Whole-Body Heat Exchange during Heat Acclimation and Its Decay

Myths and methodologies: Considerations for evaluating the time course of thermoregulatory adaptation during heat acclimation

Since the early 1900s, repeated heat exposure has been used as a method to induce physiological adaptations that enhance our ability to tolerate heat stress during athletic and occupational pursuits. Much of this work has been dedicated to quantifying the time course of adaptation and identifying the minimum duration of acclimation required to optimise performance or enhance safety. To achieve this, investigators have typically applied classical (constant load) heat acclimation, whereby 60-90 min exercise is performed at the same absolute or relative intensity in a hot environment for 3-24 days, with adaptations evaluated using an identical forcing function test before and after. This approach has provided a foundation from which to develop our understanding of changes in thermoregulatory function, but it has several, frequently overlooked shortcomings, which have resulted in misconceptions concerning the time course of adaptation. It is frequently suggested that most of the thermoregulatory adaptations during heat acclimation occur within a week, but this is an oversimplification and a predictable artefact of the experimental designs used. Consequently, the time course of complete human adaptation to heat remains poorly understood and appears to vary considerably due to numerous individual factors. The purpose of this communication is to highlight the key methodological considerations required when interpreting the existing literature documenting adaptation over time. We also propose potential means by which to improve the way we induce and quantify the magnitude of adaptation to expedite discovery.

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.

A critical review of the effectiveness of electric fans as a personal cooling intervention in hot weather and heatwaves

Health agencies worldwide have historically cautioned that electric fans accelerate body-heat gain during hot weather and heatwaves (typically in air temperatures ≥35°C). However, guidance published since 2021 has suggested that fans can still cool the body in air temperatures up to 40°C by facilitating sweat evaporation, and therefore are an inexpensive yet sustainable alternative to air conditioning. In a critical analysis of the reports cited to support this claim, we found that although fan use improves sweat evaporation, these benefits are of insufficient magnitude to exert meaningful reductions in body core temperature in air temperatures exceeding 35°C. Health agencies should continue to advise against fan use in air temperatures higher than 35°C, especially for people with compromised sweating capacity (eg, adults aged 65 years or older). Improving access to ambient cooling strategies (eg, air conditioning or evaporative coolers) and minimising their economic and environmental costs through policy initiatives, efficient cooling technology, and combined use of low-cost personal interventions (eg, skin wetting or fan use) are crucial for climate adaptation.

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.

Induction and decay of seasonal acclimatization on whole body heat loss responses during exercise in a hot humid environment with different air velocities

Whether whole body heat loss and thermoregulatory function (local sweat rate and skin blood flow) are different between summer and autumn and between autumn and winter seasons during exercise with different air flow in humid heat remain unknown. We therefore tested the hypotheses that whole body sweat rate (WBSR), evaporated sweat rate, and thermoregulatory function during cycling exercise in autumn would be higher than in winter but would be lower than in summer under hot-humid environment (32 C, 75% RH). We also tested the hypothesis that the increase of air velocity would enhance evaporated sweat rate and sweating efficiency across winter, summer, and autumn seasons. Eight males cycled for 1 h at 40% V̇o_2max in winter, summer, and autumn seasons. Using an electric fan, air velocity increased from 0.2 m/s to 1.1 m/s during the final 20 min of cycling. The autumn season resulted in a lower WBSR, unevaporated sweat rate, and a higher sweating efficiency compared with summer (all P ≤ 0.05) but WBSR and unevaporated sweat rate in autumn were higher than in winter and thus sweating efficiency was lower when compared with winter only at the air velocity of 0.2 m/s (All P ≤ 0.05). Furthermore, evaporated sweat rate and core temperature (T_core) were not different among winter, summer, and autumn seasons (All P > 0.19). In conclusion, changes in WBSR across different seasons do not alter T_core during exercise in a hot humid environment. Furthermore, increasing air velocity enhances evaporated sweat rate and sweating efficiency across all seasons.

How humans adapt to hot climates learned from the recent research on tropical indigenes

This review mainly aimed to introduce the findings of research projects comparing the responses of tropical and temperate indigenes to heat. From a questionnaire survey on thermal sensation and comfort of Indonesians and Japanese, we found that the thermal descriptor “cool” in tropical indigenes connotes a thermally comfortable feeling, suggesting that linguistic heat acclimatization exists on a cognitive level. Ten male students born and raised in Malaysia were invited to Fukuoka, Japan, and compared their responses with 10 Japanese male students with matched physical fitness and morphological characteristics. Cutaneous thermal sensitivity: The sensitivities were measured at 28 °C. The forehead warm sensitivity was significantly blunted in Malaysians. The less sensitivity to the warmth of tropical indigenes is advantageous in respect to withstanding heat stress with less discomfort and a greater ability to work in hot climates. Passive heat stress: Thermoregulatory responses, especially sweating, were investigated, during the lower leg hot bathing (42 °C for 60 min). The rectal temperature at rest was higher in Malaysians and increased smaller during immersion. There was no significant difference in the total amount of sweating between the two groups, while the local sweating on the forehead and thighs was lesser in Malaysians, suggesting distribution of sweating was different from Japanese. Exercise: Malaysian showed a significantly smaller increase in their rectal temperature during 55% maximal exercise for 60 min in heat (32 °C 70% relative humidity), even with a similar sweating and skin blood flow response in Japanese. The better heat tolerance in Malaysians could be explained by the greater convective heat transfer from the body core to the skin due to the greater core-to-skin temperature gradient. In addition, when they were hydrated, Malaysian participants showed better body fluid regulation with smaller reduction in plasma volume at the end of the exercise compared to the non-hydrated condition, whereas Japanese showed no difference between hydration conditions. We further investigated the de-acclimatization of heat adaptation by longitudinal observation on the heat tolerance of international students who had moved from tropical areas to Fukuoka for several years.

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.

Effects of Heat Acclimation Following Heat Acclimatization on Whole Body Heat Exchange in Trained Endurance Athletes

