The Effects of Heat Adaptation on Physiology, Perception and Exercise Performance in the Heat: A Meta-Analysis

Self-reported exertional heat illness and risk factors among collegiate marching band artists

Marching band (MB) artists are often part of the general student population and not required to complete a pre-participation health screening to identify predisposing medical conditions or risks for injury/illness. Anecdotally, exertional heat illnesses (EHI) are a concern for MB artists. As more athletic trainers provide MB healthcare, research is needed on EHI occurrence and MB associated EHI risk factors. We utilized an exploratory cross-sectional study design to determine EHI risk factors, including previous EHI occurrence, among collegiate MB artists. MB artists (n ​= ​1 207; age ​= ​[19.6 ​± ​1.3] years) actively participating in their college/university's MB during the 2019 football season completed an online survey to characterize demographics, medical history, medication and supplement use, and nutrition behaviors. Chi-square and binomial logistical regressions assessed associations between categorical variables. Previous EHI was reported by 50.6% of MB artists, with 466 (76.3%) experiencing exertional heat exhaustion and 31 (5.1%) exertional heat stroke. More females reported exertional heat exhaustion overall (68.2%, p ​< ​0.001) and in the previous year (73.3%, p ​< ​0.001). Experiencing a previous EHI was significantly associated with having a mood/neurological condition (63.5%, p ​< ​0.001), diagnosed (74.3%, p ​= ​0.004) or perceived eating disorder (66.7%, p ​< ​0.001), and taking prescription medications (59.4%, p ​< ​0.001), over-the-counter medications (58.9%, p ​= ​0.002), and supplements (55.4%, p ​= ​0.037). Half of collegiate MB artists reported experiencing previous EHIs and engaged in behaviors known to increase EHI risk. MB artists should complete pre-participation examinations to identify pre-existing medical conditions and risks for adverse medical events. Healthcare providers working with MB artists should develop policies and procedures to mitigate EHI risks and occurrence.

Effect of condensed heat acclimation on thermophysiological adaptations, hypoxic cross-tolerance, exercise performance, and deacclimation

Short duration heat acclimation (HA) (≤5 daily heat exposures) elicits incomplete adaptation compared with longer interventions, possibly due to the lower accumulated thermal "dose." It is unknown if matching thermal "dose" over a shorter timescale elicits comparable adaptation to a longer intervention. Using a parallel-groups design, we compared: 1) "condensed" HA (CHA; n = 17 males) consisting of 4 × 75 min·day-1 heat exposures [target rectal temperature (Trec) = 38.5 °C] for two consecutive days, with 2) "traditional" HA (THA; n = 15 males) consisting of 1 × 75 min·day-1 heat exposure (target Trec = 38.5°C) for eight consecutive days. Physiological responses to exercise heat stress, hypoxia, and normoxic exercise performance were evaluated pre- and postintervention. Thermal (Trec over final 45 min: CHA = 38.45 ± 0.17°C, THA = 38.53 ± 0.13°C, P = 0.126) and cardiovascular strain were not different during interventions, indicating similar thermal "dose," although CHA had lower sweating rate, higher starting Trec, and greater inflammation, gastrointestinal permeability, and renal stress (P < 0.05). However, CHA elicited an array of thermophysiological adaptations that did not differ from THA [reduced indices of peak thermal (e.g., Δ peak Trec CHA = -0.28 ± 0.26°C, THA = -0.36 ± 0.17°C, P = 0.303) and cardiovascular strain, inflammation, and renal stress; blood and plasma volume expansion; improved perceptual indices], although improvements in resting thermal strain (e.g., Δ resting Trec CHA = -0.14 ± 0.21°C, THA = -0.35 ± 0.29°C, P = 0.027) and sweating rate were less with CHA. Both interventions improved aspects of hypoxic tolerance, but effects on temperate normoxic exercise indices were limited. The diminished thermal strain was well-maintained over a 22-day decay period. In conclusion, CHA could represent a viable acclimation option for time-restricted young healthy males preparing for a hot, and possibly high-altitude, environment.NEW & NOTEWORTHY This study has shown, for the first time, that a novel condensed heat acclimation program can elicit an array of thermophysiological adaptations, many of which do not differ from traditional heat acclimation. These findings suggest that accumulated thermal "dose" is an important factor contributing to the adaptive responses to heat stress and that condensed heat acclimation may represent a viable option for time-restricted individuals (e.g., military personnel, firefighters, and athletes) preparing to enter a hot environment.

Effect of Heat Acclimatization, Heat Acclimation, and Intermittent Heat Training on Maximal Oxygen Uptake

BACKGROUND

Maximal oxygen uptake (VO2max) is an important determinant of endurance performance. Heat acclimation/acclimatization (HA/HAz) elicits improvements in endurance performance. Upon heat exposure reduction, intermittent heat training (IHT) may alleviate HA/HAz adaptation decay; however, corresponding VO2max responses are unknown.

HYPOTHESIS

VO2max is maintained after HAz/HA; IHT mitigates decrements in aerobic power after HAz/HA.

STUDY DESIGN

Interventional study.

LEVEL OF EVIDENCE

Level 3.

METHODS

A total of 27 male endurance runners (mean ± SD; age, 36 ± 12 years; body mass, 73.03 ± 8.97 kg; height, 178.81 ± 6.39 cm) completed VO2max testing at 5 timepoints; baseline, post-HAz, post-HA, and weeks 4 and 8 of IHT (IHT4, IHT8). After baseline testing, participants completed HAz, preceded by 5 days of HA involving exercise to induce hyperthermia for 60 minutes in the heat (ambient temperature, 39.13 ± 1.37°C; relative humidity, 51.08 ± 8.42%). Participants were assigned randomly to 1 of 3 IHT groups: once-weekly, twice-weekly, or no IHT. Differences in VO2max, velocity at VO2max (vVO2), and maximal heart rate (HRmax) at all 5 timepoints were analyzed using repeated-measure analyses of variance with Bonferroni corrections post hoc.

RESULTS

No significant VO2max or vVO2 differences were observed between baseline, post-HAz, or post-HA (P = 0.36 and P = 0.09, respectively). No significant group or time effects were identified for VO2max or vVO2 at post-HA, IHT4, and IHT8 (P = 0.67 and P = 0.21, respectively). Significant HRmax differences were observed between baseline and post-HA tests (P < 0.01). No significant group or time HRmax differences shown for post-HA, IHT4, and IHT8 (P = 0.59).

CONCLUSION

VO2max was not reduced among endurance runners after HA/HAz and IHT potentially due to participants' similar aerobic training status and high aerobic fitness levels.

CLINICAL RELEVANCE

HAz/HA and IHT maintain aerobic power in endurance runners, with HAz/HA procuring reductions in HRmax.

Outpacing climate change: adaptation to heatwaves in Europe

Current predictions of climate change impacts rely on conservative assumptions about a lack of adaptation, projecting significantly increased heatwave mortality. However, long-term studies have shown a decline in actual heatwave deaths, raising questions about the underlying mechanisms. We combined Eurostat weekly mortality data (baseline extracted via Seasonal-Trend decomposition by Loess and smoothed through Principal Component Analysis dimension reduction and reconstruction) with economic indicators, Copernicus temperature data since 1950, and ENTSO-E electricity demand data. Panel regression analyzed mortality patterns during weeks with daily temperatures exceeding 22 °C for 2000-2022. During the analyzed period, Europe outpaced climate change, with the capacity to tolerate an additional 1 °C rise every 17.9 years [95% CI 15.3-22.7]. Extending the temperature indicators beyond the prior 3 years did not enhance predictive accuracy, suggesting swift adaptations and historical climate lacked any predictive value. Additionally, increasing economic output, likely driven by infrastructural improvements, especially greater affordability of air conditioning, enabled tolerating each additional 1 °C due to a per capita GDP increase of 19.7 thousand euros [95% CI 14.6-30.3]. Consistently, the increase in cooling energy demand was the strongest in eastern Europe. The findings shed light on the mechanisms driving the observed reduction in heatwave mortality despite the warming climate trend, offering a more plausible basis for extrapolation than assuming a lack of adaptation. The model emphasizes the role of long term economic growth and addressing energy poverty.

Passive heating in sport: context-specific benefits, detriments, and considerations

Exercise and passive heating share some acute physiological responses. These include increases in body temperature, sweat rate, blood flow, heart rate, and redistribution of plasma and blood volume. These responses can vary depending on the heating modality or dose (e.g., temperature, duration, body coverage) and are beneficial to athletes in specific scenarios. These scenarios include being applied to increase muscle or force production, induce rapid weight loss, stimulate thermoregulatory or cardiovascular adaptation, or to accelerate recovery. The rationale being to tailor the specific passive heating protocol to target the desired physiological response. However, some acute responses to passive heating may also be detrimental to sporting outcomes, such as exercising in the heat, having unintended residual negative effects on performance or perceptions of fatigue, or even resulting in hospitalisation if implemented inappropriately. Accordingly, the effects of passive heating should be carefully considered prior to implementation by athletes, coaches, and support staff. Therefore, the purpose of this review is to evaluate the physiological responses to different modes and doses of passive heating and explore the various sport contexts where these effects may either benefit or hinder athletes. Understanding these responses can aid the implementation of passive heating in sport and identify potential recommended heating protocols in each given scenario.

