Elevated Core Temperature Research Articles

Exercise and Heat Stress in Well-Healed Burn Survivors: Effects of Cooling Modalities on Thermal and Perceptual Responses.

Burn injuries that require grafting impair thermoregulation, which may dissuade individuals with such injuries from being physically active. We tested the hypothesis that cooling modalities attenuate core temperature elevations and perceptions of heat stress during physical activity in the heat among adults with well-healed burn injuries. Adults with no burn injuries (non-burned), 20-40% body surface area burn injuries (moderate burn), and > 40% body surface area burn injuries (large burn) performed 1 hour of moderate intensity exercise (2.5 ± 0.2 mph and 2% grade) on four different occasions in two environmental conditions (30 °C & 39 °C, 40% relative humidity). Within each environmental condition, we applied one of the following cooling modalities, random assigned, for each visit: no cooling (control), fan at 4 m/s (fan), water spray every 5 min (water spray; scaled to burn area size), or a combination of water spray + fan. In 30 °C, perceptual strain index (PeSI) was reduced in the non-burned and moderate burn groups with water spray + fan, whereas PeSI was reduced with all cooling modalities in the large burn group. The cooling modalities did not affect core temperature responses. In the 39 °C environment, water spray and water spray + fan attenuated the elevation in core temperature (p ≤ 0.007) only in the large burn group. In the moderate burn group, PeSI was decreased with water spray + fan (p = 0.017). In the large burn group, both water spray alone and water spray + fan (p ≤ 0.041) lowered PeSI. For both environments across burn groups, the applied cooling modalities were generally more effective at reducing indices of perceptual strain relative to indices of thermal strain (e.g., core temperature).

Impact of a simulated multiday heatwave on nocturnal physiology, behavior, and sleep: a 10-day confinement study.

This study investigated the impact of a multiday heatwave on nocturnal physiology, behavior, and sleep under controlled conditions with comprehensive monitoring of environmental factors and participant activities. Seven young healthy males were confined for 10 days in controlled conditions that ranged between hot-to-warm (day: 35.4°C, night: 26.3°C) during nights 4-6 and temperate (day: 25.4°C, night: 22.3°C) before (nights 1-3) and after (nights 7-10) the heatwave. Measurements included core and skin temperatures, heart rate, sympathovagal balance, vasomotion indicators, urine samples, blanket coverage, subjective sleep assessments, and partial polysomnography. The average nocturnal core temperature was 0.2°C higher during and after the heatwave compared to the pre-heatwave period, with this difference being more pronounced (+0.3°C) in the first 2 h of sleep (p<0.001). For every 0.1°C rise in overnight core temperature, the total sleep time decreased by 14 min (pseudo-R2=0.26, p=0.01). The elevated core temperatures occurred despite the participants exhibiting evident thermoregulatory behavior, as they covered 30% less body surface during the heatwave compared to pre- and post-heatwave periods (p<0.001). During the heatwave, mean skin temperature at bedtime was 1.3°C higher than pre-heatwave and 0.8°C higher than post-heatwave periods (p<0.001). No differences in other responses, including heart rate and vasomotion indicators, were observed. The paper details a 20-min sleepwalking episode that was coupled with marked changes in sleepwalker's thermophysiological responses. In conclusion, the simulated heatwave resulted in higher overnight core temperature which was associated with reduced total sleep time. Behavioral thermoregulation during sleep may serve as a defense against these effects, though more research is needed.

Skin Wetting Attenuates the Thermal Strain of Older Adults Exposed to Very Hot and Dry Heat with Accompanying Activities of Daily Living

