris, 2014). It is expected that, owing to climate change, the number of heat-related diseases and even deaths could increase by 3-6%, resulting in an additional 38,000 heat-related deaths over the next 30 years, or even an additional 100,000 deaths over the next 50 years (WHO, 2014). ...
The urban heat island (UHI) phenomenon has been gaining intensity because of population growth, continuous urbanization and inappropriate urban planning and design. Geospatial models can importantly contribute to solve the negative impacts left behind by these developments in the urban environment. Here, the influence of different weather patterns (anti‐cyclonic, cyclonic and advective), occurring as a consequence of regional synoptic condition, on the UHI intensity and morphology was studied. The previously published geographically weighted regression UHI model was used in order to predict the behavior of the urban‐rural temperature anomaly in different weather patterns in a small city named Ljutomer (8 km2 of built‐up area occupied by 3500 inhabitants) located in the NE part of Slovenia. As expected, UHI intensity was significantly higher in the anti‐cyclonic weather pattern compared to cyclonic and advective conditions. The urban‐rural temperature contrast was at least recognizable under a turbulent windy atmosphere. Surprisingly, Ljutomer’s UHI can be most accurately modeled in the cyclonic weather pattern, followed by advective and anti‐cyclonic conditions, and statistically significant cold spots can be better predicted, with selected predictor variables, than hot‐spots. This article is protected by copyright. All rights reserved.
... Indeed, it is well known that aging is associated with a reduced thermoregulatory capacity, especially when work or exercise in hot environments [3,[12][13][14][15][16]. Moreover, heat exposure negatively affects a number of body tissues and systems and there are distinct sex differences in thermoregulatory capacity for individuals resting or exercising in the heat [12,13,17,18]. However, our knowledge on these issues is derived primarily from studies assessing the effects of daytime temperature on human physiology and health, while, further research is required to elucidate if night-time temperature affects mortality rates in elderly individuals. ...
Background: The relationship between non-thermoneutral zone temperatures with increased mortality in elderly individuals is well established. However, less is known regarding the effect of night-time temperature on mortality in elderly individuals. The aim of this study was to investigate the association between night-time temperature and mortality in elderly people (≥ 70 years) in Cyprus. Methods: All-cause mortality data covering the period 2004-2014 were retrieved from the Health Monitoring Unit of the Cypriot Ministry of Health. Midnight (00:00) temperature data from weather stations across the island of Cyprus covering the same time period were collected from www.wunderground.com. Night-time temperatures were categorized into six 5-degree Celsius categories (≤ 8 ℃, 9-13 ℃, 14-18 ℃, 19-23 ℃, 24-28 ℃, and ≥ 29 ℃). The 19-23 ℃ category was defined as thermoneutral zone. Results: A total of 43,107 elderly individuals died during the monitored period and the most prevalent cause of death was "diseases of the circulatory system" (41.5%; p < 0.001). Mortality due to diseases of the circulatory system was significantly reduced when night-time temperature was at the thermoneutral zone during the previous night (p < 0.05). The prevalence of deaths due to circulatory causes was higher for females compared to males (p < 0.001) (Cohen's d = 0.34). Furthermore, there was higher prevalence of deaths during extreme night time temperatures compared with thermoneutral zone (0.24 ≥ Cohen's d ≤ 1.01). Conclusion: Mortality due to circulatory causes, the most prevalent cause of death in Cyprus, is increased when night-time temperature is above or below the thermoneutral zone.
... Recently, the importance of comfort and hence its evaluation for these fabrics is gaining significance. Although several studies have been carried out investigating the comfort properties of the personal protective fabrics, especially for firefighters (Atalay, Kursun Bahadir, & Kalaoglu, 2015;Cui & Zhang, 2009;Kenny & Flouris, 2014;Nayak, Houshyar, & Padhye, 2014;Song, Cao, & Gholamreza, 2011;, there are relatively few studies concerning the comfort properties of other protective clothing such as ballistic and workwear (Barker, An, Shalev, & Scruggs, 1992;Mahbub, Ratnapandian, Wang, & Arnold, 2013). The most common problem with protective clothing is lack of comfort. ...
Protective clothing protects the body from external influence like heat, chemicals, mechanical hazards, bad weather, etc. by shielding the human body from harsh environmental effects. The maintenance of thermal balance is one of the most important aspects of protective clothing. The study aims to investigate the thermal comfort properties of woven hybrid fabrics produced with high performance core spun yarns. For this purpose, meta-aramid, e-glass, Technora® and Dyneema® fibres were combined using core yarn spinning method in order to enhance the protective performance characteristics. The effects of the core/sheath ratio and type of core materials on the thermal comfort characteristics were investigated and evaluated statistically. The results revealed that, core/sheath ratio, types of core materials have significant effects on thermal comfort characteristics of the fabrics. As the core ratio increases from 19 to 56%, the air permeability of the fabrics increases whereas their thermal conductivity and thermal absorptivity properties decrease. Meta-aramid/E-glass core fabrics can be preferable for protective clothing due to high air permeability and thermal conductivity values.
... Importantly, this study reported that the differences in the capacity to dissipate heat were evident as early as 15 min into the work bout and continued into the recovery period. These findings have particularly important implications for occupations with physically demanding jobs since several lines of evidence demonstrate that hyperthermia and heat stress lead to attenuated work output in individuals of all ages (Flouris 2011;Kenny and Flouris 2014). In addition, these effects may be exacerbated in occupations where protective clothing is worn (e.g., mining, armed forces, automotive/athletic settings) as the clothing will further impair the rate of heat loss. ...
The proportion of older workers has increased substantially in recent years, with over 25% of the Canadian labour force aged ≥55 years. Along with chronological age comes age-related declines in functional capacity associated with impairments to the cardiorespiratory and muscular systems. As a result, older workers are reported to exhibit reductions in work output and in the ability to perform and/or sustain the required effort when performing work tasks. However, research has presented some conflicting views on the consequences of aging in the workforce, as physically demanding occupations can be associated with improved or maintained physical function. Furthermore, the current methods for evaluating physical function in older workers often lack specificity and relevance to the actual work tasks, leading to an underestimation of physical capacity in the older worker. Nevertheless, industry often lacks the appropriate information and/or tools to accommodate the aging workforce, particularly in the context of physical employment standards. Ultimately, if appropriate workplace strategies and work performance standards are adopted to optimize the strengths and protect against the vulnerability of the aging workers, they can perform as effectively as their younger counterparts. Our aim in this review is to evaluate the impact of different individual (including physiological decline, chronic disease, lifestyle, and physical activity) and occupational (including shift work, sleep deprivation, and cold/heat exposure) factors on the physical decline of older workers, and therefore the risk of work-related injuries or illness.
... Thermal stress generally occurs due to a failure of the thermoregulatory system to keep body core temperature within its boundaries [2]. Thermal stress is not caused by any diseases but results from a combination of various factors such as extra metabolic heat generation within the body during exercise, prolonged exposure to an extremely hot thermal environment (high air and radiant temperature and high humidity), low air velocity, and reduced evaporation of sweat [3]. When the body becomes unable to cool itself, heatinduced illness such as heat stress and heat exhaustion may result. ...
Fabric assemblies for firefighting clothing have been tested for heat protection and comfort. The constituent materials and fabric structures have been specifically selected and tailored for firefighters’ clothing. In order to do this, four types of outer shell fabrics, four types of moisture barrier fabrics, and four types of heat barriers with different weights and material compositions were used to make a multilayered fabric assembly. Heat transfer (flame), heat transfer (radiant), and water vapour resistance tests were conducted according to the latest EN469 test standard which also recommends washing tests. These tests reveal that material content and material brand have considerable effect on the required performance levels of heat protection. In addition, while washing tests have improved water vapor transfer properties, they have a deteriorating effect on heat protection performance. Considering heat protection and moisture comfort properties, the optimal assemblies are thereby identified.
... The heart strives to maintain perfusion to vital organs and exercising muscles whilst also perusing significantly dilated vessels in the skin. Blood flow to the skin can reach 7.5 L/min −1 (from a resting base of 300 mL·min −1 ) which requires about half the total cardiac output [98]. Meanwhile, exercising muscles also demand significantly increased supply of oxygenated blood. ...
Human thermoregulation and acclimatization are core components of the human coping mechanism for withstanding variations in environmental heat exposure. Amidst growing recognition that curtailing global warming to less than two degrees is becoming increasing improbable, human survival will require increasing reliance on these mechanisms. The projected several fold increase in extreme heat events suggests we need to recalibrate health protection policies and ratchet up adaptation efforts. Climate researchers, epidemiologists, and policy makers engaged in climate change adaptation and health protection are not commonly drawn from heat physiology backgrounds. Injecting a scholarly consideration of physiological limitations to human heat tolerance into the adaptation and policy literature allows for a broader understanding of heat health risks to support effective human adaptation and adaptation planning. This paper details the physiological and external environmental factors that determine human thermoregulation and acclimatization. We present a model to illustrate the interrelationship between elements that modulate the physiological process of thermoregulation. Limitations inherent in these processes, and the constraints imposed by differing exposure levels, and thermal comfort seeking on achieving acclimatization, are then described. Combined, these limitations will restrict the likely contribution that acclimatization can play in future human adaptation to global warming. We postulate that behavioral and technological adaptations will need to become the dominant means for human individual and societal adaptations as global warming progresses.
... Furthermore, local changes of the skin temperature may cause local autonomic reactions such as modifying the sweating rate or the local vasodilatation (Fiala et al. 2001).Besides these active control mechanisms, a thermoregulation model essentially consists of a passive system which models the heat exchange between body and environment and the heat transfer within tissues including blood circulation. In the literature, a number of models have been proposed such as twonode models (Gagge 1973) and multi-segment models (Fiala et al. 2001, Gordon 1974, Huizenga et al. 2001, Tanabe et al. 2002, Werner 1988, Wissler 1985), which are mainly founded on the work of Stolwijk (1971). Most of them decompose of the body into a set of segments which are partitioned into concentric layers and sectors. ...
