Sex Differences in Human Thermoregulation: Relevance for 2020 and Beyond

Nocturnal heat exposure and stroke risk

BACKGROUND AND AIMS

In recent decades, nighttime temperatures have increased faster than daytime temperatures. The increasing prevalence of nocturnal heat exposure may pose a significant risk to cardiovascular health. This study investigated the association between nighttime heat exposure and stroke risk in the region of Augsburg, Germany, and examined its temporal variations over 15 years.

METHODS

Hourly meteorological parameters, including mean temperature, relative humidity, and barometric pressure, were acquired from a local meteorological station. A data set was obtained consisting of 11 037 clinical stroke cases diagnosed during warmer months (May to October) between the years 2006 and 2020. The average age of cases was 71.3 years. Among these cases, 642 were identified as haemorrhagic strokes, 7430 were classified as ischaemic strokes, and 2947 were transient ischaemic attacks. A time-stratified case-crossover analysis with a distributed lag non-linear model was used to estimate the stroke risk associated with extreme nighttime heat, as measured by the hot night excess (HNE) index after controlling for the potential confounding effects of daily maximum temperature and other climatic variables. Subgroup analyses by age group, sex, stroke subtype, and stroke severity were performed to identify variations in susceptibility to nighttime heat.

RESULTS

Results suggested a significant increase in stroke risk on days with extreme nighttime heat (97.5% percentile of HNE) (odds ratio 1.07, 95% confidence interval 1.01-1.15) during the full study period. When comparing the results for 2013-20 with the results for 2006-12, there was a significant increase (P < .05) in HNE-related risk for all strokes and specifically for ischaemic strokes during the more recent period. Furthermore, older individuals, females, and patients with mild stroke symptoms exhibited a significantly increased vulnerability to nighttime heat.

CONCLUSIONS

This study found nocturnal heat exposure to be related to elevated stroke risk after controlling for maximum daytime temperature, with increasing susceptibility between 2006 and 2020. These results underscore the importance of considering nocturnal heat as a critical trigger of stroke events in a warming climate.

Heat Exposure and Cause-Specific Hospital Admissions in Spain: A Nationwide Cross-Sectional Study

BACKGROUND

More frequent and intense exposure to extreme heat conditions poses a serious threat to public health. However, evidence on the association between heat and specific diagnoses of morbidity is still limited. We aimed to comprehensively assess the short-term association between cause-specific hospital admissions and high temperature, including the added effect of temperature variability and heat waves and the effect modification by humidity and air pollution.

METHODS

We used data on cause-specific hospital admissions, weather (i.e., temperature and relative humidity), and air pollution [i.e., fine particulate matter with aerodynamic diameter ≤ 2.5 μ m (PM 2.5 ), fine particulate matter with aerodynamic diameter ≤ 10 μ m (PM 10 ), NO 2 , and ozone (O 3 )] for 48 provinces in mainland Spain and the Balearic Islands between 1 January 2006 and 31 December 2019. The statistical analysis was performed for the summer season (June-September) and consisted of two steps. We first applied quasi-Poisson generalized linear regression models in combination with distributed lag nonlinear models (DLNM) to estimate province-specific temperature-morbidity associations, which were then pooled through multilevel univariate/multivariate random-effect meta-analysis.

RESULTS

High temperature had a generalized impact on cause-specific hospitalizations, while the added effect of temperature variability [i.e., diurnal temperature range (DTR)] and heat waves was limited to a reduced number of diagnoses. The strongest impact of heat was observed for metabolic disorders and obesity [relative risk (RR) = 1.978; 95% empirical confidence interval (eCI): 1.772, 2.208], followed by renal failure (1.777; 95% eCI: 1.629, 1.939), urinary tract infection (1.746; 95% eCI: 1.578, 1.933), sepsis (1.543; 95% eCI: 1.387, 1.718), urolithiasis (1.490; 95% eCI: 1.338, 1.658), and poisoning by drugs and nonmedicinal substances (1.470; 95% eCI: 1.298, 1.665). We also found differences by sex (depending on the diagnosis of hospitalization) and age (very young children and the elderly were more at risk). Humidity played a role in the association of heat with hospitalizations from acute bronchitis and bronchiolitis and diseases of the muscular system and connective tissue, which were higher in dry days. Moreover, heat-related effects were exacerbated on high pollution days for metabolic disorders and obesity (PM 2.5 ) and diabetes (PM 10 , O 3 ).

DISCUSSION

Short-term exposure to heat was found to be associated with new diagnoses (e.g., metabolic diseases and obesity, blood diseases, acute bronchitis and bronchiolitis, muscular and connective tissue diseases, poisoning by drugs and nonmedicinal substances, complications of surgical and medical care, and symptoms, signs, and ill-defined conditions) and previously identified diagnoses of hospital admissions. The characterization of the vulnerability to heat can help improve clinical and public health practices to reduce the health risks posed by a warming planet. https://doi.org/10.1289/EHP13254.

Impact of temperature on physical and cognitive performance in elite female football players during intermittent exercise

There is limited research on female football players, especially related to their physical and cognitive performance under different climactic conditions. We analyzed the impact of a hot environmental temperature on physical performance and anticipation in elite female football players during a fatigue-inducing intermittent protocol. Elite female players (n = 21) performed the countermovement jump (CMJ) and responded to filmed sequences of offensive play under two distinct environmental temperatures (i.e., mild environment temperature- 20°C and 30% rh versus hot environment temperature- 38°C and 80% rh), interspersed by 1-week interval. Linear mixed models were used. CMJ performance declined following the intermittent protocol on both temperature conditions (p < 0.05). Moreover, there were significant main effects for protocol on CMJ speed (m/s) (p = 0.001; ηp 2 = 0.12), CMJ power (p = 0.002; ηp 2 = 0.11), and CMJ Heightmax (p = 0.002; ηp 2 = 0.12). After performing the intermittent protocol, exposure to a hot temperature caused a greater decline in anticipation accuracy (mild temperature = 64.41% vs. hot temperature = 53.44%; p < 0.001). Our study shows impaired performance in elite female football players following an intermittent protocol under hot compared with mild environmental conditions. We report decreased performance in both CMJ and anticipation performance under hotter conditions. The results reveal that exposure to hot temperatures had a negative effect on the accuracy of their anticipatory behaviors. We consider the implication of the work for research and training interventions.

Modeling thermoregulatory responses during high-intensity exercise in warm environments

The six cylinder thermoregulatory model (SCTM) has been validated thoroughly for resting humans. This type of modeling is helpful to predict and develop guidance for safe performance of work and recreational activities. In the context of a warming global climate, updating the accuracy of the model for intense exercise in warm environments will help a wide range of individuals in athletic, recreational, and military settings. Three sets of previously collected data were used to determine SCTM accuracy. Dataset 1: two groups [large (LG) 91.5 kg and small (SM) 67.7 kg] of individuals performed 60 min of semirecumbent cycling in temperate conditions (25.1°C) at metabolic rates of 570-700 W. Dataset 2: two LG (100 kg) and SM (65.8 kg) groups performed 60 min of semirecumbent cycling in warm/hot environmental conditions (36.2°C) at metabolic rates of 590-680 W. Dataset 3: seven volunteers completed 8-km track trials (∼30 min) in cool (17°C) and warm (30°C) environments. The volunteers' metabolic rates were estimated to be 1,268 W and 1,166 W, respectively. For all datasets, SCTM-predicted core temperatures were found to be similar to the observed core temperatures. The root mean square deviations (RMSDs) ranged from 0.06 to 0.46°C with an average of 0.2°C deviation, which is less than the acceptance threshold of 0.5°C. Thus, the present validation shows that SCTM predicts core temperatures with acceptable accuracy during intense exercise in warm environments and successfully captures core temperature differences between large and small individuals.NEW & NOTEWORTHY The SCTM has been validated thoroughly for resting humans in warm and cold environments and during water immersion. The present study further demonstrated that SCTM predicts core temperatures with acceptable accuracy during intense exercise up to 1,300 W in temperate and warm environments and captures core temperature differences between large and small individuals. SCTM is potentially useful to develop guidance for safe operation in athletic, military, and occupational settings during exposure to warm or hot environments.

Female thermal sensitivity and behaviour across the lifespan: A unique journey

Women are a group of individuals that undergo unique anatomical, physiological and hormonal changes across the lifespan. For example, consider the impact of the menstrual cycle, pregnancy and menopause, all of which are accompanied by both short- and long-term effects on female body morphology (e.g., changes in breast size) and temperature regulation, heat tolerance, thermal sensitivity and comfort. However, empirical evidence on how skin thermal and wetness sensitivity might change across the lifespan of women, and the implications that this has for female-specific thermal behaviours, continues to be lacking. This paper is based on a symposium presentation given at Physiology 2023 in Harrogate, UK. It aims to review new evidence on anatomical and physiological mechanisms underpinning differences in skin thermal and wetness sensitivity amongst women varying in breast size and age, in addition to their role in driving female thermal behaviours. It is hoped that this brief overview will stimulate the development of testable hypotheses to increase our understanding of the behavioural thermal physiology of women across the lifespan and at a time of climate change.