Heat waves cause more deaths and hospitalizations in the elderly relative to any other age group. Indoor cooling with an air conditioner is an effective strategy to prevent the deleterious health effects of heat. However, air conditioning may not be available to some due to limited access, rising energy costs, power outages, and/or industry/government-imposed rolling blackouts. Thus, there is a clear need to identify non-air conditioning dependent cooling strategies to attenuate excessive elevations in body temperature and associated physiological strain in the elderly during heat wave conditions. Purpose: We tested the hypothesis that supplemental skin wetting via water spray attenuates elevations in core temperature and the accompanying physiological strain in older adults exposed to very hot and dry heat with accompanying activities of daily living. Methods: In a randomized order, 10 older adults (4 Females/6 Males; 74 ± 5 years) were exposed to 3-hours of ambient heating, replicating peak environmental conditions during the 2018 Los Angeles heat wave (47 ºC, 15% relative humidity), with (water spray) or without (control) skin-wetting via tap temperature water (~40 g applied every 10-minutes). To simulate heat generation associated with activities of daily living, participants performed six 5-minute periods of light exercise (~3 METS; metabolic heat production: 2.93 ± 0.42 W/kg in control and 2.95 ± 0.44 W/kg in water spray) dispersed throughout the heat exposure. To maintain euhydration, participants consumed 3 mL/kg/hr of tap-temperature water. We assessed core and skin temperatures, heart rate, whole-body sweat rate, and forearm blood flow (as a surrogate of changes in skin blood flow). We compared data between control and water spray trials using one-tailed paired t-tests. Results: When compared to control, water spray reduced the increase in core temperature by 0.33 ± 0.43 °C (p=0.020), ending skin temperature by 1.83 ± 0.73 °C (p&lt;0.001), and ending heart rate by 6 ± 9 bpm (p=0.033). Whole-body sweat rate was 2.12 ± 1.08 g/min lower with water spray compared to control (p&lt;0.001). However, the increase in forearm blood flow was not different between control (154 ± 74 mL/min) and water spray (149 ± 93 mL/min) trials (p=0.400). Conclusion: These preliminary data show that supplemental skin-wetting via water spray attenuates the thermal (core and skin temperature) and associated physiological strain (heart rate and fluid-loss) of older adults exposed to extreme heat with accompanying activities of daily living. The downstream impact of this reduced thermal strain on cardiac and renal stress should be examined, as these are two of the main causes of heat-related morbidity and mortality. Supported by NIH R01AG069005 (CGC) and AHA 23POST1023065 (ZJM). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

DETERMINANTS OF PHYSIOLOGICAL RESPONSE TO EXERTIONAL HEAT STRESS IN HUMANS

INTRODUCTION AND AIMS Previous field studies of exercise undertaken in hot conditions reported dissociation between heat-related symptoms and body core temperature (Tc) elevation. This prompted us to examine physiological mechanisms underpinning responses to exertional heat exposure in a controlled laboratory experiment. METHODS Thirty-eight apparently healthy participants had body composition (DEXA) and fitness (V̇O2peak) measured in a preliminary visit. Tc, heart rate (HR), exercise intensity (V̇O2) and echocardiographic measures of end-diastolic volume (EDV), stroke volume (SV) and cardiac output (Q̇) were measured at rest and at 30-minute intervals throughout a two-hour walk (5km/h and 2% gradient) in a climate-controlled chamber (40⸰C and 50% relative-humidity). RESULTS Twenty-seven participants completed the experiment (Completers; Com), 7 were stopped due to Tc exceeding 39⸰C (Hyperthermics; Hyp). Four participants did not complete the protocol due to adverse symptoms and were excluded from this analysis. Hyp had significantly greater Tc at 60-minutes (38.6±0.5⸰C vs. 38.0±0.3⸰C; P=&amp;lt;0.001) and 90-minutes (39.0±0.3⸰C vs. 38.3±0.3⸰C; P=&amp;lt;0.001) than Com. Baseline V̇O2peak (ml.kg-1.min-1) did not differ between Hyp and Com (P=0.248). Whilst body weight and lean body mass did not significantly differ between Hyp and Com, visceral adipose tissue (VAT) volume (463.2±194.3cm3 vs. 259.9±284.4cm3 P=0.014) and mass (437.0±183.1g vs. 245.2±268.2g P=0.013) were significantly greater in Hyp vs Com. In Hyp, HR change from baseline was greater at 60-minutes (Δ57±15bpm vs. Δ36±18bpm; P=0.01) and 90-minutes (Δ63±20bpm vs. Δ42±19bpm; P=0.017), whilst change in EDV (Δ-22.7±11.4ml vs. Δ-6.92±11.2ml; P=0.016) and SV (Δ-19.4±6.1ml vs. Δ-0.7±7.5ml; P=&amp;lt;0.001) were significantly reduced in Hyp compared to Com. CONCLUSION During walking in the heat, subjects who became hyperthermic had significantly higher baseline visceral fat mass and early signs of exaggerated haemodynamic burden during exertional heat exposure. This study has implications for the identification of appropriate variables for establishing safe work limits during heat exposure in military and work-related contexts.