Association between physiological responses and heat shock protein 70 (HSP70) expressions in the vulnerable populations of Kuala Lumpur

Continued heat exposure can cause physiological and cellular responses. This study investigated the association between physiological responses and heat shock protein 70 (HSP70) expressions in Kuala Lumpur's urban vulnerable population. We conducted a cross-sectional study involving 54 participants from four areas classified as experiencing moderate to strong heat stress. Physiological measurements included core body temperature, heart rate, and diastolic and systolic blood pressure. RT-qPCR and ELISA were also performed on blood samples to assess HSP70 gene and protein expressions. Despite indoor heat stress, participants maintained normal physiological parameters while there were significant indications of HSP70 expression at both the gene and protein levels. However, our study found no significant correlation (p > 0.05) between physiological responses and HSP70 expressions. This study shows no interaction between physiological responses and HSP70 expressions in the study population, revealing the complex mechanisms of indoor heat stress in vulnerable individuals.

Quantifying the time course of changes in maximal skin wettedness with 7 days of heat acclimation

The aim of the present study was to quantify the time course of changes in maximum skin wettedness (ωmax)-that is, the proportion of skin surface area covered in sweat at the point of uncompensable heat stress, throughout seven consecutive days of heat acclimation. Nine adults (6 M, 3 F) completed a humidity-ramp protocol (RAMP) on days 1, 3, 5, and 7 of seven consecutive days of heat acclimation. In each RAMP trial, participants cycled continuously at 275 W·m-2 for 120 min at 37°C: 60 min at a vapor pressure of 2.05 kPa followed by 60 min with vapor pressure increased by 0.045 kPa·min-1. An upward inflection in esophageal temperature (Teso) signaled a transition to uncompensable heat stress with the critical water vapor pressure at that point used to calculate ωmax. In days between RAMP assessments, participants cycled for 90 min at 75% HRmax at 37°C, 60% RH. Teso, whole body sweat rate (WBSR), local sweat rates on the back and forearm (LSRback and LSRarm, respectively), and activated sweat gland density (ASGD) were measured throughout. ωmax was progressively and significantly greater from day 1 (0.68 ± 0.10) to day 3 (0.75 ± 0.10; P = 0.002), to day 5 (0.79 ± 0.10; P = 0.004), and to day 7 (0.87 ± 0.06; P = 0.009). WBSR was higher on days 5 (1.11 ± 0.30 L·h-1; P = 0.01) and 7 (1.12 ± 0.19 L·h-1; P < 0.001) compared with day 1 (0.94 ± 0.21 L·h-1). ASGD was higher on day 5 (78 ± 15 glands·cm-2; P < 0.001) and day 7 (81 ± 17 glands·cm-2; P = 0.001) compared with day 1 (65 ± 12 glands·cm-2). There were no observed differences in sweat gland output (P = 0.21). In conclusion, ωmax significantly increased throughout 7 days of heat acclimation. These progressive increases in ωmax were predominantly mediated by an increase in the number of active sweat glands, not the output per gland.NEW & NOTEWORTHY Significant increases in ωmax were observed as early as 3 days into a 7-day heat acclimation protocol. These data are the first to report dynamic changes in ωmax with progressive heat acclimation and provide new information about ωmax levels between the standard "unacclimated" and "acclimated" states in existing heat stress models.

Effects of dynamic thermal conditioning on cognitive load and performance in an office environment

In the design of buildings with minimal environmental impact, the incorporation of higher energy flexibility is becoming increasingly relevant. This approach is associated with dynamic modulations in setpoint temperatures. Until now, a link between indoor temperatures and cognitive performance of workers has been assumed, leading to high energy consumption and overcooling of office environments in summer conditions. However, research focusing on the relationship between thermal indoor environments and cognitive performance has rarely considered the influence of dynamic temperatures or temporal effects. This is the first experimental study aiming to understand the impact of temperature in relation to time of day on the subjective perception of cognitive load and performance under various thermal conditions in real-world office environments. The results indicated no observable relationship between temperature setpoints (25-30 °C) and cognitive performance. Instead, the temporal dynamics of cooling rather than fixed and static temperature setpoints appeared to have an impact.

Effects of Precooling on Endurance Exercise Performance in the Heat: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

An increasing number of studies have explored the effects of precooling on endurance exercise performance in the heat, yet the available results remain inconsistent. Therefore, this study aimed to investigate the effects of different precooling strategies on endurance exercise performance in the heat. A comprehensive search was conducted across PubMed, Web of Science, Cochrane, Scopus, and EBSCO database. The Cochrane risk assessment tool was employed to evaluate the methodological quality of the included studies. A meta-analysis was subsequently conducted to quantify the standardized mean difference (SMD) and 95% confidence interval for the effects of precooling on endurance exercise performance in the heat. Out of the initially identified 6982 search records, 15 studies were deemed eligible for meta-analysis. Our results showed that precooling significantly improved time trial (TT) performance (SMD, -0.37, p < 0.01, I2 = 0%) and time to exhaustion (TTE) performance in the heat (SMD, 0.73, p < 0.01, I2 = 50%). Further subgroup analyses revealed that external precooling is more effective in improving TT performance (SMD, -0.43, p = 0.004, I2 = 0%) and TTE performance (SMD, 1.01, p < 0.001, I2 = 48%), particularly in running-based performances (TT, SMD, -0.41, p = 0.02, I2 = 0%; TTE, SMD, 0.85, p = 0.0001, I2 = 31%). Precooling is an effective approach to improve endurance exercise performance in the heat. External precooling is more effective in improving endurance exercise performance, particularly in running-based performance.

The Role of Ion-Transporting Proteins on Crosstalk Between the Skeletal Muscle and Central Nervous Systems Elicited by Physical Exercise

A paradigm shift in the understanding of bidirectional interactions between peripheral and central nervous systems is essential for development of rehabilitation and preventive interventions based on physical exercise. Although a causal relationship has not been completely established, modulation of voltage-dependent ion channels (Ca2+, Cl-, K+, Na+, lactate-, H+) in skeletal and neuronal cells provides opportunities to maintain force production during exercise and reduce the risk of disease. However, there are caveats to consider when interpreting the effects of physical exercise on this bidirectional axis, since exercise protocol details (e.g., duration and intensity) have variable effects on this crosstalk. Therefore, an integrative perspective of the skeletal muscle and brain's communication pathway is discussed, and the role of physical exercise on such communication highway is explained in this review.

Impact of Physical Activity on Cellular Metabolism Across Both Neurodegenerative and General Neurological Conditions: A Narrative Review

BACKGROUND

Regular physical activity plays a crucial role in modulating cellular metabolism and mitigating the progression of neurodegenerative diseases such as Alzheimer's, Parkinson's, and Multiple Sclerosis.

OBJECTIVE

The objective of this review is to evaluate the molecular mechanisms by which exercise influences cellular metabolism, with a focus on its potential as a therapeutic intervention for neurological disorders.

METHODS

A comprehensive literature review was conducted using peer-reviewed scientific articles, with a focus on the period between 2015 and 2024, to analyze the effects of exercise on mitochondrial function, oxidative stress, and metabolic health.

RESULTS

The findings indicate that exercise promotes mitochondrial biogenesis, enhances oxidative phosphorylation, and reduces reactive oxygen species, contributing to improved energy production and cellular resilience. These metabolic adaptations are associated with delayed disease progression and reduced symptoms in patients with neurodegenerative conditions. Additionally, integrating exercise with nutritional strategies may further enhance therapeutic outcomes by addressing metabolic disturbances comprehensively.

CONCLUSIONS

This review concludes that personalized exercise protocols should be developed to optimize metabolic benefits for patients with neurological diseases, while future research should focus on biomarker development for individualized treatment approaches. These findings highlight the importance of non-pharmacological interventions in managing neurodegenerative diseases.

The Efficacy of Nutritional Strategies and Ergogenic Aids on Acute Responses and Chronic Adaptations to Exertional-Heat Exposure: A Narrative Review

Global warming is attributed to an increased frequency of high ambient temperatures and humidity, elevating the prevalence of high-temperature-related illness and death. Evidence over recent decades highlights that tailored nutritional strategies are essential to improve performance and optimise health during acute and chronic exertional-heat exposure. Therefore, the purpose of this review is to discuss the efficacy of various nutritional strategies and ergogenic aids on responses during and following acute and chronic exertional-heat exposure. An outline is provided surrounding the application of various nutritional practices (e.g., carbohydrate loading, fluid replacement strategies) and ergogenic aids (e.g., caffeine, creatine, nitrate, tyrosine) to improve physiological, cognitive, and recovery responses to acute exertional-heat exposure. Additionally, this review will evaluate if the magnitude and time course of chronic heat adaptations can be modified with tailored supplementation practices. This review highlights that there is robust evidence for the use of certain ergogenic aids and nutritional strategies to improve performance and health outcomes during exertional-heat exposure. However, equivocal findings across studies appear dependent on factors such as exercise testing modality, duration, and intensity; outcome measures in relation to the ergogenic aid's proposed mechanism of action; and sex-specific responses. Collectively, this review provides evidence-based recommendations and highlights areas for future research that have the potential to assist with prescribing specific nutritional strategies and ergogenic aids in populations frequently exercising in the heat. Future research is required to establish dose-, sex-, and exercise-modality-specific responses to various nutritional practices and ergogenic aid use for acute and chronic exertional-heat exposure.

Implementation of a mixed-methods heat acclimation programme in a professional soccer referee before the 2022 FIFA world cup in Qatar: a case study

Objectives

This case study reports the real-world practicalities of implementing a mixed-methods heat acclimation (HA) programme before the 2022 FIFA World Cup.

Methods

One male English soccer referee (age: 44 years; height: 1.82 m; body mass: 76.0 kg) who had officiated professionally for over 17 years and had over 10 years' experience officiating in European and international matches undertook an 11-session HA programme over 22 days. On days 1 and 22, a 30 min fixed-intensity heat tolerance test (9 km.h-1, 2% gradient, 40°C, 40% relative humidity) was performed, and physiological and perceptual responses were measured. A mixed-methods HA approach was used, including environmental chamber isothermic training, post-temperate training saunas and hot water immersion.