Relation of body surface area-to-mass ratio to risk of exertional heat stroke in healthy men and women

Risk of exertional heat stroke (EHS) is an ongoing challenge for United States military personnel, for athletes and for individuals with occupational stressors that involve prolonged activity in hot environments. Higher body mass index (BMI) is significantly associated with increased risk for EHS in activity duty U.S. Soldiers. During exercise, heat is generated primarily by contracting skeletal muscle (and other metabolically active body mass) and dissipated based on body surface area (BSA). Thus, in compensable environments, a higher BSA·mass-1 may be a benefit to heat dissipation and decrease the risk of EHS. The purpose of the present analysis was to test the hypothesis that BSA·mass-1 ratio is an important biophysical characteristic contributing to the risk of EHS. We employed a matched case-control approach, where each individual with a diagnosis of EHS was matched to five controls who were never diagnosed with EHS but were in the same unit and had the same job title. We used a multivariate conditional logistic regression model including variables of BSA·mass-1, sex, age, military rank, and race. BSA·mass-1 significantly predicted EHS risk (P = 0.006), such that people with higher BSA·mass-1 were at lower risk of developing EHS when controlling for other potential factors such as age and race. This relationship persisted after adjustment for other anthropometric measures of body size including weight, BMI, and BSA. These data suggest that biophysical factors play an important role in EHS risk, particularly in a healthy military-aged cohort of men and women.NEW & NOTEWORTHY With the impacts of climate change yielding higher average ambient temperatures over time, the incidence of EHS for individuals participating in outdoor activities may consequently increase. With the larger sample size in this study compared with prior research in this field, we were able to use various methods that had not been applied before. For example, we were able to mutually adjust for different measurements of body size to understand which metric had the highest association with EHS risk. Understanding factors that may be modifiable may be important for developing interventions to counteract the increased risk of EHS associated with climate change.

Ambient temperature and risk of cardiovascular and respiratory adverse health outcomes: a nationwide cross-sectional study from Spain

AIMS

We assessed the association of temperature and temperature variability with cause-specific emergency hospitalizations and mortality from cardiovascular and respiratory diseases in Spain, as well as the effect modification of this association by individual and contextual factors.

METHODS AND RESULTS

We collected data on health (hospital admissions and mortality), weather (temperature and relative humidity), and relevant contextual indicators for 48 Spanish provinces during 2004-2019. The statistical analysis was separately performed for the summer (June-September) and winter (December-March) seasons. We first applied a generalized linear regression model with quasi-Poisson distribution to estimate daily province-specific temperature-health associations, and then we fitted multilevel multivariate meta-regression models to the evaluate effect modification of the contextual characteristics on heat- and cold-related risks. High temperature increased the risk of mortality across all cardiovascular and respiratory diseases, with the strongest effect for hypertension (relative risk (RR) at 99th temperature percentile vs. optimum temperature: 1.510 [95% empirical confidence interval {eCI} 1.251 to 1.821]), heart failure (1.528 [1.353 to 1.725]), and pneumonia (2.224 [1.685 to 2.936]). Heat also had an impact on all respiratory hospitalization causes (except asthma), with similar risks between pneumonia (1.288 [1.240 to 1.339]), acute bronchitis and bronchiolitis (1.307 [1.219 to 1.402]), and chronic obstructive pulmonary disease (1.260 [1.158 to 1.372]). We generally found significant risks related to low temperature for all cardiovascular and respiratory causes, with heart failure (RR at 1st temperature percentile vs. optimum temperature: 1.537 [1.329 to 1.779]) and chronic obstructive pulmonary disease (1.885 [1.646 to 2.159]) exhibiting the greatest risk for hospitalization, and acute myocardial infarction (1.860 [1.546 to 2.238]) and pneumonia (1.734 [1.219 to 2.468]) for mortality. Women and the elderly were more vulnerable to heat, while people with secondary education were less susceptible to cold compared to those not achieving this educational stage. Results from meta-regression showed that increasing heating access to the highest current provincial value (i.e. 95.6%) could reduce deaths due to cold by 59.5% (57.2 to 63.5).

CONCLUSION

Exposure to low and high temperatures was associated with a greater risk of morbidity and mortality from multiple cardiovascular and respiratory conditions, and heating was the most effective societal adaptive measure to reduce cold-related mortality.

Hot water immersion: Maintaining core body temperature above 38.5°C mitigates muscle fatigue

PURPOSE

Hot water immersion (HWI) has gained popularity to promote muscle recovery, despite limited data on the optimal heat dose. The purpose of this study was to compare the responses of two exogenous heat strains on core body temperature, hemodynamic adjustments, and key functional markers of muscle recovery following exercise-induced muscle damage (EIMD).

METHODS

Twenty-eight physically active males completed an individually tailored EIMD protocol immediately followed by one of the following recovery interventions: HWI (40°C, HWI40 ), HWI (41°C, HWI41 ) or warm water immersion (36°C, CON36 ). Gastrointestinal temperature (Tgi ), hemodynamic adjustments (cardiac output [CO], mean arterial pressure [MAP], and systemic vascular resistance [SVR]), pre-frontal cortex deoxyhemoglobin (HHb), ECG-derived respiratory frequency, and subjective perceptual measures were tracked throughout immersion. In addition, functional markers of muscle fatigue (maximal concentric peak torque [Tpeak ]) and muscle damage (late-phase rate of force development [RFD100-200 ]) were measured prior to EIMD (pre-), 24 h (post-24 h), and 48 h (post-48 h) post-EIMD.

RESULTS

By the end of immersion, HWI41 led to significantly higher Tgi values than HWI40 (38.8 ± 0.1 vs. 38.0°C ± 0.6°C, p < 0.001). While MAP was well maintained throughout immersion, only HWI41 led to increased (HHb) (+4.2 ± 1.47 μM; p = 0.005) and respiratory frequency (+4.0 ± 1.21 breath.min-1 ; p = 0.032). Only HWI41 mitigated the decline in RFD100-200 at post-24 h (-7.1 ± 31.8%; p = 0.63) and Tpeak at post-48 h (-3.1 ± 4.3%, p = 1).

CONCLUSION

In physically active males, maintaining a core body temperature of ~25 min within the range of 38.5°C-39°C has been found to be effective in improving muscle recovery, while minimizing the risk of excessive physiological heat strain.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

Advances in Non-Electrochemical Sensing of Human Sweat Biomarkers: From Sweat Sampling to Signal Reading

Sweat, commonly referred to as the ultrafiltrate of blood plasma, is an essential physiological fluid in the human body. It contains a wide range of metabolites, electrolytes, and other biologically significant markers that are closely linked to human health. Compared to other bodily fluids, such as blood, sweat offers distinct advantages in terms of ease of collection and non-invasive detection. In recent years, considerable attention has been focused on wearable sweat sensors due to their potential for continuous monitoring of biomarkers. Electrochemical methods have been extensively used for in situ sweat biomarker analysis, as thoroughly reviewed by various researchers. This comprehensive review aims to provide an overview of recent advances in non-electrochemical methods for analyzing sweat, including colorimetric methods, fluorescence techniques, surface-enhanced Raman spectroscopy, and more. The review covers multiple aspects of non-electrochemical sweat analysis, encompassing sweat sampling methodologies, detection techniques, signal processing, and diverse applications. Furthermore, it highlights the current bottlenecks and challenges faced by non-electrochemical sensors, such as limitations and interference issues. Finally, the review concludes by offering insights into the prospects for non-electrochemical sensing technologies. By providing a valuable reference and inspiring researchers engaged in the field of sweat sensor development, this paper aspires to foster the creation of innovative and practical advancements in this domain.

Ambient temperature and seasonal variation in inpatient mortality from respiratory diseases: a retrospective observational study

Background

The seasonal fluctuation in mortality and hospital admissions from respiratory diseases, with a winter peak and a summer trough, is widely recognized in extratropical countries. However, little is known about the seasonality of inpatient mortality and the role of ambient temperature remains uncertain. We aimed to analyse the association between ambient temperature and in-hospital mortality from respiratory diseases in the provinces of Madrid and Barcelona, Spain.

Methods

We used data on daily hospitalisations, weather (ie, temperature and relative humidity) and air pollutants (ie, PM2.5, PM10, NO2 and O3) for the Spanish provinces of Madrid and Barcelona during 2006-2019. We applied a daily time-series quasi-Poisson regression in combination with distributed lag non-linear models (DLNM) to assess, on the one hand, the seasonal variation in fatal hospitalisations and the contribution of ambient temperature, and on the other hand, the day-to-day association between temperature and fatal hospital admissions. The analyses were stratified by sex, age and primary diagnostic of hospitalisation.