Relationship Between Running Biomechanics and Core Temperature Across a Competitive Road Race.

Outdoor races introduce environmental stressors to runners, and core temperature changes may influence runners' movement patterns. This study assessed changes and determined relationships between sensor-derived running biomechanics and core temperature among runners across an 11.27-km road race. Core temperatures would increase significantly across the race, related to changes in spatiotemporal biomechanical measures. Cross-sectional cohort study. Level 3. Twenty runners (9 female, 11 male; age, 48 ± 12 years; height, 169.7 ± 9.1 cm; mass, 71.3 ± 13.4 kg) enrolled in the 2022 Falmouth Road Race were recruited. Participants used lightweight technologies (ingestible thermistors and wearable sensors) to monitor core temperature and running biomechanics throughout the race. Timestamps were used to align sensor-derived measures for 7 race segments. Observations were labeled as core temperatures generally within normal limits (<38°C) or at elevated core temperatures (≥38°C). Multivariate repeated measures analyses of variance were used to assess changes in sensor-derived measures across the race, with Bonferroni post hoc comparisons for significant findings. Pearson's r correlations were used to assess the relationship between running biomechanics and core temperature measures. Eighteen participants developed hyperthermic core temperatures (39.0°C ± 0.5°C); core temperatures increased significantly across the race (P < 0.01). Kinetic measures obtained from the accelerometers, including shock, impact, and braking g, all significantly increased across the race (P < 0.01); other sensor-derived biomechanical measures did not change significantly. Core temperatures were weakly associated with biomechanics (|r range|, 0.02-0.16). Core temperatures and kinetics increased significantly across a race, yet these outcomes were not strongly correlated. The observed kinetic changes may have been attributed to fatigue-related influences over the race. Clinicians may not expect changes in biomechanical movement patterns to signal thermal responses during outdoor running in a singular event.

Greater hyperthermia in men with type 2 diabetes does not lead to higher serum levels of cellular stress biomarkers following exercise-heat stress.

Type 2 diabetes (T2D) is associated with worsening age-related impairments in heat loss, causing higher core temperature during exercise. We evaluated whether these thermoregulatory impairments occur with altered serum protein responses to heat stress by measuring cytoprotection, inflammation, and tissue damage biomarkers in middle-aged-to-older men (50-74 years) with (n=16) and without (n=14) T2D following exercise in 40°C. There were no changes in irisin, klotho, HSP70, sCD14, TNF-α, and IL-6, whereas NGAL (+539 pg/mL, p=0.002) and iFABP (+250 pg/mL, p<0.001) increased similarly across groups. These similar response patterns occurred despite elevated core temperature in individuals with T2D, suggesting greater heat vulnerability.

Characteristics and Distribution of Heat-Related Illnesses during Hajj 2022

Background: Heat-related illnesses (HRI) are defined as elevated body core temperatures when exposed to an intense high temperature. HRIs include but are not limited to heat exhaustion (HE) and heat stroke (HS). HE and HS are among the leading causes of morbidity and mortality among pilgrims when the Hajj season occurs during the hot cycle of the year in Makkah. Methods: A retrospective cross-sectional data analysis was conducted on HRI cases during Hajj 2022/1443 Hijri. The data were extracted from the Hajj electronic health records system software immediately at the end of the Hajj period. Results: A total of 200 patients presented with HRI symptoms during the study period. Over half of the HRI patients were male (n=116, 58%). The mean age was 44 ± 14.6 years. One hundred forty-three patients (71.5%) were diagnosed with HE, while 57 (28.5%) were diagnosed with HS. A statistically significant association between age and HRIs was observed. Conclusion: More HRIs are expected due to the gradual elevation of ambient temperature and the global warming phenomenon. This study emphasizes the importance of education and awareness regarding preventive measures in the general population, especially among the most vulnerable subpopulations.