We address the scale-adaptive coupling between computational codes for integrated indoor thermal comfort analysis. The paper describes the related developed subsystems such as a thermal zone model, a window model with raytracing algorithm to track short wave radiation, a radiosity solver for treating diffuse short and long wave radiation, a convective heat transfer model, and an interface to a human thermoregulation model. We demonstrate the application of the zone model to predict the whole year-related thermal quality performance of a building using an adaptive thermal comfort envelope. Having detected critical intervals in terms of overheating risk, the model resolution is refined in space and time and a multi-segment thermal manikin model interfacing with a thermoregulation model is applied for studying transient and local effects of thermal sensation. Skin and surface temperatures are visualized and correlated with local thermal sensation votes, which are based on empirical correlations from experiments with human subjects. Further work focuses on the development of a middleware concept for coupling the described engineering models with special emphasis on the issue of scale-adaptivity.
... 22,26,27 In addition to heat production, endotherms employ also heat conservation (sympathetic cutaneous vasoconstriction and blood redistribution), heat loss (cholinergic cutaneous vasodilation and eccrine sweating), and behavioral thermoregulation (conscious decisions directly aiming at Tb regulation) mechanisms (Fig. 2). These powerful mechanisms (reviewed in detail elsewhere) [28][29][30][31] allow endotherms to maintain their internal environment at a metabolically favorable temperature. ...
While the link between thermoregulation and aging is generally accepted, much further research, reflection, and debate is required to elucidate the physiological and molecular pathways that generate the observed thermal-induced changes in lifespan. Our aim in this review is to present, discuss, and scrutinize the thermoregulatory mechanisms that are implicated in the aging process in endotherms and ectotherms. Our analysis demonstrates that low body temperature benefits lifespan in both endothermic and ectothermic organisms. Research in endotherms has delved deeper into the physiological and molecular mechanisms linking body temperature and longevity. While research in ectotherms has been steadily increasing during the past decades, further mechanistic work is required in order to fully elucidate the underlying phenomena. What is abundantly clear is that both endotherms and ectotherms have a specific temperature zone at which they function optimally. This zone is defended through both physiological and behavioral means and plays a major role on organismal senescence. That low body temperature may be beneficial for lifespan is contrary to conventional medical theory where reduced body temperature is usually considered as a sign of underlying pathology. Regardless, this phenomenon has been targeted by scientists with the expectation that advancements may compress morbidity, as well as lower disease and mortality risk. The available evidence suggests that lowered body temperature may prolong life span, yet finding the key to temperature regulation remains the problem. While we are still far from a complete understanding of the mechanisms linking body temperature and longevity, we are getting closer.
... Indeed, an elevation in Tm is observed as early as the first 3 s of muscle contraction due to the heat produced by metabolic reactions required for ATP resynthesis (22). As exercise continues, the overall efficiency of muscle contraction is reduced at a rate proportional to the increase in heat production (22,28). Therefore, knowledge of Tm is vital for performance enhancement (41). ...
We introduced non-invasive and accurate techniques to estimate muscle temperature (Tm) of vastus lateralis (VL), triceps brachii (TB), and trapezius (TRAP) during rest, exercise, and post-exercise recovery using the insulation disk (iDISK) technique. Thirty-six volunteers (24 men; 12 women; 73.0±12.2 kg; 1.75±0.07 m; 24.4±5.5 years; 49.2±6.8 peak oxygen uptake) underwent periods of rest, cycling exercise at 40% of peak oxygen uptake, and post-exercise recovery in three environments: NORMAL (24°C, 56% relative humidity), HOT-HUMID (30°C, 60% relative humidity), and HOT-DRY (40°C, 24% relative humidity). Participants were randomly allocated into the 'model' and the 'validation' groups. Results in the model group demonstrated that Tm (VL: 36.65±1.27°C; TB: 35.76±1.73°C; TRAP: 36.53±0.96°C) was increased compared to iDISK (VL: 35.67±1.71°C; TB: 34.77±2.27°C; TRAP: 35.98±1.34°C) across all environments (p<0.001). Stepwise regression analysis generated models that accurately predicted Tm (predTm) of VL (R(2)=0.73-0.91), TB (R(2)=0.85-0.93), and TRAP (R(2)=0.84-0.86) using iDISK and the difference between the current iDISK temperature and that recorded between one to four minutes before. Cross-validation analyses in the validation group demonstrated small differences (p<0.05) of no physiological significance, small effect size of the differences, and strong associations (r=0.85-0.97; p<0.001) between Tm and predTm. Moreover, narrow 95% limits of agreement and low percent coefficient of variation were observed between Tm and predTm. It is concluded that the developed non-invasive, practical, and inexpensive techniques provide accurate estimations of VL, TB, and TRAP Tm during rest, cycling exercise, and post-exercise recovery.
Copyright © 2014, Journal of Applied Physiology.
... Most models use a decomposition of the human body into layers and segments for the passive system which are in thermodynamic contact. The 8-segment model of Stolwijk (1971) has been enhanced by Gordon (1974), Wissler (1985), Werner (1988), Fiala (1998) and Tanabe (2002). An active thermoregulatory system captures the four essential human responses to thermal influences: vasoconstriction, vasodilatation, sweating and shivering. ...
We outline the current state of the development of a computational steering environment (CSE) for the interactive simulation and local assessment of indoor thermal comfort. The system consists of a parallel CFD kernel, a fast 3D mesh generator and a virtual reality-based visualization component. The numeri- cal method is based on a lattice Boltzmann algorithm with extensions for simulations of turbulent convec- tive flows. Utilizing high-performance supercompu- ting facilities, the CSE allows for modifying both the geometric model and the boundary conditions during runtime coupled with the immediate update of results. This is made possible by a space-tree based partitioning algorithm that facilitates the meshing of arbitrarily shaped, complex facet models in a matter of just a few seconds computing time. Ongoing developments focus on the integration of a radiation solver, a human thermoregulation model and a local thermal comfort model. Our first step was therefore to develop a prototype for computing resultant surface temperatures mapped for the surface of a numerical manikin. Results are compared with measurement data.
Aging is associated with attenuated sweat gland function, which has been suggested to occur in a peripheral-to-central manner. However, evidence supporting this hypothesis remains equivocal. We revisited this hypothesis by evaluating the sweat rate across the limbs and trunk in young and older men during whole-body, passive heating. A water-perfused suit was used to raise and clamp esophageal temperature at 0.6°C (low-heat strain) and 1.2°C (moderate-heat strain) above baseline in 14 young (24 (SD 5) years) and 15 older (69 (4) years) men. Sweat rate was measured at multiple sites on the trunk (chest, abdomen) and limbs (biceps, forearm, quadriceps, calf) using ventilated capsules (3.8 cm2 ). Sweat rates, expressed as the average of 5 min of stable sweating at low- and moderate-heat strain, were compared between groups (young, older) and regions (trunk, limbs) within each level of heat strain using a linear mixed-effects model with nested intercepts (sites nested within region nested within participant). At low-heat strain, the age-related reduction in sweat rate (older-young values) was greater at the trunk (0.65 mg/cm2 /min [95% CI 0.44, 0.86]) compared to the limbs (0.42 mg/cm2 /min [0.22, 0.62]; interaction: p = 0.010). At moderate-heat strain, sweat rate was lower in older compared to young (main effect: p = 0.025), albeit that reduction did not differ between regions (interaction: p = 0.888). We conclude that, contrary to previous suggestions, the age-related decline in sweat rate was greater at the trunk compared to the limbs at low-heat strain, with no evidence of regional variation in that age-related decline at moderate-heat strain.
Anthropometric parameters for the estimation of body composition are variables commonly used in the identification of cardiovascular risk factors (CVRFs). In this regard, infrared thermography (IT) could be used as a tool that provides relevant information in the CVRFs assessment by estimating skin surface temperature and its relationship with the anthropometric measures associated with these factors. The aim of this study was to determine the relationship of skin surface temperature in adult men with anthropometric variables associated with CVRFs. The study gathered sixty-four healthy men aged 26.4 ± 7.8 years, 76.1 ± 13.3 kg body mass, 171.3 ± 7.2 cm height and 25.9 ± 3.7 kg/m2 body mass index (BMI). Anthropometric assessments of 6 skinfolds (tricipital, subscapular, suprascapular, supraspinal, abdominal, mid-thigh, and calf), waist and hip circumference were conducted, as well as IT surface temperature measurements at the measurement areas. Significant negative relationships were found between surface temperature and skinfold measurements at the subscapular, supraspinal, thigh and calf levels (p < 0.01; r = -0.39 to -0.55). The average IT temperature of the 6- folds and all individual folds IT had significant negative correlations with BMI, waist circumference, waist hip ratio, waist height ratio, and the sum of 6 skinfolds (p < 0.05; r = -0.35 to -0.65). There is a relationship between skin surface temperature and some body composition anthropometric parameters that indicate and association with CVRFs, therefore, IT may be a useful tool to complement the assessment of these parameters.