The purpose of this study was to examine the changes in metabolic heat production (Hprod), evaporative heat loss (Hevap), and dry heat loss (Hdry), following heat acclimatization (HAz) and heat acclimation (HA). Twenty-two male endurance athletes (mean ± standard deviation; age, 37 ± 12 y; body mass, 73.4 ± 8.7 kg; height, 178.7 ± 6.8 cm; and VO2max, 57.1 ± 7.2 mL·kg−1·min−1) completed three trials (baseline; post-HAz; and post-HA), which consisted of 60 min steady state exercise at 59 ± 2% velocityVO2max in the heat (ambient temperature [Tamb], 35.2 ± 0.6 °C; relative humidity [%rh] 47.5 ± 0.4%). During the trial, VO2 and RER were collected to calculate Hprod, Hevap, and Hdry. Following the baseline trial, participants completed self-directed outdoor summer training followed by a post-HAz trial. Then, five days of HA were completed over eight days in the heat (Tamb, 38.7 ± 1.1 °C; %rh, 51.2 ± 2.3%). During the HA sessions, participants exercised to maintain hyperthermia (38.50 °C and 39.75 °C) for 60 min. Then, a post-HA trial was performed. There were no differences in Hprod between the baseline (459 ± 59 W·m−2), post-HAz (460 ± 61 W·m−2), and post-HA (464 ± 55 W·m−2, p = 0.866). However, Hevap was significantly increased post-HA (385 ± 84 W·m−2) compared to post-HAz (342 ± 86 W·m−2, p = 0.043) and the baseline (332 ± 77 W·m−2, p = 0.037). Additionally, Hdry was significantly lower at post-HAz (125 ± 8 W·m−2, p = 0.013) and post-HA (121 ± 10 W·m−2, p < 0.001) compared to the baseline (128 ± 7 W·m−2). Hdry at post-HA was also lower than post-HAz (p = 0.049). Hprod did not change following HAz and HA. While Hdry was decreased following HA, the decrease in Hdry was smaller than the increases in Hevap. Adaptations in body heat exchange can occur by HA following HAz.

Effect of Handgrip Training in Extreme Heat on the Development of Handgrip Maximal Isometric Strength among Young Males

The aim of this study was to evaluate the acute and adaptive effects of passive extreme heat (100 ± 3 °C) exposition in combination with a strength training protocol on maximal isometric handgrip strength. Fifty-four untrained male university students participated in this investigation. Twenty-nine formed the control group (NG) and 25 the heat-exposed group (HG). All the participants performed a 3-week isotonic handgrip strength training program twice a week with a training volume of 10 series of 10 repetitions with 45-s rest between series, per session. All the subjects only trained their right hand, leaving their left hand untrained. HG performed the same training protocol in hot (100 ± 3 °C) conditions in a dry sauna. Maximal isometric handgrip strength was evaluated each training day before and after the session. NG participants did not experience any modifications in either hand by the end of the study while HG increased maximal strength values in both hands (p < 0.05), decreased the difference between hands (p < 0.05), and recorded higher values than the controls in the trained (p < 0.05) and untrained (p < 0.01) hands after the intervention period. These changes were not accompanied by any modification in body composition in either group. The performance of a unilateral isotonic handgrip strength program in hot conditions during the three weeks induced an increase in maximal isometric handgrip strength in both hands without modifications to bodyweight or absolute body composition.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Change in Exercise Performance and Markers of Acute Kidney Injury Following Heat Acclimation with Permissive Dehydration

Implementing permissive dehydration (DEH) during short-term heat acclimation (HA) may accelerate adaptations to the heat. However, HA with DEH may augment risk for acute kidney injury (AKI). This study investigated the effect of HA with permissive DEH on time-trial performance and markers of AKI. Fourteen moderately trained men (age and VO_2max = 25 ± 0.5 yr and 51.6 ± 1.8 mL·kg⁻¹·min⁻¹) were randomly assigned to DEH or euhydration (EUH). Time-trial performance and VO_2max were assessed in a temperate environment before and after 7 d of HA. Heat acclimation consisted of 90 min of cycling in an environmental chamber (40 °C, 35% RH). Neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (KIM-1) were assessed pre- and post-exercise on day 1 and day 7 of HA. Following HA, VO_2max did not change in either group (p = 0.099); however, time-trial performance significantly improved (3%, p < 0.01) with no difference between groups (p = 0.485). Compared to pre-exercise, NGAL was not significantly different following day 1 and 7 of HA (p = 0.113) with no difference between groups (p = 0.667). There was a significant increase in KIM-1 following day 1 and 7 of HA (p = 0.002) with no difference between groups (p = 0.307). Heat acclimation paired with permissive DEH does not amplify improvements in VO_2max or time-trial performance in a temperate environment versus EUH and does not increase markers of AKI.

Effects of short-term heat acclimation on whole-body heat exchange and local nitric oxide synthase- and cyclooxygenase-dependent heat loss responses in exercising older men

NEW FINDINGS

What is the central question of this study? Does short-term heat acclimation enhance whole-body evaporative heat loss and augment nitric oxide synthase (NOS)-dependent cutaneous vasodilatation and NOS- and cyclooxygenase (COX)-dependent sweating, in exercising older men? What is the main finding and its importance? Our preliminary data (n = 8) demonstrated that short-term heat acclimation improved whole-body evaporative heat loss, but it did not influence the effects of NOS and/or COX inhibition on cutaneous vasodilatation or sweating in older men during an exercise-heat stress. These outcomes might imply that although short-term heat acclimation enhances heat dissipation in older men, it does not modulate NOS- and COX-dependent control of cutaneous vasodilatation or sweating on the forearm.

ABSTRACT

Ageing is associated with decrements in whole-body heat loss (evaporative + dry heat exchange), which might stem from alterations in nitric oxide synthase (NOS)- and cyclooxygenase (COX)-dependent cutaneous vasodilatation and sweating. We evaluated whether short-term heat acclimation would (i) enhance whole-body heat loss primarily by increasing evaporative heat loss, and (ii) augment NOS-dependent cutaneous vasodilatation and NOS- and COX-dependent sweating, in exercising older men. Eight older men [mean (SD) age, 59 (8) years] completed a calorimetry and microdialysis trial before and after 7 days of exercise-heat acclimation. For the calorimetry trials, whole-body evaporative and dry heat exchange were assessed using direct calorimetry during 30 min bouts of cycling at light, moderate and vigorous metabolic heat productions (150, 200 and 250 W/m² , respectively) in dry heat (40°C, 20% relative humidity). For the microdialysis trials, local cutaneous vascular conductance and sweat rate were assessed during 60 min exercise in the heat (35°C, 20% relative humidity) at four dorsal forearm skin sites treated with lactated Ringer solution (control), NOS inhibitor, COX inhibitor or combined NOS and COX inhibitors, via microdialysis. Evaporative heat loss during moderate (P = 0.036) and vigorous (P = 0.021) exercise increased after acclimation. Inhibition of NOS alone reduced cutaneous vascular conductance to a similar extent before and after acclimation (P < 0.040), whereas separate and combined NOS and COX inhibition had no significant effects on sweating relative to the control site (P = 0.745). Our preliminary results might suggest that short-term heat acclimation improves evaporative heat loss, but does not significantly modulate the contributions of NOS or COX to cutaneous vasodilatation or sweating on the forearm in older men during an exercise-heat stress.

