Integrated Physiological Mechanisms of Exercise Performance, Adaptation, and Maladaptation to Heat Stress

The Gastrointestinal Exertional Heat Stroke Paradigm: Pathophysiology, Assessment, Severity, Aetiology and Nutritional Countermeasures

Exertional heat stroke (EHS) is a life-threatening medical condition involving thermoregulatory failure and is the most severe condition along a continuum of heat-related illnesses. Current EHS policy guidance principally advocates a thermoregulatory management approach, despite growing recognition that gastrointestinal (GI) microbial translocation contributes to disease pathophysiology. Contemporary research has focused to understand the relevance of GI barrier integrity and strategies to maintain it during periods of exertional-heat stress. GI barrier integrity can be assessed non-invasively using a variety of in vivo techniques, including active inert mixed-weight molecular probe recovery tests and passive biomarkers indicative of GI structural integrity loss or microbial translocation. Strenuous exercise is strongly characterised to disrupt GI barrier integrity, and aspects of this response correlate with the corresponding magnitude of thermal strain. The aetiology of GI barrier integrity loss following exertional-heat stress is poorly understood, though may directly relate to localised hyperthermia, splanchnic hypoperfusion-mediated ischemic injury, and neuroendocrine-immune alterations. Nutritional countermeasures to maintain GI barrier integrity following exertional-heat stress provide a promising approach to mitigate EHS. The focus of this review is to evaluate: (1) the GI paradigm of exertional heat stroke; (2) techniques to assess GI barrier integrity; (3) typical GI barrier integrity responses to exertional-heat stress; (4) the aetiology of GI barrier integrity loss following exertional-heat stress; and (5) nutritional countermeasures to maintain GI barrier integrity in response to exertional-heat stress.

Per-Cooling (Using Cooling Systems during Physical Exercise) Enhances Physical and Cognitive Performances in Hot Environments. A Narrative Review

There are many important sport events that are organized in environments with a very hot ambient temperature (Summer Olympics, FIFA World Cup, Tour de France, etc.) and in hot locations (e.g., Qatar). Additionally, in the context of global warming and heat wave periods, athletes are often subjected to hot ambient temperatures. It is known that exercising in the heat induces disturbances that may provoke premature fatigue and negatively affects overall performance in both endurance and high intensity exercises. Deterioration in several cognitive functions may also occur, and individuals may be at risk for heat illnesses. To train, perform, work and recover and in a safe and effective way, cooling strategies have been proposed and have been routinely applied before, during and after exercise. However, there is a limited understanding of the influences of per-cooling on performance, and it is the subject of the present review. This work examines the influences of per-cooling of different areas of the body on performance in terms of intense short-term exercises ("anaerobic" exercises), endurance exercises ("aerobic" exercises), and cognitive functioning and provides detailed strategies that can be applied when individuals train and/or perform in high ambient temperatures.

The Influence of Heat Acclimation and Hypohydration on Post-Weight-Loss Exercise Performance

PURPOSE

To examine the influence of fluid intake on heat acclimation and the subsequent effects on exercise performance following acute hypohydration.

METHODS

Participants were randomly assigned to 1 of 2 groups, either able to consume water ad libitum (n = 10; age 23 [3] y, height 1.81 [0.09] m, body mass 87 [13] kg; HAW) or not allowed fluid (n = 10; age 26 [5] y, height 1.76 [0.05] m, body mass 79 [10] kg; HANW) throughout 12 × 1.5-h passive heat-acclimation sessions. Experimental trials were completed on 2 occasions before (2 baseline trials) and 1 following the heat-acclimation sessions. These sessions involved 3 h of passive heating (45°C, 38% relative humidity) to induce hypohydration followed by 3 h of ad libitum food and fluid intake after which participants performed a repeat sled-push test to assess physical performance. Urine and blood samples were collected before, immediately, and 3 h following hypohydration to assess hydration status. Mood was also assessed at the same time points.

RESULTS

No meaningful differences in physiological or performance variables were observed between HANW and HAW at any time point. Using pooled data, mean sprint speed was significantly (P < .001) faster following heat acclimation (4.6 [0.7] s compared with 5.1 [0.8] s). Furthermore, heat acclimation appeared to improve mood following hypohydration.

CONCLUSIONS

Results suggest that passive heat-acclimation protocols may be effective at improving short-duration repeat-effort performance following acute hypohydration.

Heat Acclimation Does Not Modify Q 10 and Thermal Cardiac Reactivity

Heat acclimation (HA) is an essential modifier of physiological strain when working or exercising in the heat. It is unknown whether HA influences the increase of energy expenditure (Q ₁₀ effect) or heart rate (thermal cardiac reactivity TCR) due to increased body temperature. Therefore, we studied these effects using a heat strain database of climatic chamber experiments performed by five semi-nude young males in either non-acclimated or acclimated state. Measured oxygen consumption rate (VO₂), heart rate (HR), and rectal temperature (T _re) averaged over the third hour of exposure were obtained from 273 trials in total. While workload (walking 4 km/h on level) was constant, heat stress conditions varied widely with air temperature 25-55°C, vapor pressure 0.5-5.3 kPa, and air velocity 0.3-2 m/s. HA was induced by repeated heat exposures over a minimum of 3 weeks. Non-acclimated experiments took place in wintertime with a maximum of two exposures per week. The influence of T _re and HA on VO₂ and HR was analyzed separately with mixed model ANCOVA. Rising T _re significantly (p < 0.01) increased both VO₂ (by about 7% per degree increase of T _re) and HR (by 39-41 bpm per degree T _re); neither slope nor intercept depended significantly on HA (p > 0.4). The effects of T _re in this study agree with former outcomes for VO₂ (7%/°C increase corresponding to Q ₁₀ = 2) and for HR (TCR of 33 bpm/°C in ISO 9886). Our results indicate that both relations are independent of HA with implications for heat stress assessment at workplaces and for modeling heat balance.

Heat acclimation-induced intracellular HSP70 in humans: a meta-analysis

Heat acclimation (HA) in humans promotes thermoregulatory adaptations that support management of core temperature in hot environments and reduces the likelihood of heat related illness. Another adaptation to HA is thermotolerance through induction of the heat shock protein (HSP) stress system, which provides protection against thermal insult. However, whether or not HA leads to upregulation of the intracellular HSP system, namely intracellular HSP70 (HSP70), is unclear in humans. Therefore, the purposes of this meta-analysis were to determine if HA leads to HSP70 induction among humans and to evaluate how methodological differences among HA studies influence findings regarding HA-induced HSP70 accumulation. Several databases were searched to identify studies that measured HSP70 (protein and mRNA) changes in response to HA among humans. The effect of HA on HSP70 was analyzed. Differences in the effect of HA were assessed between protein and mRNA. The moderating effect of several independent variables (HA frequency, HA duration, core temperature, exercise intensity) on HSP70 was also evaluated. Data were extracted from 12 studies including 118 participants (mean age 24 years, 98% male). There was a significant effect of HA on HSP70 expression, g = 0.97 (95% CI, 0.08-1.89). The effect of HA was different between subgroups (protein vs. mRNA), g = 1.51 (95% CI, 0.71-2.31), and g = - 0.39 (95% CI, - 1.36), respectively. The frequency of HA (in days) moderated HSP70 protein expression. There was a significant effect of heat acclimation on HSP70 induction in humans. The only factor among identified studies that may moderate this response was the frequency (number of days) of heat exposure.