On the roadmap to building completely autonomous artificial bio-robots, all major aspects of robotic functions, namely, energy generation, processing, sensing, and actuation, need to be self-sustainable and function in the biological realm. Microbial Fuel Cells (MFCs) provide a platform technology for achieving this goal. In a series of experiments, we demonstrate that MFCs can be used as living, autonomous sensors in robotics. In this work, we focus on thermal sensing that is akin to thermoreceptors in mammalian entities. We therefore designed and tested an MFC-based thermosensor system for utilization within artificial bio-robots such as EcoBots. In open-loop sensor characterization, with a controlled load resistance and feed rate, the MFC thermoreceptor was able to detect stimuli of 1 min directed from a distance of 10 cm causing a temperature rise of ∼1°C at the thermoreceptor. The thermoreceptor responded to continuous stimuli with a minimum interval of 384 s. In a practical demonstration, a mobile robot was fitted with two artificial thermosensors, as environmental thermal detectors for thermotactic application, mimicking thermotaxis in biology. In closed-loop applications, continuous thermal stimuli were detected at a minimum time interval of 160 s, without the need for complete thermoreceptor recovery. This enabled the robot to detect thermal stimuli and steer away from a warmer thermal source within the rise of 1°C. We envision that the thermosensor can be used for future applications in robotics, including as a potential sensor mechanism for maintaining thermal homeostasis.
Introduction: Climate change predictions indicate that global temperatures are likely to exceed those seen in the last 200,000 years, rising by around 4°C above pre-industrial levels by 2100 (without effective mitigation of current emission rates). In regions of the world set to experience extreme temperatures, women often work outside in agriculture even during pregnancy. The implications of heat strain in pregnancy on maternal health and pregnancy outcome are not well understood. This protocol describes a study to assess the physiological response of pregnant women to environmental heat stress and the immediate effect this has on fetal wellbeing.
Methods and analysis: The study will be performed in West Kiang district, The Gambia; a semi-arid zone in West Africa with daily maximum temperatures ranging from approximately 32 to 40°C. We will recruit 125 pregnant women of all ages who perform agricultural work during their pregnancy. Participants will be followed every two months until delivery. At each study visit fetal growth will be measured by ultrasound scan. During the course of their working day we will take the following measurements: continuous maternal physiological measurements (heart rate, respiratory rate, chest skin temperature and tri-axis accelerometer data); intermittent maternal tympanic core temperature, four point skin temperature, blood pressure; intermittent fetal heart rate and, if eligible, umbilical artery doppler; intermittent environmental measurements of air temperature, humidity, solar radiation and wind speed. Venous blood and urine will be collected at beginning and end of day for biomarkers of heat strain or fetal distress and hydration status.
Introduction: Climate change predictions indicate that global temperatures are likely to exceed those seen in the last 200,000 years, rising by around 4°C above pre-industrial levels by 2100 (without effective mitigation of current emission rates). In regions of the world set to experience extreme temperatures, women often work outside in agriculture even during pregnancy. The implications of heat strain in pregnancy on maternal health and pregnancy outcome are not well understood. This protocol describes a study to assess the physiological response of pregnant women to environmental heat stress and the immediate effect this has on fetal wellbeing.
Methods and analysis: The study will be performed in West Kiang district, The Gambia; a semi-arid zone in West Africa with daily maximum temperatures ranging from approximately 32 to 40°C. We will recruit 125 pregnant women of all ages who perform agricultural work during their pregnancy. Participants will be followed every two months until delivery. At each study visit fetal growth will be measured by ultrasound scan. During the course of their working day we will take the following measurements: continuous maternal physiological measurements (heart rate, respiratory rate, chest skin temperature and tri-axis accelerometer data); intermittent maternal tympanic core temperature, four point skin temperature, blood pressure; intermittent fetal heart rate and, if eligible, umbilical artery doppler; intermittent environmental measurements of air temperature, humidity, solar radiation and wind speed. Venous blood and urine will be collected at beginning and end of day for biomarkers of heat strain or fetal distress and hydration status.
This chapter presents an overview of the general principles of human thermoregulation at the systems level (i.e. the level of architecture and organization) as well as the relevant afferent and efferent thermoeffector pathways in the central and peripheral nervous systems. It focuses primarily on systems and pathways related to autonomic (i.e. involuntary) and behavioural (i.e. voluntary) thermoregulation. These are two entirely different branches of the human thermoregulatory system, sharing the same sensors for detecting changes in thermal homeostasis but having discrete pathways for afferent and efferent information relay, as well as for central (i.e., brain) information processing. The chapter also presents in detail the four major architectural concepts of human thermoregulation that have been proposed since the 1960s and discusses the advantages and limitations of each one for explaining the various phenomena of human thermoregulation.
INTRODUCTION: The aim of this study was to investigate the impact of heat acclimatization on athletic performance during the 7-day Marathon des Sables (MdS) which takes place in the Sahara Desert. METHODS: Anonymous data for nationality and average running speed (km/h) of all runners who ran the MdS during the period 2000–2015 were collected from the official website of the race and other related websites. Average maximum temperature for each runner’s country during the month preceding the MdS was collected from www.weatherbase.com. Athletes were divided into two Torigin groups as follows: -5 to 15˚C (i.e., cold countries) and 15 to 35˚C (i.e., warm countries). RESULTS: Overall, 12 467 (10 828 men; 1639 women) athletes from 78 countries (37 cold; 41 warm) participated in the MdS during the 16-year study period. The ambient temperature of these countries one month prior to the MdS ranged from 24.2 to 34.4˚C. Athletes’ average running speed during the MdS ranged from 2.9 to 13.4 km/h. Moreover, athletes who originated from warm countries ran the MdS 10.7% faster compared to athletes from cold countries. CONCLUSION: The natural heat acclimatization achieved by living in warmer countries seems to provide an advantage during the MdS.
The human capacity to perform prolonged exercise is impaired in hot environments. To address this issue, a number of studies have investigated behavioral aspects of thermoregulation that are recognized as important factors in determining performance. In this review, we evaluated and interpreted the available knowledge regarding the voluntary control of exercise work rate in hot environments. Our analysis indicated that: (a) Voluntary reductions in exercise work rate in uncompensable heat aid thermoregulation and are, therefore, thermoregulatory behaviors. (b) Unlike thermal behavior during rest, the role of thermal comfort as the ultimate mediator of thermal behavior during exercise in the heat remains uncertain. By contrast, the rating of perceived exertion appears to be the key perceptual controller under such conditions, with thermal perception playing a more modulatory role. (c) Prior to increases in core temperature (when only skin temperature is elevated), reductions in self-selected exercise work rate in the heat are likely mediated by thermal perception (thermal comfort and sensation) and its influence on the rating of perceived exertion. (d) However, when both core and skin temperatures are elevated, factors associated with cardiovascular strain likely dictate the rate of perceived exertion response, thereby mediating such voluntary reductions in exercise work rate.
© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
Older adults typically experience greater levels of thermal strain during physical efforts in the heat compared to young individuals. While this may be related to an age-dependent reduction in whole-body sweating, no study has clearly delineated at what age this occurs. In the present study, we report direct measurements of human heat dissipation during physical activity in the heat in males ranging in age from 20-70 years. Eighty-five males performed four 15-min bouts of cycling separated by 15-min rest periods, in a calorimeter regulated to 35°C and 20% relative humidity. Direct calorimetry was used to measure total heat loss (whole-body evaporative heat loss and dry heat exchange). We also used indirect calorimetry as a continuous measure of metabolic heat production. Body heat storage was calculated as the temporal summation of heat production and total heat loss over the experimental session. Whole-body sweat rate (WBSR) was calculated from measurements of evaporative heat loss. Males were divided into five age categories for the analysis of WBSR and body heat storage: 20-31 years (n = 18), 40-44 years (n = 15), 45-49 years (n = 15), 50-55 years (n = 21) and 56-70 years (n = 16). Relative to young males, WBSR was reduced in males aged 56-70 during each exercise (all P<0.05), in males aged 50-55 during the second (P = 0.031) and third exercises (P = 0.028) and in males aged 45-49 during the final exercise bout (P = 0.046). Although not significantly different, 40-44 years old males also had a lower rate of heat loss compared to younger males. Over the sum of two hours, the change in body heat content was greater in males 40-70 years compared to young males (all P<0.05). Our findings suggest that middle-aged and older adults have impairments in heat dissipation when doing physical activity in the heat, thus possibly increasing their risk of heat-related illness under such conditions.
Abstract Chronic medical conditions such as type 2 diabetes may alter the body's normal response to heat. Evidence suggests that the local heat loss response of skin blood flow (SkBF) is affected by diabetes-related impairments in both endothelium-dependent and non-endothelium-dependent mechanisms, resulting in lower elevations in SkBF in response to a heat or pharmacological stimulus. Thermoregulatory sweating may also be diminished by type 2 diabetes, impairing the body's ability to transfer heat from its core to the environment. Diabetes-associated co-morbidities and the medications (particularly those affecting fluid balance) required to treat these conditions may exacerbate the risk of heat-related illness by decreasing SkBF and sweating further. Unfortunately, the majority of studies measure local heat loss responses in the hands and feet and lack measures of core temperature. Therefore, the impact of these impairments on whole-body heat loss remains unknown. This review addresses heat-related vulnerability in individuals with type 2 diabetes by examining the literature related to heat loss responses in this population. Type 2 diabetes, its associated co-morbidities, and the medications required in their treatment may cause dehydration, lower SkBF, and reduced sweating, which could consequently impair thermoregulation. This effect is most evident in individuals with poor blood glucose control. Although type 2 diabetes can be associated with impairments in SkBF and sweating, more physically active individuals requiring fewer medications and having good blood glucose control may be able to tolerate heat as well as those of similar age and body composition.