Characteristics and Distribution of Heat-Related Illnesses during Hajj 2022

Background: Heat-related illnesses (HRI) are defined as elevated body core temperatures when exposed to an intense high temperature. HRIs include but are not limited to heat exhaustion (HE) and heat stroke (HS). HE and HS are among the leading causes of morbidity and mortality among pilgrims when the Hajj season occurs during the hot cycle of the year in Makkah. Methods: A retrospective cross-sectional data analysis was conducted on HRI cases during Hajj 2022/1443 Hijri. The data were extracted from the Hajj electronic health records system software immediately at the end of the Hajj period. Results: A total of 200 patients presented with HRI symptoms during the study period. Over half of the HRI patients were male (n=116, 58%). The mean age was 44 ± 14.6 years. One hundred forty-three patients (71.5%) were diagnosed with HE, while 57 (28.5%) were diagnosed with HS. A statistically significant association between age and HRIs was observed. Conclusion: More HRIs are expected due to the gradual elevation of ambient temperature and the global warming phenomenon. This study emphasizes the importance of education and awareness regarding preventive measures in the general population, especially among the most vulnerable subpopulations.

Exposures to Elevated Core Temperatures during Football Training: The Impact on Autonomic Nervous System Recovery and Function.

Exercising with elevated core temperatures may negatively affect autonomic nervous system (ANS) function. Additionally, longer training duration under higher core temperatures may augment these negative effects. This study evaluated the relationship between exercise training duration and 24 h ANS recovery and function at ≥37 °C, ≥38 °C and ≥39 °C core temperature thresholds in a sample of male Division I (D1) collegiate American football athletes. Fifty athletes were followed over their 25-week season. Using armband monitors (Warfighter MonitorTM, Tiger Tech Solutions, Inc., Miami, FL, USA), core temperature (°C) and 24 h post-exercise baseline heart rate (HR), HR recovery and heart rate variability (HRV) were measured. For HRV, two time-domain indices were measured: the root mean square of the standard deviation of the NN interval (rMSSD) and the standard deviation of the NN interval (SDNN). Linear regression models were performed to evaluate the associations between exercise training duration and ANS recovery (baseline HR and HRV) and function (HR recovery) at ≥37 °C, ≥38 °C and ≥39 °C core temperature thresholds. On average, the athletes were 21.3 (± 1.4) years old, weighed 103.0 (±20.2) kg and had a body fat percentage of 15.4% (±7.8%, 3.0% to 36.0%). The duration of training sessions was, on average, 161.1 (±40.6) min and they ranged from 90.1 to 339.6 min. Statistically significant associations between training duration and 24 h ANS recovery and function were observed at both the ≥38.0 °C (baseline HR: β = 0.10 ± 0.02, R2 = 0.26, p < 0.0000; HR recovery: β = -0.06 ± 0.02, R2 = 0.21, p = 0.0002; rMSSD: β = -0.11 ± 0.02, R2 = 0.24, p < 0.0000; and SDNN: β = -0.16 ± 0.04, R2 = 0.22, p < 0.0000) and ≥39.0 °C thresholds (β = 0.39 ± 0.05, R2 = 0.62, p < 0.0000; HR recovery: β = -0.26 ± 0.04, R2 = 0.52, p < 0.0000; rMSSD: β = -0.37 ± 0.05, R2 = 0.58, p < 0.0000; and SDNN: β = -0.67 ± 0.09, R2 = 0.59, p < 0.0000). With increasing core temperatures, increases in slope steepness and strengths of the associations were observed, indicating accelerated ANS deterioration. These findings demonstrate that exercise training under elevated core temperatures (≥38 °C) may negatively influence ANS recovery and function 24 h post exercise and progressively worsen.

Diagnosing and managing heat exhaustion: insights from a systematic review of cases in the desert climate of Mecca.