Ultra-endurance athletic performance suggests that energetics drive human morphological thermal adaptation

Both extinct and extant hominin populations display morphological features consistent with Bergmann's and Allen's Rules. However, the functional implications of the morphologies described by these ecological laws are poorly understood. We examined this through the lens of endurance running. Previous research concerning endurance running has focused on locomotor energetic economy. We considered a less-studied dimension of functionality, thermoregulation. The performance of male ultra-marathon runners (n = 88) competing in hot and cold environments was analysed with reference to expected thermoregulatory energy costs and the optimal morphologies predicted by Bergmann's and Allen's Rules. Ecogeographical patterning supporting both principles was observed in thermally challenging environments. Finishers of hot-condition events had significantly longer legs than finishers of cold-condition events. Furthermore, hot-condition finishers had significantly longer legs than those failing to complete hot-condition events. A degree of niche-picking was evident; athletes may have tailored their event entry choices in accordance with their previous race experiences. We propose that the interaction between prolonged physical exertion and hot or cold climates may induce powerful selective pressures driving morphological adaptation. The resulting phenotypes reduce thermoregulatory energetic expenditure, allowing diversion of energy to other functional outcomes such as faster running.

Heat Exposure and Occupational Injuries: Review of the Literature and Implications

PURPOSE OF REVIEW

The burden of heat-related adverse occupational health effects, as well as traumatic injuries, is already substantial. Projected increases in mean temperatures and extreme events may increase the risk of adverse heat health effects and enhance disparities among exposed workers. This article reviews the emerging literature on the relationship between heat exposure and occupational traumatic injuries and discusses implications of this work.

RECENT FINDINGS

A recent meta-analysis of three case-crossover and five time series studies in industrialized settings reported an association of increasing occupational injuries with increasing heat exposure, with increased effect estimates for male gender and age less than 25 years, although heterogeneity in exposure metrics and sources of bias were demonstrated to varying degrees across studies. A subsequent case-crossover study in outdoor construction workers reported a 0.5% increase in the odds of traumatic injuries per 1 °C increase in maximum daily humidex (odds ratio 1.005 [95% CI 1.003-1.007]). While some studies have demonstrated reversed U-shaped associations between heat exposure and occupational injuries, different risk profiles have been reported in different industries and settings. Studies conducted primarily in industrialized settings suggest an increased risk of traumatic injury with increasing heat exposure, though the exact mechanisms of heat exposure's effects on traumatic injuries are still under investigation. The effectiveness of heat-related injury prevention approaches has not yet been established. To enhance the effectiveness of prevention efforts, prioritization of approaches should take into account not only the hierarchy of controls, social-ecological models, community and stakeholder participation, and tailoring of approaches to specific local work settings, but also methods that reduce local and global disparities and better address the source of heat exposure, including conservation-informed land-use planning, built environment, and prevention through design approaches. Participation of occupational health experts in transdisciplinary development and integration of these approaches is needed.

Hydration Status After an Ironman Triathlon: A Meta-Analysis

The Ironman is one of the most popular triathlon events in the world. Such a race involves a great number of tactical decisions for a healthy finish and best performance. Dehydration is widely postulated to decrease performance and is known as a cause of dropouts in Ironman. Despite the importance of hydration status after an Ironman triathlon, there is a clear lack of review and especially meta-analysis studies on this topic. Therefore, the objective was to systematically review the literature and carry out a meta-analysis investigating the hydration status after an Ironman triathlon. We conducted a systematic review of the literature up to June 2016 that included the following databases: PubMed, SCOPUS, Science Direct and Web of Science. From the initial 995 references, we included 6 studies in the qualitative analysis and in the meta-analysis. All trials had two measures of hydration status after a full Ironman race. Total body water, blood and urine osmolality, urine specific gravity and sodium plasma concentration were considered as hydration markers. Three investigators independently abstracted data on the study design, sample size, participants’ and race characteristics, outcomes, and quantitative data for the meta-analysis. In the pooled analysis, it seems that the Ironman event led to a moderate state of dehydration in comparison to baseline values (SMD 0.494; 95% CI 0.220 to 0.767; p = 0.001). Some evidence of heterogeneity and consistency was also observed: Q = 19.6; I² = 28.5%; τ² = 2.39. The results suggest that after the race athletes seem to be hypo-hydrated in comparison to baseline values.

Performance Changes Following Heat Acclimation and the Factors That Influence These Changes: Meta-Analysis and Meta-Regression

Heat acclimation (HA) is the process of intentional and consistent exercise in the heat that results in positive physiological adaptations, which can improve exercise performance both in the heat and thermoneutral conditions. Previous research has indicated the many performance benefits of HA, however, a meta-analysis examining the magnitude of different types of performance improvement is absent. Additionally, there are several methodological discrepancies in the literature that could lead to increased variability in performance improvement following HA and no previous study has examined the impact of moderators on performance improvement following HA. Therefore, the aim of this study was two-fold; (1) to perform a meta-analysis to examine the magnitude of changes in performance following HA in maximal oxygen consumption (VO2max), time to exhaustion, time trial, mean power, and peak power tests; (2) to determine the impact of moderators on results of these performance tests. Thirty-five studies met the inclusion/exclusion criteria with 23 studies that assessed VO2max (n = 204), 24 studies that assessed time to exhaustion (n = 232), 10 studies that performed time trials (n = 101), 7 studies that assessed mean power (n = 67), and 10 papers that assessed peak power (n = 88). Data are reported as Hedge's g effect size (ES), and 95% confidence intervals (95% CI). Statistical significance was set to p < 0.05, a priori. The magnitude of change following HA was analyzed, with time to exhaustion demonstrating the largest performance enhancement (ES [95% CI], 0.86 [0.71, 1.01]), followed by time trial (0.49 [0.26, 0.71]), mean power (0.37 [0.05, 0.68]), VO2max (0.30 [0.07, 0.53]), and peak power (0.29 [0.09, 0.48]) (p < 0.05). When all of the covariates were analyzed as individual models, induction method, fitness level, heat index in time to exhaustion (coefficient [95% CI]; induction method, -0.69 [-1.01, -0.37], p < 0.001; fitness level, 0.04 [0.02, 0.06], p < 0.001; heat index, 0.04 [0.02, 0.07], p < 0.0001) and induction length in mean power (coefficient [95% CI]; induction length 0.15 [0.05, 0.25], p = 0.002) significantly impacted the magnitude of change. Sport scientists and researchers can use the findings from this meta-analysis to customize HA induction. For time to exhaustion improvements, HA implementation should focus on induction method and baseline fitness, while the training and recovery balance could lead to optimal time trial performance.