The sections in this article are:
The sections in this article are:
A rethinking of current heat-acclimation strategies is required as most research and advice for improving physiological strain in the heat includes maintaining hydration using long-term acclimation protocols (>10 days). Furthermore, these strategies have tended to use untrained and moderately trained participants. Therefore, the aims of this review were to (i) investigate the effectiveness of short-term heat acclimation (STHA) with moderately and highly trained athletes; (ii) determine the importance of fluid regulatory strain, which has a thermally independent role in heat adaptation; (iii) assess the impact of STHA on a marker of thermotolerance (inducible heat-shock protein 70 [HSP70]); and (iv) provide further information on the decay of acclimation to heat. The review suggests that 5-day STHA is effective, and adaptations may be more pronounced after fluid regulatory strain from a dehydration-acclimation regimen. Furthermore, highly trained athletes may have similar physiological gains to those who are less trained using STHA. However, research has tended to focus on untrained or moderately trained participants and more information is required for highly trained populations. HSP70 response is upregulated across STHA. This indicates increased thermotolerance and protective adaptive change that may indicate HSP70 response as a useful marker of heat acclimation. Physiological adaptations after heat acclimation are relatively short term and may vanish only a few days or weeks after removal from heat exposure. From a practical perspective 5-day STHA may be the preferred acclimation regimen for moderately and highly trained athletes as it has been shown to be effective, less expensive and less likely to disrupt the tapering for competition in elite performers. Furthermore, updated information on the time course of acclimation decay may allow a reliable estimate of how long individuals can be free from heat exposure before reacclimation is required. This is particularly pertinent in present times as many athletes, civilians and military personnel increasingly have to relocate to different climates of the world, often within a short period of time.
The purpose of this work was to investigate adaptation and decay from short-term (5-day) heat acclimation (STHA). Ten moderately trained males (mean ± SD age 28 ± 7 years; body mass 74.6 ± 4.4 kg; \( \dot{V}_{{{\text{O}}_{ 2{\text{peak}}} }} \) 4.26 ± 0.37 l min−1) underwent heat acclimation (Acc) for 90-min on 5-days consecutively (T
a = 39.5°C, 60% RH), under controlled hyperthermia (rectal temperature 38.5°C). Participants completed a heat stress test (HST) 1 week before acclimation (Acc), then on the 2nd and 8th day (1 week) following Acc (T
a = 35°C, 60% RH). Seven participants completed HSTs 2 and 3 weeks after Acc. HST consisted of 90-min cycling at 40% peak power output before an incremental performance test. Rectal temperature at rest (37.1 ± 0.4°C) was not lowered by Acc (95% CI −0.3 to 0.2°C), after 90-min exercise (38.6 ± 0.5°C) it reduced 0.3°C (−0.5 to −0.1°C) and remained at this level 1 week later (−0.5 to −0.1°C), but not two (0.1°C −0.4 to 0.5°C; n = 7) or 3 weeks. Similarly, heart rate after 90-min exercise (146 ± 21 b min−1) was reduced (−13: −6 to −20 b min−1) and remained at this level after 1 week (−13: −6 to −20 b min−1) but not two (−9: 6 to −23 b min−1; n = 7) or 3 weeks. Performance (746 s) increased 106 s: 59 to 152 s after Acc and remained higher after one (76 s: 31 to 122) but not two (15 s: −88 to 142 s; n = 7) or 3 weeks. Therefore, STHA (5-day) induced adaptations permitting increased heat loss and this persisted 1 week but not 2 weeks following Acc.
Scientists haven predicted that extremes in climate are likely to increase in frequency and severity.(1) These changes may have a direct impact on population health, as heat waves can exceed the physiological adaptive capacity of vulnerable population groups. Individuals over the age of 60 years are consistently the most vulnerable,(2-4) with 82%-92% of excess mortality occurring in this group.(5) Risks for heat-related illness or injuries are compounded for people with obesity,(6,7) cardiovascular disease,(8-10) respiratory disease(8-10) and diabetes mellitus.(4,8,9) These conditions decrease the body's ability to adapt to changes in environmental conditions.(11) When people must perform physical work in the heat, the occurrence of heat-related morbidity and mortality is likely to be more frequent.(12) Although these trends in heat-related morbidity and mortality are evident, there has been little research to explain the causes of increased susceptibility within vulnerable populations. In this review, we describe the effects of heat on human physiology and the factors that increase the risk of heat stress. The methods used in preparing this review are summarized in Box 1 and are described in greater detail in Appendix 1 (available at www.cmaj.ca/cgi/content/full/cmaj.081050/DC1).
This study examined the independent and combined importance of aerobic fitness and body fatness on physiological tolerance and exercise time during weight-bearing exercise while wearing a semipermeable protective ensemble. Twenty-four men and women were matched for aerobic fitness and body fatness in one of four groups (4 men and 2 women in each group). Aerobic fitness was expressed per kilogram of lean body mass (LBM) to eliminate the influence of body fatness on the expression of fitness. Subjects were defined as trained (T; regularly active with a peak aerobic power of 65 ml · kg LBM ⁻¹ · min ⁻¹ ) or untrained (UT; sedentary with a peak aerobic power of 53 ml · kg LBM ⁻¹ · min ⁻¹ ) with high (High; 20%) or low (Low; 11%) body fatness. Subjects exercised until exhaustion or until rectal temperature reached 39.5°C or heart rate reached 95% of maximum. Exercise times were significantly greater in T Low (116 ± 6.5 min) compared with their matched sedentary (UT Low ; 70 ± 3.6 min) or fatness (T High ; 82 ± 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness. However, similar effects were not evident between T High and UT High (74 ± 4.1 min) or between the UT groups (UT Low and UT High ). The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9°C), whereas both body fatness and rate of heat storage affected the exercise time as independent factors.
The distribution of cutaneous thermosensitivity has not been determined in humans for the control of autonomic or behavioural thermoregulation under open-loop conditions. We therefore examined local cutaneous warm and cool sensitivities for sweating and whole-body thermal discomfort (as a measure of alliesthesia). Thirteen males rested supine during warming (+4 degrees C), and mild (-4 degrees C) and moderate (-11 degrees C) cooling of ten skin sites (274 cm(2)), whilst the core and remaining skin temperatures were clamped above the sweat threshold using a water-perfusion suit and climate chamber. Local thermosensitivities were calculated from changes in sweat rates (pooled from sweat capsules on all limbs) and thermal discomfort, relative to the changes in local skin temperature. Thermosensitivities were examined across local sites and body segments (e.g. torso, limbs). The face displayed stronger cold (-11 degrees C) sensitivity than the forearm, thigh, leg and foot (P = 0.01), and was 2-5 times more thermosensitive than any other segment for both sudomotor and discomfort responses (P = 0.01). The face also showed greater warmth sensitivity than the limbs for sudomotor control and discomfort (P = 0.01). The limb extremities ranked as the least thermosensitive segment for both responses during warming, and for discomfort responses during moderate cooling (-11 degrees C). Approximately 70% of the local variance in sudomotor sensitivity was common to the alliesthesial sensitivity. We believe these open-loop methods have provided the first dear evidence for a greater facial thermosensitivity for sweating and whole-body thermal discomfort.
Individualized model of human thermoregulation for the simulation of heat stress response. J Appl Physiol 90: 1943-1954, 2001. - A population-based dynamic model of human thermoregulation was expanded with control equations incorporating the individual person's characteristics (body surface area, mass, fat%, maximal O2 uptake, acclimation). These affect both the passive (heat capacity, insulation) and active systems (sweating and skin blood flow function). Model parameters were estimated from literature data. Other data, collected for the study of individual differences {working at relative or absolute workloads in hot-dry [45°C, 20% relative humidity (rh)], warm-humid [35°C, 80% rh], and cool [21°C, 50% rh] environments}, were used for validation. The individualized model provides an improved prediction [mean core temperature error, -0.21 → -0.07°C (P < 0.001); mean squared error, 0.40 → 0.16°C, (P < 0.001)]. The magnitude of improvement varies substantially with the climate and work type. Relative to an empirical multiple-regression model derived from these specific data sets, the analytical simulation model has between 54 and 89% of its predictive power, except for the cool climate, in which this ratio is zero. In conclusion, individualization of the model allows improved prediction of heat strain, although a substantial error remains.
During prolonged exercise or work in the heat, human thermal homeostasis is first challenged, and eventually lost, as one moves from a compensable state through to uncompensable heat stress. During the first week of such exposure, work and athletic performance is most affected, and the threat of heat illness is greatest. However, given adequate time, the body will undergo a three-phase adaptation to better tolerate the heat. In this review, the principles and practices of the six primary methods by which such heat adaptation may be achieved are evaluated. One technique involves repeated exposure to both heat and exercise, and is designed to elevate and maintain a target body temperature, by varying the intensity of the work rate during the acclimation period: the controlled-hyperthermia (isothermal) technique. It is recommended that this method provides the most dependable, and least hazardous, means of adapting workers and athletes for heat stress.
The changes in esophageal and mean skin temperatures did not show a marked difference between active limb movement (ALM) and passive limb movement (PLM). Increase in heart rate was significantly greater during ALM than PLM at 30 and 60 rpm (P < 0.05). Sweating rate on the chest and forearm were significantly greater during ALM than PLM at each pedalling frequency (P < 0.05). The result suggests that the greater increase in sweating rate during ALM relative to PLM may in part be of a consequence of non-thermal factors.
Introduction:
Aging is associated with a reduction in the body's capacity to dissipate heat. To date, few studies have examined age-related changes in thermoregulatory function during short exercise periods in the heat in older females.
Purpose:
This study aimed to investigate the effects of age on whole-body heat loss during intermittent exercise in the heat in young and older females.
Methods:
Direct and indirect calorimetry was used to measure whole-body evaporative heat loss (EHL), change in body heat content, and metabolic heat production. Eleven young (Y) (mean ± SD age = 24 ± 4 yr) and 13 older (O) (51 ± 8 yr) females matched for body surface area (Y, 1.72 ± 0.15; O, 1.75 ± 0.12 m²) and fitness (V(˙)O(2max)) (Y, 36.7 ± 6.8 mL O₂·kg⁻¹·min⁻¹; O, 33.8 ± 8.0 mL O₂·kg⁻¹·min⁻¹) performed four bouts of 15-min cycling (Ex1, Ex2, Ex3, and Ex4) at a constant rate of heat production (300 W) at 35°C and 20% relative humidity. Each exercise bout was separated by 15 min of rest.