Methods for improving thermal tolerance in military personnel prior to deployment

Acute exposure to heat, such as that experienced by people arriving into a hotter or more humid environment, can compromise physical and cognitive performance as well as health. In military contexts heat stress is exacerbated by the combination of protective clothing, carried loads, and unique activity profiles, making them susceptible to heat illnesses. As the operational environment is dynamic and unpredictable, strategies to minimize the effects of heat should be planned and conducted prior to deployment. This review explores how heat acclimation (HA) prior to deployment may attenuate the effects of heat by initiating physiological and behavioural adaptations to more efficiently and effectively protect thermal homeostasis, thereby improving performance and reducing heat illness risk. HA usually requires access to heat chamber facilities and takes weeks to conduct, which can often make it impractical and infeasible, especially if there are other training requirements and expectations. Recent research in athletic populations has produced protocols that are more feasible and accessible by reducing the time taken to induce adaptations, as well as exploring new methods such as passive HA. These protocols use shorter HA periods or minimise additional training requirements respectively, while still invoking key physiological adaptations, such as lowered core temperature, reduced heart rate and increased sweat rate at a given intensity. For deployments of special units at short notice (< 1 day) it might be optimal to use heat re-acclimation to maintain an elevated baseline of heat tolerance for long periods in anticipation of such an event. Methods practical for military groups are yet to be fully understood, therefore further investigation into the effectiveness of HA methods is required to establish the most effective and feasible approach to implement them within military groups.

Individual Responses to Heat Stress: Implications for Hyperthermia and Physical Work Capacity

Background

Extreme heat events are increasing in frequency, severity, and duration. It is well known that heat stress can have a negative impact on occupational health and productivity, particularly during physical work. However, there are no up-to-date reviews on how vulnerability to heat changes as a function of individual characteristics in relation to the risk of hyperthermia and work capacity loss. The objective of this narrative review is to examine the role of individual characteristics on the human heat stress response, specifically in relation to hyperthermia risk and productivity loss in hot workplaces. Finally, we aim to generate practical guidance for industrial hygienists considering our findings. Factors included in the analysis were body mass, body surface area to mass ratio, body fat, aerobic fitness and training, heat adaptation, aging, sex, and chronic health conditions.

Findings

We found the relevance of any factor to be dynamic, based on the work-type (fixed pace or relative to fitness level), work intensity (low, moderate, or heavy work), climate type (humidity, clothing vapor resistance), and variable of interest (risk of hyperthermia or likelihood of productivity loss). Heat adaptation, high aerobic fitness, and having a large body mass are the most protective factors during heat exposure. Primary detrimental factors include low fitness, low body mass, and lack of heat adaptation. Aging beyond 50 years, being female, and diabetes are less impactful negative factors, since their independent effect is quite small in well matched participants. Skin surface area to mass ratio, body composition, hypertension, and cardiovascular disease are not strong independent predictors of the heat stress response.

Conclusion

Understanding how individual factors impact responses to heat stress is necessary for the prediction of heat wave impacts on occupational health and work capacity. The recommendations provided in this report could be utilized to help curtail hyperthermia risk and productivity losses induced by heat.

Novel Acclimatization and Acclimation Strategies for Hot Climates

Exercising in hot, humid temperatures increases the risk for heat-related illnesses, ranging from mild heat edema to severe heat stroke. With increasing globalization in the world of sports, athletes are sometimes expected to compete in unforgiving conditions that expose them to these risks. In an effort to improve exercise capacity and reduce the risk of serious heat injury, many athletes are recommended to undergo heat acclimatization program prior to competing in climates with elevated average temperature. This article will look at current recommendations as well as studies on differing techniques for acclimatization and acclimation, with hopes to provide guidance for the modern-day clinician and athletes.

Effect of regular precooling on adaptation to training in the heat

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Differing Physiological Adaptations Induced by Dry and Humid Short-Term Heat Acclimation

PURPOSE

To investigate the effect of a 5-day short-term heat acclimation (STHA) protocol in dry (43°C and 20% relative humidity) or humid (32°C and 80% relative humidity) environmental conditions on endurance cycling performance in temperate conditions (21°C).

METHODS

In a randomized, cross-over design, 11 cyclists completed each of the two 5-day blocks of STHA matched for heat index (44°C) and total exposure time (480 min), separated by 30 days. Pre- and post-STHA temperate endurance performance (4-min mean maximal power, lactate threshold 1 and 2) was assessed; in addition, a heat stress test was used to assess individual levels of heat adaptation.

RESULTS

Differences in endurance performance were unclear. Following dry STHA, gross mechanical efficiency was likely reduced (between-condition effect size dry vs humid -0.59; 90% confidence interval, -1.05 to -0.15), oxygen uptake was likely increased for a given workload (0.64 [0.14 to 1.07]), and energy expenditure likely increased (0.59 [0.17 to 1.03]). Plasma volume expansion at day 5 of acclimation was similar (within-condition outcome 4.6% [6.3%] and 5.3% [5.1%] dry and humid, respectively) but was retained for 3 to 4 days longer after the final humid STHA exposure (-0.2% [8.1%] and 4.5% [4.2%] dry and humid, respectively). Sweat rate was very likely increased during dry STHA (0.57 [0.25 to 0.89]) and possibly increased (0.18 [-0.15 to 0.50]) during humid STHA.

CONCLUSION

STHA induced divergent adaptations between dry and humid conditions, but did not result in differences in temperate endurance performance.

Exercise Heat Stress in Patients With and Without Type 2 Diabetes

This physiology study assesses whole-body heat loss in physically active middle-aged and older men with vs without type 2 diabetes after aerobic cycling to evaluate whether type 2 diabetes impairs heat loss and by what mechanism.

Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations That Are Retained for at Least Two Weeks

Heat acclimation by post-exercise hot water immersion (HWI) on six consecutive days reduces thermal strain and improves exercise performance during heat stress. However, the retention of adaptations by this method remains unknown. Typically, adaptations to short-term, exercise-heat-acclimation (<7 heat exposures) decay rapidly and are lost within 2 weeks. Short-term protocols should therefore be completed within 2 weeks of relocating to the heat; potentially compromising pre-competition/deployment training. To establish whether adaptations from post-exercise HWI are retained for up to 2 weeks, participants completed a 40-min treadmill run at 65% _max in the heat (33°C, 40% RH) before (PRE) and 24 h after (POST) the HWI intervention (n = 13) and then at 1 week (WK 1) and 2 weeks (WK 2) after the HWI intervention (n = 9). Heat acclimation involved a 40-min treadmill run (65% _max) on six consecutive days in temperate conditions (20°C), followed by ≤40 min HWI (40°C). Post-exercise HWI induced heat acclimation adaptations that were retained for at least 2 weeks, evidenced by reductions from PRE to WK 2 in: resting rectal core temperature (T _re, -0.36 ± 0.25°C), T _re at sweating onset (-0.26 ± 0.24°C), and end-exercise T _re (-0.36 ± 0.37°C). Furthermore, mean skin temperature (T _sk) (-0.77 ± 0.70°C), heart rate (-14 ± 10 beats⋅min^-1), rating of perceived exertion (-1 ± 2), and thermal sensation (-1 ± 1) were reduced from PRE to WK 2 (P < 0.05). However, PRE to POST changes in total hemoglobin mass, blood volume, plasma volume, the drive for sweating onset, sweating sensitivity and whole body sweating rate did not reach significance (P > 0.05). As such, the reduction in thermal strain during exercise-heat stress appears likely due to the reduction in resting T _re evident at POST, WK 1, and WK 2. In summary, 6 days of post-exercise HWI is an effective, practical and accessible heat acclimation strategy that induces adaptations, which are retained for at least 2 weeks. Therefore, post-exercise HWI can be completed during an athlete's pre-taper phase and does not suffer from the same practical limitations as short-term, exercise-heat-acclimation.

Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations That Are Retained for at Least Two Weeks

Heat acclimation by post-exercise hot water immersion (HWI) on six consecutive days reduces thermal strain and improves exercise performance during heat stress. However, the retention of adaptations by this method remains unknown. Typically, adaptations to short-term, exercise-heat-acclimation (<7 heat exposures) decay rapidly and are lost within 2 weeks. Short-term protocols should therefore be completed within 2 weeks of relocating to the heat; potentially compromising pre-competition/deployment training. To establish whether adaptations from post-exercise HWI are retained for up to 2 weeks, participants completed a 40-min treadmill run at 65% _max in the heat (33°C, 40% RH) before (PRE) and 24 h after (POST) the HWI intervention (n = 13) and then at 1 week (WK 1) and 2 weeks (WK 2) after the HWI intervention (n = 9). Heat acclimation involved a 40-min treadmill run (65% _max) on six consecutive days in temperate conditions (20°C), followed by ≤40 min HWI (40°C). Post-exercise HWI induced heat acclimation adaptations that were retained for at least 2 weeks, evidenced by reductions from PRE to WK 2 in: resting rectal core temperature (T _re, -0.36 ± 0.25°C), T _re at sweating onset (-0.26 ± 0.24°C), and end-exercise T _re (-0.36 ± 0.37°C). Furthermore, mean skin temperature (T _sk) (-0.77 ± 0.70°C), heart rate (-14 ± 10 beats⋅min^-1), rating of perceived exertion (-1 ± 2), and thermal sensation (-1 ± 1) were reduced from PRE to WK 2 (P < 0.05). However, PRE to POST changes in total hemoglobin mass, blood volume, plasma volume, the drive for sweating onset, sweating sensitivity and whole body sweating rate did not reach significance (P > 0.05). As such, the reduction in thermal strain during exercise-heat stress appears likely due to the reduction in resting T _re evident at POST, WK 1, and WK 2. In summary, 6 days of post-exercise HWI is an effective, practical and accessible heat acclimation strategy that induces adaptations, which are retained for at least 2 weeks. Therefore, post-exercise HWI can be completed during an athlete's pre-taper phase and does not suffer from the same practical limitations as short-term, exercise-heat-acclimation.

Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations That Are Retained for at Least Two Weeks

Heat acclimation by post-exercise hot water immersion (HWI) on six consecutive days reduces thermal strain and improves exercise performance during heat stress. However, the retention of adaptations by this method remains unknown. Typically, adaptations to short-term, exercise-heat-acclimation (<7 heat exposures) decay rapidly and are lost within 2 weeks. Short-term protocols should therefore be completed within 2 weeks of relocating to the heat; potentially compromising pre-competition/deployment training. To establish whether adaptations from post-exercise HWI are retained for up to 2 weeks, participants completed a 40-min treadmill run at 65% _max in the heat (33°C, 40% RH) before (PRE) and 24 h after (POST) the HWI intervention (n = 13) and then at 1 week (WK 1) and 2 weeks (WK 2) after the HWI intervention (n = 9). Heat acclimation involved a 40-min treadmill run (65% _max) on six consecutive days in temperate conditions (20°C), followed by ≤40 min HWI (40°C). Post-exercise HWI induced heat acclimation adaptations that were retained for at least 2 weeks, evidenced by reductions from PRE to WK 2 in: resting rectal core temperature (T _re, -0.36 ± 0.25°C), T _re at sweating onset (-0.26 ± 0.24°C), and end-exercise T _re (-0.36 ± 0.37°C). Furthermore, mean skin temperature (T _sk) (-0.77 ± 0.70°C), heart rate (-14 ± 10 beats⋅min^-1), rating of perceived exertion (-1 ± 2), and thermal sensation (-1 ± 1) were reduced from PRE to WK 2 (P < 0.05). However, PRE to POST changes in total hemoglobin mass, blood volume, plasma volume, the drive for sweating onset, sweating sensitivity and whole body sweating rate did not reach significance (P > 0.05). As such, the reduction in thermal strain during exercise-heat stress appears likely due to the reduction in resting T _re evident at POST, WK 1, and WK 2. In summary, 6 days of post-exercise HWI is an effective, practical and accessible heat acclimation strategy that induces adaptations, which are retained for at least 2 weeks. Therefore, post-exercise HWI can be completed during an athlete's pre-taper phase and does not suffer from the same practical limitations as short-term, exercise-heat-acclimation.

Variation in renal responses to exercise in the heat with progressive acclimatisation

OBJECTIVES

To investigate changes in renal status from exercise in the heat with acclimatisation and to evaluate surrogates markers of Acute Kidney Injury.

DESIGN

Prospective observational cohort study.

METHODS

20 male volunteers performed 60 min standardised exercise in the heat, at baseline and on four subsequent occasions during a 23-day acclimatisation regimen. Blood was sampled before and after exercise for serum creatinine, copeptin, interleukin-6, normetanephrine and cortisol. Fractional excretion of sodium was calculated for corresponding urine samples. Ratings of Perceived Exertion were reported every 5 min during exercise. Acute Kidney Injury was defined as serum creatinine rise ≥26.5 μmol L^-1 or fall in estimated glomerular filtration rate >25%. Predictive values of each candidate marker for developing Acute Kidney Injury were determined by ROC analysis.

RESULTS

From baseline to Day 23, serum creatinine did not vary at rest, but showed a significant (P<0.05) reduction post-exercise (120 [102, 139] versus 102 [91, 112] μmol L^-1). Acute Kidney Injury was common (26/100 exposures) and occurred most frequently in the unacclimatised state. Log-normalised fractional excretion of sodium showed a significant interaction (exercise by acclimatization day), with post-exercise values tending to rise with acclimatisation. Ratings of Perceived Exertion predicted AKI (AUC 0.76, 95% confidence interval 0.65-0.88), performing at least as well as biochemical markers.

CONCLUSIONS

Heat acclimatization is associated with reduced markers of renal stress and AKI incidence, perhaps due to improved regional perfusion. Acclimatisation and monitoring Ratings of Perceived Exertion are practical, non-invasive measures that could help to reduce renal injury from exercise in the heat.

Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations in Endurance Trained and Recreationally Active Individuals

Hot water immersion (HWI) after exercise on 6 consecutive days in temperate conditions has been shown to provide heat acclimation adaptations in a recreationally active population. Endurance athletes experience frequent, sustained elevations in body temperature during training and competition; as a consequence, endurance athletes are considered to be partially heat acclimatized. It is therefore important to understand the extent to which endurance trained individuals may benefit from heat acclimation by post-exercise HWI. To this end, we compared the responses of eight endurance trained and eight recreationally active males (habitual weekly endurance exercise: 9 h vs. 3 h) to a 6-day intervention involving a daily treadmill run for 40 min (65% O_2max) in temperate conditions followed immediately by HWI (≤40 min, 40°C). Before (PRE) and after the intervention (POST), hallmark heat acclimation adaptations were assessed during a 40-min treadmill run at 65% O_2max in the heat (33°C, 40% RH). The 6 day, post-exercise HWI intervention induced heat acclimation adaptations in both endurance trained and recreationally active individuals. Training status did not significantly influence the magnitude of heat acclimation adaptations from PRE to POST (interactions P > 0.05) for: the reduction in end-exercise rectal core temperature (T _re, mean, endurance trained -0.36°C; recreationally active -0.47°C); the reduction in resting T _re (endurance trained -0.17°C; recreationally active -0.23°C); the reduction in T _re at sweating onset (endurance trained -0.22°C; recreationally active -0.23°C); and, the reduction in mean skin temperature (endurance trained -0.67°C; recreationally active -0.75°C: PRE to POST P < 0.01). Furthermore, training status did not significantly influence the observed reductions in mean O₂, mean metabolic energy expenditure, end-exercise physiological strain index, perceived exertion or thermal sensation (PRE to POST P < 0.05). Only end-exercise heart rate was influenced by training status (P < 0.01, interaction); whereby, recreationally active but not endurance trained individuals experienced a significant reduction in end-exercise heart rate from PRE to POST (P < 0.01). In summary, these findings demonstrate that post-exercise HWI presents a practical strategy to reduce thermal strain during exercise-heat-stress in endurance trained and recreationally active individuals.

Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations in Endurance Trained and Recreationally Active Individuals

Hot water immersion (HWI) after exercise on 6 consecutive days in temperate conditions has been shown to provide heat acclimation adaptations in a recreationally active population. Endurance athletes experience frequent, sustained elevations in body temperature during training and competition; as a consequence, endurance athletes are considered to be partially heat acclimatized. It is therefore important to understand the extent to which endurance trained individuals may benefit from heat acclimation by post-exercise HWI. To this end, we compared the responses of eight endurance trained and eight recreationally active males (habitual weekly endurance exercise: 9 h vs. 3 h) to a 6-day intervention involving a daily treadmill run for 40 min (65% O_2max) in temperate conditions followed immediately by HWI (≤40 min, 40°C). Before (PRE) and after the intervention (POST), hallmark heat acclimation adaptations were assessed during a 40-min treadmill run at 65% O_2max in the heat (33°C, 40% RH). The 6 day, post-exercise HWI intervention induced heat acclimation adaptations in both endurance trained and recreationally active individuals. Training status did not significantly influence the magnitude of heat acclimation adaptations from PRE to POST (interactions P > 0.05) for: the reduction in end-exercise rectal core temperature (T_re, mean, endurance trained -0.36°C; recreationally active -0.47°C); the reduction in resting T_re (endurance trained -0.17°C; recreationally active -0.23°C); the reduction in T_re at sweating onset (endurance trained -0.22°C; recreationally active -0.23°C); and, the reduction in mean skin temperature (endurance trained -0.67°C; recreationally active -0.75°C: PRE to POST P < 0.01). Furthermore, training status did not significantly influence the observed reductions in mean O₂, mean metabolic energy expenditure, end-exercise physiological strain index, perceived exertion or thermal sensation (PRE to POST P < 0.05). Only end-exercise heart rate was influenced by training status (P < 0.01, interaction); whereby, recreationally active but not endurance trained individuals experienced a significant reduction in end-exercise heart rate from PRE to POST (P < 0.01). In summary, these findings demonstrate that post-exercise HWI presents a practical strategy to reduce thermal strain during exercise-heat-stress in endurance trained and recreationally active individuals.

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

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

Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis

Background

Although the acquisition of heat acclimation (HA) is well-documented, less is known about HA decay (HAD) and heat re-acclimation (HRA). The available literature suggests 1 day of HA is lost following 2 days of HAD. Understanding this relationship has the potential to impact upon the manner in which athletes prepare for major competitions, as a HA regimen may be disruptive during final preparations (i.e., taper).

Objective

The aim of this systematic review and meta-analysis was to determine the rate of HAD and HRA in three of the main physiological adaptations occurring during HA: heart rate (HR), core temperature (T_c), and sweat rate (SR).

Data Sources

Data for this systematic review were retrieved from Scopus and critical review of the cited references.

Study Selection

Studies were included when they met the following criteria: HA, HAD, and HRA (when available) were quantified in terms of exposure and duration. HA had to be for at least 5 days and HAD for at least 7 days for longitudinal studies. HR, T_c, or SR had to be monitored in human participants.

Study Appraisal

The level of bias in each study was assessed using the McMaster critical review form. Multiple linear regression techniques were used to determine the dependency of HAD in HR, T_c, and SR from the number of HA and HAD days, daily HA exposure duration, and intensity.

Results

Twelve studies met the criteria and were systematically reviewed. HAD was quantified as a percentage change relative to HA (0% = HA, 100% = unacclimated state). Adaptations in end-exercise HR decreased by 2.3% (P < 0.001) for every day of HAD. For end-exercise T_c, the daily decrease was 2.6% (P < 0.01). The adaptations in T_c during the HA period were more sustainable when the daily heat exposure duration was increased and heat exposure intensity decreased. The decay in SR was not related to the number of decay days. However, protracted HA-regimens seem to induce longer-lasting adaptations in SR. High heat exposure intensities during HA seem to evoke more sustained adaptations in SR than lower heat stress. Only eight studies investigated HRA. HRA was 8–12 times faster than HAD at inducing adaptations in HR and T_c, but no differences could be established for SR.

Limitations

The available studies lacked standardization in the protocols for HA and HAD.

Conclusions

HAD and HRA differ considerably between physiological systems. Five or more HA days are sufficient to cause adaptations in HR and T_c; however, extending the daily heat exposure duration enhances T_c adaptations. For every decay day, ~ 2.5% of the adaptations in HR and T_c are lost. For SR, longer HA periods are related to better adaptations. High heat exposure intensities seem beneficial for adaptations in SR, but not in T_c. HRA induces adaptations in HR and T_c at a faster rate than HA. HRA may thus provide a practical and less disruptive means of maintaining and optimizing HA prior to competition.