Indices of physiological strain for firefighters of the Australian Defence Forces

High levels of exertion and physiological strain are the leading cause of fireground injuries. The Physiological Strain Index (PSI) provides a rating of strain based on body core temperature and heart rate; however, it may underestimate the strain of workers in protective clothing as skin temperature may be elevated. This study aimed to examine the relationship between the PSI and an Adaptive Physiological Strain Index (aPSI) that incorporates skin temperature, among firefighters wearing protective clothing. Nine male firefighters of the Australian Defence Force volunteered to participate. Participants conducted scenario-based activities while wearing turnout gear and breathing apparatus. Working in teams of four, participants would respond to a situation around and within a small building with several rooms that could be filled with smoke, however, no live fire was present. Heart rate, gastrointestinal temperature, and skin temperature were monitored throughout work and rehabilitation. Physiological strain was estimated via the PSI and aPSI. Absolute peak PSI and aPSI ratings were significantly different during work (PSI: 7.3 ± 1.6; aPSI 8.2 ± 2.0; p < 0.001). The aPSI produced higher ratings of physiological strain, >0.5 above PSI, progressively from a moderate level of strain (>6). The aPSI may provide a more accurate indication of a level of "maximal strain" for encapsulated workers than the original PSI, coincident with the occupational limits for body core temperature of 38.0 °C for general occupational groups, or 38.5 °C for selected and acclimatised personnel.

Controversies in exertional heat stroke diagnosis, prevention, and treatment

During the past several decades, the incidence of exertional heat stroke (EHS) has increased dramatically. Despite an improved understanding of this syndrome, numerous controversies still exist within the scientific and health professions regarding diagnosis, pathophysiology, risk factors, treatment, and return to physical activity. This review examines the following eight controversies: 1) reliance on core temperature for diagnosing and assessing severity of EHS; 2) hypothalamic damage induces heat stroke and this mediates “thermoregulatory failure” during the immediate recovery period; 3) EHS is a predictable condition primarily resulting from overwhelming heat stress; 4) heat-induced endotoxemia mediates systemic inflammatory response syndrome in all EHS cases; 5) nonsteroidal anti-inflammatory drugs for EHS prevention; 6) EHS shares similar mechanisms with malignant hyperthermia; 7) cooling to a specific body core temperature during treatment for EHS; and 8) return to physical activity based on physiological responses to a single-exercise heat tolerance test. In this review, we present and discuss the origins and the evidence for each controversy and propose next steps to resolve the misconception.

Heat alleviation strategies for athletic performance: A review and practitioner guidelines

International competition inevitably presents logistical challenges for athletes. Events such as the Tokyo 2020 Olympic Games require further consideration given historical climate data suggest athletes will experience significant heat stress. Given the expected climate, athletes face major challenges to health and performance. With this in mind, heat alleviation strategies should be a fundamental consideration. This review provides a focused perspective of the relevant literature describing how practitioners can structure male and female athlete preparations for performance in hot, humid conditions. Whilst scientific literature commonly describes experimental work, with a primary focus on maximizing magnitudes of adaptive responses, this may sacrifice ecological validity, particularly for athletes whom must balance logistical considerations aligned with integrating environmental preparation around training, tapering and travel plans. Additionally, opportunities for sophisticated interventions may not be possible in the constrained environment of the athlete village or event arenas. This review therefore takes knowledge gained from robust experimental work, interprets it and provides direction on how practitioners/coaches can optimize their athletes' heat alleviation strategies. This review identifies two distinct heat alleviation themes that should be considered to form an individualized strategy for the athlete to enhance thermoregulatory/performance physiology. First, chronic heat alleviation techniques are outlined, these describe interventions such as heat acclimation, which are implemented pre, during and post-training to prepare for the increased heat stress. Second, acute heat alleviation techniques that are implemented immediately prior to, and sometimes during the event are discussed. Abbreviations: CWI: Cold water immersion; HA: Heat acclimation; HR: Heart rate; HSP: Heat shock protein; HWI: Hot water immersion; LTHA: Long-term heat acclimation; MTHA: Medium-term heat acclimation; ODHA: Once-daily heat acclimation; RH: Relative humidity; RPE: Rating of perceived exertion; STHA: Short-term heat acclimation; TCORE: Core temperature; TDHA: Twice-daily heat acclimation; TS: Thermal sensation; TSKIN: Skin temperature; V̇O2max: Maximal oxygen uptake; WGBT: Wet bulb globe temperature.

The Use of Technology to Protect the Health of Athletes During Sporting Competitions in the Heat

During the 2019 IAAF World Championships in Athletics in Doha and the 2020 Olympic Games in Tokyo, minimum daily temperatures are expected to be in excess of 30°C. Due to the metabolic demands of the sporting events and the high environmental temperatures, the risk of exertional heat stroke (EHS) is high. Careful planning by event organizers are needed to ensure that athletes are protected from irreversible long-term health damage, or even death during sporting competitions in the heat. Efforts typically have included standard medical plans, equipment, protocols, and expert medical teams. In addition, the importance of responding quickly to a hyperthermic athlete cannot be understated, as minimizing treatment time will greatly improve the chances of full recovery. Treatment time can be minimized by notifying medical personnel about the health status of the athlete and the extent of any pre-competition heat acclimatization. Technology that allows the live transmission of physiological, biomechanical, and performance data to alert medical personnel of potential indicators of EHS should be considered. Real time monitoring ecosystems need to be developed that integrate information from numerous sensors such as core temperature-monitoring “pills” to relay information on how an athlete is coping with competing in intense heat. Medical/support staff would be alerted if an athlete's responses were indicating signs of heat stress or EHS signs and the athlete could be withdrawn under exceptional circumstances. This technology can also help provide more rapid, accurate and dignified temperature assessment at the road/track side in medical emergencies.

Heat acclimation attenuates the increased sensations of fatigue reported during acute exercise-heat stress

Athletes exercising in heat stress experience increased perceived fatigue acutely, however it is unknown whether heat acclimation (HA) reduces the magnitude of this perceptual response and whether different HA protocols influence the response. This study investigated sensations of fatigue following; acute exercise-heat stress; short- (5-sessions) and medium-term (10-sessions) HA; and between once- (ODHA) and twice-daily HA (TDHA) protocols. Twenty male participants (peak oxygen uptake: 3.75 ± 0.47 L·min-1) completed 10 sessions (60-min cycling at ~2 W·kg-1, 45°C/20% relative humidity) of ODHA (n = 10) or non-consecutive TDHA (n = 10). Sensations of fatigue (General, Physical, Emotional, Mental, Vigor and Total Fatigue) were assessed using the multi-dimensional fatigue scale inventory-short form pre and post session 1, 5 and 10. Heat adaptation was induced following ODHA and TDHA, with reductions in resting rectal temperature and heart rate, and increased plasma volume and sweat rate (P < 0.05). General, Physical and Total Fatigue increased from pre-to-post for session 1 within both groups (P < 0.05). Increases in General, Physical and Total Fatigue were attenuated in session 5 and 10 vs. session 1 of ODHA (P < 0.05). This change only occurred at session 10 of TDHA (P < 0.05). Whilst comparative heat adaptations followed ODHA and TDHA, perceived fatigue is prolonged within TDHA.