Results:
EHL was reduced in O compared with Y during Ex1 (O, 199 ± 6 W; Y, 240 ± 9 W; P = 0.001), Ex2 (O, 238 ± 4 W; Y, 261 ± 9 W, P = 0.023), and Ex3 (O, 249 ± 4 W; Y, 274 ± 11 W; P = 0.040). EHL was not different between groups during Ex4 or during the recovery periods. Older females had a greater change in body heat content compared with young females (O, 270 ± 20 kJ; Y, 166 ± 20 kJ; P = 0.001).
Conclusion:
These findings suggest that older females have a lower capacity for whole-body EHL compared with younger females during short intermittent exercise in the heat performed at a fixed rate of metabolic heat production.
Introduction:
It is unknown if diabetes-related reductions in local skin blood flow (SkBF) and sweating (LSR) measured during passive heat stress translate into greater heat storage during exercise in the heat in individuals with type 2 diabetes (T2D) compared with nondiabetic control (CON) subjects.
Purpose:
This study aimed to examine the effects of T2D on whole-body heat exchange during exercise in the heat.
Methods:
Ten adults (6 males and 4 females) with T2D and 10 adults (6 males and 4 females) without diabetes matched for age, sex, body surface area, and body surface area and aerobic fitness cycled continuously for 60 min at a fixed rate of metabolic heat production (∼370 W) in a whole-body direct calorimeter (30°C and 20% relative humidity). Upper back LSR, forearm SkBF, rectal temperature, and heart rate were measured continuously. Whole-body heat loss and changes in body heat content (ΔHb) were determined using simultaneous direct whole-body and indirect calorimetry.
Results:
Whole-body heat loss was significantly attenuated from 15 min throughout the remaining exercise with the differences becoming more pronounced over time for T2D relative to CON (P = 0.004). This resulted in a significantly greater ΔHb in T2D (367 ± 35; CON, 238 ± 25 kJ, P = 0.002). No differences were measured during recovery (T2D, -79 ± 23; CON, -132 ± 23 kJ, P = 0.083). By the end of the 60-min recovery, the T2D group lost only 21% (79 kJ) of the total heat gained during exercise, whereas their nondiabetic counterparts lost in excess of 55% (131 kJ). No difference were observed in LSR, SkBF, rectal temperature or heart rate during exercise. Similarly, no differences were measured during recovery with the exception that heart rate was elevated in the T2D group relative to CON (p=0.004).
Conclusion:
Older adults with T2D have a reduced capacity to dissipate heat during exercise, resulting in a greater heat storage and therefore level of thermal strain.
Although the requirements for heat dissipation during exercise are determined by the necessity for heat balance, few studies have considered them when examining sweat production and its potential modulators. Rather, the majority of studies have used an experimental protocol based on a fixed percent of maximum oxygen uptake (%VO2max). Using multiple regression analysis, we examined the independent contribution of the evaporative requirement for heat balance (Ereq) and %VO2max to whole-body sweat rate (WBSR) during exercise. We hypothesised that WBSR would be determined by Ereq and not by %VO2max. A total of 23 males performed two separate experiments during which they exercised for 90 min at different rates of metabolic heat production (200, 350, 500 W) at a fixed air temperature (30°C, n=8), or at a fixed rate of metabolic heat production (290 W) at different air temperatures (30, 35, 40°C, n=15 and 45°C, n=7). Whole-body evaporative heat loss was measured by direct calorimetry and used to calculate absolute WBSR in g·min-1. The conditions employed resulted in a wide range of Ereq (131 to 487 W) and %VO2max (15 to 55%). The individual variation in non steady-state (0-30 min) and steady-state (30-90 min) WBSR correlated significantly with Ereq (p<0.001). In contrast, %VO2max correlated negatively with the residual variation in WBSR not explained by Ereq, and marginally increased (~2%) the amount of total variability in WBSR described by Ereq alone (non steady-state: R2 = 0.885; steady-state: R2 = 0.930). These data provide clear evidence that absolute WBSR during exercise is determined by Ereq, not by %VO2max. Future studies should therefore use an experimental protocol which ensures a fixed Ereq when examining absolute WBSR between individuals, irrespective of potential differences in relative exercise intensity.
Observations were made on American marathon record holder Alberto Salazar during a climatic chamber trial, heat acclimatization training, and the 1984 Olympic Marathon. Blood samples and rectal temperature data showed that hormonal and thermoregulatory mechanisms were normal. However, measurements of a very high sweat rate (2.79 liter/hr and 3.06 liter/hr) indicated that dehydration was a potentially serious problem. In fact, Salazar lost 5.43 kg (-8.1 percent) during the Olympic marathon, in 134.3 minutes of running. Although Salazar's decreased rectal temperature was desireable, his increased sweat rate was an unnecessary physiological adaptation to training in the heat. Keywords include: Exercise, physical training, gastric emptying, body weight, and iron deficiency.
The current study aimed to determine if a peripheral modulation of sweating contributes to the lower sudomotor thermosensitivity previously observed in females during exercise. We examined dose-response relationships in 12 males and 12 females to incremental doses of acetylcholine (ACh) and methylcholine (MCh) for sweating (ventilated capsule), as well as to ACh and sodium nitroprusside (SNP) for cutaneous vasodilation (laser-Doppler). All drugs were infused using intradermal microdialysis. On a separate day, potential sex-differences in the onset threshold and/or thermosensitivity of heat loss responses were assessed during progressive increases in mean body temperature elicited by passive heating. Increases in sweating as a function of increasing concentration of ACh (p=0.008) and MCh (p=0.046) significantly differed between males and females. Although the concentration eliciting 50% of the maximal sweating response did not differ between sexes for either agonist (p>0.1), maximum values were lower in females in response to ACh (0.34 ± 0.12 vs. 0.59 ± 0.19 mg•min(-1)•cm(-2), p=0.04) and MCh (0.48 ± 0.12 vs. 0.78 ± 0.26 mg•min(-1)•cm(-2), p=0.05). This observation was paralleled by a lower thermosensitivity of sudomotor activity in females during passive heating (1.29 ± 0.34 vs. 1.83 ± 0.33 mg•min(-1)•cm(-2)•°C(-1), p=0.03), with no significant differences in the onset threshold (0.85 ± 0.19 vs. 0.67 ± 0.13°C, p=0.10). No sex-differences in cutaneous vasodilation were observed in response to ACh and SNP, as well as during passive heating (all p>0.1). These findings provide direct evidence for a peripheral modulation of sudomotor activity in females. In contrast, sex does not modulate cutaneous vasodilation.
The sections in this article are:
Human body temperature control is characterized by a local dependence of system variables and parameters: receptors are thought to be distributed locally throughout the body. The integrative control centres appear to extend quasicontinuously from the spinal cord up to the hypothalamus. Finally, control actions obviously affect the passive system with distinct local dependence. The essential properties of non-linear distributed parameter control of human body temperature are demonstrated by way of experimental and mathematical results.With regard to the control strategy there are evidently five different concepts which claim to answer the question: ‘What is regulated and how?’ A control concept based on spatial integration of temperatures plus local effector actuation seems to be the one which is most adequately supported by system-theoretical as well as by experimental results.The discussion about ‘reference’ and ‘set-point’ of temperature regulation is still going on, and concentrates on four concepts. The concept of balance of passive and active processes, i.e. of controlling and of controlled subsystems, defines the steady-state as the only compatible operating point of the open loop functions in the closed control loop. It does not need an explicit neuronal reference input or difference of controller inputs and solves simultaneously the so-called problem of changing set-point due to fever or to circadian rhythm. This is explained as an adjustment of control parameters requiring another balance of controller and passive system.
To assess potential mechanisms responsible for the lower sudomotor thermosensitivity in women during exercise, we examined sex differences in sudomotor function and skin blood flow (SkBF) during exercise performed at progressive increases in the requirement for heat loss. Eight men and eight women cycled at rates of metabolic heat production of 200, 250, and 300 W/m(2) of body surface area, with each rate being performed sequentially for 30 min. The protocol was performed in a direct calorimeter to measure evaporative heat loss (EHL) and in a thermal chamber to measure local sweat rate (LSR) (ventilated capsule), SkBF (laser-Doppler), sweat gland activation (modified iodine-paper technique), and sweat gland output (SGO) on the back, chest, and forearm. Despite a similar requirement for heat loss between the sexes, significantly lower increases in EHL and LSR were observed in women (P ≤ 0.001). Sex differences in EHL and LSR were not consistently observed during the first and second exercise periods, whereas EHL (348 ± 13 vs. 307 ± 9 W/m(2)) and LSR on the back (1.61 ± 0.07 vs. 1.20 ± 0.09 mg·min(-1)·cm(-2)), chest (1.33 ± 0.06 vs. 1.08 ± 0.09 mg·min(-1)·cm(-2)), and forearm (1.53 ± 0.07 vs. 1.20 ± 0.06 mg·min(-1)·cm(-2), men vs. women) became significantly greater in men during the last exercise period (P < 0.05). At each site, differences in LSR were solely due to a greater SGO in men, as opposed to differences in sweat gland activation. In contrast, no sex differences in SkBF were observed throughout the exercise period. The present study demonstrates that sex differences in sudomotor function are only evidenced beyond a certain requirement for heat loss, solely through differences in SGO. In contrast, the lower EHL and LSR in women are not paralleled by a lower SkBF response.