Short-Term Heat Acclimation and Precooling, Independently and Combined, Improve 5-km Time Trial Performance in the Heat

James, CA, Richardson, AJ, Watt, PW, Willmott, AGB, Gibson, OR, and Maxwell, NS. Short-term heat acclimation and precooling, independently and combined, improve 5-km time trial performance in the heat. J Strength Cond Res 32(5): 1366-1375, 2018-Following heat acclimation (HA), endurance running performance remains impaired in hot vs. temperate conditions. Combining HA with precooling (PC) demonstrates no additive benefit in intermittent sprint, or continuous cycling exercise protocols, during which heat strain may be less severe compared to endurance running. This study investigated the effect of short-term HA (STHA) combined with mixed methods PC, on endurance running performance and directly compared PC and HA. Nine amateur trained runners completed 5-km treadmill time trials (TTs) in the heat (32° C, 60% relative humidity) under 4 conditions; no intervention (CON), PC, short-term HA (5 days-HA) and STHA with PC (HA + PC). Mean (±SD) performance times were; CON 1,476 (173) seconds, PC 1,421 (146) seconds, HA 1,378 (116) seconds and HA + PC 1,373 (121) seconds. This equated to the following improvements versus CON; PC -3.7%, HA -6.6% and HA + PC -7.0%. Statistical differences were only observed between HA and CON (p = 0.004, d = 0.68, 95% CI [-0.27 to 1.63]) however, similar effect sizes were observed for HA + PC vs. CON (d = 0.70, 95% CI [-0.25 to 1.65]), with smaller effects between PC vs. CON (d = 0.34, 95% CI [-0.59 to 1.27]), HA vs. PC (d = 0.33, 95% CI [-0.60 to 1.26]) and HA + PC vs. PC (d = 0.36, 95% CI [-0.57 to 1.29]). Pilot testing revealed a TT typical error of 16 seconds (1.2%). Precooling offered no further benefit to performance in the acclimated individual, despite modest alleviation of physiological strain. Maintenance of running speed in HA + PC, despite reduced physiological strain, may indicate an inappropriate pacing strategy therefore, further familiarization is recommended to optimize a combined strategy. Finally, these data indicate HA, achieved through cycle training, yields a larger ergogenic effect than PC on 5-km running performance in the heat, although PC remains beneficial when HA is not possible.

Acetaminophen (Paracetamol) Induces Hypothermia During Acute Cold Stress

BACKGROUND

Acetaminophen is an over-the-counter drug used to treat pain and fever, but it has also been shown to reduce core temperature (T _c) in the absence of fever. However, this side effect is not well examined in humans, and it is unknown if the hypothermic response to acetaminophen is exacerbated with cold exposure.

OBJECTIVE

To address this question, we mapped the thermoregulatory responses to acetaminophen and placebo administration during exposure to acute cold (10 °C) and thermal neutrality (25 °C).

METHODS

Nine healthy Caucasian males (aged 20-24 years) participated in the experiment. In a double-blind, randomised, repeated measures design, participants were passively exposed to a thermo-neutral or cold environment for 120 min, with administration of 20 mg/kg lean body mass acetaminophen or a placebo 5 min prior to exposure. T _c, skin temperature (T _sk), heart rate, and thermal sensation were measured every 10 min, and mean arterial pressure was recorded every 30 min. Data were analysed using linear mixed effects models. Differences in thermal sensation were analysed using a cumulative link mixed model.

RESULTS

Acetaminophen had no effect on T _c in a thermo-neutral environment, but significantly reduced T _c during cold exposure, compared with a placebo. T _c was lower in the acetaminophen compared with the placebo condition at each 10-min interval from 80 to 120 min into the trial (all p < 0.05). On average, T _c decreased by 0.42 ± 0.13 °C from baseline after 120 min of cold exposure (range 0.16-0.57 °C), whereas there was no change in the placebo group (0.01 ± 0.1 °C). T _sk, heart rate, thermal sensation, and mean arterial pressure were not different between conditions (p > 0.05).

CONCLUSION

This preliminary trial suggests that acetaminophen-induced hypothermia is exacerbated during cold stress. Larger scale trials seem warranted to determine if acetaminophen administration is associated with an increased risk of accidental hypothermia, particularly in vulnerable populations such as frail elderly individuals.

Post Junctional Sudomotor and Cutaneous Vascular Responses in Noninjured Skin Following Heat Acclimation in Burn Survivors

Thermal tolerance is improved in burn survivors following 7 days of exercise heat acclimation. It is unknown whether post junctional sudomotor and/or cutaneous vascular adaptations in noninjured skin contribute to this improvement. Thirty-three burn survivors were stratified into moderately (17-40% BSA grafted, n = 19) and highly (>40% BSA grafted, n = 14) skin-grafted groups. Nine nonburned subjects served as controls. All subjects underwent a 7-day heat acclimation protocol, which improved thermal tolerance in all groups. Before and after this heat acclimation protocol, post junctional cutaneous vascular responses were assessed by administering increasing doses of sodium nitroprusside (SNP) and methacholine (MCh) using intradermal microdialysis in noninjured skin. MCh infusion was also used to assess post junctional responses in sudomotor function in noninjured skin. Cutaneous vascular responses to SNP and MCh were not different between pre- and post heat acclimation in either group of burn survivors (both P > .05). The maximal sweating rate to MCh increased post acclimation in the control group (0.41 ± 0.20 to 0.54 ± 0.21 mg·min·cm; P = .016) but was unchanged in both groups of burn survivors (both P > .05). The number of sweat glands activated during the highest dose of MCh was elevated in the >40% BSA-grafted group (49 ± 16 to 56 ± 18 glands·cm; P = .005) but was unchanged in control subjects and the <40% BSA-grafted group (both P > .05). Given that post junctional administration of MCh and SNP did not alter sweating or skin blood flow from noninjured skin of burn survivors, improved thermal tolerance in these individuals following heat acclimation is more likely a result of either an increased sweating efficiency or an increased neural drive for sweating.

Heart rate variability responses to acute and repeated postexercise sauna in trained cyclists

Short- to medium-term (i.e., 4-14 days) heating protocols induce physiological adaptations including improved cardiac autonomic modulations, as assessed using heart rate variability, which may contribute to greater exercise performance. Whether similar cardiac autonomic changes occur during an intense heating protocol (sauna) reported to increase plasma volume in athletes remains to be confirmed. This study examined changes in heart rate and its variability during a single extreme heat (sauna) exposure and repeated exposures in athletes. Six well-trained male cyclists undertook sauna bathing (30 min, 87 °C, 11% relative humidity) immediately after normal training over 10 consecutive days. Heart rate recordings were obtained during each sauna bout. Heart rate and its variability (natural logarithm of root mean square of successive differences, lnRMSSD) were analysed during 10-min periods within the first bout, and changes in heart rate and lnRMSSD were analysed during each bout via magnitude-based inferences. During the first sauna bout, heart rate was almost certainly increased (∼32%, effect size 1.68) and lnRMSSD was almost certainly reduced (∼62%, effect size -5.21) from the first to the last 10-min period, indicating reduced parasympathetic and (or) enhanced sympathetic modulations. Acute exposure to extreme heat stress via sauna produced alterations in heart rate and cardiac autonomic modulations with successive postexercise heat exposures producing unclear changes over a 10-day period. The physiological benefits of intense heating via sauna on cardiac control in athletes remain to be elucidated.