Findings

The study analysed 1 710 012 emergency hospital admissions for respiratory diseases (mean [SD] age, 60.4 [31.0] years; 44.2% women), from which 103 845 resulted in in-hospital death (81.4 [12.3] years; 45.1%). We found a strong seasonal fluctuation in in-hospital mortality from respiratory diseases. While hospital admissions were higher during the cold season, the maximum incidence of inpatient mortality was during the summer and was strongly related to high temperatures. When analysing the day-to-day association between temperature and in-hospital mortality, we only found an effect for high temperatures. The relative risk (RR) of fatal hospitalisation at the 99th percentile of the distribution of daily temperatures vs the minimum mortality temperature (MMT) was 1.395 (95% eCI: 1.211-1.606) in Madrid and 1.612 (1.379-1.885) in Barcelona. In terms of attributable burden, summer temperatures (June-September) were responsible for 16.2% (8.8-23.3) and 22.3% (15.4-29.2) of overall fatal hospitalisations from respiratory diseases in Madrid and Barcelona, respectively. Women were more vulnerable to heat than men, whereas the results by diagnostic of admission showed heat effects for acute bronchitis and bronchiolitis, pneumonia and respiratory failure.

Interpretation

Unless effective adaptation measures are taken in hospital facilities, climate warming could exacerbate the burden of inpatient mortality from respiratory diseases during the warm season.

Funding

European Research Council Consolidator Grant EARLY-ADAPT, European Research Council Proof-of-Concept Grants HHS-EWS and FORECAST-AIR.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

Effect of acute heat exposure on the determination of critical power and W' in women and men

The goal of this study was to investigate to what extent acute heat exposure would affect the parameters of the power-duration relationship, i.e. CP and W', using multiple constant workload tests to task failure, in women and men. Twenty four young physically active participants (12 men, 12 women) performed 3-5 constant load tests to determine CP and W', both in temperate (TEMP; 18°C) and hot (HOT; 36°C) environmental conditions. A repeated-measures ANOVA was executed to find differences between TEMP and HOT, and between women and men. In HOT, CP was reduced by 6.5% (227 ± 50 vs. 212 ± 47 W), while W' increased 12.4% (16.4 ± 4.4 vs. 18.5 ± 5.6 kJ). No significant two-way sex × temperature interactions were observed, indicating that the environmental conditions did not have a different effect in men compared with women. The intersection of the average curvatures in TEMP and HOT occurred at 137 s and 280 W in women, and 153 s and 397 W in men. Acute heat exposure had an impact on the parameters CP and W', i.e. CP decreased whereas W' increased. The increase in W' might be a consequence of the mathematical modelling for the used test methodology, rather than a physiological accurate value of W' in HOT. No differences induced by heat exposure were observed between women and men.

Cognitive Function among Young Women's Football Players in the Summer Heat

Recently, there has been a growing focus on studies related to women's football. However, the cognitive function of female football players has not been extensively characterized. Thus, we explored how the cognitive function of female football players was altered during a series of matches in summer and examined day-to-day variations in cognitive function with regard to dehydration status. Resting cognitive function was assessed from 17 young women football players during the Japan Club Youth Women's football tournament, which spanned eight consecutive days. Cognitive function initially improved, with this improvement sustained throughout the tournament. It is worth noting that ten participants experienced symptoms of dehydration at least once during the tournament; however, these symptoms were not found to be linked to impaired cognitive function, suggesting that resting cognitive function remains unaffected during summer matches, even in the presence of dehydration symptoms.

To the extreme! How biological anthropology can inform exercise physiology in extreme environments

The fields of biological anthropology and exercise physiology are closely related and can provide mutually beneficial insights into human performance. These fields often use similar methods and are both interested in how humans function, perform, and respond in extreme environments. However, these two fields have different perspectives, ask different questions, and work within different theoretical frameworks and timescales. Biological anthropologists and exercise physiologists can greatly benefit from working together when examining human adaptation, acclimatization, and athletic performance in the extremes of heat, cold, and high-altitude. Here we review the adaptations and acclimatizations in these three different extreme environments. We then examine how this work has informed and built upon exercise physiology research on human performance. Finally, we present an agenda for moving forward, hopefully, with these two fields working more closely together to produce innovative research that improves our holistic understanding of human performance capacities informed by evolutionary theory, modern human acclimatization, and the desire to produce immediate and direct benefits.

Non-Conventional Risk Factors: "Fact" or "Fake" in Cardiovascular Disease Prevention?

Cardiovascular diseases (CVDs), such as arterial hypertension, myocardial infarction, stroke, heart failure, atrial fibrillation, etc., still represent the main cause of morbidity and mortality worldwide. They significantly modify the patients' quality of life with a tremendous economic impact. It is well established that cardiovascular risk factors increase the probability of fatal and non-fatal cardiac events. These risk factors are classified into modifiable (smoking, arterial hypertension, hypercholesterolemia, low HDL cholesterol, diabetes, excessive alcohol consumption, high-fat and high-calorie diet, reduced physical activity) and non-modifiable (sex, age, family history, of previous cardiovascular disease). Hence, CVD prevention is based on early identification and management of modifiable risk factors whose impact on the CV outcome is now performed by the use of CV risk assessment models, such as the Framingham Risk Score, Pooled Cohort Equations, or the SCORE2. However, in recent years, emerging, non-traditional factors (metabolic and non-metabolic) seem to significantly affect this assessment. In this article, we aim at defining these emerging factors and describe the potential mechanisms by which they might contribute to the development of CVD.

Gender inequalities in heat-related mortality in the Czech Republic

It is acknowledged that climate change exacerbates social inequalities, and women have been reported as more vulnerable to heat than men in many studies in Europe, including the Czech Republic. This study aimed at investigating the associations between daily temperature and mortality in the Czech Republic in the light of a sex and gender perspective, taking into account other factors such as age and marital status. Daily mean temperature and individual mortality data recorded during the five warmest months of the year (from May to September) over the period 1995-2019 were used to fit a quasi-Poisson regression model, which included a distributed lag non-linear model (DLNM) to account for the delayed and non-linear effects of temperature on mortality. The heat-related mortality risks obtained in each population group were expressed in terms of risk at the 99th percentile of summer temperature relative to the minimum mortality temperature. Women were found generally more at risk to die because of heat than men, and the difference was larger among people over 85 years old. Risks among married people were lower than risks among single, divorced, and widowed people, while risks in divorced women were significantly higher than in divorced men. This is a novel finding which highlights the potential role of gender inequalities in heat-related mortality. Our study underlines the relevance of including a sex and gender dimension in the analysis of the impacts of heat on the population and advocates the development of gender-based adaptation policies to extreme heat.

Cold water immersion of the hand and forearm during half-time improves intermittent exercise performance in the heat

The present study aimed to investigate the effect of cold water immersion of the hand and forearm during half-time (HT) on intermittent exercise performance and thermoregulation by imitating intermittent athletic games in the heat. In a randomized crossover design, 11 physically active men performed the first half (first and second block) and second half (third and fourth block) intermittent cycling exercise protocol, which consisted of a 5-s maximal power pedalling (body weight × 0.075 kp) every minute separated by 25-s of unloaded pedalling and rest (30 s) in the heat (33°C, 50% relative humidity). The two-halves were separated by a 15-min HT. During HT, the participants were assigned to the CON (sedentary resting) or COOL (immersion of hands and forearms in cold water at 15-17°C) condition. The mean power output in the second half was significantly greater (third and fourth block: p < 0.05) in the COOL than in the CON condition. Moreover, there was a significant decrease in the rectal (0.54 ± 0.17°C, p < 0.001) and mean skin (1.86 ± 0.34°C, p < 0.05) temperatures of the COOL condition during HT. Furthermore, the heart rate (16 ± 7 bpm, p < 0.05) and skin blood flow (40.2 ± 10.5%, p < 0.001) decreased at the end of HT in the COOL condition. In the second half, thermal sensation was more comfortable in the COOL condition (p < 0.001). Cold water immersion of the hand and forearm during HT improved physiological and reduced perceived heat stress. Moreover, it prevented a reduction in intermittent exercise performance in the second half.

Limitations associated with thermoregulation and cardiovascular research assessing laborers performing work in the heat

PURPOSE

To quantify the current literature and limitations associated with research examining thermoregulatory and cardiovascular strain in laborers working in the heat.

METHODS

PubMed, SCOPUS, and SPORTDiscus were searched for terms related to the cardiovascular system, heat stress, and physical work. Qualifying studies included adult participants (18-65 years old), a labor-intensive environment or exercise protocol simulating a labor environment, a minimum duration of 120 min of physical work, and environmental heat stress (ambient temperature ≥26.0°C and ≥30% relative humidity). Studies included at least one of the following outcomes: pre- and peak physical work, core temperature, heart rate (HR), systolic blood pressure, diastolic blood pressure, HR variability, and rate pressure product.