Results

Compared with the pre-test, peak core temperature reduced by 0.40°C (38.4 vs 38.0°C; minimal detectable change (MDC) = 0.34°C), peak skin temperature reduced by 0.5°C (36.7 vs 36.2°C; MDC=0.28°C) and peak heart rate reduced by 5 b·min-1 (167 vs. 162 b·min-1; MDC=4 b·min-1) in the post-test. In the post-test, the sweat rate increased by 17% (1.94 vs 2.27 L.h-1; MDC=0.42 L.h-1). Peak thermal sensation (7 = 'hot') and the rating of perceived exertion (3 = 'moderate') were unchanged between the tests. However, peak thermal comfort (3 = 'slightly uncomfortable' vs 2 = 'uncomfortable') was rated lower in the post-test.

Conclusion

The HA programme elicited positive physiological but indifferent perceptual responses, highlighting that mixed-methods HA can be implemented when a referee still has officiating, travel and training responsibilities during the HA window.

The repeatability of a cycling exercise-heat stress test in a male population

INTRODUCTION

There is considerable inter-individual variability in the physiological responses to environmental stressors and so to accurately assess and monitor changes in an individual's ability to cope with exercise-heat stress, a reliable protocol is required. The aim of this study was to examine the repeatability of a 90-min steady-state heat exercise bout with physiological and subjective variables, and performance during an incremental test to exhaustion post 90-min steady-state exercise.

METHOD

Sixteen mixed ability males (Age: 39 ± 15yrs; Height: 176.5 ± 4.8 cm; BM: 79.7 ± 10.3 kg; V˙O2peak: 46.2 ± 8.6 ml/kg/min; PPO: 309 ± 39W) who trained at least three times a week undertook two 90-min steady-state - followed by an incremental protocol to exhaustion - cycling heat stress tests (HSTs) in a hot-humid environment (35 °C, 60%RH). Heart rate (HR), rectal (T‾re) and skin temperature (T‾sk), rating of perceived exertion (RPE), thermal sensation (TS), and thermal comfort (TC) were measured throughout. Data was analysed using Intraclass Correlation Coefficients (ICC), technical error of measurement (TEM), Bland-Altman plots, t-tests, and Cohen's d to indicate magnitude of change.

RESULTS

Physiological variables indicated good repeatability evident through moderate to strong ICC ratings, low magnitudes of change (d), lower mean biases compared to their respective calculated TEMs, and statistical non-significance, except HR90, ₸sk90, and ₸sk. Hydration status showed good repeatability except for urine osmolality (osmu90) and resting urine colour (colu). Perceptual variables showed encouraging repeatability apart from resting TS and mean TS. Performance data showed good repeatability overall, however 11 participants progressed to the incremental test to exhaustion in the second visit compared to 7 in the first.

CONCLUSION

Current data demonstrated favourable physiological, perceptual, and performance repeatability during repeated cycling HSTs in hot-humid conditions. However, given more participants progressed to the incremental trial to exhaustion protocol in the second visit, at least one familiarisation trial may improve the reliability of exercise capacity assessment.

Chasing Gold: Heat Acclimation in Elite Handcyclists with Spinal Cord Injury

Thermoregulation is impaired in individuals with a spinal cord lesion (SCI), affecting sweat capacity, heat loss, and core temperature. This can be particularly problematic for athletes with SCI who exercise in hot and humid conditions, like those during the Tokyo 2020 Paralympic Games. Heat acclimation can support optimal preparation for exercise in such challenging environments, but evidence is limited in endurance athletes with SCI. We evaluated whether seven consecutive days of exercise in the heat would result in heat acclimation. Five elite para-cycling athletes with SCI participated (two females, three males, median (Q1-Q3) 35 (31-51) years, four with paraplegia and one with tetraplegia). All tests and training sessions were performed in a heat chamber (30°C and 75% relative humidity). A time-to-exhaustion test was performed on day 1 (pretest) and day 7 (posttest). On days 2-6, athletes trained daily for one hour at 50-60% of individual peak power (PPeak). Comparing pretest and posttest, all athletes increased their body mass loss (p=0.04), sweat rate (p=0.04), and time to exhaustion (p=0.04). Effects varied between athletes for core temperature and heart rate. All athletes appeared to benefit from our heat acclimation protocol, helping to optimize their preparation for the Tokyo 2020 Paralympic Games.

Coping with extreme heat: current exposure and implications for the future

A preview of how effective behavioral, biological and technological responses might be in the future, when outdoor conditions will be at least 2°C hotter than current levels, is available today from studies of individuals already living in extreme heat. In areas where high temperatures are common-particularly those in the hot and humid tropics-several studies report that indoor temperatures in low-income housing can be significantly hotter than those outdoors. A case study indicates that daily indoor heat indexes in almost all the 123 slum dwellings monitored in Kolkata during the summer were above 41°C (106°F) for at least an hour. Economic constraints make it unlikely that technological fixes, such as air conditioners, will remedy conditions like these-now or in the future. People without access to air conditioning will have to rely on behavioral adjustments and/or biological/physiological acclimatization. One important unknown is whether individuals who have lived their entire lives in hot environments without air conditioning possess natural levels of acclimatization greater than those indicated by controlled laboratory studies. Answering questions about the future will require more studies of heat conditions experienced by individuals, more information on indoor versus outdoor heat conditions, and a greater understanding of the behavioral and biological adjustments made by people living today in extremely hot conditions.

Time-course for onset and decay of physiological adaptations in endurance trained athletes undertaking prolonged heat acclimation training

Short-term heat acclimation (HA) appears adequate for maximizing sudomotor adaptations and enhancing thermal resilience in trained athletes. However, for enhanced erythropoiesis and transfer effects to exercise capacity in cooler environments, prolonged HA appears necessary. To establish the time-course for physiological adaptations and performance effects, 20 male elite cyclists were divided into an intervention group (HEAT; n = 10) completing 5 weeks of HA (six one-hour HA-training sessions per week) and control (n = 10) tested pre and post in hot (40°C) and cool conditions (20°C). HEAT completed tests at 40°C every week during HA with measures of sweat rate and [Na+] and a decay test 2 weeks after termination of HA. HEAT improved time for exhaustion by 15 min (p < 0.001) in the 40°C test, increased sweat rate by 0.44 L/hour (p < 0.001), and lowered sweat sodium concentration [Na+] by 14.1 mmol/L (p = 0.006) from pre- to post-HA, with performance returning to pre-HA levels in the 2-week decay test. Total hemoglobin mass (tHbmass) was increased by 30 grams (+3%, p = 0.048) after 3 weeks and 40 grams (+4%, p = 0.038) after 5 weeks in HEAT but returned to pre-HA levels at the 2-week decay test. HEAT improved incremental peak power output (+12 W, p = 0.001) without significant changes in maximal oxygen uptake (p = 0.094). In conclusion, improvements in heat exercise tolerance and sudomotor adaptations materialized during the first ~3 weeks and the entire 5 weeks of HA augmented both cool exercise capacity and tHbmass. However, the 2-week post-HA evaluation demonstrated a rapid decay of physiological adaptations and exercise capacity in the heat.

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.

Environmental challenges facing athletes, stakeholders and spectators at Paris 2024 Olympic and Paralympic Games: an evidence-based review of mitigation strategies and recommendations

The upcoming Paris 2024 Olympic and Paralympic Games could face environmental challenges related to heat, air quality and water quality. These challenges will pose potential threats to athletes and impact thousands of stakeholders and millions of spectators. Recognising the multifaceted nature of these challenges, a range of strategies will be essential for mitigating adverse effects on participants, stakeholders and spectators alike. From personalised interventions for athletes and attendees to comprehensive measures implemented by organisers, a holistic approach is crucial to address these challenges and the possible interplay of heat, air and water quality factors during the event. This evidence-based review highlights various environmental challenges anticipated at Paris 2024, offering strategies applicable to athletes, stakeholders and spectators. Additionally, it provides recommendations for Local Organising Committees and the International Olympic Committee that may be applicable to future Games. In summary, the review offers solutions for consideration by the stakeholders responsible for and affected by the anticipated environmental challenges at Paris 2024.

Reliability and validity of the MX3 portable sweat sodium analyser during exercise in warm conditions

PURPOSE

Accurately measuring sweat sodium concentration ([Na+]) in the field is advantageous for coaches, scientists, and dieticians looking to tailor hydration strategies. The MX3 hydration testing system is a new portable analyser that uses pre-calibrated biosensors to measure sweat [Na+]. This study aimed to assess the validity and reliability of the MX3 hydration testing system.

METHODS

Thirty-one (11 females) recreationally active participants completed one experimental trial. During this trial, participants exercised at a self-selected pace for 45 min in a warm environment (31.5 ± 0.8 °C, 63.2 ± 1.3% relative humidity). Sweat samples were collected from three measurement sites using absorbent patches. The samples were then analysed for sweat [Na+] using both the MX3 hydration testing system and the Horiba LAQUAtwin-NA-11. The reliability of the MX3 hydration testing system was determined following two measurements of the same sweat sample.

RESULTS

The mean difference between measurements was 0.1 mmoL·L-1 (95% limits of agreement (LoA): - 9.2, 9.4). The analyser demonstrated a coefficient of variation (CV) of 5.6% and the standard error of measurement was 3.3 mmoL·L-1. When compared to the Horiba LAQUAtwin-NA-11, there was a mean difference of - 1.7 mmoL·L-1 (95% LoA: - 0.25 X ¯ , 0.25 X ¯ ) and the CV was 9.8%.