The sections in this article are:
• To investigate the effect of the muscle metaboreflex on the thermoregulatory sweating response in humans, eight healthy male subjects performed sustained isometric handgrip exercise in an environmental chamber (35 °C and 50% relative humidity) at 30 or 45% maximal voluntary contraction (MVC), at the end of which the blood circulation to the forearm was occluded for 120 s. The environmental conditions were such as to produce sweating by increase in skin temperature without a marked change in oesophageal temperature. • During circulatory occlusion after handgrip exercise at 30% MVC for 120 s or at 45% MVC for 60 s, the sweating rate (SR) on the chest and forearm (hairy regions), and the mean arterial blood pressure were significantly above baseline values ( P • During the occlusion after handgrip exercise at 30% MVC for 60 s and during the occlusion alone, none of the measured parameters differed from baseline values. • It is concluded that, under mildly hyperthermic conditions, the thermoregulatory sweating response on the hairy regions is modulated by afferent signals from muscle metaboreceptors.
The present study investigated the influence of maximal oxygen uptake (V(O2 max)) on local steady-state sudomotor responses to exercise, independently of evaporative requirements for heat balance (E(req)). Eleven fit (F; (V(O2 max))61.9 ± 6.0 ml kg(-1) min(-1)) and 10 unfit men (UF; (V(O2 max)) 40.4 ± 3.8 ml kg(-1) min(-1)) cycled for 60 min at an air temperature of 24.5 ± 0.8°C and ambient humidity of 0.9 ± 0.3 kPa at a set metabolic heat production per unit surface area, producing the same E(req) in all participants (BAL trial) and, in a second trial, at 60% of (V(O2 max)). During the BAL trial, absolute power (F 107 ± 2 and UF 102 ± 2 W; P = 0.126), E(req) (F 175 ± 5 and UF 176 ± 9 W m(-2); P = 0.855), steady-state whole-body sweat rate (F 0.44 ± 0.02 and UF 0.47 ± 0.02 mg cm(-2) min(-1); P = 0.385) and local sweat rate on the arm (F 0.29 ± 0.03 and UF 0.35 ± 0.03 mg cm(-2) min(-1); P = 0.129) were not different between groups; however, local sweat rate on the forehead in UF (1.67 ± 0.20 mg cm(-2) min(-1)) was almost double (P = 0.002) that of F (0.87 ± 0.11 mg cm(-2) min(-1)). Heart rate, ratings of perceived exertion and relative exercise intensity were also significantly greater in UF (P < 0.05). There was a trend towards an elevated minute ventilation in UF (P = 0.052), while end-tidal P(CO2) was significantly lower in UF (P = 0.028). At 60% (V(O2 max)), absolute power (F 174 ± 6 and UF 110 ± 5 W; P < 0.001), E(req) (F 291 ± 14 and UF 190 ± 17 W m(-2); P < 0.001), steady-state whole-body sweat rate (F 0.84 ± 0.05 and UF 0.53 ± 0.03 mg cm(-2) min(-1); P < 0.001) and local sweat rate on the arm (F 0.75 ± 0.04 and UF 0.35 ± 0.03 mg cm(-2) min(-1); P < 0.001) and on the forehead (F 2.92 ± 0.42 and UF 1.68 ± 0.23 mg cm(-2) min(-1); P = 0.022) were all significantly greater in F compared with UF. Heart rate and ratings of perceived exertion were similar at all time points (P > 0.05). Significantly greater minute ventilation (P < 0.001) and end-tidal P (CO2) responses (P = 0.017) were found in F. In conclusion, aerobic fitness alters local sweating on the forehead, but not the forearm, independently of evaporative requirements for heat balance, and may be the result of differential control of sweating in these skin areas associated with the relative intensity of exercise.
Plasma hyperosmolality and baroreceptor unloading have been shown to independently influence the heat loss responses of sweating and cutaneous vasodilation. However, their combined effects remain unresolved. On four separate occasions, eight males were passively heated with a liquid-conditioned suit to 1.0°C above baseline core temperature during a resting isosmotic state (infusion of 0.9% NaCl saline) with (LBNP) and without (CON) application of lower-body negative pressure (-40 cmH2O) and during a hyperosmotic state (infusion of 3.0% NaCl saline) with (LBNP + HYP) and without (HYP) application of lower-body negative pressure. Forearm sweat rate (ventilated capsule) and skin blood flow (laser-Doppler), as well as core (esophageal) and mean skin temperatures, were measured continuously. Plasma osmolality increased by ∼10 mosmol/kgH2O during HYP and HYP + LBNP conditions, whereas it remained unchanged during CON and LBNP (P ≤ 0.05). The change in mean body temperature (0.8 × core temperature + 0.2 × mean skin temperature) at the onset threshold for increases in cutaneous vascular conductance (CVC) was significantly greater during LBNP (0.56 ± 0.24°C) and HYP (0.69 ± 0.36°C) conditions compared with CON (0.28 ± 0.23°C, P ≤ 0.05). Additionally, the onset threshold for CVC during LBNP + HYP (0.88 ± 0.33°C) was significantly greater than CON and LBNP conditions (P ≤ 0.05). In contrast, onset thresholds for sweating were not different during LBNP (0.50 ± 0.18°C) compared with CON (0.46 ± 0.26°C, P = 0.950) but were elevated (P ≤ 0.05) similarly during HYP (0.91 ± 0.37°C) and LBNP + HYP (0.94 ± 0.40°C). Our findings show an additive effect of hyperosmolality and baroreceptor unloading on the onset threshold for increases in CVC during whole body heat stress. In contrast, the onset threshold for sweating during heat stress was only elevated by hyperosmolality with no effect of the baroreflex.
The relative influence of muscle metabo- and baroreflex activity on heat loss responses during post-isometric handgrip (IHG) exercise ischemia remains unknown, particularly under heat stress. Therefore, we examined the separate and integrated influences of metabo- and baroreceptor-mediated reflex activity on sweat rate and cutaneous vascular conductance (CVC) under increasing levels of hyperthermia. Twelve men performed 1 min of IHG exercise at 60% of maximal voluntary contraction followed by 2 min of ischemia with simultaneous application of lower body positive pressure (LBPP, +40 mmHg), lower body negative pressure (LBNP, -20 mmHg), or no pressure (control) under no heat stress. On separate days, trials were repeated under heat stress conditions of 0.6°C (moderate heat stress) and 1.4°C (high heat stress) increase in esophageal temperature. For all conditions, mean arterial pressure was greater with LBPP and lower with LBNP than control during ischemia (all P ≤ 0.05). No differences in sweat rate were observed between pressure conditions, regardless of the level of hyperthermia (P > 0.05). Under moderate heat stress, no differences in CVC were observed between pressure conditions. However, under high heat stress, LBNP significantly reduced CVC by 21 ± 4% (P ≤ 0.05) and LBPP significantly elevated CVC by 14 ± 5% (P ≤ 0.05) relative to control. These results show that sweating during post-IHG exercise ischemia is activated by metaboreflex stimulation, and not by baroreflexes. In contrast, our results suggest that baroreflexes can influence the metaboreflex modulation of CVC, but only at greater levels of hyperthermia.
The purpose of the present study was to determine the separate and combined effects of aerobic fitness, short-term heat acclimation, and hypohydration on tolerance during light exercise while wearing nuclear, biological, and chemical protective clothing in the heat (40 degrees C, 30% relative humidity). Men who were moderately fit [(MF); <50 ml . kg-1 . min-1 maximal O2 consumption; n = 7] and highly fit [(HF); >55 ml . kg-1 . min-1 maximal O2 consumption; n = 8] were tested while they were euhydrated or hypohydrated by approximately 2.5% of body mass through exercise and fluid restriction the day preceding the trials. Tests were conducted before and after 2 wk of daily heat acclimation (1-h treadmill exercise at 40 degrees C, 30% relative humidity, while wearing the nuclear, biological, and chemical protective clothing). Heat acclimation increased sweat rate and decreased skin temperature and rectal temperature (Tre) in HF subjects but had no effect on tolerance time (TT). MF subjects increased sweat rate but did not alter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased Tre and heart rate and decreased the respiratory exchange ratio and the TT regardless of acclimation state. Overall, the rate of rise of skin temperature was less, while DeltaTre, the rate of rise of Tre, and the TT were greater in HF than in MF subjects. It was concluded that exercise-heat tolerance in this uncompensable heat-stress environment is not influenced by short-term heat acclimation but is significantly improved by long-term aerobic fitness.
Multiple heterogeneous groups of subjects (both sexes and a wide range of maximal oxygen uptake VO2max, body mass, body surface area (AD),% body fat, and AD/mass coefficient) exercised on a cycle ergometer at a relative (%VO2max, REL) or an absolute (60 W) exercise intensity in a cool (CO 21 degrees C, 50% relative humidity), warm humid (WH 35 degrees C, 80%) and a hot dry (HD 45 degrees C, 20%) environment. Rectal temperature (Tre) responses were analysed for the influence of the individual's characteristics, environment and exercise intensity. Exposures consisted of 30-min rest, followed by 60-min exercise. The Tre was negatively correlated with mass in all conditions. Body mass acted as a passive heat sink in all the conditions tested. While negatively correlated with VO2max and VO2max per kilogram body mass in most climates, Tre was positively correlated with VO2max and VO2max per kilogram body mass in the WH/REL condition. Thus, when evaporative heat loss was limited as in WH, the higher heat production of the fitter subjects in the REL trials determined Tre and not the greater efficiency for heat loss associated with high VO2max. Body fatness significantly affected Tre only in the CO condition, where, with low skin blood flows (measured as increases in forearm blood flow), the insulative effect of fat was pronounced. In the warmer environments, high skin blood flows offset the resistance offered by peripheral adipose tissue. Contrary to other studies, Tre was positively correlated with AD/mass coefficient for all conditions tested. For both exercise types used, being big (a high heat loss area and heat capacity) was apparently more beneficial from a heat strain standpoint than having a favourable AD/mass coefficient (high in small subjects). The total amount of variance in Tre responses which could be attributed to individual characteristics was dependent on the climate and the type of exercise. Though substantial for absolute exercise intensities (52%-58%) the variance explained in Tre differed markedly for relative intensities: 72% for the WH climate with its limited evaporative capacity, and only 10%-26% for the HD and CO climates. The results showed that individual characteristics play a significant role in determining the responses of body core temperature in all conditions tested, but their contribution was low for relative exercise intensities when evaporative heat loss was not restricted. This study demonstrated that effects of individual characteristics on human responses to heat stress cannot be interpreted without taking into consideration both the heat transfer properties of the environment and the metabolic heat production resulting from the exercise type and intensity chosen. Their impact varies substantially among conditions.