ABBREVIATIONS

∆: Change; ANOVA: Analysis of variance; HA: Heat acclimation; HR: Heart rate; IL-6: Interleukin-6; MFS-SF: Multi-dimensional fatigue symptom inventory-short form (MFSI-SF); MTHA: Medium-term heat acclimation; Na+: Sodium; ODHA: Once daily heat acclimation; PV: Plasma volume; RH: Relative humidity; RPE: Rating of perceived exertion; SD: Standard deviation; SE: Standard error of the slope coefficient or intercept; SEE : Standard error of the estimate for the regression equation; STHA: Short-term heat acclimation; TDHA: Twice daily heat acclimation; TC: Thermal Comfort; Tre: Rectal temperature; TSS: Thermal sensation; V̇O2peak: Peak oxygen uptake; WBSL: whole-body sweat loss.

The time course of adaptations in thermoneutral maximal oxygen consumption following heat acclimation

PURPOSE

This study investigated the effects of a 10-day heat acclimation (HA) programme on the time course of changes in thermoneutral maximal oxygen uptake ([Formula: see text]O_2max) during and up to 10 days post-HA.

METHODS

Twenty-two male cyclists were assigned to a HA or control (Con) training group following baseline ramp tests of thermoneutral [Formula: see text]O_2max. Ten days of fixed-intensity (50% baseline [Formula: see text]O_2max) indoor cycling was performed in either ~ 38.0 °C (HA) or ~ 20 °C (Con). [Formula: see text]O_2max was re-tested on HA days 5, 10 and post-HA days 1, 2, 3, 4, 5 and 10.

RESULTS

[Formula: see text]O_2max initially declined across time in both groups during training (P < 0.05), before increasing in the post-HA period in both groups (P < 0.05). However, [Formula: see text]O_2max was higher than control by post-HA day 4 in the HA group (P = 0.046).

CONCLUSIONS

The non-linear time course of [Formula: see text]O_2max adaptation suggests that post-testing should be performed 96-h post-training to identify the maximal change for most individuals. In preparation for training or testing, athletes can augment their aerobic power in thermoneutral environments by performing 10 days HA, but the full effects will manifest at varying stages of the post-HA period.

Association between work in deforested, compared to forested, areas and human heat strain: An experimental study in a rural tropical environment

BACKGROUND

With climate change, adverse human health effects caused by heat exposure are of increasing public health concern. Forests provide beneficial ecosystem services for human health, including local cooling. Few studies have assessed the relationship between deforestation and heat-related health effects in tropical, rural populations. We sought to determine whether deforested compared to forested landscapes are associated with increased physiological heat strain in a rural, tropical environment.

METHODS

We analyzed data from 363 healthy adult participants from ten villages who participated in a two-by-two factorial, randomized study in East Kalimantan, Indonesia from 10/1/17 to 11/6/17. Using simple randomization, field staff allocated participants equally to different conditions to conduct a 90-minute outdoor activity, representative of typical work. Core body temperature was estimated at each minute during the activity using a validated algorithm from baseline oral temperatures and sequential heart rate data, measured using chest band monitors. We used linear regression models, clustered by village and with a sandwich variance estimator, to assess the association between deforested versus forested conditions and the number of minutes each participant spent above an estimated core body temperature threshold of 38.5°C.

RESULTS

Compared to those in the forested condition (n=172), participants in the deforested condition (n=159) spent an average of 3.08 (95% CI 0.57, 5.60) additional minutes with an estimated core body temperature exceeding 38.5°C, after adjustment for age, sex, body mass index, and experiment start time, with a larger difference among those who began the experiment after 12 noon (5.17 [95% CI 2.20, 8.15]).

CONCLUSIONS

In this experimental study in a tropical, rural setting, activity in a deforested versus a forested setting was associated with increased objectively measured heat strain. Longer durations of hyperthermia can increase the risk of serious health outcomes. Land use decisions should consider the implications of deforestation on local heat exposure and health as well as on forest services, including carbon storage functions that impact climate change mitigation.

Physiology of sweat gland function: The roles of sweating and sweat composition in human health

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

The Physiological Strain Index Modified for Trained Heat-Acclimatized Individuals in Outdoor Heat

Purpose: To determine if the Physiological Strain Index (PSI), in original or modified form, can evaluate heat strain on a 0-10 scale, in trained and heat-acclimatized men undertaking a competitive half-marathon run in outdoor heat. Methods: Core (intestinal) temperature (TC) and heart rate (HR) were recorded continuously in 24 men (mean [SD] age = 26 [3] y, VO₂peak = 59 [5] mL·kg·min^-1). A total of 4 versions of the PSI were computed: original PSI with upper constraints of TC 39.5°C and HR 180 beats·min^-1 (PSI_39.5/180) and 3 modified versions of PSI with each having an age-predicted maximal HR constraint and graded TC constraints of 40.0°C (PSI_40.0/PHRmax), 40.5°C (PSI_40.5/PHRmax), and 41.0°C (PSI_41.0/PHRmax). Results: In a warm (26.1-27.3°C) and humid (79-82%) environment, all runners finished the race asymptomatic in 107 (10) (91-137) min. Peak TC and HR were 39.7°C (0.5°C) (38.5-40.7°C) and 186 (6) (175-196) beats·min^-1, respectively. In total, 63% exceeded TC 39.5°C, 71% exceeded HR 180 beats·min^-1, and 50% exceeded both of the original PSI upper TC and HR constraints. The computed heat strain was significantly greater with PSI_39.5/180 than all other methods (P < .003). PSI >10 was observed in 63% of runners with PSI_39.5/180, 25% for PSI_40.0/PHRmax, 8% for PSI_40.5/PHRmax, and 0% for PSI_41.0/PHRmax. Conclusions: The PSI was able to quantify heat strain on a 0-10 scale in trained and heat-acclimatized men undertaking a half-marathon race in outdoor heat, but only when the upper TC and HR constraints were modified to 41.0°C and age-predicted maximal HR, respectively.

Independent or simultaneous lowering of core and skin temperature has no impact on self-paced intermittent running performance in hot conditions

Purpose

To investigate the effects of lowering core (T_gi) and mean skin temperature (T_sk) concomitantly and independently on self-paced intermittent running in the heat.