CONCLUSION

The MX3 hydration testing system demonstrated very good single-trial reliability, moderate agreement and a very good CV relative to the Horiba LAQUAtwin-Na-11. To further validate its performance, the MX3 hydration testing system should be compared with analytical techniques known for superior reliability and validity.

The effects of cold water immersion and partial body cryotherapy on subsequent exercise performance and thermoregulatory responses in hot conditions

This study investigated the effects of cold water immersion (CWI) and partial body cryotherapy (PBC) applied within a 15-min post-exercise recovery period on thermoregulatory responses, subjective perceptions, and exercise performance under hot conditions (39 °C). Twelve male soccer players participated in team-sports-specific assessments, including Agility T-test (T-test), 20-m sprint test (20M-ST), and Yo-Yo Intermittent Endurance Test Level 1 (YY-T), during two exercise bouts (1st bout and 2nd bout) with a 15-min post-exercise recovery period. Within the recovery period, a 3-min of PBC at -110 °C or CWI at 15 °C or a seated rest (CON) was performed. Mean skin temperature (Tskin) decreased by 4.3 ± 1.08°C (p < 0.001) immediately after PBC, while CWI induced a reduction of 2.5 ± 0.21°C (p < 0.01). Furthermore, PBC and CWI consistently reduced Tskin for 15 and 33 min, respectively (p < 0.05). During the 2nd bout, core temperature (Tcore) was significantly lower in PBC compared to CON (p < 0.05). Heart rate (HR) was significantly lower in CWI compared to CON and PBC during the intervention period. Thermal sensation (TS) was significantly greater in PBC compared to CON and CWI (p < 0.05). Compared to the 1st bout, PBC alleviated the declines in T-test (p < 0.05) and 20M-ST (p < 0.05), while CWI alleviated the decreases in T-test (p < 0.05) and YY-T (p < 0.05), concurrently significantly enhancing 20M-ST (p < 0.05). 20M-ST and YY-T was greater from PBC (p < 0.05) and CWI (p < 0.05) compared with CON in 2nd bout. Additionally, the T-test in CWI was significantly greater than CON (p < 0.05). These results indicate that both PBC and CWI, performed between two exercise bouts, have the potential to improve thermoregulatory strain, reduce thermal perceptual load, and thereby attenuate the subsequent decline in exercise performance.

The roles of ACE I/D and ACTN3 R577X gene variants in heat acclimation

Roles of genes in heat acclimation (HA, repeated exercise-heat exposures) had not been explored. ACE I/D and ACTN3 R577X genetic polymorphisms are closely associated with outstanding exercise performances. This study investigated whether the two polymorphisms influenced the response to HA. Fifty young Han nationality male subjects were selected and conducted HA for 2 weeks. Exercise indicators (5-km run, push-up and 100-m run) were tested and rest aural thermometry (RTau) was measured before and after HA. ACE gene was grouped by I homozygote and D carrier, and ACTN3 gene was grouped by R homozygote and X carrier. Results showed that there were no differences between groups in age, body mass index, exercise indicators and RTau before HA. After HA, RTau of ACE I homozygote was lower than that of D carrier [F (1, 48) = 9.12, p = 0.004, η = 0.40]. Compared with RTau before HA, that of I homozygote decreased after HA (Δ = -0.26 °C, 95 % CI -0.34-0.18, p < 0.001), while that of D carrier did not change. There was a ACE gene × HA interaction in RTau [F (1, 48) = 14.26, p < 0.001, η = 0.48]. No effect of ACTN3 gene on RTau was observed. For exercise indicators, there were no differences between groups after HA, and no gene × HA interactions were observed. There may be a strong interaction of ACE gene and HA in the change of rest core temperature. I homozygote may have an advantage on improving heat tolerance.

Implications of Heat Stress-induced Metabolic Alterations for Endurance Training

Inducing a heat-acclimated phenotype via repeated heat stress improves exercise capacity and reduces athletes̓ risk of hyperthermia and heat illness. Given the increased number of international sporting events hosted in countries with warmer climates, heat acclimation strategies are increasingly popular among endurance athletes to optimize performance in hot environments. At the tissue level, completing endurance exercise under heat stress may augment endurance training adaptation, including mitochondrial and cardiovascular remodeling due to increased perturbations to cellular homeostasis as a consequence of metabolic and cardiovascular load, and this may improve endurance training adaptation and subsequent performance. This review provides an up-to-date overview of the metabolic impact of heat stress during endurance exercise, including proposed underlying mechanisms of altered substrate utilization. Against this metabolic backdrop, the current literature highlighting the role of heat stress in augmenting training adaptation and subsequent endurance performance will be presented with practical implications and opportunities for future research.

Eccentric muscle-damaging exercise in the heat lowers cellular stress prior to and immediately following future exertional heat exposure

Muscle-damaging exercise (e.g., downhill running [DHR]) or heat exposure bouts potentially reduce physiological and/or cellular stress during future exertional heat exposure; however, the true extent of their combined preconditioning effects is unknown. Therefore, this study investigated the effect of muscle-damaging exercise in the heat on reducing physiological and cellular stress during future exertional heat exposure. Ten healthy males (mean ± Standard Definition; age, 23 ± 3 years; body mass, 78.7 ± 11.5 kg; height, 176.9 ± 4.7 cm) completed this study. Participants were randomly assigned into two preconditioning groups: (a) DHR in the heat (ambient temperature [Tamb], 35 °C; relative humidity [RH], 40%) and (b) DHR in thermoneutral (Tamb, 20 °C; RH, 20%). Seven days following DHR, participants performed a 45-min flat run in the heat (FlatHEAT [Tamb, 35 °C; RH, 40%]). During exercise, heart rate and rectal temperature (Trec) were recorded at baseline and every 5-min. Peripheral blood mononuclear cells were isolated to assess heat shock protein 72 (Hsp72) concentration between conditions at baseline, immediately post-DHR, and immediately pre-FlatHEAT and post-FlatHEAT. Mean Trec during FlatHEAT between hot (38.23 ± 0.38 °C) and thermoneutral DHR (38.26 ± 0.38 °C) was not significantly different (P = 0.68), with no mean heart rate differences during FlatHEAT between hot (172 ± 15 beats min-1) and thermoneutral conditions (174 ± 8 beats min-1; P = 0.58). Hsp72 concentration change from baseline to immediately pre-FlatHEAT was significantly lower in hot (-51.4%) compared to thermoneutral (+24.2%; P = 0.025) DHR, with Hsp72 change from baseline to immediately post-FlatHEAT also lower in hot (-52.6%) compared to thermoneutral conditions (+26.3%; P = 0.047). A bout of muscle-damaging exercise in the heat reduces cellular stress levels prior to and immediately following future exertional heat exposure.

'Super boots' for soldiers: theoretical ergogenic and thermoprotective benefits of energetically optimised military combat boots

Soldiers typically perform physically demanding tasks while wearing military uniforms and tactical footwear. New research has revealed a substantial increase of ~10% in energetic cost of walking when wearing modern combat boots versus running shoes. One approach to mitigating these costs is to follow in the footsteps of recent innovations in athletic footwear that led to the development of 'super shoes', that is, running shoes designed to lower the energetic cost of locomotion and maximise performance. We modelled the theoretical effects of optimised combat boot construction on physical performance and heat strain with the intent of spurring similarly innovative research and development of 'super boots' for soldiers. We first assessed the theoretical benefits of super boots on 2-mile run performance in a typical US Army soldier using the model developed by Kipp and colleagues. We then used the Heat Strain Decision Aid thermoregulatory model to determine the metabolic savings required for a physiologically meaningful decrease in heat strain in various scenarios. Combat boots that impart a 10% improvement in running economy would result in 7.9%-15.1% improvement in 2-mile run time, for faster to slower runners, respectively. Our thermal modelling revealed that a 10% metabolic savings would more than suffice for a 0.25°C reduction in heat strain for the vast majority of work intensities and durations in both hot-dry and hot-humid environments. These findings highlight the impact that innovative military super boots would have on physical performance and heat strain in soldiers, which could potentially maximise the likelihood of mission success in real-world scenarios.

An expert review of environmental heat exposure and stillbirth in the face of climate change: Clinical implications and priority issues

Exposure to extreme heat in pregnancy increases the risk of stillbirth. Progress in reducing stillbirth rates has stalled, and populations are increasingly exposed to high temperatures and climate events that may further undermine health strategies. This narrative review summarises the current clinical and epidemiological evidence of the impact of maternal heat exposure on stillbirth risk. Out of 20 studies, 19 found an association between heat and stillbirth risk. Recent studies based in low- to middle-income countries and tropical settings add to the existing literature to demonstrate that all populations are at risk. Additionally, both short-term heat exposure and whole-pregnancy heat exposure increase the risk of stillbirth. A definitive threshold of effect has not been identified, as most studies define exposure as above the 90th centile of the usual temperature for that population. Therefore, the association between heat and stillbirth has been found with exposures from as low as >12.64°C up to >46.4°C. The pathophysiological pathways by which maternal heat exposure may lead to stillbirth, based on human and animal studies, include both placental and embryonic or fetal impacts. Although evidence gaps remain and further research is needed to characterise these mechanistic pathways in more detail, preliminary evidence suggests epigenetic changes, alteration in imprinted genes, congenital abnormalities, reduction in placental blood flow, size and function all play a part. Finally, we explore this topic from a public health perspective; we discuss and evaluate the current public health guidance on minimising the risk of extreme heat in the community. There is limited pregnancy-specific guidance within heatwave planning, and no evidence-based interventions have been established to prevent poor pregnancy outcomes. We highlight priority research questions to move forward in the field and specifically note the urgent need for evidence-based interventions that are sustainable.