Sweating as a heat loss thermoeffector

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

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Heart rate variability and plasma nephrines in the evaluation of heat acclimatisation status

Purpose

Heat adaptation (HA) is critical to performance and health in a hot environment. Transition from short-term heat acclimatisation (STHA) to long-term heat acclimatisation (LTHA) is characterised by decreased autonomic disturbance and increased protection from thermal injury. A standard heat tolerance test (HTT) is recommended for validating exercise performance status, but any role in distinguishing STHA from LTHA is unreported. The aims of this study were to (1) define performance status by serial HTT during structured natural HA, (2) evaluate surrogate markers of autonomic activation, including heart rate variability (HRV), in relation to HA status.

Methods

Participants (n = 13) were assessed by HTT (60-min block-stepping, 50% VO₂peak) during STHA (Day 2, 6 and 9) and LTHA (Day 23). Core temperature (Tc) and heart rate (HR) were measured every 5 min. Sampling for HRV indices (RMSSD, LF:HF) and sympathoadrenal blood measures (cortisol, nephrines) was undertaken before and after (POST) each HTT.

Results

Significant (P < 0.05) interactions existed for Tc, logLF:HF, cortisol and nephrines (two-way ANOVA; HTT by Day). Relative to LTHA, POST results differed significantly for Tc (Day 2, 6 and 9), HR (Day 2), logRMSSD (Day 2 and Day 6), logLF:HF (Day 2 and Day 6), cortisol (Day 2) and nephrines (Day 2 and Day 9). POST differences in HRV (Day 6 vs. 23) were + 9.9% (logRMSSD) and − 18.6% (logLF:HF).

Conclusions

Early reductions in HR and cortisol characterised STHA, whereas LTHA showed diminished excitability by Tc, HRV and nephrine measures. Measurement of HRV may have potential to aid real-time assessment of readiness for activity in the heat.

Electronic supplementary material

The online version of this article (10.1007/s00421-017-3758-y) contains supplementary material, which is available to authorized users.

Power Relative to Body Mass Best Predicts Change in Core Temperature During Exercise-Heat Stress

Gibson, OR, Willmott, AGB, James, CA, Hayes, M, and Maxwell, NS. Power relative to body mass best predicts change in core temperature during exercise-heat stress. J Strength Cond Res 31(2): 403-414, 2017-Controlling internal temperature is crucial when prescribing exercise-heat stress, particularly during interventions designed to induce thermoregulatory adaptations. This study aimed to determine the relationship between the rate of rectal temperature (Trec) increase, and various methods for prescribing exercise-heat stress, to identify the most efficient method of prescribing isothermic heat acclimation (HA) training. Thirty-five men cycled in hot conditions (40° C, 39% R.H.) for 29 ± 2 minutes. Subjects exercised at 60 ± 9% V[Combining Dot Above]O2peak, with methods for prescribing exercise retrospectively observed for each participant. Pearson product moment correlations were calculated for each prescriptive variable against the rate of change in Trec (° C·h), with stepwise multiple regressions performed on statistically significant variables (p ≤ 0.05). Linear regression identified the predicted intensity required to increase Trec by 1.0-2.0° C between 20- and 45-minute periods and the duration taken to increase Trec by 1.5° C in response to incremental intensities to guide prescription. Significant (p ≤ 0.05) relationships with the rate of change in Trec were observed for prescriptions based on relative power (W·kg; r = 0.764), power (%Powermax; r = 0.679), rating of perceived exertion (RPE) (r = 0.577), V[Combining Dot Above]O2 (%V[Combining Dot Above]O2peak; r = 0.562), heart rate (HR) (%HRmax; r = 0.534), and thermal sensation (r = 0.311). Stepwise multiple regressions observed relative power and RPE as variables to improve the model (r = 0.791), with no improvement after inclusion of any anthropometric variable. Prescription of exercise under heat stress using power (W·kg or %Powermax) has the strongest relationship with the rate of change in Trec with no additional requirement to correct for body composition within a normal range. Practitioners should therefore prescribe exercise intensity using relative power during isothermic HA training to increase Trec efficiently and maximize adaptation.

Restoration of thermoregulation after exercise

Performing exercise, especially in hot conditions, can heat the body, causing significant increases in internal body temperature. To offset this increase, powerful and highly developed autonomic thermoregulatory responses (i.e., skin blood flow and sweating) are activated to enhance whole body heat loss; a response mediated by temperature-sensitive receptors in both the skin and the internal core regions of the body. Independent of thermal control of heat loss, nonthermal factors can have profound consequences on the body's ability to dissipate heat during exercise. These include the activation of the body's sensory receptors (i.e., baroreceptors, metaboreceptors, mechanoreceptors, etc.) as well as phenotypic factors such as age, sex, acclimation, fitness, and chronic diseases (e.g., diabetes). The influence of these factors extends into recovery such that marked impairments in thermoregulatory function occur, leading to prolonged and sustained elevations in body core temperature. Irrespective of the level of hyperthermia, there is a time-dependent suppression of the body's physiological ability to dissipate heat. This delay in the restoration of postexercise thermoregulation has been associated with disturbances in cardiovascular function which manifest most commonly as postexercise hypotension. This review examines the current knowledge regarding the restoration of thermoregulation postexercise. In addition, the factors that are thought to accelerate or delay the return of body core temperature to resting levels are highlighted with a particular emphasis on strategies to manage heat stress in athletic and/or occupational settings.

Local versus whole-body sweating adaptations following 14 days of traditional heat acclimation

The purpose of this study was to examine if local changes in sweat rate following 14 days of heat acclimation reflect those that occur at the whole-body level. Both prior to and following a 14-day traditional heat acclimation protocol, 10 males exercised in the heat (35 °C, ∼20% relative humidity) at increasing rates of heat production equal to 300 (Ex1), 350 (Ex2), and 400 (Ex3) W·m−2. A 10-min recovery period followed Ex1, while a 20-min recovery period separated Ex2 and Ex3. The exercise protocol was performed in a direct calorimeter to measure whole-body sweat rate and, on a separate day, in a thermal chamber to measure local sweat rate (LSR), sweat gland activation (SGA), and sweat gland output (SGO) on the upper back, chest, and mid-anterior forearm. Post-acclimation, whole-body sweat rate was greater during each exercise bout (Ex1: 14.3 ± 0.9; Ex2: 17.3 ± 1.2; Ex3: 19.4 ± 1.3 g·min−1, all p ≤ 0.05) relative to pre-acclimation (Ex1: 13.1 ± 0.6; Ex2: 15.4 ± 0.8; Ex3: 16.5 ± 1.3 g·min−1). In contrast, only LSR on the forearm increased with acclimation, and this increase was only observed during Ex2 (Post: 1.32 ± 0.33 vs. Pre: 1.06 ± 0.22 mg·min−1·cm−2, p = 0.03) and Ex3 (Post: 1.47 ± 0.41 vs. Pre: 1.17 ± 0.23 mg·min−1·cm−2, p = 0.05). The greater forearm LSR post-acclimation was due to an increase in SGO, as no changes in SGA were observed. Overall, these data demonstrate marked regional variability in the effect of heat acclimation on LSR, such that not all local measurements of sweat rate reflect the improvements observed at the whole-body level.