Methods

10 males (30.5 ± 5.8 years, 73.2 ± 14.5 kg, 176.9 ± 8.0 cm, 56.2 ± 6.6 ml/kg/min) completed four randomised 46-min self-paced intermittent protocols on a non-motorised treadmill in 34.4 ± 1.4 °C, 36.3 ± 4.6% relative humidity. 30-min prior to exercise, participants were cooled via either ice slurry ingestion (INT); a cooling garment (EXT); mixed-cooling (ice slurry and cooling garment concurrently) (MIX); or no-cooling (CON).

Results

At the end of pre-cooling and the start of exercise T_gi were lower during MIX (36.11 ± 1.3 °C) compared to CON (37.6 ± 0.5 °C) and EXT (36.9 ± 0.5 °C, p < 0.05). Throughout pre-cooling T_sk and thermal sensation were lower in MIX compared to CON and INT, but not EXT (p < 0.05). The reductions in thermophysiological responses diminished within 10–20 min of exercise. Despite lowering T_gi, T_sk, body temperature (T_b), and thermal sensation prior to exercise, the distances covered were similar (CON: 6.69 ± 1.08 km, INT: 6.96 ± 0.81 km, EXT: 6.76 ± 0.65 km, MIX 6.87 ± 0.70 km) (p > 0.05). Peak sprint speeds were also similar between conditions (CON: 25.6 ± 4.48 km/h, INT: 25.4 ± 3.6 km/h, EXT: 26.0 ± 4.94 km/h, MIX: 25.6 ± 3.58 km/h) (p > 0.05). Blood lactate, heart rate and RPE were similar between conditions (p > 0.05).

Conclusion

Lowering T_gi and T_sk prior to self-paced intermittent exercise did not improve sprint, or submaximal running performance.

Electronic supplementary material

The online version of this article (10.1007/s00421-019-04173-y) contains supplementary material, which is available to authorized users.

Validity of a noninvasive estimation of deep body temperature when wearing personal protective equipment during exercise and recovery

BACKGROUND

Deep body temperature is a critical indicator of heat strain. However, direct measures are often invasive, costly, and difficult to implement in the field. This study assessed the agreement between deep body temperature estimated from heart rate and that measured directly during repeated work bouts while wearing explosive ordnance disposal (EOD) protective clothing and during recovery.

METHODS

Eight males completed three work and recovery periods across two separate days. Work consisted of treadmill walking on a 1% incline at 2.5, 4.0, or 5.5 km/h, in a random order, wearing EOD protective clothing. Ambient temperature and relative humidity were maintained at 24 °C and 50% [Wet bulb globe temperature (WBGT) (20.9 ± 1.2) °C] or 32 °C and 60% [WBGT (29.0 ± 0.2) °C] on the separate days, respectively. Heart rate and gastrointestinal temperature (T_GI) were monitored continuously, and deep body temperature was also estimated from heart rate (ECTemp).

RESULTS

The overall systematic bias between T_GI and ECTemp was 0.01 °C with 95% limits of agreement (LoA) of ±0.64 °C and a root mean square error of 0.32 °C. The average error statistics among participants showed no significant differences in error between the exercise and recovery periods or the environmental conditions. At T_GI levels of (37.0-37.5) °C, (37.5-38.0) °C, (38.0-38.5) °C, and > 38.5 °C, the systematic bias and ± 95% LoA were (0.08 ± 0.58) °C, (- 0.02 ± 0.69) °C, (- 0.07 ± 0.63) °C, and (- 0.32 ± 0.56) °C, respectively.

CONCLUSIONS

The findings demonstrate acceptable validity of the ECTemp up to 38.5 °C. Conducting work within an ECTemp limit of 38.4 °C, in conditions similar to the present study, would protect the majority of personnel from an excessive elevation in deep body temperature (> 39.0 °C).

The Effect of a Resistance Training Session on Physiological and Thermoregulatory Measures of Sub-maximal Running Performance in the Heat in Heat-Acclimatized Men

BACKGROUND

The current study examined the acute effects of a lower body resistance training (RT) session on physiological and thermoregulatory measures during a sub-maximal running protocol in the heat in heat-acclimatized men. Ten resistance-untrained men (age 27.4 ± 4.1 years; height 1.78 ± 0.06 m; body mass 76.8 ± 9.9 kg; peak oxygen uptake 48.2 ± 7.0 mL kg^-1 min^-1) undertook a high-intensity RT session at six-repetition maximum. Indirect muscle damage markers (i.e., creatine kinase [CK], delayed-onset muscle soreness [DOMS], and countermovement jump [CMJ]) were collected prior to, immediately post and 24 and 48 h after the RT session. The sub-maximal running protocol was performed at 70% of the ventilatory threshold, which was conducted prior to and 24 and 48 h following the RT session to obtain physiological and thermoregulatory measures.

RESULTS

The RT session exhibited significant increases in DOMS (p < 0.05; effect size [ES]: 1.41-10.53), whilst reduced CMJ (p < 0.05; ES: - 0.79-1.41) for 48 h post-exercise. There were no differences in CK (p > 0.05), although increased with moderate to large ES (0.71-1.12) for 48 h post-exercise. The physiological cost of running was increased for up to 48 h post-exercise (p < 0.05) with moderate to large ES (0.50-0.84), although no differences were shown in thermoregulatory measures (p > 0.05) with small ES (0.33).

CONCLUSION

These results demonstrate that a RT session impairs sub-maximal running performance for several days post-exercise, although thermoregulatory measures are unperturbed despite elevated muscle damage indicators in heat-acclimatized, resistance untrained men. Accordingly, whilst a RT session may not increase susceptibility to heat-related injuries in heat-acclimatized men during sub-maximal running in the heat, endurance sessions should be undertaken with caution for at least 48 h post-exercise following the initial RT session in resistance untrained men.

The Current State of Weight-Cutting in Combat Sports-Weight-Cutting in Combat Sports

In combat sports, athletes are divided into categories based on gender and body mass. Athletes attempt to compete against a lighter opponent by losing body mass prior to being weighed (i.e., 'weight-cutting'). The purpose of this narrative review was to explore the current body of literature on weight-cutting and outline gaps for further research. Methods of weight-loss include energy intake restriction, total body fluid reduction and pseudo extreme/abusive medical practice (e.g., diuretics). The influence of weight-cutting on performance is unclear, with studies suggesting a negative or no effect. However, larger weight-cuts (~5% of body mass in <24 h) do impair repeat-effort performance. It is unclear if the benefit from competing against a smaller opponent outweighs the observed reduction in physical capacity. Many mechanisms have been proposed for the observed reductions in performance, ranging from reduced glycogen availability to increased perceptions of fatigue. Athletes undertaking weight-cutting may be able to utilise strategies around glycogen, total body water and electrolyte replenishment to prepare for competition. Despite substantial discussion on managing weight-cutting in combat sports, no clear solution has been offered. Given the prevalence of weight-cutting, it is important to develop a deeper understanding of such practices so appropriate advice can be given.