Heat exhaustion (HE) is a common, yet obscure, heat-related illness that affects millions of people yearly and its burden is projected to rise due to climate change. A comprehensive literature synthesis is lacking despite previous studies on various HE aspects. This systematic review aims to fill this gap by identifying and synthesizing available evidence on the risk factors, symptoms, biomarkers, treatment options, and outcomes for HE. The review focused on HE during the Muslim (Hajj) pilgrimage where the condition is endemic. We conducted a structured search of MEDLINE/PubMed, Embase, Web of Science Core Collection, SCOPUS, and CINAHL databases. We summarized the data from eligible studies and synthesized them in narrative form using pooled descriptive statistics. Ten studies were included between 1980 and 2019, reporting over 1,194 HE cases. HE cases presented with elevated core temperature (up to 40°C) and mainly affected older males from the Middle East and North Africa region, with overweight individuals at a higher risk. Clinical symptoms included hyperventilation, fatigue, dizziness, headaches, nausea, and vomiting, but not central nervous system disturbances. HE was associated with cardiac stress, and with water, electrolyte, and acid-base alterations. Cooling and hydration therapy were the primary management strategies, leading to a low mortality rate (pooled case fatality rate=0.11 % [95 % CI: 0.01, 0.3]). Most cases recovered within a few hours without complications. HE is associated with cardiac stress and changes in homeostasis, leading to distinct clinical symptoms. Early diagnosis and treatment of HE are crucial in reducing the risk of complications and mortality. The review provides insights into the pathophysiology and outcomes of HE, adding to the scarce literature on the subject. Prospero registration number: CRD42022325759.

Onset of cardiovascular drift during progressive heat stress in young adults (PSU HEAT project).

With climate change, humans are at a greater risk for heat-related morbidity and mortality, often secondary to increased cardiovascular strain associated with an elevated core temperature (Tc). Critical environmental limits (i.e., the upper limits of compensable heat stress) have been established based on Tc responses for healthy, young individuals. However, specific environmental limits for the maintenance of cardiovascular homeostasis have not been investigated in the context of thermal strain during light activity. Therefore, the purposes of this study were to 1) identify the specific environmental conditions (combinations of ambient temperature and water vapor pressure) at which cardiovascular drift [i.e., a continuous rise in heart rate (HR)] began to occur and 2) compare those environments to the environmental limits for the maintenance of heat balance. Fifty-one subjects (27 F; 23 ± 4 yr) were exposed to progressive heat stress across a wide range of environmental conditions in an environmental chamber at two low metabolic rates reflecting minimal activity (MinAct; 159 ± 34 W) or light ambulation (LightAmb; 260 ± 55 W). Whether systematically increasing ambient temperature or humidity, the onset of cardiovascular drift occurred at lower environmental conditions compared with Tc inflection points at both intensities (P < 0.05). Furthermore, the time at which cardiovascular drift began preceded the time of Tc inflection (MinAct P = 0.01; LightAmb P = 0.0002), and the difference in time between HR and Tc inflection points did not differ (MinAct P = 0.08; LightAmb P = 0.06) across environmental conditions for either exercise intensity. These data suggest that even in young adults, increases in cardiovascular strain precede the point at which heat stress becomes uncompensable during light activity.NEW & NOTEWORTHY To our knowledge, this study is the first to 1) identify the specific combinations of temperature and humidity at which an increase in cardiovascular strain (cardiovascular drift) occurs and 2) compare those environments to the critical environmental limits for the maintenance of heat balance. We additionally examined the difference in time between the onset of increased cardiovascular strain and uncompensable heat stress. We show that an increase in cardiovascular strain systematically precedes sustained heat storage in young adults.

A Systematic Review of Post-Work Core Temperature Cooling Rates Conferred by Passive Rest.