A Novel Conceptual Model for Human Heat Tolerance

Human "heat tolerance" has no accepted definition or physiological underpinnings; rather, it is almost always discussed in relative or comparative terms. We propose to use environmental limits to heat balance accounting for metabolic rate and clothing, that is, the environments for which heat stress becomes uncompensable for a specified metabolic rate and clothing, as a novel metric for quantifying heat tolerance.

Quantifying Exercise Heat Acclimatisation in Athletes and Military Personnel: A Systematic Review and Meta-analysis

BACKGROUND

Athletes and military personnel are often expected to compete and work in hot and/or humid environments, where decrements in performance and an increased risk of exertional heat illness are prevalent. A physiological strategy for reducing the adverse effects of heat stress is to acclimatise to the heat.

OBJECTIVE

The aim of this systematic review was to quantify the effects of relocating to a hotter climate to undergo heat acclimatisation in athletes and military personnel.

ELIGIBILITY CRITERIA

Studies investigating the effects of heat acclimatisation in non-acclimatised athletes and military personnel via relocation to a hot climate for < 6 weeks were included.

DATA SOURCES

MEDLINE, SPORTDiscus, CINAHL Plus with Full Text and Scopus were searched from inception to June 2022.

RISK OF BIAS

A modified version of the McMaster critical review form was utilised independently by two authors to assess the risk of bias.

DATA SYNTHESIS

A Bayesian multi-level meta-analysis was conducted on five outcome measures, including resting core temperature and heart rate, the change in core temperature and heart rate during a heat response test and sweat rate. Wet-bulb globe temperature (WBGT), daily training duration and protocol length were used as predictor variables. Along with posterior means and 90% credible intervals (CrI), the probability of direction (Pd) was calculated.

RESULTS

Eighteen articles from twelve independent studies were included. Fourteen articles (nine studies) provided data for the meta-analyses. Whilst accounting for WBGT, daily training duration and protocol length, population estimates indicated a reduction in resting core temperature and heart rate of - 0.19 °C [90% CrI: - 0.41 to 0.05, Pd = 91%] and - 6 beats·min-1 [90% CrI: - 16 to 5, Pd = 83%], respectively. Furthermore, the rise in core temperature and heart rate during a heat response test were attenuated by - 0.24 °C [90% CrI: - 0.67 to 0.20, Pd = 85%] and - 7 beats·min-1 [90% CrI: - 18 to 4, Pd = 87%]. Changes in sweat rate were conflicting (0.01 L·h-1 [90% CrI: - 0.38 to 0.40, Pd = 53%]), primarily due to two studies demonstrating a reduction in sweat rate following heat acclimatisation.

CONCLUSIONS

Data from athletes and military personnel relocating to a hotter climate were consistent with a reduction in resting core temperature and heart rate, in addition to an attenuated rise in core temperature and heart rate during an exercise-based heat response test. An increase in sweat rate is also attainable, with the extent of these adaptations dependent on WBGT, daily training duration and protocol length.

PROSPERO REGISTRATION

CRD42022337761.

Shifting focus: Time to look beyond the classic physiological adaptations associated with human heat acclimation

Planet Earth is warming at an unprecedented rate and our future is now assured to be shaped by the consequences of more frequent hot days and extreme heat. Humans will need to adapt both behaviorally and physiologically to thrive in a hotter climate. From a physiological perspective, countless studies have shown that human heat acclimation increases thermoeffector output (i.e., sweating and skin blood flow) and lowers cardiovascular strain (i.e., heart rate) during heat stress. However, the mechanisms mediating these adaptations remain understudied. Furthermore, several possible benefits of heat acclimation for other systems and functions involved in maintaining health and performance during heat stress remain to be elucidated. This review summarizes recent advances in human heat acclimation, with emphasis on recent studies that (1) advanced our understanding of the mechanisms mediating improved thermoeffector output and (2) investigated adaptations that go beyond those classically associated with heat acclimation. We highlight that these studies have contributed to a better understanding of the integrated physiological responses underlying human heat acclimation while leaving key unanswered questions that will need to be addressed in the future.

Cross-adaptation from heat stress to hypoxia: A systematic review and exploratory meta-analysis

Cross-adaptation (CA) refers to the successful induction of physiological adaptation under one environmental stressor (e.g., heat), to enable subsequent benefit in another (e.g., hypoxia). This systematic review and exploratory meta-analysis investigated the effect of heat acclimation (HA) on physiological, perceptual and physical performance outcome measures during rest, and submaximal and maximal intensity exercise in hypoxia. Database searches in Scopus and MEDLINE were performed. Studies were included when they met the Population, Intervention, Comparison, and Outcome criteria, were of English-language, peer-reviewed, full-text original articles, using human participants. Risk of bias and study quality were assessed using the COnsensus based Standards for the selection of health status Measurement INstruments checklist. Nine studies were included, totalling 79 participants (100 % recreationally trained males). The most common method of HA included fixed-intensity exercise comprising 9 ± 3 sessions, 89 ± 24-min in duration and occurred within 39 ± 2 °C and 32 ± 13 % relative humidity. CA induced a moderate, beneficial effect on physiological measures at rest (oxygen saturation: g = 0.60) and during submaximal exercise (heart rate: g = -0.65, core temperature: g = -0.68 and skin temperature: g = -0.72). A small effect was found for ventilation (g = 0.24) and performance measures (peak power: g = 0.32 and time trial time: g = -0.43) during maximal intensity exercise. No effect was observed for perceptual outcome measures. CA may be appropriate for individuals, such as occupational or military workers, whose access to altitude exposure prior to undertaking submaximal activity in hypoxic conditions is restricted. Methodological variances exist within the current literature, and females and well-trained individuals have yet to be investigated. Future research should focus on these cohorts and explore the mechanistic underpinnings of CA.

High or hot-Perspectives on altitude camps and heat-acclimation training as preparation for prolonged stage races

Adaptation to heat stress and hypoxia are relevant for athletes participating in Tour de France or similar cycling races taking place during the summertime in landscapes with varying altitude. Both to minimize detrimental performance effects associated with arterial desaturation occurring at moderate altitudes in elite athletes, respectively, reduce the risk of hyperthermia on hot days, but also as a pre-competition acclimatization strategy to boost blood volume in already highly adapted athletes. The hematological adaptations require weeks of exposure to manifest, but are attractive as an augmented hemoglobin mass may improve arterial oxygen delivery and hence benefit prolonged performances. Altitude training camps have in this context a long history in exercise physiology and are still common practice in elite cycling. However, heat-acclimation training provides an attractive alternative for some athletes either as a stand-alone approach or in combination with altitude. The present paper provides an update and practical perspectives on the potential to utilize hypoxia and heat exposure to optimize hematological adaptations. Furthermore, we will consider temporal aspects both in terms of onset and decay of the adaptations relevant for improved thermoregulatory capacity and respiratory adaptations to abate arterial desaturation during altitude exposure. From focus on involved physiological mechanisms, time course, and responsiveness in elite athletes, we will provide guidance based on our experience from practical implementation in cyclists preparing for prolonged stage races such as the Tour de France.

The reliability of a portable steam sauna pod for the whole-body passive heating of humans

INTRODUCTION

Passive heating is receiving increasing attention within human performance and health contexts. A low-cost, portable steam sauna pod may offer an additional tool for those seeking to manipulate physiological (cardiovascular, thermoregulatory and sudomotor) and perceptual responses for improving sporting or health profiles. This study aimed to 1) report the different levels of heat stress and determine the pods' inter-unit reliability, and 2) quantify the reliability of physiological and perceptual responses to passive heating.

METHOD

In part 1, five pods were assessed for temperature and relative humidity (RH) every 5 min across 70 min of heating for each of the 9 settings. In part 2, twelve males (age: 24 ± 4 years) completed two 60 min trials of passive heating (3 × 20 min at 44 °C/99% RH, separated by 1 week). Heart rate (HR), rectal (Trectal) and tympanic temperature (Ttympanic) were recorded every 5 min, thermal comfort (Tcomfort) and sensation (Tsensation) every 10 min, mean arterial pressure (MAP) at each break period and sweat rate (SR) after exiting the pod.

RESULTS

In part 1, setting 9 provided the highest temperature (44.3 ± 0.2 °C) and longest time RH remained stable at 99% (51±7 min). Inter-unit reliability data demonstrated agreement between pods for settings 5-9 (intra-class correlation [ICC] >0.9), but not for settings 1-4 (ICC <0.9). In part 2, between-visits, high correlations, and low typical error of measurement (TEM) and coefficient of variation (CV) were found for Trectal, HR, MAP, SR, and Tcomfort, but not for Ttympanic or Tsensation. A peak Trectal of 38.09 ± 0.30 °C, HR of 124 ± 15 b min-1 and a sweat loss of 0.73 ± 0.33 L were reported. No between-visit differences (p > 0.05) were observed for Trectal, Ttympanic, Tsensation or Tcomfort, however HR (+3 b.min-1) and MAP (+4 mmHg) were greater in visit 1 vs. 2 (p < 0.05).

CONCLUSION

Portable steam sauna pods generate reliable heat stress between-units. The highest setting (44 °C/99% RH) also provides reliable but modest adjustments in physiological and perceptual responses.