Nine-, but Not Four-Days Heat Acclimation Improves Self-Paced Endurance Performance in Females

Although emerging as a cost and time efficient way to prepare for competition in the heat, recent evidence indicates that "short-term" heat acclimation (<7 days) may not be sufficient for females to adapt to repeated heat stress. Furthermore, self-paced performance following either short-term, or longer (>7 days) heat acclimation has not been examined in a female cohort. Therefore, the aim of this study was to investigate self-paced endurance performance in hot conditions following 4- and 9-days of a high-intensity isothermic heat acclimation protocol in a female cohort. Eight female endurance athletes (mean ± SD, age 27 ± 5 years, mass 61 ± 5 kg, VO2peak 47 ± 6 ml⋅kg⋅min-1) performed 15-min self-paced cycling time trials in hot conditions (35°C, 30%RH) before (HTT1), and after 4-days (HTT2), and 9-days (HTT3) isothermic heat acclimation (HA, with power output manipulated to increase and maintain rectal temperature (T rec) at ∼38.5°C for 90-min cycling in 40°C, 30%RH) with permissive dehydration. There were no significant changes in distance cycled (p = 0.47), mean power output (p = 0.55) or cycling speed (p = 0.44) following 4-days HA (i.e., from HTT1 to HTT2). Distance cycled (+3.2%, p = 0.01; +1.8%, p = 0.04), mean power output (+8.1%, p = 0.01; +4.8%, p = 0.05) and cycling speed (+3.0%, p = 0.01; +1.6%, p = 0.05) were significantly greater in HTT3 than in HTT1 and HTT2, respectively. There was an increase in the number of active sweat glands per cm2 in HTT3 as compared to HTT1 (+32%; p = 0.02) and HTT2 (+22%; p < 0.01), whereas thermal sensation immediately before HTT3 decreased ("Slightly Warm," p = 0.03) compared to ratings taken before HTT1 ("Warm") in 35°C, 30%RH. Four-days HA was insufficient to improve performance in the heat in females as observed following 9-days HA.

Influence of Sodium Citrate Supplementation after Dehydrating Exercise on Responses of Stress Hormones to Subsequent Endurance Cycling Time-Trial in the Heat

Background and objectives: In temperate environments, acute orally induced metabolic alkalosis alleviates exercise stress, as reflected in attenuated stress hormone responses to relatively short-duration exercise bouts. However, it is unknown whether the same phenomenon occurs during prolonged exercise in the heat. This study was undertaken with aim to test the hypothesis that ingestion of an alkalizing substance (sodium citrate; CIT) after dehydrating exercise would decrease blood levels of stress hormones during subsequent 40 km cycling time-trial (TT) in the heat. Materials and Methods: Male non-heat-acclimated athletes (n = 20) lost 4% of body mass by exercising in the heat. Then, during a 16 h recovery period prior to TT in a warm environment (32 °C), participants ate the prescribed food and ingested CIT (600 mg·kg-1) or placebo (PLC) in a double-blind, randomized, crossover manner with 7 days between the two trials. Blood aldosterone, cortisol, prolactin and growth hormone concentrations were measured before and after TT. Results: Total work performed during TT was similar in the two trials (p = 0.716). In CIT compared to PLC trial, lower levels of aldosterone occurred before (72%) and after (39%) TT (p ˂ 0.001), and acute response of aldosterone to TT was blunted (29%, p ˂ 0.001). Lower cortisol levels in CIT than in PLC trial occurred before (13%, p = 0.039) and after TT (14%, p = 0.001), but there were no between-trial differences in the acute responses of cortisol, prolactin or growth hormone to TT, or in concentrations of prolactin and growth hormone before or after TT (in all cases p > 0.05). Conclusions: Reduced aldosterone and cortisol levels after TT and blunted acute response of aldosterone to TT indicate that CIT ingestion during recovery after dehydrating exercise may alleviate stress during the next hard endurance cycling bout in the heat.

Exertional Heat Stroke within Secondary School Athletics

Exertional heat stroke (EHS) remains one of the leading causes of sudden death in sport despite clear evidence showing 100% survivability with the proper standards of care in place and utilized. Of particular concern are student athletes competing at the secondary school level, where the extent of appropriate health care services remains suboptimal compared with organized athletics at the collegiate level and higher. While rapid recognition and rapid treatment of EHS ensures survival, the adoption and implementation of these lifesaving steps within secondary school athletics warrant further discussion within the sports medicine community. Establishing proper policies regarding the prevention and care of EHS coupled with utilizing an interdisciplinary care approach is essential for 1) minimizing risk and 2) guaranteeing optimal outcomes for the patient.

Subjective thermal strain impairs endurance performance in a temperate environment

PURPOSE

The aim of this study was to test the hypothesis that subjective thermal strain can reduce endurance performance independently from the general physiological strain normally associated with impaired endurance performance in the heat.

METHODS

In 20 °C and 44% relative humidity, 12 endurance-trained athletes (1♀ 11♂; mean ± SD; age: 27 ± 6 y; VO_2max: 61 ± 6 ml/kg/min) performed a time to exhaustion (TTE) test in two different experimental conditions: with an electric heat pad applied to the subjects' upper back (HP) and control (CON: without heat pad). In both conditions, subjects cycled to volitional exhaustion at 70% of their VO_2max. Cardiorespiratory, metabolic, thermoregulatory and perceptual responses were measured throughout the TTE test and compared at 0%, 50% and 100% isotime and at exhaustion.

RESULTS

TTE was reduced by 9% in HP (2092 ± 305 s) compared to CON (2292 ± 344 s; p = .023). The main effect of condition on thermal discomfort at isotime (p = .002), the effect of condition on thermal sensation at 0% isotime (p = .004) and the condition by isotime interaction on rating of perceived exertion (p = .036) indicated higher subjective thermal strain in HP compared to CON. None of the measured cardiorespiratory, metabolic and thermoregulatory variables differed significantly between conditions.

CONCLUSION

Our novel experimental manipulation (HP) was able to induce significant subjective thermal strain and reduce endurance performance in a temperate environment without inducing the general physiological strain normally associated with impaired endurance performance in the heat. These results suggest that subjective thermal strain is an important and independent mediator of the heat-induced impairment in endurance performance.

Special Environments: Altitude and Heat

High-level athletes are always looking at ways to maximize training adaptations for competition performance, and using altered environmental conditions to achieve this outcome has become increasingly popular by elite athletes. Furthermore, a series of potential nutrition and hydration interventions may also optimize the adaptation to altered environments. Altitude training was first used to prepare for competition at altitude, and it still is today; however, more often now, elite athletes embark on a series of altitude training camps to try to improve sea-level performance. Similarly, the use of heat acclimation/acclimatization to optimize performance in hot/humid environmental conditions is a common practice by high-level athletes and is well supported in the scientific literature. More recently, the use of heat training to improve exercise capacity in temperate environments has been investigated and appears to have positive outcomes. This consensus statement will detail the use of both heat and altitude training interventions to optimize performance capacities in elite athletes in both normal environmental conditions and extreme conditions (hot and/or high), with a focus on the importance of nutritional strategies required in these extreme environmental conditions to maximize adaptations conducive to competitive performance enhancement.