RESULTS

Twenty-one out of 1559 potential studies qualified from our search. There was a total of 598 participants (mean = 28 ± 50 participants per study, range = 4-238 participants per study), which included 51 females (8.5%) and 547 males (91.5%). Of the participants, 3.8% had cardiovascular risk factors (diabetes: n = 10; hypertension: n = 13) and 96.2% were characterized as "healthy". Fifty-seven percent of the included studies were performed in a laboratory setting.

CONCLUSIONS

Studies were predominantly in men (91.5%), laboratory settings (57%), and "healthy" individuals (96.2%). To advance equity in protection against occupational heat stress and better inform future heat safety recommendations to protect all workers, future studies must focus on addressing these limitations. Employers, supervisors, and other safety stakeholders should consider these limitations while implementing current heat safety recommendations.

The Effect of Remimazolam Compared to Sevoflurane on Postoperative Shivering in Patients Undergoing Laparoscopic Gynecologic Surgery under General Anesthesia: A Prospective Randomized Controlled Trial

Background and objectives: Anesthesia maintenance agents affect the incidence of postoperative shivering (PS) after general anesthesia. This study compared the effects of remimazolam with sevoflurane on PS in patients undergoing laparoscopic gynecologic surgery under general anesthesia. Materials and methods: Seventy-four patients were allocated into one of two groups. In anesthesia maintenance, group S received sevoflurane and remifentanil, and group R received remimazolam and remifentanil. Results: The incidence and severity of postoperative shivering, mean arterial pressure (MAP), heart rate (HR), core body temperature, and the association of PS with hypothermia, MAP, or HR in the post-anesthesia care unit (PACU) were measured. Group R had significantly lower rates of perioperative hypothermia (58.8 vs. 27.8%, p = 0.009) and postoperative shivering (41.2 vs. 19.4%, p = 0.047). The severity of PS was also lower in group R than in group S (p = 0.034). Core body temperature was significantly higher in group R than in group S from 10 min after induction (p = 0.047) to the PACU (p = 0.009). MAP and HR were significantly higher in group R than in group S from 20 min after induction (p = 0.047) to the PACU (p = 0.009). In group S, the correlation between the severity of PS and the incidence of hypothermia (φc = 0.414, p = 0.121) was moderate but not significant. In group R, the correlation between PS severity and hypothermia (φc = 0.418, p = 0.043) was moderate and significant. Conclusions: Remimazolam showed better results than sevoflurane in anesthesia maintenance regarding hypothermia and PS.

Three dimensional models of human thermoregulation: A review

Numerous human thermoregulatory models have been developed and widely used in various applications such as aerospace, medicine, public health, and physiology research. This paper is a review of three dimensional (3D) models for human thermoregulation. This review begins with a short introduction of thermoregulatory model development followed by key principles for mathematical description of human thermoregulation systems. Different representations of 3D human bodies are discussed with respect to their detail and prediction capability. The human body was divided into fifteen layered cylinders in early 3D models (cylinder model). Recent 3D models have utilized medical image datasets to develop geometrically correct human models (realistic geometry model). The finite element method is mostly used to solve the governing equations and get numerical solutions. The realistic geometry models provide a high degree of anatomical realism and predict whole-body thermoregulatory responses at high resolution and at organ and tissue levels. Thus, 3D models extend to a wide range of applications where temperature distribution is critical, such as hypothermia/hyperthermia therapy and physiology research. The development of thermoregulatory models will continue with the growth in computational power, advancement in numerical methods and simulation software, advances in modern imaging techniques, and progress in the basic science of thermal physiology.

Is There Evidence for the Development of Sex-Specific Guidelines for Ultramarathon Coaches and Athletes? A Systematic Review

BACKGROUND

There is evidence of sex differences in the physiology of endurance exercise, yet most of the advice and guidelines on training, racing, nutrition, and recovery for ultramarathons are based on research that has largely excluded female athletes. The objective was therefore to review the current knowledge of sex differences in ultramarathon runners and determine if sufficient evidence exists for providing separate guidelines for males and females.

METHODS

This systematic review was carried out in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Three databases were searched for studies investigating differences in elite and recreational male and female ultramarathon runners. Studies were included if they compared males and females and looked at outcomes relating to the performance or health of ultramarathon runners. The quality of the included studies was determined using the Grading of Recommendations Assessment Development and Evaluation (GRADE) approach.

RESULTS

The search strategy identified 45 studies that met the inclusion criteria. Most studies were observational in design, with only three papers based on randomised controlled trials. The overall quality of the evidence was low. Sex differences in the predictors of ultramarathon performance; physiological responses to training, racing, and recovery; chronic and acute health issues; and pacing strategies were found. There were areas with contradictory findings, and very few studies examined specific interventions.

CONCLUSION

The results from this review suggest that the development of sex-specific guidelines for ultramarathon coaches and athletes could have a significant effect on the performance and health of female runners. At present, there is insufficient high-quality evidence on which to formulate these guidelines, and further research is required.

Growth hormone receptor (GHR) in AgRP neurons regulates thermogenesis in a sex-specific manner

Evidence for hypothalamic regulation of energy homeostasis and thermoregulation in brown adipose tissue (BAT) during aging has been well recognized, yet the central molecular mediators involved in this process are poorly understood. The arcuate hypothalamus, orexigenic agouti-related peptide (AgRP) neurons control nutrient intake, energy homeostasis, and BAT thermogenesis. To determine the roles of growth hormone receptor (GHR) signaling in the AgRP neurons, we used mice with the AgRP-specific GHR deletion (AgRP^ΔGHR). We found that female AgRP^ΔGHR mice were resistant to temperature adaptation, and their body core temperature remained significantly lower when held at 10 °C, 22 °C, or 30 °C, compared to control mice. Low body core temperature in female AgRP^ΔGHR mice has been associated with significant reductions in Ucp1 and Pgc1α expression in the BAT. Further, neuronal activity in AgRP in response to cold exposure was blunted in AgRP^ΔGHR female mice, while the number of Fos⁺ AgRP neurons was increased in female controls exposed to cold. Global transcriptome from BAT identified increased the expression of genes related to immune responses and chemokine activity and decreased the expression of genes involved in triglyceride synthesis and metabolic pathways in AgRP^ΔGHR female mice. Importantly, these were the same genes that are downregulated by thermoneutrality in control mice but not in the AgRP^ΔGHR animals. Collectively, these data demonstrate a novel sex-specific role for GHR signaling in AgRP neurons in thermal regulation, which might be particularly relevant during aging.

Body mass index, but not sex, influences exertional heat stroke risk in young healthy men and women

Exertional heat stroke (EHS) remains a persistent threat for individuals working or playing in the heat, including athletes and military and emergency service personnel. However, influence of biological sex and/or body mass index (BMI) on the risk of EHS remain poorly understood. The purpose of this study was to retrospectively assess the influence of sex and BMI on risk of EHS in the active-duty US Army. We analyzed data from 2016 to 2021, using a matched case-control approach, where each individual with a diagnosis of EHS was matched to five controls based on calendar time, unit ID, and job category, to capture control individuals who were matched to EHS events by location, time, and activity. We used a multivariate logistic regression model mutually adjusted for sex, BMI, and age to compare 745 (n = 61 F) individuals (26 ± 7 yr) with a diagnosed EHS to 4,290 (n = 384 F) case controls (25 ± 5 yr). Group average BMI were similar: 26.6 ± 3.1 (EHS) and 26.5 ± 3.6 kg/m² (CON). BMI was significantly (P < 0.0001) associated with higher risk of EHS with a 3% increase in risk of EHS for every unit increase in BMI. Notably, sex was not associated with any difference in risk for EHS (P = 0.54). These data suggest that young healthy people with higher BMI have significantly higher risk of EHS, but, contrary to what some have proposed, this risk was not higher in young women.

Finite element model of female thermoregulation with geometry based on medical images

INTRODUCTION

this study describes the development of a female finite element thermoregulatory model (FETM) METHOD: the female body model was developed from medical image datasets of a median U.S. female and was constructed to be anatomically correct. The body model preserves the geometric shapes of 13 organs and tissues, including skin, muscles, fat, bones, heart, lungs, brain, bladder, intestines, stomach, kidneys, liver, and eyes. Heat balance within the body is described by the bio-heat transfer equation. Heat exchange at the skin surface includes conduction, convection, radiation, and sweat evaporation. Vasodilation, vasoconstriction, sweating, and shivering are controlled by afferent and efferent signals to and from the skin and hypothalamus.