Characteristics of hyperthermia-induced hyperventilation in humans

In humans, hyperthermia leads to activation of a set of thermoregulatory responses that includes cutaneous vasodilation and sweating. Hyperthermia also increases ventilation in humans, as is observed in panting dogs, but the physiological significance and characteristics of the hyperventilatory response in humans remain unclear. The relative contribution of respiratory heat loss to total heat loss in a hot environment in humans is small, and this hyperventilation causes a concomitant reduction in arterial CO₂ pressure (hypocapnia), which can cause cerebral hypoperfusion. Consequently, hyperventilation in humans may not contribute to the maintenance of physiological homeostasis (i.e., thermoregulation). To gain some insight into the physiological significance of hyperthermia-induced hyperventilation in humans, in this review, we discuss 1) the mechanisms underlying hyperthermia-induced hyperventilation, 2) the factors modulating this response, and 3) the physiological consequences of the response.

The effects of single versus twice daily short term heat acclimation on heat strain and 3000m running performance in hot, humid conditions

Endurance performances are impaired under conditions of elevated heat stress. Short term heat acclimation (STHA) over 4-6 days can evoke rapid adaptation, which mitigate decrements in performance and alleviate heat strain. This study investigated the efficacy of twice daily heat acclimation (TDHA) compared to single session per day heat acclimation (SDHA) and normothermic training, at inducing heat acclimation phenotype and its impact upon running performance in hot, humid conditions. Twenty one, moderately trained males were matched and assigned to three groups; SDHA (mean±SD) (peak oxygen consumption [V̇O2peak] 45.8±6.1mLkg(-1)min(-1), body mass 81.3±16.0kg, stature 182±3cm), TDHA (46.1±7.0mLkg(-1)min(-1), 80.1±11.9kg, 178±4cm) or control (CON) (47.1±3.5mLkg(-1)min(-1), 78.6±16.7kg, 178±4cm). Interventions consisted of 45min cycling at 50% V̇O2peak, once daily for 4d (SDHA) and twice daily for 2d (TDHA), in 35°C, 60% relative humidity (RH), and once daily for 4 days (CON) in 21°C, 40% RH. Participants completed a pre- and post-intervention 5km treadmill run trial in 30°C, 60% RH, where the first 2km were fixed at 40% V̇O2peak and the final 3km was self-paced. No statistically significant interaction effects occurred within- or between-groups over the 2-4 days intervention. While within-group differences were found in physiological and perceptual measures during the fixed intensity trial post-intervention, they did not statistically differ between-groups. Similarly, TDHA (-36±34s [+3.5%]) and SDHA (-26±28s [+2.8%]) groups improved 3km performances (p=0.35), but did not differ from CON (-6±44s [+0.6%]). This is the first study to investigate the effects of HA twice daily and compare it with traditional single session per day STHA. These STHA protocols may have the ability to induce partial adaptive responses to heat stress and possibly enhance performance in environmentally challenging conditions, however, future development is warranted to optimise the administration to provide a potent stimuli for heat adaptation in athletic and military personnel within a rapid regime.

Heat acclimation attenuates physiological strain and the HSP72, but not HSP90α, mRNA response to acute normobaric hypoxia

Heat acclimation (HA) attenuates physiological strain in hot conditions via phenotypic and cellular adaptation. The aim of this study was to determine whether HA reduced physiological strain, and heat shock protein (HSP) 72 and HSP90α mRNA responses in acute normobaric hypoxia. Sixteen male participants completed ten 90-min sessions of isothermic HA (40°C/40% relative humidity) or exercise training [control (CON); 20°C/40% relative humidity]. HA or CON were preceded (HYP1) and proceeded (HYP2) by a 30-min normobaric hypoxic exposure [inspired O2 fraction = 0.12; 10-min rest, 10-min cycling at 40% peak O2 uptake (V̇o2 peak), 10-min cycling at 65% V̇o2 peak]. HA induced greater rectal temperatures, sweat rate, and heart rates (HR) than CON during the training sessions. HA, but not CON, reduced resting rectal temperatures and resting HR and increased sweat rate and plasma volume. Hemoglobin mass did not change following HA nor CON. HSP72 and HSP90α mRNA increased in response to each HA session, but did not change with CON. HR during HYP2 was lower and O2 saturation higher at 65% V̇o2 peak following HA, but not CON. O2 uptake/HR was greater at rest and 65% V̇o2 peak in HYP2 following HA, but was unchanged after CON. At rest, the respiratory exchange ratio was reduced during HYP2 following HA, but not CON. The increase in HSP72 mRNA during HYP1 did not occur in HYP2 following HA. In CON, HSP72 mRNA expression was unchanged during HYP1 and HYP2. In HA and CON, increases in HSP90α mRNA during HYP1 were maintained in HYP2. HA reduces physiological strain, and the transcription of HSP72, but not HSP90α mRNA in acute normobaric hypoxia.

Effect of short-term exercise-heat acclimation on ventilatory and cerebral blood flow responses to passive heating at rest in humans

Hyperthermia induces hyperventilation and cerebral hypoperfusion in resting humans. We tested the hypothesis that short-term exercise-heat acclimation would alleviate those effects. Twenty healthy male subjects were divided into two groups that performed exercise training in the heat (TR-HEAT, n = 10) or cold (TR-COLD, n = 10). Before and after the training, the subjects in both groups participated in passive-heat tests at rest. Training was performed at 37°C (TR-HEAT) or 10°C (TR-COLD) and entailed four 20-min bouts of cycling at 50% peak oxygen uptake separated by 10-min recoveries daily for 6 consecutive days. After TR-HEAT, esophageal temperature was lowered when measured before and during passive heating, as was the esophageal temperature threshold for cutaneous active vasodilation, whereas plasma volume was increased (all P < 0.05). These traditional indices of successful heat acclimation were not all induced by TR-COLD (all P > 0.05). TR-HEAT had no significant effect on passive heating-induced increases in minute ventilation, even when evaluated as the esophageal temperature threshold for increases in minute ventilation and the slope relating minute ventilation to esophageal temperature (all P > 0.05). By contrast, TR-HEAT attenuated the passive heating-induced reduction in the cerebral vascular conductance index (middle cerebral artery mean blood velocity/mean arterial pressure) (all P < 0.05). TR-COLD did not attenuate the increase in minute ventilation or the decrease in the cerebral vascular conductance index observed during passive heating (all P > 0.05). These data suggest that in resting heated humans, short-term heat acclimation achieved through moderate-intensity exercise training (i.e., 50% peak oxygen uptake) in the heat does not influence hyperthermia-induced hyperventilation, but it does potentially attenuate cerebral hypoperfusion.