Acute taurine supplementation enhances thermoregulation and endurance cycling performance in the heat

This study investigated the effects of oral taurine supplementation on cycling time to exhaustion at a fixed-intensity and thermoregulation in the heat. In a double-blind, randomised crossover design, 11 healthy males participated in a time to exhaustion test in the heat (35°C, 40% RH), cycling at the power output associated with ventilatory threshold, 2 h after ingesting: Taurine (50 mg kg^-1) or placebo (3 mg kg^-1 maltodextrin). Core and mean skin temperature, mean sweat rate, heart rate, rating of perceived exertion (RPE), thermal comfort and thermal sensation were measured during exercise and blood lactate concentration (B[La]) was measured after exercise. Taurine supplementation increased time to exhaustion by 10% (25.16 min vs. 22.43 min, p = 0.040), end sweat rate by 12.7% (687 nL min^-1 vs. 600 nL min^-1, p = 0.034) and decreased B[La] by 16.5% (5.75 mmol L^-1 vs. 6.85 mmol L^-1, p = 0.033). Core temperature was lower in the final 10% of the time to exhaustion (38.5°C vs. 38.1°C, p = 0.049). Taurine supplementation increased time to exhaustion and local sweating, while decreasing RPE and core temperature in the later stages of exercise, as well as reducing post-exercise B[La]. This study provides the evidence of taurine's role in thermoregulatory processes. These findings have implications for the short-term preparation strategies of individuals exercising in the heat. Based on these findings, a single dose of taurine 2 h prior to training or competition would provide an ergogenic and thermoregulatory effect.

A controlled heat stress during late gestation affects thermoregulation, productive performance, and metabolite profiles of primiparous sow

Heat stress (HS) alters metabolic parameters and reduces productive performance in lactating sows. However, the impact of HS on metabolomic profiles of sows during late gestation is not fully understood. We present here, a study investigating the productive performance and metabolic responses in sows when exposed to HS during late gestation. Twelve first-parity Landrace × Large White F1 sows were randomly assigned into two environmental treatments including the thermoneutral (TN) (18-22 °C; n = 6) and HS (28-32 °C; n = 6) conditions from 85 d of gestation until farrowing. Rectal temperature (RT), respiration rates (RR), and surface temperature (ST) were measured every 4 h from 0800 h to 2000 h during the 2nd week. Farrowing and litter Data, as well as duration of eating, were monitored to assess sows' productive performance. Blood biochemical parameters and urinary metabolomic profiles were measured on d107 of gestation to analyze the host metabolic responses. Our results show that HS increased RT, RR, and ST (P < 0.0001). Duration of parturition was prolonged during the delivery in HS group (P < 0.05). Piglet body weight (BW) at d 10 and weaning were reduced by 18% and 17% respectively due to maternal HS (P < 0.001). Duration of eating increased as a result of HS (P < 0.001), consistent with the significant changes observed in serum ghrelin (P < 0.05). Moreover, serum ACTH, cortisol, insulin, creatinine, and BUN saw increase as well (P < 0.05). Plasma NEFA were elevated by HS (P < 0.001). Additionally, HS elevated (VIP>1, log₂fold change>0.585, and P < 0.05) the relative concentrations of 5-aminovaleric acid, β-alanine, cysteine, isoleucine, glyceric acid, erythronic acid, mannitol, erythritol, 2-methyl-1,3-butanediol, and pantothenic acid in urine. These ten metabolites mainly affected the pantothenate and CoA biosynthesis, β-alanine metabolism, and glycerolipid metabolism in pregnant sows. In summary, our study suggests that the controlled HS during late gestation elevates thermal responses, reduces productive performance, and more importantly, enhances the catabolism of lipid and protein of first-parity pregnant sow.

A new occupational heat tolerance test: A feasibility study

Heat tolerance tests identify those susceptible to heat illnesses and monitor heat adaptations. Currently, tolerance tests do not replicate the uncompensable heat strain environments experienced in some occupations. In addition, tests can take up to 2 h to complete, and cannot offer intra and inter individual comparisons, due to the use of a fixed exercise intensity. This study aimed to assess the validity and reliability of a new heat occupational tolerance test (HOTT: 40 min at 6 W kg^-1 metabolic heat production, 50 °C 10% RH, in protective clothing) to the standard heat tolerance test (HTT: 2 h walk at 5 km h^-1 1% gradient, 40 °C 40% RH, in shorts and t-shirt). Eighteen participants (age: 21 ± 3 yrs, body mass: 81.3 ± 5.9 kg) completed trials to assess the validity and/or reliability of the HOTT. Peak rectal temperature (T_re) displayed strong agreement and low measurement error (0.19 °C) between HTT (38.7 ± 0.4 °C) and HOTT (38.6 ± 0.4 °C). Strong agreement was also displayed for physiological and perceptual measures between the two HOTT trials, including peak T_re (38.5 ± 0.4 °C vs. 38.5 ± 0.4 °C) and peak heart rate (182 ± 20 b min^-1 vs. 182 ± 21 b min^-1). The HOTT is the first tolerance test that assesses individuals' responses whilst wearing protective clothing in high temperatures. It can consistently identify individuals' levels of heat tolerance within a reduced time frame. In addition, it allows for participant monitoring over time and comparisons between individuals to be made. A continuum based approach is recommended when assessing individuals' responses to the HOTT.

Once- and twice-daily heat acclimation confer similar heat adaptations, inflammatory responses and exercise tolerance improvements

This experiment aimed to investigate the efficacy of twice-daily, nonconsecutive heat acclimation (TDHA) in comparison to once-daily heat acclimation (ODHA) and work matched once- or twice-daily temperate exercise (ODTEMP, TDTEMP) for inducing heat adaptations, improved exercise tolerance, and cytokine (immune) responses. Forty males, matched biophysically and for aerobic capacity, were assigned to ODHA, TDHA, ODTEMP, or TDTEMP. Participants completed a cycling-graded exercise test, heat acclimation state test, and a time to task failure (TTTF) at 80% peak power output in temperate (TTTFTEMP : 22°C/40% RH) and hot conditions (TTTFHOT : 38°C/20% RH), before and after 10-sessions (60 min of cycling at ~2 W·kg-1 ) in 45°C/20% RH (ODHA and TDHA) or 22°C/40% RH (ODTEMP or TDTEMP). Plasma IL-6, TNF-α, and cortisol were measured pre- and postsessions 1, 5, and 10. ODHA and TDHA induced equivalent heat adaptations (P < 0.05) (resting rectal temperature [-0.28 ± 0.22, -0.28 ± 0.19°C], heart rate [-10 ± 3, -10 ± 4 b·min-1 ], and plasma volume expansion [+10.1 ± 5.6, +8.5 ± 3.1%]) and improved heat acclimation state (sweat set point [-0.22 ± 0.18, -0.22 ± 0.14°C] and gain [+0.14 ± 0.10, +0.15 ± 0.07 g·sec-1 ·°C-1 ]). TTTFHOT increased (P < 0.001) following ODHA (+25 ± 4%) and TDHA (+24 ± 10%), but not ODTEMP (+5 ± 14%) or TDTEMP (+5 ± 17%). TTTFTEMP did not improve (P > 0.05) following ODHA (+14 ± 4%), TDHA (14 ± 8%), ODTEMP (9 ± 10%) or TDTEMP (8 ± 13%). Acute (P < 0.05) but no chronic (P > 0.05) increases were observed in IL-6, TNF-α, or cortisol during ODHA and TDHA, or ODTEMP and TDTEMP. Once- and twice-daily heat acclimation conferred similar magnitudes of heat adaptation and exercise tolerance improvements, without differentially altering immune function, thus nonconsecutive TDHA provides an effective, logistically flexible method of HA, benefitting individuals preparing for exercise-heat stress.