RESULTS

the model was validated with measured physiological data during exercise and rest in thermoneutral, hot, and cold conditions. Validations show the model predicted the core temperature (rectal and tympanic temperatures) and mean skin temperatures with acceptable accuracy (within 0.5 °C and 1.6 °C, respectively) CONCLUSION: this female FETM predicted high spatial resolution temperature distribution across the female body, which provides quantitative insights into human thermoregulatory responses in females to non-uniform and transient environmental exposure.

Ecological rules for global species distribution also predict performance variation in Ironman triathletes

Bergmann's and Allen's rules predict changes in body size and appendage length across temperature gradients for species with broad geographic distributions. Larger bodies and longer limbs facilitate cooling whereas smaller bodies and compact limbs limit heat loss. Although these patterns are highly repeatable (hence "rules" of ecology) the patterns and underlying mechanisms are less-well understood in humans. Here I show that variation in running performance among human male triathletes is consistent with both Bergmann's and Allen's rules. Males (but not females) with relatively larger body size and longer limbs performed better at hot compared to cold race venues and vice-versa. Consistent with results in other taxa, sex-specificity may reflect selection for sexual dimorphism. Results suggest that ecological patterns detected over large-spatial scales may arise from fine-scale variation in locomotor performance.

Bioimpedance-Derived Membrane Capacitance: Clinically Relevant Sources of Variability, Precision, and Reliability

Membrane capacitance (CM), a bioimpedance-derived measure of cell membrane health, has been suggested as an indicator of health status. However, there are few published data to support its use in clinical settings. Hence, this study evaluated clinically relevant sources of variation, precision, and reliability of CM measurements. This longitudinal study included 60 premenopausal women. Sources of variability (e.g., demographics, body composition, serum measures, diet) were identified by stepwise regression. Precision and reliability were assessed by the coefficient of variation (CV), intraclass correlation coefficients (ICC), and technical error of the measurement (TEM) for intra-day (30 min apart) and inter-day measurements (7-14 days apart). Body composition, temperature, and metabolic activity were identified as sources of variability. CM measurements had high precision (CV = 0.42%) and high reliability for intra-day (ICC = 0.996) and inter-day (ICC = 0.959) measurements, independent of menstrual cycle and obesity status. Our results showed that CM measurements are sensitive to clinical factors and have high precision and reliability. The results of this study suggest that CM is sufficiently reliable for health status monitoring in conditions with variation in body composition, metabolic activity, or body temperature among premenopausal women.

The Role of Sports Clothing in Thermoregulation, Comfort, and Performance During Exercise in the Heat: A Narrative Review

The aims of this review are to (1) summarise the current research of sports clothing as it relates to thermoregulation, comfort, and performance during exercise in the heat, (2) identify methodological limitations and gaps in the knowledge base of sports clothing, and (3) provide recommendations for exercise testing protocols to accurately assess the impact of sports clothing in athletic populations during exercise in the heat. Sports clothing consists of lightweight and breathable fabrics, surface treatments, and various designs which aim to enhance sweat evaporation and comfort during exercise in the heat. Sports clothing comprised of natural, synthetic, and chemically treated fabrics has been investigated during exercise of varying durations (15–120 min), intensities (20–70% VO₂ max) and types (fixed intensity, incremental, self-paced), and in an array of climatic conditions (18–40 °C, 20–60% relative humidity). To date, few studies have identified significant differences in thermo-physiological, perceptual, and performance measures between natural and synthetic fabrics or compared the effect of chemical treatments to their non-treated equivalent on such measures during exercise. Collectively, previous wearer trials have failed to replicate the upper limit of training and competition demands when assessing sports clothing in endurance-trained individuals who regularly train and compete in hot and humid climates. Clothing comfort has also been evaluated using simple scales which fail to capture intricate detail pertaining to psychological and sensorial parameters. The incorporation of protocols using hot and humid climates (≥ 30 °C, ≥ 70% relative humidity) and longer exercise durations (> 45 min) is warranted. Future research should also consider exploring the effect of sports clothing on thermal, physiological, perceptual, and performance measures between males and females, and assessing clothing comfort using a multi-dimensional approach.

Temperature change in children undergoing magnetic resonance imaging-An observational cohort study

AIM

An increasing number of children undergo magnetic resonance imaging requiring anesthesia or sedation to ensure their immobility; however, magnetic resonance imaging may increase body temperature whereas sedation or anesthesia may decrease it. We investigated changes in body temperature in children who underwent sedation or anesthesia for magnetic resonance imaging.

METHODS

Children aged 12 weeks-12 years undergoing anesthesia and magnetic resonance imaging were included in this prospective observational study. Tympanic body temperature was measured before and after magnetic resonance imaging, and the difference between measurements was calculated. Associations between the temperature difference and patient- or procedure-related factors were evaluated with linear and logistic regression analysis.

RESULTS

A total of 74 children were included, of whom 5 (7%) had a temperature increase ≥0.5°C. Mean temperature difference was -0.24°C (SD 0.48) for the entire group and -0.28°C for the youngest children (0-2 years). The temperature difference correlated positively with the duration of imaging (unadjusted coefficient 0.26, 95% confidence interval (CI), (0.01; 0.52)).

CONCLUSION

In this study of sedated or anesthetized children undergoing magnetic resonance imaging, clinically relevant increases in body temperature above 0.5°C were only found in a few patients. However, longer imaging duration tended to be associated with increased body temperature.

Heat exposure limits for young unacclimatized males and females at low and high humidity

Little is known about the separate and combined influences of humidity conditions, sex, and aerobic fitness on heat tolerance in unacclimatized males and females. The purpose of the current study was to describe heat tolerance, in terms of critical WBGT (WBGT_crit), in unacclimatized young males and females in hot-dry (HD) and warm-humid (WH) environments. Eighteen subjects (9 M/9F; 21 ± 2 yr) were tested during exercise at 30% V̇O_2max in a controlled environmental chamber. Progressive heat stress exposures were performed with either (1) constant dry-bulb temperature (T_db) of 34 and 36 °C and increasing ambient water vapor pressure (P_a) (P_crit trials; WH); or (2) constant P_a of 12 and 16 mmHg and increasing T_db (T_crit trials; HD). Chamber T_db and P_a, and subject esophageal temperature (T_es), were continuously monitored throughout each trial. After a 30-min equilibration period, progressive heat stress continued until subject heat balance could no longer be maintained and a clear rise in T_es was observed. Absolute WBGT_crit and WBGT_crit adjusted to a metabolic rate of 300 W (WBGT₃₀₀), and the difference between WBGT_crit and occupational exposure limits (OEL; ΔOEL) was assessed. WBGT_crit, WBGT₃₀₀, and ΔOEL were higher in WH compared to HD (p < 0.0001) for females but were the same between environments for males (p ≥ 0.21). WBGT_crit was higher in females compared to males in WH (p < 0.0001) but was similar between sexes in HD (p = 0.44). When controlling for metabolic rate, WBGT₃₀₀ and ΔOEL were higher in males compared to females in WH and HD (both p < 0.0001). When controlling for sex, V̇O_2max was not associated with WBGT₃₀₀ or ΔOEL for either sex (r ≤ 0.12, p ≥ 0.49). These findings suggest that WBGT_crit is higher in females compared to males in WH, but not HD, conditions. Additionally, the WBGT_crit is lower in females, but not males, in HD compared to WH conditions.

Efficiency of three cooling methods for hyperthermic military personnel linked to water availability

PURPOSE

Three feasible cooling methods for treatment of hyperthermic individuals in the military, that differed considerably in water volume needed (none to ~80 L), were evaluated.

METHODS

Ten male soldiers were cooled following exercise-induced hyperthermia (rectal temperature (T_re) ∼39.5 °C) using ventilation by fanning (1.7 m s^-1), ventilation by fanning (1.7 m s^-1) while wearing a wet t-shirt (250 mL-27 °C water) and tarp assisted cooling with oscillations (80 L of 27.2 ± 0.5 °C water; TACO).

RESULTS

Cooling rates were higher using TACO (0.116 ± 0.032 °C min^-1) compared to ventilation (0.065 ± 0.011 °C min^-1, P<0.001) and ventilation in combination with a wet t-shirt (0.074 ± 0.020 °C min^-1, P=0.002). Time to cool (TTC) to T_re=38.2 °C for TACO was shorter (14 ± 4 min) compared to ventilation only (20 ± 5 min; P=0.018), but not to ventilation while wearing a wet t-shirt (18 ± 6 min; P=0.090).

CONCLUSIONS

TACO may be an acceptable, efficient and feasible cooling method in case of exertional heat stroke. However, in case of limited water availability, transportat should be prioritized, and cooling of any form should be implemented while waiting for and during transport.

The influence of sex and biological maturation on the sudomotor response to exercise-heat stress: Are girls disadvantaged?