Living in Western Australia induces some physiological adaptations of seasonal acclimatisation in the surgical burns team

Seasonal acclimatization is known to result in adaptations that can improve heat tolerance. Staff who operate on burn injuries are exposed to thermally stressful conditions and seasonal acclimatization may improve their thermoeffector responses during surgery. Therefore, the aim of this study was to assess the physiological and perceptual responses of staff who operate on burn injuries during summer and winter, to determine whether they become acclimatized to the heated operating theater. Eight staff members had physiological and perceptual responses compared during burn surgeries conducted in thermoneutral (CON: 24.1 ± 1.2°C, 45 ± 7% relative humidity [RH]) and heated (HOT: 31.3 ± 1.6°C, 44 ± 7% RH) operating theaters, in summer and winter. Physiological parameters that were assessed included core temperature, heart rate, total sweat loss, sweat rate, and urinary specific gravity. Perceptual responses included ratings of thermal sensation and comfort. In summer, CON compared to winter CON, baseline (85 ± 15 bpm VS 94 ± 18 bpm), mean (84 ± 16 bpm VS 93 ± 18 bpm), and peak HR (94 ± 17 bpm VS 105 ± 19 bpm) were lower (p < 0.05), whereas core temperature was not different between seasons in either condition (p > 0.05). In HOT, ratings of discomfort were higher in summer (15 ± 3) than winter (13 ± 3; p > 0.05), but ratings of thermal sensation and sweat rate were similar between seasons (p > 0.05). The surgical team in burns in Western Australia can obtain some of the physiological adaptations that result from seasonal acclimatization, but not all. That is most likely due to a lower than required amount of outdoor heat exposure in summer, to induce all physiological and perceptual adaptations.

Core Body Temperatures in Intermittent Sports: A Systematic Review

BACKGROUND

Hyperthermia (and associated health and performance implications) can be a significant problem for athletes and teams involved in intermittent sports. Quantifying the highest thermal strain (i.e. peak core body temperature [peak Tc]) from a range of intermittent sports would enhance our understanding of the thermal requirements of sport and assist in making informed decisions about training or match-day interventions to reduce thermally induced harm and/or performance decline.

OBJECTIVE

The objective of this systematic review was to synthesise and characterise the available thermal strain data collected in competition from intermittent sport athletes.

METHODS

A systematic literature search was performed on Web of Science, MEDLINE, and SPORTDiscus to identify studies up to 17 April 2023. Electronic databases were searched using a text mining method to provide a partially automated and systematic search strategy retrieving terms related to core body temperature measurement and intermittent sport. Records were eligible if they included core body temperature measurement during competition, without experimental intervention that may influence thermal strain (e.g. cooling), in healthy, adult, intermittent sport athletes at any level. Due to the lack of an available tool that specifically includes potential sources of bias for physiological responses in descriptive studies, a methodological evaluation checklist was developed and used to document important methodological considerations. Data were not meta-analysed given the methodological heterogeneity between studies and therefore were presented descriptively in tabular and graphical format.

RESULTS

A total of 34 studies were selected for review; 27 were observational, 5 were experimental (2 parallel group and 3 repeated measures randomised controlled trials), and 2 were quasi-experimental (1 parallel group and 1 repeated measures non-randomised controlled trial). Across all included studies, 386 participants (plus participant numbers not reported in two studies) were recruited after accounting for shared data between studies. A total of 4 studies (~ 12%) found no evidence of hyperthermia, 24 (~ 71%) found evidence of 'modest' hyperthermia (peak Tc between 38.5 and 39.5 °C), and 6 (~ 18%) found evidence of 'marked' hyperthermia (peak Tc of 39.5 °C or greater) during intermittent sports competition.

CONCLUSIONS

Practitioners and coaches supporting intermittent sport athletes are justified to seek interventions aimed at mitigating the high heat strain observed in competition. More research is required to determine the most effective interventions for this population that are practically viable in intermittent sports settings (often constrained by many competing demands). Greater statistical power and homogeneity among studies are required to quantify the independent effects of wet bulb globe temperature, competition duration, sport and level of competition on peak Tc, all of which are likely to be key modulators of the thermal strain experienced by competing athletes.

REGISTRATION

This systematic review was registered on the Open Science Framework ( https://osf.io/vfb4s ; https://doi.org/10.17605/OSF.IO/EZYFA , 4 January 2021).

Molecular biomarkers for assessing the heat-adapted phenotype: a narrative scoping review

Heat acclimation/acclimatisation (HA) mitigates heat-related decrements in physical capacity and heat-illness risk and is a widely advocated countermeasure for individuals operating in hot environments. The efficacy of HA is typically quantified by assessing the thermo-physiological responses to a standard heat acclimation state test (i.e. physiological biomarkers), but this can be logistically challenging, time consuming, and expensive. A valid molecular biomarker of HA would enable evaluation of the heat-adapted state through the sampling and assessment of a biological medium. This narrative review examines candidate molecular biomarkers of HA, highlighting the poor sensitivity and specificity of these candidates and identifying the current lack of a single 'standout' biomarker. It concludes by considering the potential of multivariable approaches that provide information about a range of physiological systems, identifying a number of challenges that must be overcome to develop a valid molecular biomarker of the heat-adapted state, and highlighting future research opportunities.

Distribution of upper limit of the prescriptive zone values for acclimatized and unacclimatized individuals

Heat stress has an adverse impact on worker health and well-being, and the effects will increase with more frequent and severe heat events associated with global warming. Acclimatization to heat stress is widely considered to be a critical mitigation strategy and wet bulb globe temperature- (WBGT-) based occupational standards and guidelines contain adjustments for acclimatization. The purpose here was to 1) compare the mean values for the upper limit of the prescriptive zone (ULPZ, below which the rise in core temperature is minimal) between unacclimatized and acclimatized men and women; 2) demonstrate that the change in the occupational exposure limit (ΔOEL) due to acclimatization is independent of metabolic rate; 3) examine the relation between ΔOEL and body surface area (BSA); and 4) compare the exposure-response curves between unacclimatized and acclimatized populations. Empirically derived ULPZ data for unacclimatized participants from Pennsylvania State University (PSU) and acclimatized participants from University of South Florida (USF) were used to explore the difference between unacclimatized and acclimatized heat exposure limits. The findings provide support for a constant 3°C WBGT OEL decrease to account for unacclimatized workers. Body surface area explained part of the difference in ULPZ values between men and women. In addition, the pooled PSU and USF data provide insight into the distribution of individual values for the ULPZ among young, healthy unacclimatized and acclimatized populations in support of occupational heat stress guidelines.NEW & NOTEWORTHY Occupational exposure limit guidelines using wet bulb globe temperature (WBGT) distinguish between acclimatized and unacclimatized workers with about a 3°C difference between them. For the first time, empirical data from two laboratories provide support for acclimatization state adjustments. Using a constant difference rather than increasing differences with metabolic rate better describes the limit for unacclimatized participants. Furthermore, the lower upper limit of the prescriptive zone (ULPZ) values set forth for women do not relate to fitness level but are partly explained by their smaller body surface area (BSA). An examination of individual ULPZ values suggests that many unacclimatized individuals should be able to sustain safe work at the exposure limit for acclimatized workers.

Diver Underwater Cycling Endurance After Short-Term Warm and Hot Water Acclimation

INTRODUCTION

It is unclear whether immersion heat acclimation benefits exercise in warm water conditions. This study examined the effects of heat acclimation strategies on heart rate (HR), core temperature, and time to exhaustion (TTE) during cycling exercise in varying warm water conditions.

METHODS

Twenty male divers completed this study at the Navy Experimental Diving Unit. Subjects were randomly assigned to one of two 9-day heat acclimation groups. The first group (WARM; n = 10) cycled for 2 hours at 50 W in 34.4 °C water, while the second group (HOT; n = 10) cycled for 1 hour against minimal resistance in 36.7 °C water. Following acclimation, TTE was tested by underwater cycling (30 W) in 35.8 °C, 37.2 °C, and 38.6 °C water.

RESULTS

Throughout acclimation, the rate of core temperature rise in the first 30 minutes of exercise increased (P = .02), but the maximum core temperature reached was not different for either group. Time to exhaustion (TTE) was reduced, and the rate of core temperature rise during performance testing increased (both P < .001) with increasing water temperature but was not different between groups. Core temperature and HR increased throughout performance testing in each water condition and were lower in the HOT compared to the WARM acclimation group (all P < .05) with the exception of core temperature in the 37.2 °C condition.

CONCLUSIONS

Underwater exercise performance did not differ between the two acclimation strategies. This study suggests that passive acclimation to a higher water temperature may improve thermoregulatory and cardiovascular responses to exercise in warm water. Hot water immersion adaptations are dependent on exercise intensity and water temperature.

Study on thermal health and its safety management mode for the working environment

Thermal health concerns have gained significant attention due to the heightened health risks faced by workers who are exposed to extreme thermal environments for prolonged periods. To ensure the occupational health and safety of such workers, and to enhance work efficiency, it is imperative to examine the characteristics of thermal health in the working environment. This study proposes three key elements of thermal health in the working environment, namely thermal health states, absence of heat-related illnesses, and heat adaptability, which can be used to develop a safety management framework for thermal health. By exploring the interconnections between these elements, the study summarizes their features and outlines the necessary precautions to safeguard them. The PDCA (plan/do/check/action) cycle management mode is utilized as a framework, with the three components of thermal health forming the core, to establish a safety management mode for thermal health. To ensure that employees work in a safe, healthy, comfortable, and productive environment, the assessment and control objectives of the thermal environment are regularly revised through the use of labor protection technology and thermal environment control technology. This paper presents a PDCA cycle safety management mode based on the characteristics of thermal health, which offers novel insights and approaches for assessing and managing workers' thermal health.

Heat Adaptation for Females: A Systematic Review and Meta-Analysis of Physiological Adaptations and Exercise Performance in the Heat

BACKGROUND

Heat adaptation regimes are used to prepare athletes for exercise in hot conditions to limit a decrement in exercise performance. However, the heat adaptation literature mostly focuses on males, and consequently, current heat adaptation guidelines may not be optimal for females when accounting for the biological and phenotypical differences between sexes.