Physical work increases energy expenditure, requiring a considerable elevation of metabolic rate, which causes body heat production that can cause heat stress, heat strain, and hyperthermia in the absence of adequate cooling. Given that passive rest is often used for cooling, a systematic search of literature databases was conducted to identify studies that reported post-work core temperature cooling rates conferred by passive rest, across a range of environmental conditions. Data regarding cooling rates and environmental conditions were extracted, and the validity of key measures was assessed for each study. Forty-four eligible studies were included, providing 50 datasets. Eight datasets indicated a stable or rising core temperature in participants (range 0.000 to +0.028 °C min-1), and forty-two datasets reported reducing core temperature (-0.002 to -0.070 °C min-1) during passive rest, across a range of Wet-Bulb Globe Temperatures (WBGT). For 13 datasets where occupational or similarly insulative clothing was worn, passive rest resulted in a mean core temperature decrease of -0.004 °C min-1 (-0.032 to +0.013 °C min-1). These findings indicate passive rest does not reverse the elevated core temperatures of heat-exposed workers in a timely manner. Climate projections of higher WBGT are anticipated to further marginalise the passive rest cooling rates of heat-exposed workers, particularly when undertaken in occupational attire.

Does increased core temperature alter cognitive performance during exercise-induced heatstrain? A narrative review.

Research to date, provides equivocal evidence regarding the influence of heat stress, heat strain, and more specifically, elevated exercise-induced core temperature on cognitive performance. This review sought to identify differences in how specific cognitive tasks were affected by increases in core body temperatures. Included papers (N = 31) measured cognitive performance and core temperature during exercise, while experiencing heightened thermal stress. Cognitive tasks were classified as: cognitive inhibition, working memory, or cognitive flexibility tasks. Independently, core temperature changes were not sufficient predictors of cognitive performance. However, reaction time, memory recall, and Stroop tasks appeared to be most effective at identifying cognitive changes during heightened thermal strain. Alterations in performance were more likely to arise under increased thermal loads, which were typically associated with cumulative physiological stressors, such as elevated core temperatures, occurring alongside dehydration, and prolonged exercise durations. Future experimental designs should consider the relevance, or futility of assessing cognitive performance in activities that do not elicit a considerable degree of heat strain, or physiological load.

Nerve growth factor mediates activation of transient receptor potential vanilloid 1 in neurogenic pruritus of psoriasis

Pruritus is a common and painful symptom in psoriasis with profoundly negative impacts on quality of life. The underlying mechanisms of pruritus are complex and multifactorial, and accumulating evidence suggests that pruritus induced by neurogenic inflammation predominates in psoriasis. Nerve growth factor (NGF) -mediated transient receptor potential vanilloid receptor 1(TRPV1) pathway has emerged as a crucial node in the regulation of neurogenic pruritus. TRPV1 appears coupled to most pruritus-specific molecules via the neuro-immune axis. While the modes of regulation differ for each axis, TRPV1 is involved in substantial biochemical crosstalk-causing feedback loops with significant effects on neurogenic pruritus. Therefore, TRPV1 has emerged as a target molecular in drug development for pruritus in psoriasis. However, no significant clinical progress occurred in the development of systemic TRPV1 antagonists due to elevated core temperature. Thus, topical application of TRPV1 antagonists and interference with mediators linked to the TRPV1 activation pathway may be promising therapeutic options to ameliorate pruritus. This Review focuses on recent advances in complicated regulation of NGF-mediated TRPV1 pathway in psoriatic neurogenic pruritus, as well as the therapeutic options that arise from the dissection of the pathway.

Effects of cooling glove on the human body's recovery after exercise and improvement of exercise ability.

After high-intensity exercises, the body's core temperature increases, affecting the body's metabolism, increasing thermal stress and muscle fatigue. The most popular technique to maximize post-workout recovery is cryotherapy. However, the cooling effect may vary depending on the body part being cooled since body tissues do not process the same perfusion. This study investigates the effects of hand cooling on human body functional recovery and exercise ability improvement by comparing normal rest and rest with hand cooling gloves after high-intensity exercise. Thirty healthy subjects participated in this study wherein they exercised and used normal rest for one session and hand cooling rest for the next. Blood lactate concentration, heart rate recovery rate, VO2 max measurement, and the degree of recovery of muscle strength, muscular endurance, and muscle fatigue were investigated in both groups to determine the efficacy of hand cooling gloves for postexercise recovery. When hands were cooled after exercise, blood lactate concentration and body temperature significantly decreased, and cardiopulmonary function, muscle strength, and muscular endurance significantly recovered. Using hand cooling gloves after exercise could attenuate core temperature elevation and improve postexercise recovery. It could also effectively improve athletic performance without using large-scale facilities.