During direct passive heating in young men, a dramatic increase in skin blood flow is achieved by a rise in cardiac output (Q(c)) and redistribution of flow from the splanchnic and renal vascular beds. To examine the effect of age on these responses, seven young (Y; 23 ± 1 yr) and seven older (O; 70 ± 3 yr) men were passively heated with water-perfused suits to their individual limit of thermal tolerance. Measurements included heart rate (HR), Q(c) (by acetylene rebreathing), central venous pressure (via peripherally inserted central catheter), blood pressures (by brachial auscultation), skin blood flow (from increases in forearm blood flow by venous occlusion plethysmography), splanchnic blood flow (by indocyanine green clearance), renal blood flow (by p-aminohippurate clearance), and esophageal and mean skin temperatures. Q(c) was significantly lower in the older than in the young men (11.1 ± 0.7 and 7.4 ± 0.21/min in Y and O, respectively, at the limit of thermal tolerance; P < 0.05), despite similar increases in esophageal and mean skin temperatures and time to reach the limit of thermal tolerance. A lower stroke volume (99 ± 7 and 68 ± 4 ml/beat in Y and O, respectively, P < 0.05), most likely due to an attenuated increase in inotropic function during heating, was the primary factor for the lower Q(c) observed in the older men. Increases in HR were similar in the young and older men; however, when expressed as a percentage of maximal HR, the older men relied on a greater proportion of their chronotropic reserve to obtain the same HR response (62 ± 3 and 75 ± 4% maximal HR in Y and O, respectively, P < 0.05). Furthermore, the older men redistributed less blood flow from the combined splanchnic and renal circulations at the limit of thermal tolerance (960 ± 80 and 720 ± 100 ml/min in Y and O, respectively, P < 0.05). As a result of these combined attenuated responses, the older men had a significantly lower increase in total blood flow directed to the skin.
Previous studies have suggested that greater core temperatures during intermittent exercise (Ex) are due to attenuated sweating [upper back sweat rate (SR)] and skin blood flow (SkBF) responses. We evaluated the hypothesis that heat loss is not altered during exercise-rest cycles (ER). Ten male participants randomly performed four 120-min trials: 1) 60-min Ex and 60-min recovery (60ER); 2) 3 × 20-min Ex separated by 20-min recoveries (20ER); 3) 6 × 10-min Ex separated by 10-min recoveries (10ER), or 4) 12 × 5-min Ex separated by 5-min recoveries (5ER). Exercise was performed at a workload of 130 W at 35°C. Whole body heat exchange was determined by direct calorimetry. Core temperature, SR (by ventilated capsule), and SkBF (by laser-doppler) were measured continuously. Evaporative heat loss (EHL) progressively increased with each ER, such that it was significantly greater (P ≤ 0.05) at the end of the last compared with the first Ex for 5ER (299 ± 39 vs. 440 ± 41 W), 10ER (425 ± 51 vs. 519 ± 45 W), and 20ER (515 ± 63 vs. 575 ± 74 W). The slope of the EHL response against esophageal temperature significantly increased from the first to the last Ex within the 10ER (376 ± 56 vs. 445 ± 89 W/°C, P ≤ 0.05) and 20ER (535 ± 85 vs. 588 ± 28 W/°C, P ≤ 0.05) conditions, but not during 5ER (296 ± 96 W/°C vs. 278 ± 95 W/°C, P = 0.237). In contrast, the slope of the SkBF response against esophageal temperature did not significantly change from the first to the last Ex (5ER: 51 ± 23 vs. 54 ± 19%/°C, P = 0.848; 10ER: 53 ± 8 vs. 56 ± 21%/°C, P = 0.786; 20ER: 44 ± 20 vs. 50 ± 27%/°C, P = 0.432). Overall, no differences in body heat content and core temperature were observed. These results suggest that altered local and whole body heat loss responses do not explain the previously observed greater core temperatures during intermittent exercise.
Please cite this paper as: Flouris and Cheung (2011). Thermal Basis of Finger Blood Flow Adaptations During Abrupt Perturbations in Thermal Homeostasis. Microcirculation18(1), 56–62.
The objective of this experiment was to assess whether reflex alterations in finger blood flow during repetitive hot and cold water immersion are associated with changes in rectal, tympanic, mean body temperature or heat storage. Fifteen healthy adults (eight males) volunteered. Following a 15-minute baseline period, participants were immersed in 42°C water and passively rested until their rectal temperature was raised by 0.5°C above baseline. Thereafter, they were immersed in a different water tank maintained at 12°C water temperature until their rectal temperature was decreased by 0.5°C below baseline. This procedure was conducted twice. Auto-Regressive Integrated Moving Average analysis showed that fluctuations in finger blood flow were associated with changes in mean body temperature (Ljung-Box statistic >0.05; R2 = 0.67) and body heat storage (Ljung-Box statistic >0.05; R2 = 0.70), but not with rectal (Ljung-Box statistic <0.05; R2 = 0.54) or tympanic (Ljung-Box statistic <0.05; R2 = 0.49) temperatures. It is concluded that reflex alterations in finger blood flow during repetitive hot and cold water immersions are associated with mean body temperature and the rate of body heat storage, but not with rectal and tympanic temperatures.
The combined metabolic and thermoregulatory demands of exercise in the heat place an exceptional burden on the circulation, more than can be met through cardiac output and blood flow redistribution. Blood flow to muscle is not reduced by heat stress in exercise and cardiac output is insufficient to meet competing demands from skin and muscle. Skin blood flow during exercise in the heat is limited in several ways. Dynamic exercise causes a cutaneous vasoconstriction at exercise onset through increased vasoconstrictor activity, both in cool and warm conditions. As exercise continues, internal temperature reaches a threshold for increased active vasodilator activity that is elevated by exercise, but reduced by high skin temperature. Beyond that threshold, skin blood flow is limited well below what would be achieved at rest with the same thermal drive through a limit to the active vasodilator system. This combination of restraints on cutaneous vasodilator function compromises temperature regulation. Internal temperature rises to levels that limit exercise through central thermal effects, rather than loss of blood pressure or a reduction in blood flow to active muscle.
Caloric restriction (CR) causes a reduction in body temperature (T(b)) which is suggested to contribute to changes that increase lifespan. Moreover, low T(b) has been shown to improve health and longevity independent of CR. In this review we examine the connections between CR, T(b) and mechanisms that influence longevity and ageing. Recent findings regarding the overlapping mechanisms of CR and T(b) that benefit longevity are discussed, including changes in body composition, hormone regulation, and gene expression, as well as reductions in low-level inflammation and reactive oxygen species-induced molecular damage. This information is summarized in a model describing how CR and low T(b), both synergistically and independently, increase lifespan. Moreover, the nascent notion that the rate of ageing may be pre-programmed in response to environmental influences at critical periods of early development is also considered. Based on current evidence, it is concluded that low T(b) plays an integral role in mediating the effects of CR on health and longevity, and that low T(b) may exert independent biological changes that increase lifespan. Our understanding of the overlap between CR- and T(b)-mediated longevity remains incomplete and should be explored in future research.
Exposure to excessive heat is a physical hazard that threatens Canadian workers. As patterns of global climate change suggest an increased frequency of heat waves, the potential impact of these extreme climate events on the health and well-being of the Canadian workforce is a new and growing challenge. Increasingly, industries rely on available technology and information to ensure the safety of their workers. Current Canadian labor codes in all provinces employ the guidelines recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) that are Threshold Limit Values (TLVs) based upon Wet Bulb Globe Temperature (WBGT). The TLVs are set so that core body temperature of the workers supposedly does not exceed 38.0 degrees C. Legislation in most Canadian provinces also requires employers to install engineering and administrative controls to reduce the heat stress risk of their working environment should it exceed the levels permissible under the WBGT system. There are however severe limitations using the WGBT system because it only directly evaluates the environmental parameters and merely incorporates personal factors such as clothing insulation and metabolic heat production through simple correction factors for broadly generalized groups. An improved awareness of the strengths and limitations of TLVs and the WGBT index can minimize preventable measurement errors and improve their utilization in workplaces. Work is on-going, particularly in the European Union to develop an improved individualized heat stress risk assessment tool. More work is required to improve the predictive capacity of these indices.
This review focuses on the characteristics of heat-loss responses during exercise with respect to non-thermal factors. In addition, the effects of physical training on non-thermal heat-loss responses are discussed. When a subject is already sweating the sweating rate increases at the onset of dynamic exercise without changes in core temperature, while cutaneous vascular conductance (skin blood flow) is temporarily decreased. Although exercise per se does not affect the threshold for the onset of sweating, it is possible that an increase in exercise intensity induces a higher sensitivity of the sweating response. Exercise increases the threshold for cutaneous vasodilation, and at higher exercise intensities, the sensitivity of the skin-blood-flow response decreases. Facilitation of the sweating response with increased exercise intensity may be due to central command, peripheral reflexes in the exercising muscle, and mental stimuli, whereas the attenuation of skin-blood-flow responses with decreased cutaneous vasodilation is related to many non-thermal factors. Most non-thermal factors have negative effects on magnitude of cutaneous vasodilation; however, several of these factors have positive effects on the sweating response. Moreover, thermal and non-thermal factors interact in controlling heat-loss responses, with non-thermal factors having a greater impact until core temperature elevations become significant, after which core temperature primarily would control heat loss. Finally, as with thermally induced sweating responses, physical training seems to also affect sweating responses governed by non-thermal factors.