Both adult females and children have been reported to have a lower sweating capacity and thus reduced evaporative heat loss potential which may increase their susceptibility to exertional hyperthermia in the heat. Compared to males, females have a lower maximal sweat rate and thus a theoretically lower maximum skin wettedness, due to a lower sweat output per gland. Similarly, children have been suggested to be disadvantaged in high ambient temperatures due to a lower sweat production and therefore reduced evaporative capacity, despite modifications of heat transfer due to physical attributes and possible evaporative efficiency. The reported reductions in sudomotor activity of females and children suggests a lower sweating capacity in girls. However, due to the complexities of isolating sex and maturation from the confounding effects of morphological differences (e.g., body surface area-to-mass ratio) and metabolic heat production, limited evidence exists supporting whether children and, more specifically, girls are at a thermoregulatory disadvantage. Furthermore, a limited number of child-adult comparison studies involve females and very few studies have directly compared regional and whole-body sudomotor activity between boys and girls. This mini review highlights the exercise-induced sudomotor response of females and children, summarises previous research investigating the sudomotor response to exercise in girls and suggests important areas for further research.

Heat tolerance during uncompensable heat stress in men and women wearing firefighter personal protective equipment

Firefighters run a risk of heat strain during occupational tasks. The number of female firefighters has been increasing, but research relevant to this group is still scarce. We aimed to investigate whether there are any sex differences in heat tolerance or physiological responses during uncompensable heat stress while wearing firefighter personal protective equipment. Twelve female (28 ± 7 years, 66 ± 5 kg, 51.7 ± 4.7 mL kg^-1 min^-1) and 12 male (27 ± 7 years, 83 ± 8 kg, 58.8 ± 7.5 mL kg^-1 min^-1) participants performed walking (maximum of 60 min) at 6W·kg^-1, 40 °C, and 14% relative humidity. No differences were observed between groups in heat tolerance, rectal temperature, heart rate, percent body mass loss, thermal sensation, and rate of perceived exertion. Thus, when personnel are selected using gender-neutral physical employment standards, sex is not an independent factor influencing heat tolerance when wearing firefighter personal protective equipment during uncompensable heat stress.

Sex Differences in VO2max and the Impact on Endurance-Exercise Performance

It was not until 1984 that women were permitted to compete in the Olympic marathon. Today, more women than men participate in road racing in all distances except the marathon where participation is near equal. From the period of 1985 to 2004, the women’s marathon record improved at a rate three times greater than men’s. This has led many to question whether women are capable of surpassing men despite the fact that there remains a 10–12% performance gap in all distance events. The progressive developments in sports performance research and training, beginning with A.V. Hill’s establishment of the concept of VO_2max, have allowed endurance athletes to continue performance feats previously thought to be impossible. However, even today women are significantly underrepresented in sports performance research. By focusing more research on the female physiology and sex differences between men and women, we can better define how women differ from men in adapting to training and potentially use this information to improve endurance-exercise performance in women. The male advantage in endurance-exercise performance has commonly been attributed to their higher VO_2max, even when expressed as mL/kg/min. It is widely known that oxygen delivery is the primary limiting factor in elite athletes when it comes to improving VO_2max, but little research has explored the sex differences in oxygen delivery. Thus, the purpose of this review is to highlight what is known about the sex differences in the physiological factors contributing to VO_2max, more specifically oxygen delivery, and the impacts on performance.

Ambient Temperature and Field-Based Cycling Performance: Insights From Male and Female Professional Cyclists

PURPOSE

Ambient temperature affects endurance exercise performance. However, most research has been conducted in a laboratory-based setting, and whether there are sex-specific trends remains unclear. The present study aimed to analyze the influence of ambient temperature on cycling performance in male and female professional cyclists using field-based data collected during both training and racing.

METHODS

A total of 74 cyclists (48 male and 26 female; age 29 [5] y, 8 [5] y of experience in the professional category) were included in the analyses. We registered the participants' record power profile using data from both training and competitions over 8 years (2013-2020; 8 [5] seasons per cyclist). We analyzed their mean maximal power (MMP) values attained for efforts lasting 5 seconds, 30 seconds, 5 minutes, and 20 minutes at ambient temperatures ranging from <5°C to >35°C.

RESULTS

A significant influence of ambient temperature on MMP values was found in male and female cyclists (P < .001 for both), with no significant differences between sexes (P = .512). Cyclists attained the highest MMP values at temperate conditions (10-30°C in males and 5-25°C in females), whereas an impairment in performance was found at colder and hotter temperatures, particularly for the more extreme conditions (performance impairment at <5°C and >35°C of -18% to -9% and -16% to -9%, respectively).

CONCLUSIONS

Ambient temperature influences field-based cycling performance, following a reverse U-shaped relationship, with the highest MMP values attained in the range of ∼10°C to 25°C and with no major differences between sexes.

The Effectiveness of a Standardized Ice-Sheet Cooling Method Following Exertional Hyperthermia

INTRODUCTION

Exertional heat illnesses remain a major threat to military service members in the United States and around the world. Exertional heat stroke (EHS) is the most severe heat illness, characterized by core hyperthermia and central nervous system dysfunction. Per current Army regulations, iced-sheet cooling (ISC) is the recommended immediate treatment for heat casualties in the field, but concerns have been raised regarding the efficacy of this approach. Thus, the purpose of this study was to quantify the cooling rate of ISC following exertional hyperthermia.

MATERIALS AND METHODS

We utilized a randomized crossover design with 2 experimental trials. In both trials, exertional hyperthermia was induced by walking (3.5 mph at 5% grade) on a treadmill in an environmental chamber (40 °C, 30% RH) for up to 3 hours or until core body temperature reached 39.2 °C. After the walking portion, individuals either received ISC (experimental trial) or cooling and rested supine in the same environmental conditions for 30 minutes with no ISC (control trial). For ISC, bed sheets soaked in ice water were applied (per Army guidance) at the neck, chest, and groin with another sheet covering the body. Sheets were rotated and resoaked every 3 minutes until core temperature decreased to <38.0 °C.

RESULTS

By design, participants finished exercise with increased core temperature (38.8 ± 0.39 °C vs. 38.90 ± 0.34 °C, ISC and control trials, P = 1.00). The ISC trial provided significantly (P = .023) greater cooling rates, 0.068 °C/min 95% confidence interval [CI; 0.053, 0.086], compared to the control trial, 0.047 °C/min 95% CI [0.038, 0.056]. Additionally, the time to decrease to less than 38.0 °C was significantly (P = .018) faster in the ISC trial (median = 9.3 minutes) compared to the control trial (median = 26.6 minutes).

CONCLUSION

ISC increases the cooling rate of those recovering from exertional hyperthermia. With the observed cooling rate, we can extrapolate that ISC would reduce core temperature by ∼2 °C within 30 minutes during a case of EHS. We conclude that ISC provides a safe and effective alternative for the field where cold water immersion resources may not be readily available.

Heat tolerance and the validity of occupational heat exposure limits in women during moderate-intensity work

To mitigate excessive rises in core temperature (>1°C) in non heat-acclimatized workers, the American Conference of Governmental Industrial Hygienists (ACGIH) provide heat stress limits (Action Limit Values; ALV), defined by the wet-bulb globe temperature (WBGT) and a worker's metabolic rate. However, since these limits are based on data from men, their suitability for women remains unclear. We therefore assessed core temperature and heart rate in men (n=19; body surface area-to-mass ratio: 250 (SD 17) cm2/kg) and women (n=15; body surface area-to-mass ratio: 268 (SD 24) cm2/kg) aged 18-45 years during 180-min walking at a moderate metabolic rate (200 W/m2) in WBGTs below (16 and 24°C) and above (28 and 32°C) ACGIH ALV. Sex did not significantly influence (i) rises in core temperature, irrespective of WBGT, (ii) the proportion of participants with rises in core temperature >1°C in environments below ACGIH limits, and (iii) work duration before rises in core temperature exceeded 1°C or volitional termination in environments above ACGIH limits. Although further studies are needed, these findings indicate that for the purpose of mitigating rises in core temperature exceeding recommended limits (>1°C), ACGIH guidelines have comparable effectiveness in non heat-acclimatized men and women when working at a moderate metabolic rate. Novelty points • Sex did not appreciably influence thermal strain nor the proportion of participants with core temperatures exceeding recommended limits. • Sex did not significantly influence tolerance to uncompensable heat stress • Despite originating from data obtained in only men, current occupational heat stress guidance offered comparable effectiveness in men and women.