Critical Environmental Limits for Human Thermoregulation in the Context of a Changing Climate.

Human-caused climate change has increased the average temperature of the Earth by over 1°C since the 19th century with larger increases expected by 2100 due to continued human influence. This change in mean ambient temperature has had nonlinear effects, resulting in more high temperature extremes, i.e., heat waves, that have increased in frequency, duration, and magnitude. Additional occurrences of humid heatwaves have significantly affected human health due to the physiological strain associated with a relative inability for evaporative cooling. Inability to efficaciously cool the body, whether during passive heat exposure or physical activity, not only leads to elevated core temperatures but also places strain on the cardiovascular system, often exacerbating age-related co-morbidities. As part of the PSU HEAT (Pennsylvania State University - Human Environmental Age Thresholds) Project, a progressive environmental strain protocol has been developed to determine critical environmental limits - combinations of ambient temperature and humidity -- associated with uncompensable heat stress and intractable rises in core temperature (Tc). These human heat balance thresholds, well below those originally theorized by climatologists, have been surpassed in recent heatwaves and be exceeded on a more regular basis in the future, providing additional impetus to the urgency of adaptative measures and climate change mitigation.

Concurrent validity of the CORE wearable sensor with BodyCap temperature pill to assess core body temperature during an elite women’s field hockey heat training camp

ABSTRACT Wearable temperature sensors offer the potential to overcome several limitations associated with current laboratory- and field-based methods for core temperature assessment; however, their ability to provide accurate data at elevated core temperatures (Tc) has been questioned. Therefore, this investigation aimed to determine the concurrent validity of a wearable temperature sensor (CORE) compared to a reference telemetric temperature pill (BodyCAP) during a team-sport heat training camp prior to the 2020 Olympic Games. Female field hockey players (n = 19) in the Australian national squad completed 4 sessions in hot conditions where their temperature was monitored via CORE and BodyCAP. Concurrent validity of the wearable CORE device was determined with reference to the ingested BodyCAP pill. Lin’s Concordance Correlation Coefficients determined there was “poor” agreement between devices during all sessions. Mean bias demonstrated that CORE underestimated Tc in all sessions (−0.06°C to −0.34°C), with wide mean 95% confidence intervals (±0.35°C to ±0.56°C). Locally estimated scatterplot smoothing regression lines illustrated a non-linearity of error, with greater underestimation of Tc by the CORE device, as Tc increased. The two devices disagreed more than ±0.3°C for 41-60% of all data samples in each session. Our findings do not support the use of the CORE device as a valid alternative to telemetric temperature pills for Tc assessment, particularly during exercise in hot conditions where elevated Tc are expected.

Cold water ingestion ameliorates increase in core temperature and discomfort during simulated motor racing in a hot environment: a randomised trial

Introduction: Formula One drivers race for prolonged periods (>90min) in hot conditions, wearing protective apparel that impairs heat loss, thus are at risk of prolonged elevated core temperature. Ingestion of cold fluids has been shown to be beneficial in endurance activities such as running and cycling. This study aimed to determine whether the voluntary ingestion of cold fluid aided thermoregulation more effectively than the voluntary ingestion of ambient temperature fluid in a simulated motor racing environment.

The impact of castration on physiological responses to exertional heat stroke in mice.