This study compared the effect of active, passive, and inactive recoveries on whole body evaporative and dry heat loss responses during intermittent exercise at an air temperature of 30°C and a relative humidity of 20%. Nine males performed three 15-min bouts of upright seated cycling at a fixed external workload of 150 W. The exercise bouts were separated by three 15-min recoveries during which participants 1) performed loadless pedaling (active recovery), 2) had their lower limbs passively compressed with inflatable sleeves (passive recovery), or 3) remained upright seated on the cycle ergometer (inactive recovery). Combined direct and indirect calorimetry was employed to measure rates of whole body evaporative heat loss (EHL) and metabolic heat production (M-W). Mean body temperature (Tb) was calculated from esophageal and mean skin temperatures, and mean arterial pressure (MAP) was measured continuously. Active and passive recoveries both reversed the reduction in MAP associated with inactive recovery (P ≤ 0.05). This response was paralleled by greater levels of EHL during active (207 ± 53 W) and passive recoveries (203 ± 55 W) compared with the inactive condition (168 ± 53 W, P ≤ 0.05). However, the greater rate of EHL during active recovery was paralleled by a greater M-W (194 ± 16 W) compared with inactive recovery (149 ≤ 27 W, P ≤ 0.001). In contrast, M-W during passive recovery (139 ± 20 W) was not significantly different from the inactive condition (P = 0.468). Furthermore, there were no differences in Tb between inactive and passive conditions during the recovery periods (P = 0.820). As such, passive recovery resulted in greater levels of EHL for a given change in T b compared with inactive recovery (P ≤ 0.05). These results strongly suggest that the progressive increase in core temperature during successive exercise/rest cycles is primarily the result of a baroreflex-mediated attenuation of postexercise whole body evaporative heat loss.
Human thermoregulatory control during heat stress has been studied at rest, during exercise and more recently during exercise recovery. Heat balance in the body is maintained by changes in the rate of heat loss via adjustments in skin blood flow and sweating. Independent of thermal control, the actions of nonthermal factors have important consequences in the control of heat loss responses during and following exercise. While the effect of these nonthermal factors is largely considered to be an inhibitory or excitatory stimulus which displaces the set-point about which temperature is regulated, their effects on human thermoregulatory control are far reaching. Many factors can affect the relative contribution of thermal and nonthermal influences to heat balance including exercise intensity, hemodynamic status, and the level of hyperthermia imposed. This review will characterize the physiological responses associated with heat stress and discuss the thermal and nonthermal influences on sweating and skin blood flow in humans. Further, recent calorimetric evidence for the understanding of thermal and nonthermal contributions to human heat balance will also be discussed.
The most commonly studied laboratory rodents possess a specialized form of fat called brown adipose tissue (BAT) that generates heat to help maintain body temperature in cold environments. In humans, BAT is abundant during embryonic and early postnatal development, but is absent or present in relatively small amounts in adults where it is located in paracervical and supraclavicular regions. BAT cells can 'burn' fatty acid energy substrates to generate heat because they possess large numbers of mitochondria in which oxidative phosphorylation is uncoupled from ATP production as a result of a transmembrane proton leak mediated by uncoupling protein 1 (UCP1). Studies of rodents in which BAT levels are either increased or decreased have revealed a role for BAT in protection against diet-induced obesity. Data suggest that individuals with low levels of BAT are prone to obesity, insulin resistance and cardiovascular disease, whereas those with higher levels of BAT maintain lower body weights and exhibit superior health as they age. BAT levels decrease during aging, and dietary energy restriction increases BAT activity and protects multiple organ systems including the nervous system against age-related dysfunction and degeneration. Future studies in which the effects of specific manipulations of BAT levels and thermogenic activity on disease processes in animal models (diabetes, cardiovascular disease, cancers, neurodegenerative diseases) are determined will establish if and how BAT affects the development and progression of age-related diseases. Data from animal studies suggest that BAT and mitochondrial uncoupling can be targeted for interventions to prevent and treat obesity and age-related diseases. Examples include: diet and lifestyle changes; specific regimens of mild intermittent stress; drugs that stimulate BAT formation and activity; induction of brown adipose cell progenitors in muscle and other tissues; and transplantation of brown adipose cells.
On separate days, eight physically active younger (22 ± 2 years) and eight highly trained middle-aged (45 ± 4 years) men matched for physical fitness and body composition performed 90 min of semi-recumbent cycling at a constant rate of heat production (290 W) followed by 60 min of seated recovery in either a temperate (T, 30°C), warm (W, 35°C) or hot (H, 40°C) ambient condition. Rectal temperature (T
re) was measured continuously, while the rate of whole-body heat loss (H
L), as well as changes in body heat content (∆H
b) was measured simultaneously using direct whole-body and indirect calorimetry. No difference in H
L was observed between age groups for all ambient conditions. Accordingly, the average ∆H
b during the 90-min exercise was similar for the younger (+193 ± 52, 212 ± 82 and +211 ± 44 kJ for T, W and H, respectively) and middle-aged men (+192 ± 119, +225 ± 76 and +217 ± 130 kJ for T, W and H, respectively). This was paralleled by a similar increase in T
re of 0.40 ± 0.20, 0.36 ± 0.14 and 0.34 ± 0.23°C for T, W and H, respectively in the younger men and 0.37 ± 0.23, 0.32 ± 0.19 and 0.28 ± 0.14°C for T, W and H, respectively in the middle-aged men. After 60 min of recovery, ∆H
b was similar for the younger and the middle-aged men, respectively (−45 ± 52 and −38 ± 31 kJ for T; −57 ± 78 and −40 ± 25 kJ for W; and −32 ± 71 and 11 ± 96 kJ for H). End recovery T
re remained elevated to similar levels in both the younger and middle-aged men, respectively, for each of the ambient conditions (0.24 ± 019 and 0.18 ± 0.18°C for T; 0.25 ± 0.11 and 0.24 ± 0.14°C for W and 0.33 ± 0.21 and 0.33 ± 0.13°C for H). We conclude that highly trained middle-aged men demonstrate a similar capacity for heat dissipation when compared with physically active younger men.
We evaluated the hypothesis that with increasing levels of hyperthermia, thermal influences would predominate over nonthermal baroreceptor control of cutaneous vascular conductance (CVC) and local sweat rate (LSR). On separate days, eight male participants were positioned in either an upright seated posture (URS) or a 15° head-down tilt (HDT) posture in a thermoneutral condition and during passive heating, until mean body temperature (T
body) increased by 1.5°C. Hemodynamic [heart rate (HR), cardiac output, mean arterial pressure (MAP)] and thermal responses [T
re, CVC, LSR] were measured continuously. MAP showed a gradual decrease in the early- to mid-stages of heating for both HDT and URS. At a T
body > 0.6°C, MAP achieved a stable, albeit reduced level from baseline resting for the duration of the heating, whereas MAP decreased significantly throughout the heating period in the URS position (p < 0.001). CVC increased rapidly in the early stages of heating and achieved a stable elevated level in both HDT and URS at the mid-stage of heating (T
body increase ≤ 0.45°C) for the duration of the heating period (i.e., to a T
body increase of 1.5°C). A similar pattern of response was observed in LSR. A rapid increase in LSR was observed in the early- to mid-stages of heating (T
body increase ≤ 0.75°C), followed by a slower increase until the end of heating. Responses were similar between conditions. We conclude that despite a significant nonthermal drive, as evidenced by a significant difference in MAP between conditions in the late stages of heating, the thermoeffector activity governing CVC and LSR responses are primarily modulated by thermal input.
The Frank-Starling 'law of the heart' is implicated in certain types of orthostatic intolerance in humans. Environmental conditions have the capacity to modulate orthostatic tolerance, where heat stress decreases and cooling increases orthostatic tolerance. The objective of this project was to test the hypothesis that heat stress augments and cooling attenuates orthostatic-induced decreases in stroke volume (SV) via altering the operating position on a Frank-Starling curve. Pulmonary artery catheters were placed in 11 subjects for measures of pulmonary capillary wedge pressure (PCWP) and SV (thermodilution derived cardiac output/heart rate). Subjects experienced lower-body negative-pressure (LBNP) of 0, 15 and 30 mmHg during normothermia, skin-surface cooling (decrease in mean skin temperature of 4.3 +/- 0.4 degrees C (mean +/- s.e.m.) via perfusing 16 degrees C water through a tubed-lined suit), and whole-body heating (increase in blood temperature of 1.0 +/- 0.1 degrees C via perfusing 46 degrees C water through the suit). SV was 123 +/- 8, 121 +/- 10, 131 +/- 7 ml prior to LBNP, during normothermia, skin-surface cooling, and whole-body heating, respectfully (P = 0.20). LBNP of 30 mmHg induced greater decreases in SV during heating (-48.7 +/- 6.7 ml) compared to normothermia (-33.2 +/- 7.4 ml) and to cooling (-10.3 +/- 2.9 ml; all P < 0.05). Relating PCWP to SV indicated that cooling values were located on the flatter portion of a Frank-Starling curve because of attenuated decreases in SV per decrease in PCWP. In contrast, heating values were located on the steeper portion of a Frank-Starling curve because of augmented decreases in SV per decrease in PCWP. These data suggest that a Frank-Starling mechanism may contribute to improvements in orthostatic tolerance during cold stress and orthostatic intolerance during heat stress.