Sex Differences in Endurance Running

In recent years, there has been a significant expansion in female participation in endurance (road and trail) running. The often reported sex differences in maximal oxygen uptake (VO_2max) are not the only differences between sexes during prolonged running. The aim of this narrative review was thus to discuss sex differences in running biomechanics, economy (both in fatigue and non-fatigue conditions), substrate utilization, muscle tissue characteristics (including ultrastructural muscle damage), neuromuscular fatigue, thermoregulation and pacing strategies. Although males and females do not differ in terms of running economy or endurance (i.e. percentage VO_2max sustained), sex-specificities exist in running biomechanics (e.g. females have greater non-sagittal hip and knee joint motion compared to males) that can be partly explained by anatomical (e.g. wider pelvis, larger femur-tibia angle, shorter lower limb length relative to total height in females) differences. Compared to males, females also show greater proportional area of type I fibres, are more able to use fatty acids and preserve carbohydrates during prolonged exercise, demonstrate a more even pacing strategy and less fatigue following endurance running exercise. These differences confer an advantage to females in ultra-endurance performance, but other factors (e.g. lower O₂ carrying capacity, greater body fat percentage) counterbalance these potential advantages, making females outperforming males a rare exception. The present literature review also highlights the lack of sex comparison in studies investigating running biomechanics in fatigue conditions and during the recovery process.

Implications of a patent foramen ovale on environmental physiology and pathophysiology: Do we know the hole story?

The foramen ovale is an essential component of the foetal circulation contributing to oxygenation and carbon dioxide elimination that remains patent under certain circumstances, in ∼ 30% of the healthy adult population, without major negative sequelae in most. Adults with a patent foramen ovale (PFO) have a greater tendency to develop symptoms of acute mountain sickness and high-altitude pulmonary oedema upon ascent to high altitude, and PFO presence is associated with worse cardiopulmonary function in chronic mountain sickness. This increase in altitude illness prevalence may be related to dysregulated cerebral blood flow associated with altered respiratory chemoreflex sensitivity; however, the mechanisms remain to be elucidated. Interestingly, men with a PFO appear to have a shift in thermoregulatory control to higher internal temperatures, both at rest and during exercise, and they have blunted thermal tachypnea. The teleological "reason" for this thermoregulatory shift is unclear, but the shift of ∼0.5°C in core body temperature does not appear to be sufficient to have any significant negative consequences in terms of risk of heat illness. Further work in this area is needed, particularly in women, to evaluate mechanisms of heat storage and dissipation in these individuals as compared to people without a PFO. Consequences of a PFO in SCUBA divers include a greater incidence of unprovoked decompression sickness, but whether PFO is beneficial or detrimental to breath hold diving remains unexplored. Whether PFO presence will explain interindividual variability in responses to, and consequences from, other environmental stressors such as spaceflight remain entirely unknown. Abstract figure legend Associations between PFO and altitude illnesses, core body temperature and diving. This article is protected by copyright. All rights reserved.

Sex differences in temperature-related all-cause mortality in the Netherlands

PURPOSE

Over the last few decades, a global increase in both cold and heat extremes has been observed with significant impacts on human mortality. Although it is well-identified that older individuals (> 65 years) are most prone to temperature-related mortality, there is no consensus on the effect of sex. The current study investigated if sex differences in temperature-related mortality exist in the Netherlands.

METHODS

Twenty-three-year ambient temperature data of the Netherlands were combined with daily mortality data which were subdivided into sex and three age classes (< 65 years, 65-80 years,  ≥ 80 years). Distributed lag non-linear models were used to analyze the effect of ambient temperature on mortality and determine sex differences in mortality attributable to the cold and heat, which is defined as mean daily temperatures below and above the Minimum Mortality Temperature, respectively.

RESULTS

Attributable fractions in the heat were higher in females, especially in the oldest group under extreme heat (≥ 97.5th percentile), whilst no sex differences were found in the cold. Cold- and heat-related mortality was most prominent in the oldest age group (≥ 80 years) and to a smaller extent in the age group between 65-80 years. In the age group < 65 years temperature-related mortality was only significant for males in the heat.

CONCLUSION

Mortality in the Netherlands represents the typical V- or hockey-stick shaped curve with a higher daily mortality in the cold and heat than at milder temperatures in both males and females, especially in the age group ≥ 80 years. Heat-related mortality was higher in females than in males, especially in the oldest age group (≥ 80 years) under extreme heat, whilst in the cold no sex differences were found. The underlying cause may be of physiological or behavioral nature, but more research is necessary.

Applicability of Physiological Monitoring Systems within Occupational Groups: A Systematic Review

The emergence of physiological monitoring technologies has produced exceptional opportunities for real-time collection and analysis of workers' physiological information. To benefit from these safety and health prognostic opportunities, research efforts have explored the applicability of these devices to control workers' wellbeing levels during occupational activities. A systematic review is proposed to summarise up-to-date progress in applying physiological monitoring systems for occupational groups. Adhering with the PRISMA Statement, five databases were searched from 2014 to 2021, and 12 keywords were combined, concluding with the selection of 38 articles. Sources of risk of bias were assessed regarding randomisation procedures, selective outcome reporting and generalisability of results. Assessment procedures involving non-invasive methods applied with health and safety-related goals were filtered. Working-age participants from homogeneous occupational groups were selected, with these groups primarily including firefighters and construction workers. Research objectives were mainly directed to assess heat stress and physiological workload demands. Heart rate related variables, thermal responses and motion tracking through accelerometry were the most common approaches. Overall, wearable sensors proved to be valid tools for assessing physiological status in working environments. Future research should focus on conducting sensor fusion assessments, engaging wearables in real-time evaluation methods and giving continuous feedback to workers and practitioners.

Commercially available carbohydrate drink with menthol fails to improve thermal perception or cycling exercise capacity in males

The purpose of this double-blinded, crossover randomized and counterbalanced study was to compare the effects of ingesting a tepid commercially available carbohydrate-menthol-containing sports drink (menthol) and an isocaloric carbohydrate-containing sports drink (placebo) on thermal perception and cycling endurance capacity "in a simulated home virtual cycling environment". It was hypothesized that the addition of menthol would improve indicators of thermal perception and improve endurance exercise capacity. Twelve healthy, endurance-trained males (age 29 ± 5 years, height 181 ± 6 cm, body mass 79 ± 2 kg and V̇O₂max 57.3 ± 6.4 mL kg^-1 min^-1) completed two experimental trials on a stationary bicycle without external air flow. Each trial consisted of (1) cycling for 60 min at 90% of the first ventilatory threshold while receiving a fixed amount of menthol or placebo every 10 min followed immediately by (2) cycling until volitional exhaustion (TTE) at 105% of the intensity corresponding to the respiratory compensation point. TTE did not differ between both conditions (541 ± 177 and 566 ± 150 s for menthol and placebo; p > 0.05) and neither did ratings of perceived thermal comfort or thermal sensation (p > 0.05). Also, the rectal temperature at the end of TTE was comparable between menthol and placebo trials (38.7 ± 0.2°C and 38.7 ± 0.3°C, respectively; p > 0.05). The present results demonstrate that the addition of menthol to commercially available sports drink does not improve thermal comfort or endurance exercise capacity during ∼65 min of intense virtual cycling.

The interplay between heated environment and active standing test on cardiovascular autonomic control in healthy individuals

Objective.To investigate the interplay between active standing and heat stress on cardiovascular autonomic modulation in healthy individuals.Approach.Blood pressure (BP) and ECG were continuously recorded during 30 min in supine (SUP) and 6 min in orthostatic position (ORT) under thermal reference (TC; ∼24 °C) or heated environment (HOT; ∼36 °C) conditions, in a randomized order. All data collection was performed during the winter and spring seasons when typical outdoor temperatures are ∼23 °C. Spectral analysis was employed by the autoregressive model of R-R and systolic blood pressure (SBP) time series and defined, within each band, in low (LF, 0.04 to 0.15 Hz) and high (0.15-0.40 Hz) frequencies. The indices of cardiac sympathetic (LF) and cardiac parasympathetic (HF) were normalized (nu) dividing each band power by the total power subtracted the very-low component (<0.04 Hz), obtaining the cardiac autonomic balance (LF/HF) modulation. The gain of the relationship between SBP and R-R variabilities within the LF band was utilized for analysis of spontaneous baroreflex sensitivity (alpha index;αLF). Nonlinear analysis was employed through symbolic dynamics of R-R, which provided the percentage of sequences of three heart periods without changes in R-R interval (0V%; cardiac sympathetic modulation) and two significant variations (2UV% and 2LV%; cardiac vagal modulation).Main results.HOT increased 0V% and HR, and decreasedαLF and 2UV% during SUP compared to TC. During ORT, HOT provokes a greater increment on HR, LF/HF and 0V%, indexes compared to ORT under TC.Significance.At rest, heat stress influences both autonomic branches, increasing sympathetic and decreasing vagal modulation and spontaneous baroreflex sensitivity. The augmented HR during active standing under heat stress seems to be mediated by a greater increment in cardiac sympathetic modulation, showing an interplay between gravitational and thermal stimulus.