OBJECTIVES

We aimed to examine: (1) the effects of heat adaptation on physiological adaptations in females; (2) the impact of heat adaptation on performance test outcomes in the heat; and (3) the impact of various moderators, including duration (minutes and/or days), total heat dose (°C.min), exercise intensity (kcal.min-1), total energy expended (kcal), frequency of heat exposures and training status on the physiological adaptations in the heat.

METHODS

SPORTDiscus, MEDLINE Complete and Embase databases were searched to December 2022. Random-effects meta-analyses for resting and exercise core temperature, skin temperature, heart rate, sweat rate, plasma volume and performance tests in the heat were completed using Stata Statistical Software: Release 17. Sub-group meta-analyses were performed to explore the effect of duration, total heat dose, exercise intensity, total energy expended, frequency of heat exposure and training status on resting and exercise core temperature, skin temperature, heart rate and sweat rate. An explorative meta-regression was conducted to determine the effects of physiological adaptations on performance test outcomes in the heat following heat adaptation.

RESULTS

Thirty studies were included in the systematic review; 22 studies were meta-analysed. After heat adaptation, a reduction in resting core temperature (effect size [ES] =  - 0.45; 95% confidence interval [CI] - 0.69, - 0.22; p < 0.001), exercise core temperature (ES =  - 0.81; 95% CI - 1.01, - 0.60; p < 0.001), skin temperature (ES =  - 0.64; 95% CI - 0.79, - 0.48; p < 0.001), heart rate (ES =  - 0.60; 95% CI - 0.74, - 0.45; p < 0.001) and an increase in sweat rate (ES = 0.53; 95% CI 0.21, 0.85; p = 0.001) were identified in females. There was no change in plasma volume (ES = - 0.03; 95% CI - 0.31, 0.25; p = 0.835), whilst performance test outcomes were improved following heat adaptation (ES = 1.00; 95% CI 0.56, 1.45; p < 0.001). Across all moderators, physiological adaptations were more consistently observed following durations of 451-900 min and/or 8-14 days, exercise intensity ≥ 3.5 kcal.min-1, total energy expended ≥ 3038 kcal, consecutive (daily) frequency and total heat dose ≥ 23,000 °C.min. The magnitude of change in performance test outcomes in the heat was associated with a reduction in heart rate following heat adaptation (standardised mean difference =  - 10 beats.min-1; 95% CI - 19, - 1; p = 0.031).

CONCLUSIONS

Heat adaptation regimes induce physiological adaptations beneficial to thermoregulation and performance test outcomes in the heat in females. Sport coaches and applied sport practitioners can utilise the framework developed in this review to design and implement heat adaptation strategies for females.

Heat Preparation and Knowledge at the World Athletics Race Walking Team Championships Muscat 2022

PURPOSE

To assess elite racewalkers' preparation strategies, knowledge, and general practices for competition in the heat and their health status during the World Athletics Race Walking Teams Championships (WRW) Muscat 2022.

METHODS

Sixty-six elite racewalkers (male: n = 42; mean age = 25.8 y) completed an online survey prior to WRW Muscat 2022. Athletes were grouped by sex (males vs females) and climate (self-reported) they live/trained in (hot vs temperate/cold), with differences/relationships between groups assessed. Relationships between ranking (medalist/top 10 vs nonmedalist/nontop 10) and precompetition use of heat acclimation/acclimatization (HA) were assessed.

RESULTS

All surveyed medalists (n = 4) implemented, and top 10 finishers were more likely to report using (P = .049; OR = 0.25; 95% CI, 0.06%-1%), HA before the championships. Forty-three percent of athletes did not complete specific HA training. Females (8% [males 31%]) were less likely to have measured core temperature (P = .049; OR = 0.2; 95% CI, 0.041-0.99) and more likely to not know expected conditions in Muscat (42% vs 14%; P = .016; OR = 4.3; 95% CI, 1%-14%) or what wet bulb globe temperature is (83% vs 55%; P = .024; OR = 4.1; 95% CI, 1%-14%).

CONCLUSIONS

Athletes who implemented HA before the championships tended to place better than those who did not. Forty-three percent of athletes did not prepare for the expected hot conditions at the WRW Muscat 2022, primarily attributed to challenges in accessing and/or cost of equipment/facilities for HA strategies. Further efforts to bridge the gap between research and practice in this elite sport are needed, particularly in female athletes.

Short-term heat acclimation protocols for an aging population: Systematic review

INTRODUCTION

Elderly and sedentary individuals are particularly vulnerable to heat related illness. Short-term heat acclimation (STHA) can decrease both the physical and mental stress imposed on individuals performing tasks in the heat. However, the feasibility and efficacy of STHA protocols in an older population remains unclear despite this population being particularly vulnerable to heat illness. The aim of this systematic review was to investigate the feasibility and efficacy of STHA protocols (≤twelve days, ≥four days) undertaken by participants over fifty years of age.

METHODS

Academic Search Premier, CINAHL Complete, MEDLINE, APA PsycInfo, and SPORTDiscus were searched for peer reviewed articles. The search terms were; (heat* or therm*) N3 (adapt* or acclimati*) AND old* or elder* or senior* or geriatric* or aging or ageing. Only studies using primary empirical data and which included participants ≥50 years of age were eligible. Extracted data includes participant demographics (sample size, gender, age, height, weight, BMI and [Formula: see text]), acclimation protocol details (acclimation activity, frequency, duration and outcome measures taken) and feasibility and efficacy outcomes.

RESULTS

Twelve eligible studies were included in the systematic review. A total of 179 participants took part in experimentation, 96 of which were over 50 years old. Age ranged from 50 to 76. All twelve of the studies involved exercise on a cycle ergometer. Ten out of twelve protocols used a percentage of [Formula: see text] or [Formula: see text] to determine the target workload, which ranged from 30% to 70%. One study-controlled workload at 6METs and one implemented an incremental cycling protocol until Tre was reached +0.9°C. Ten studies used an environmental chamber. One study compared hot water immersion (HWI) to an environmental chamber while the remaining study used a hot water perfused suit. Eight studies reported a decrease in core temperature following STHA. Five studies demonstrated post-exercise changes in sweat rates and four studies showed decreases in mean skin temperature. The differences reported in physiological markers suggest that STHA is viable in an older population.

CONCLUSION

There remains limited data on STHA in the elderly. However, the twelve studies examined suggest that STHA is feasible and efficacious in elderly individuals and may provide preventative protection to heat exposures. Current STHA protocols require specialised equipment and do not cater for individuals unable to exercise. Passive HWI may provide a pragmatic and affordable solution, however further information in this area is required.

Heating Up to Keep Cool: Benefits and Persistence of a Practical Heat Acclimation Protocol in Elite Female Olympic Team-Sport Athletes

PURPOSE

Although recommendations for effective heat acclimation (HA) strategies for many circumstances exist, best-practice HA protocols specific to elite female team-sport athletes are yet to be established. Therefore, the authors aimed to investigate the effectiveness and retention of a passive HA protocol integrated in a female Olympic rugby sevens team training program.

METHODS

Twelve elite female rugby sevens athletes undertook 10 days of passive HA across 2 training weeks. Tympanic temperature (TTymp), sweat loss, heart rate, and repeated 6-second cycling sprint performance were assessed using a sport-specific heat stress test Pre-HA, after 3 days (Mid-HA), after 10 days (Post-HA), and 15 days post-HA (Decay).

RESULTS

Compared with Pre-HA, submaximal TTymp was lower Mid-HA and Post-HA (both by -0.2 [0.7] °C; d ≥ 0.71), while resting TTymp was lower Post-HA (by -0.3 [0.2] °C; d = 0.81). There were no differences in TTymp at Decay compared with Pre-HA, nor were there any differences in heart rate or sweat loss at any time points. Mean peak 6-second power output improved Mid-HA and Post-HA (76 [36] W; 75 [34] W, respectively; d ≥ 0.45) compared with Pre-HA. The observed performance improvement persisted at Decay by 65 (45) W (d = 0.41).

CONCLUSIONS

Ten days of passive HA can elicit some thermoregulatory and performance benefits when integrated into a training program in elite female team-sport athletes. However, such a protocol does not provide a sufficient thermal impulse for thermoregulatory adaptations to be retained after 15 days with no further heat stimulus.

Alteration of gut microbiota after heat acclimation may reduce organ damage by regulating immune factors during heat stress

Introduction

Heat-related illnesses can lead to morbidity, which are anticipated to increase frequency with predictions of increased global surface temperatures and extreme weather events. Although heat acclimation training (HAT) could prevent heat-related diseases, the mechanisms underlying HAT-promoting beneficial changes in organ function, immunity, and gut microbes remain unclear.

Methods

In the current study, we recruited 32 healthy young soldiers and randomly divided them into 4 teams to conduct HATs for 10 days: the equipment-assisted training team at high temperature (HE); the equipment-assisted training team under normal hot weather (NE); the high-intensity interval training team at high temperature (HIIT), and the control team without training. A standard heat tolerance test (HTT) was conducted before (HTT-1st) and after (HTT-2nd) the training to judge whether the participants met the heat acclimation (HA) criteria.