The capability of male mice to exercise in hot environments without succumbing to exertional heat stroke (EHS) is markedly blunted compared to females. Epidemiological evidence in humans and other mammals also suggests some degree of greater vulnerability to heat stroke in males compared to females. The origins of these differences are unknown, but testosterone has previously been shown to induce faster elevations in core temperature during acute, passive heat exposure. In this study, we tested the hypothesis that loss of testosterone and related sex hormones through castration would improve the performance and heat tolerance of male mice during EHS exposure. Twenty-four male mice were randomly divided into 3 groups, untreated EHS mice (SHAM-EHS), castrated EHS mice (CAS+EHS) and naïve exercise controls (NAIVE). Exercise performance and physiological responses in the heat were monitored during EHS and early recovery. Two weeks later, blood and tissues were collected and analyzed for biomarkers of cardiac damage and testosterone. Core temperature in CAS+EHS rose faster to 39.5°C in the early stages of the EHS trial (P<0.0001). However, both EHS groups ran similar distances, exhibited similar peak core temperatures and achieved similar exercise times in the heat, prior to symptom limitation (unconsciousness). CAS+EHS mice had ~10.5% lower body mass at the time of EHS, but this provided no apparent advantage in performance. There was no evidence of myocardial damage in any group, and testosterone levels were undetectable in CAS+EHS after gonadectomy. The results of these experiments exclude the hypothesis that reduced performance of male mice during EHS trials is due to the effects of male sex hormones or intact gonads. However, the results are consistent with a role of male sex hormones or intact gonads in suppressing the early and rapid rise in core temperature during the early stages of exercise in the heat.

The Autophagic Response to Passive and Exercise‐induced Increases in Body Core Temperature in Young Adults

Autophagy is an essential cellular survival mechanism that involves the degradation and recycling of aged or damaged organelles and protein aggregates to maintain cellular homeostasis. While autophagic dysfunction occurs with aging and is associated with many disease‐related pathologies, methods to modulate autophagic activity remain poorly understood in the context of human physiology. Recently, we demonstrated that autophagy is activated in an intensity‐dependent manner during both exercise and passive heat stress, however it is unknown if the autophagic response to elevations in body core temperature (Tcore) during passive heat exposure are comparable to those elicited through exercise. Therefore, we sought to assess the autophagic response to an elevated state of hyperthermia associated with intense exercise and compare responses to a similar rate and magnitude of increase in Tcore elicited through warm water immersion (WI). We evaluated the hypothesis that autophagic stimulation would occur during both exercise and WI, although responses would be greater during the exercise bout. To assess this hypothesis, 6 young men (mean [SD] 23 [3] years) performed 30 min of intense (70% of maximal oxygen consumption (VO2max)) semi‐recumbent cycling. On a separate day (at least 72h after the exercise bout), the same participants were immersed in warm water (up to the clavicle) for 30 min, with the water temperature adjusted to elicit the same relative increase in Tcore (as estimated by rectal temperature) as was achieved during the prior exercise session. Given Tcore during WI was matched to the prior exercise bout, all WI trials were conducted after the exercise session. Autophagic activity was assessed in peripheral blood mononuclear cells prior to, immediately post‐exercise or WI, and following a seated recovery (3 and 6h). Western blot was used to assess changes in autophagy via microtubule‐associated light chain 3 beta (LC3‐II) and normalized to β‐actin (expressed as relative fold change to the respective baseline). Area under the curve was used to assess the Tcore response between conditions and protein data were analyzed via a two‐way repeated measures ANOVA with Fisher’s LSD (α=0.05). A similar average relative increase in Tcore was observed during the exercise and WI bouts (0.65 [0.21]°C vs 0.61 [0.19]°C, respectively; p=0.80) with a return to baseline within 3h in both conditions. In response to 30‐min of exercise, LC3‐II significantly increased immediately after exercise (1.69 [0.12]; p&lt;0.01) and remained elevated above baseline at 3h (1.51 [0.23]; p&lt;0.01) and 6h (1.38 [0.16]; p&lt;0.01). A similar pattern of response was observed following WI (1.86 [0.77]; p=0.04), however, LC3‐II returned to pre‐WI levels within the first 3h of recovery. Our preliminary findings show similar increases in autophagy following a similar elevation in core temperature associated induced by exercise and WI. However, the autophagic response only remained elevated after exercise, likely caused by exercise‐associated metabolic perturbations. Further research is warranted to delineate the underlying pathways of autophagic activation during these different thermal stressors.