Effects of passive heat stress and recovery on human cognitive function: An ERP study

Using event-related potentials (ERPs), we investigated the effects of passive heat stress and recovery on the human cognitive function with Flanker tasks, involving congruent and incongruent stimuli. We hypothesized that modulation of the peak amplitude and latency of the P300 component in ERP waveforms would differ with task difficulty during passive heat stress and recovery. Subjects performed the Flanker tasks before (Pre), at the end of whole body heating (Heat: internal temperature increase of ~1.2°C from the pre-heat baseline), and after the internal temperature had returned to the pre-heat baseline (Recovery). The internal temperature was regulated by a tube-lined suit by perfusing 50°C water for heat stress and 25°C water for recovery immediately after the heat stress. Regardless of task difficulty, the reaction time (RT) was shortened during Heat rather than Pre and Recovery, and standard deviations of RT (i.e., response variability) were significantly smaller during Heat than Pre. However, the peak amplitudes of the P300 component in ERPs, which involved selective attention, expectancy, and memory updating, were significantly smaller during Heat than during Pre, suggesting the impairment of neural activity in cognitive function. Notably, the peak amplitudes of the P300 component were higher during Recovery than during Heat, indicating that the impaired neural activity had recovered after sufficient whole-body cooling. An indicator of the stimulus classification/evaluation time (peak latency of P300) and the RT were shortened during Heat stress, but such shortening was not noted after whole-body cooling. These results suggest that hyperthermia affects the human cognitive function, reflected by the peak amplitude and latency of the P300 component in ERPs during the Flanker tasks, but sufficient treatment such as whole-body cooling performed in this study can recover those functions.

Metabolism- and sex-dependent critical WBGT limits at rest and during exercise in the heat

Critical environmental limits are environmental thresholds above which heat gain exceeds heat loss and body core temperature (T_c) cannot be maintained at equilibrium. Those limits can be represented as critical wet-bulb globe temperature (WBGT_crit), a validated index that represents the overall thermal environment. Little is known about WBGT_crit at rest and during low-to-moderate intensity exercise, or sex differences in WBGT_crit, in unacclimated young adults. The following hypotheses were tested: 1) WBGT_crit progressively decreases as metabolic heat production (M_net) increases, 2) no sex differences in WBGT_crit occur at rest, and 3) WBGT_crit is lower during absolute-intensity exercise but higher at relative intensities in women than in men. Thirty-six participants [19 men (M)/17 women (W); 23 ± 4 yr] were tested at rest, during light, absolute-intensity exercise (10 W), or during moderate, relative-intensity exercise [30% maximal oxygen consumption (V̇o_2max)] in an environmental chamber. Dry-bulb temperature was clamped as relative humidity or ambient water vapor pressure was increased until an upward inflection was observed in T_c (rectal or esophageal temperature). Sex-aggregated WBGT_crit was lower during 10 W (32.9°C ± 1.7°C, P < 0.0001) and 30% V̇o_2max (31.6°C ± 1.1°C, P < 0.0001) exercise versus at rest (35.3°C ± 0.8°C), and lower at 30% V̇o_2max versus 10 W (P = 0.01). WBGT_crit was similar between sexes at rest (35.6°C ± 0.8°C vs. 35.0°C ± 0.8°C, P = 0.83), but lower during 10 W (31.9°C ± 1.7°C vs. 34.1°C ± 0.3°C, P < 0.01) and higher during 30% V̇o_2max (32.4°C ± 0.8°C vs. 30.8°C ± 0.9°C, P = 0.03) exercise in women versus men. These findings suggest that WBGT_crit decreases as M_net increases, no sex differences occur in WBGT_crit at rest, and sex differences in WBGT_crit during exercise depend on absolute versus relative intensities.

The impact of repeated, local heating-induced increases in blood flow on lower limb endothelial function in young, healthy females

PURPOSE

The purpose of the present study was to examine the effect of repeated, single leg heating on lower limb endothelial function.

METHODS

Macrovascular function was assessed with superficial femoral artery (SFA) reactive hyperemia flow-mediated dilation (RH-FMD) and sustained stimulus FMD (SS-FMD). Calf microvascular function was assessed as the peak and area under the curve of SFA reactive hyperemia (RH). Participants (n = 13 females, 23 ± 2 yrs) had one leg randomized to the single leg heating intervention (EXP; other leg: control (CON)). The EXP leg underwent 8 weeks of single leg heating via immersion in 42.5 ℃ water for five 35-min sessions/week. At weeks 0, 2, 4, 6, and 8, SFA RH-FMD, SS-FMD (shear stress increased via plantar flexion exercise), and SFA RH flow were measured.

RESULTS

None of the variables changed with repeated, single leg heating (interaction week*limb RH-FMD: p = 0.076; SS-FMD: p = 0.958; RH flow p = 0.955). Covariation for the shear stress stimulus did not alter the FMD results.

CONCLUSION

Eight weeks of single leg heating did not change SFA endothelial or calf microvascular function. These results are in contrast with previous findings that limb heating improves upper limb endothelial function.

Sex differences in the physiological responses to exercise-induced dehydration: consequences and mechanisms

Physiological strain during exercise is increased by mild dehydration (∼1%-3% body mass loss). This response may be sex-dependent, but there are no direct comparative data in this regard. This review aimed to develop a framework for future research by exploring the potential impact of sex on thermoregulatory and cardiac strain associated with exercise-induced dehydration. Sex-based comparisons were achieved by comparing trends from studies that implemented similar experimental protocols but recruited males and females separately. This revealed a higher core temperature (T_c) in response to exercise-induced dehydration in both sexes; however, it seemingly occurred at a lower percent body mass loss in females. Although less clear, similar trends existed for cardiac strain. The average female may have a lower body water volume per body mass compared with males, and therefore the same percent body mass loss between the sexes may represent a larger portion of total body water in females potentially posing a greater physiological strain. In addition, the rate at which T_c increases at exercise onset might be faster in females and induce a greater thermoregulatory challenge earlier into exercise. The T_c response at exercise onset is associated with lower sweating rates in females, which is commonly attributed to sex differences in metabolic heat production. However, a reduced sweat gland sensitivity to stimuli, lower fluid output per sweat gland, and sex hormones promoting fluid retention in females may also contribute. In conclusion, the limited evidence suggests that sex-based differences exist in thermoregulatory and cardiac strain associated with exercise-induced dehydration, and this warrants future investigations.

Roundtable on Preseason Heat Safety in Secondary School Athletics: Heat Acclimatization

OBJECTIVE

To provide best-practice recommendations for developing and implementing heat-acclimatization strategies in secondary school athletics.

DATA SOURCES

An extensive literature review on topics related to heat acclimatization and heat acclimation was conducted by a group of content experts. Using the Delphi method, action-oriented recommendations were developed.

CONCLUSIONS

A period of heat acclimatization consisting of ≥14 consecutive days should be implemented at the start of fall preseason training or practices for all secondary school athletes to mitigate the risk of exertional heat illness. The heat-acclimatization guidelines should outline specific actions for secondary school athletics personnel to use, including the duration of training, the number of training sessions permitted per day, and adequate rest periods in a cool environment. Further, these guidelines should include sport-specific and athlete-specific recommendations, such as phasing in protective equipment and reintroducing heat acclimatization after periods of inactivity. Heat-acclimatization guidelines should be clearly detailed in the secondary school's policy and procedures manual and disseminated to all stakeholders. Heat-acclimatization guidelines, when used in conjunction with current best practices surrounding the prevention, management, and care of secondary school student-athletes with exertional heat stroke, will optimize their health and safety.

Mortality attributable to heat and cold among the elderly in Sofia, Bulgaria

Although a number of epidemiological studies have examined the effects of non-optimal temperatures on mortality in Europe, evidence about the mortality risks associated with exposures to hot and cold temperatures in Bulgaria is scarce. This study provides evidence about mortality attributable to non-optimal temperatures in adults aged 65 and over in Sofia, Bulgaria, between 2000 and 2017. We quantified the relationship between the daily mean temperature and mortality in the total elderly adult population aged 65 and over, among males and females aged 65 and over, as well as individuals aged 65-84 and 85 years or older. We used a distributed lag non-linear model with a 25-day lag to fully capture the effects of both cold and hot temperatures and calculated the fractions of mortality attributable to mild and extreme hot and cold temperatures. Cold temperatures had a greater impact on mortality than hot temperatures during the studied period. Most of the temperature-attributable mortality was due to moderate cold, followed by moderate heat, extreme cold, and extreme heat. The total mortality attributable to non-optimal temperatures was greater among females compared to males and among individuals aged 85 and over compared to those aged 65 to 84. The findings of this study can serve as a foundation for future research and policy development aimed at characterizing and reducing the risks from temperature exposures among vulnerable populations in the country, climate adaptation planning and improved public health preparedness, and response to non-optimal temperatures.