Results

We found that the participants in both HE and NE teams had significantly higher acclimation rates (HA/total population) than whom in the HIIT team. The effects of HAT on the participants of the HE team outperformed that of the NE team. In the HA group, the differences of physiological indicators and plasma organ damage biomarkers (ALT, ALP, creatinine, LDH, α-HBDH and cholinesterase) before and after HTT-2nd were significantly reduced to those during HTT-1st, but the differences of immune factors (IL-10, IL-6, CXCL2, CCL4, CCL5, and CCL11) elevated. The composition, metabolism, and pathogenicity of gut microbes changed significantly, with a decreased proportion of potentially pathogenic bacteria (Escherichia-Shigella and Lactococcus) and increased probiotics (Dorea, Blautia, and Lactobacillus) in the HA group. Training for a longer time in a high temperature and humidity showed beneficial effects for intestinal probiotics.

Conclusion

These findings revealed that pathogenic gut bacteria decrease while probiotics increase following HA, with elevated immune factors and reduced organ damage during heat stress, thereby improving the body's heat adaption.

Environmental Stress Symptoms during Heat Acclimatization, Heat Acclimation, and Intermittent Heat Training

BACKGROUND

Athletes training in heat experience physiological and perceptual symptoms that risk their safety and performance without adaptation.

PURPOSE

We examined the changes in environmental symptoms, assessed with the Environmental Symptoms Questionnaire (ESQ), during heat acclimatization (HAz), heat acclimation (HA), and intermittent heat training (HT).

METHODS

Twenty-seven participants (mean ± standard deviation [M ± SD], age of 35 ± 12 y, VO2max of 57.7 ± 6.8 mL·kg-1·min-1) completed five trials involving 60 mins of running (60% vVO2max) followed by a 4 km time trial in heat (M ± SD, temperature of 35.5 ± 0.7 °C, humidity of 46.4 ± 1.5%). The trials occurred at baseline, post-HAz, post-HA, at week 4 of HT (post-HT4), and at week 8 of HT (post-HT8). The participants completed HT once/week (HTMIN), completed HT twice/week (HTMAX), or did not complete HT (HTCON). ESQ symptoms, thermal sensation (TS), and heart rate (HR) were measured pre- and post-trial.

RESULTS

Post-ESQ symptoms improved post-HA (3[0.40, 4.72], p = 0.02) and post-HAz (3[0.35, 5.05], p = 0.03) from baseline. During HT, symptoms improved in the HTMAX group and worsened in the HTMIN and HTCON groups. Symptoms improved in the HTMAX group versus the HTCON group at post-HT8 (4[1.02, 7.23], p = 0.012). Higher TS and HR values were weakly associated with ESQ symptoms during HT (r = 0.20, p = 0.04), only explaining 20% of variance.

CONCLUSIONS

ESQ symptoms improved during HAz, HA, and HT 2x/week. ESQ symptoms were not statistically correlated with HR during exercise heat stress. TS was not sensitive to detecting adaptation and did not subjectively change. The ESQ may be valuable in monitoring adaptation and may contribute to performance post-acclimation.

Heat acclimation reduces the effects of whole-body hyperthermia on knee-extensor relaxation rate, but does not affect voluntary torque production

PURPOSE

This study investigated the effects of acute hyperthermia and heat acclimation (HA) on maximal and rapid voluntary torque production, and their neuromuscular determinants.

METHODS

Ten participants completed 10 days of isothermic HA (50 °C, 50% rh) and had their knee-extensor neuromuscular function assessed in normothermic and hyperthermic conditions, pre-, after 5 and after 10 days of HA. Electrically evoked twitch and octet (300 Hz) contractions were delivered at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary contractions.

RESULTS

Acute hyperthermia reduced neural drive (EMG at MVT and during rapid voluntary contractions; P < 0.05), increased evoked torques (P < 0.05), and shortened contraction and relaxation rates (P < 0.05). HA lowered resting rectal temperature and heart rate after 10 days (P < 0.05), and increased sweating rate after 5 and 10 days (P < 0.05), no differences were observed between 5 and 10 days. The hyperthermia-induced reduction in twitch half-relaxation was attenuated after 5 and 10 days of HA, but there were no other effects on neuromuscular function either in normothermic or hyperthermic conditions.

CONCLUSION

HA-induced favourable adaptations to the heat after 5 and 10 days of exposure, but there was no measurable benefit on voluntary neuromuscular function in normothermic or hyperthermic conditions. HA did reduce the hyperthermic-induced reduction in twitch half-relaxation time, which may benefit twitch force summation and thus help preserve voluntary torque in hot environmental conditions.

Factors contributing to the change in thermoneutral maximal oxygen consumption after iso-intensity heat acclimation programmes

The factors explaining variance in thermoneutral maximal oxygen uptake (V˙O_2max) adaptation to heat acclimation (HA) were evaluated, with consideration of HA programme parameters, biophysical variables and thermo-physiological responses. Seventy-one participants consented to perform iso-intensity training (range: 45%-55% V˙O_2max) in the heat (range: 30°C-38°C; 20%-60% relative humidity) on consecutive days (range: 5-days-14-days) for between 50-min and-90 min. The participants were evaluated for their thermoneutral V˙O_2max change pre-to-post HA. Participants' whole-body sweat rate, heart rate, core temperature, perceived exertion and thermal sensation and plasma volume were measured, and changes in these responses across the programme determined. Partial least squares regression was used to explain variance in the change in V˙O_2max across the programme using 24 variables. Sixty-three percent of the participants increased V˙O_2max more than the test error, with a mean ± SD improvement of 2.6 ± 7.9%. A two-component model minimised the root mean squared error and explained the greatest variance (R²; 65%) in V˙O_2max change. Eight variables positively contributed (P < 0.05) to the model: exercise intensity (%V˙O_2max), ambient temperature, HA training days, total exposure time, baseline body mass, thermal sensation, whole-body mass losses and the number of days between the final day of HA and the post-testing day. Within the ranges evaluated, iso-intensity HA improved V˙O_2max 63% of the time, with intensity - and volume-based parameters, alongside sufficient delays in post-testing being important considerations for V˙O_2max maximisation. Monitoring of thermal sensation and body mass losses during the programme offers an accessible way to gauge the degree of potential adaptation.

The effectiveness of heat preparation and alleviation strategies for cognitive performance: A systematic review

A range of occupational and performance contexts (e.g. military personnel operations, emergency services, sport) require the critical maintenance of cognitive performance in environmentally challenging environments. Several reviews exist which evaluate the effectiveness of heat preparation strategies to facilitate physical performance. To date, no review has explored the usefulness of heat preparation strategies for cognitive performance. Therefore, this systematic review aimed to evaluate a range of interventions for the maintenance of cognitive performance, during or following active or passive heat exposure. Studies to be included were assessed by two authors reviewing title, abstract, and full-text. Forty articles were identified which met the inclusion criteria. Interventions were categorised into chronic (i.e. acclimation/acclimatisation) and acute strategies (i.e. hydration, cooling, supplementation, psychological). The results indicate that medium-term consecutive heat acclimation may mitigate some cognitive deficits under heat stress, although heat acclimation effectiveness could be influenced by age. Further, pre-cooling appears the most effective cooling method for maintaining cognitive performance under heat stress, although results were somewhat ambiguous. The hydration literature showed that the most effective hydration strategies were those which individualised electrolyte fortified fluid volumes to match for sweat loss. Limited research exploring psychological interventions indicates that motivational self-talk could be facilitative for maintaining cognitive skills following exercise in hot conditions. These findings can be used to help inform strategies for maintaining critical cognitive and decision-making skills in hot environments.

Post-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military context

To mitigate the effects of heat during operations in hot environments, military personnel will likely benefit from heat acclimation (HA) conducted prior to deployment. Using post-exercise, passive heating, 25 participants completed a 5 d HA regime in sauna (70 °C, 18% RH) or hot-water immersion (HWI) (40 °C) for ≤40 min, preceded and followed by a heat stress test (1-h walking at 5 km.h^-1 in 33 °C, 77% RH in military uniform (20 kg) before an incremental ramp to exhaustion). Fifteen completed both regimes in a randomised, cross-over manner. While performance did not significantly improve (+14%, [-1, 29], p = .079), beneficial adaptations were observed for mean exercising core temperature (-0.2 °C, [-0.2, -0.2], p <.001), skin temperature (-0.2 °C, [-0.2, -0.2], p = 035) and heart rate (-8 bpm, [-6, -10], p<.001) in both conditions. Post-exercise, passive HA of either modality may benefit military units operating in the heat.Practitioner summary: Strategies are required to prevent health and performance impairments during military operations upon arrival in hot environments. Using a randomised, cross-over design, participants completed five-day passive, post-exercise heat acclimation using sauna or hot-water immersion. Both regimes elicited beneficial albeit modest heat adaptations.Abbreviations: HA: heat acclimation; HST: heat stress test; HWI: hot-water immersion; RH: relative humidity.

IOC consensus statement on recommendations and regulations for sport events in the heat

This document presents the recommendations developed by the IOC Medical and Scientific Commission and several international federations (IF) on the protection of athletes competing in the heat. It is based on a working group, meetings, field experience and a Delphi process. The first section presents recommendations for event organisers to monitor environmental conditions before and during an event; to provide sufficient ice, shading and cooling; and to work with the IF to remove regulatory and logistical limitations. The second section summarises recommendations that are directly associated with athletes' behaviours, which include the role and methods for heat acclimation; the management of hydration; and adaptation to the warm-up and clothing. The third section explains the specific medical management of exertional heat stroke (EHS) from the field of play triage to the prehospital management in a dedicated heat deck, complementing the usual medical services. The fourth section provides an example for developing an environmental heat risk analysis for sport competitions across all IFs. In summary, while EHS is one of the leading life-threatening conditions for athletes, it is preventable and treatable with the proper risk mitigation and medical response. The protection of athletes competing in the heat involves the close cooperation of the local organising committee, the national and international federations, the athletes and their entourages and the medical team.