Cross-Adaptation: Heat and Cold Adaptation to Improve Physiological and Cellular Responses to Hypoxia

To prepare for extremes of heat, cold or low partial pressures of oxygen (O₂), humans can undertake a period of acclimation or acclimatization to induce environment-specific adaptations, e.g. heat acclimation (HA), cold acclimation (CA), or altitude training. While these strategies are effective, they are not always feasible due to logistical impracticalities. Cross-adaptation is a term used to describe the phenomenon whereby alternative environmental interventions, e.g. HA or CA, may be a beneficial alternative to altitude interventions, providing physiological stress and inducing adaptations observable at altitude. HA can attenuate physiological strain at rest and during moderate-intensity exercise at altitude via adaptations allied to improved O₂ delivery to metabolically active tissue, likely following increases in plasma volume and reductions in body temperature. CA appears to improve physiological responses to altitude by attenuating the autonomic response to altitude. While no cross-acclimation-derived exercise performance/capacity data have been measured following CA, post-HA improvements in performance underpinned by aerobic metabolism, and therefore dependent on O₂ delivery at altitude, are likely. At a cellular level, heat shock protein responses to altitude are attenuated by prior HA, suggesting that an attenuation of the cellular stress response and therefore a reduced disruption to homeostasis at altitude has occurred. This process is known as cross-tolerance. The effects of CA on markers of cross-tolerance is an area requiring further investigation. Because much of the evidence relating to cross-adaptation to altitude has examined the benefits at moderate to high altitudes, future research examining responses at lower altitudes should be conducted, given that these environments are more frequently visited by athletes and workers. Mechanistic work to identify the specific physiological and cellular pathways responsible for cross-adaptation between heat and altitude, and between cold and altitude, is warranted, as is exploration of benefits across different populations and physical activity profiles.

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.

Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives

Heat acclimation or acclimatization (HA) occurs with repeated exposure to heat inducing adaptations that enhance thermoregulatory mechanisms and heat tolerance leading to improved exercise performance in warm-to-hot conditions. HA is an essential heat safety and performance enhancement strategy in preparation for competitions in warm-to-hot conditions for both individual and team sports. Yet, some data indicate HA is an underutilized pre-competition intervention in athletes despite the well-known benefits; possibly due to a lack of practical information provided to athletes and coaches. Therefore, the aim of this review is to provide actionable evidence-based implementation strategies and protocols to induce and sustain HA. We propose the following suggestions to circumvent potential implementation barriers: 1) incorporate multiple induction methods during the initial acclimation period, 2) complete HA 1-3 weeks before competition in the heat to avoid training and logistical conflicts during the taper period, and 3) minimize adaptation decay through intermittent exercise-heat exposure or re-acclimating immediately prior to competition with 2-4 consecutive days of exercise-heat training. Use of these strategies may be desirable or necessary to optimize HA induction and retention around existing training or logistical requirements.

Uncoupling psychological from physiological markers of heat acclimatization in a military context

Heat acclimatization may help personnel who travel to areas with a hot climate (WBGT > 27 °C), making them operationally more efficient and performant through improvements in physiological and psychological parameters. Their work-related physical activities may aid active heat acclimatization. However, it is unknown whether adding physical training to improve adaptation is effective, particularly if there is sufficient time for full acclimatization, classically reached after 15 days. Thirty French soldiers (Training group, T) performed a progressive and moderate (from three to five 8-min running sets at 50-60% of their speed at VO_2max with 4-min periods of active recovery in between) aerobic training program upon arriving at their base in United Arab Emirates (~40 °C and 20% RH). A control group (30 soldiers; No Training, NT) continued to perform only their usual outdoor military activities (~5 h d^-1). A field heat stress test (HST: three 8-min running sets at 50% of the speed at VO_2max) was performed before (D0), during (D10), and after (D15) the heat acclimatization period to assess physiological and psychological changes. An 8-km trial in battledress was then performed at D17. Although physiological modifications were mostly similar (p < 0.001 for all) for both groups (rectal temperature at the end of the HST: -0.58 ± 0.51 vs -0.53 ± 0.40 °C, HR at the end of the HST: -21 ± 12 vs -19 ± 9 bpm, and sweat osmolality: -47 ± 30 vs -26 ± 32 mOsmol.l^-1 between D15 and D0 for T and NT groups, respectively), thermal discomfort (-31 ± 4 vs -11 ± 5 mm between D15 and D0, p = 0.001) and rates of perceived exertion (-3.0 ± 0.4 vs -1.4 ± 0.3 D15 and D0, p = 0.001) were much lower in the T than NT group during the HST. HST-induced modifications in facial temperature only decreased in the T group (-1.08 ± 0.28 between D15 and D0, p < 0.001). Moreover, there was a difference in perceived thermal discomfort during the 8-km trial (40 ± 20 vs 55 ± 22 mm for the T and NT groups, respectively, p = 0.010). Thus, a 15-day, low-volume training regimen during a mission in a hot and dry environment has a modest impact on physiological adaptation but strongly decreases the perceived strain of exertion and climate potentially via greater reductions in facial temperature, even during a classical operational physical task in a military context.

Optimizing Heat Acclimation for Endurance Athletes: High- Versus Low-Intensity Training

Purpose: To determine the effect of high- versus low-intensity training in the heat and ensuing taper period in the heat on endurance performance. Methods: In total, 19 well-trained triathletes undertook 5 days of normal training and a 1-wk taper including either low- (heat acclimation [HA-L], n = 10) or high-intensity (HA-H, n = 9) training sessions in the heat (30°C, 50% relative humidity). A control group (n = 10) reproduced their usual training in thermoneutral conditions. Indoor 20-km cycling time trials (35°C, 50% relative humidity) were performed before (Pre) and after the main heat exposure (Mid) and after the taper (Post). Results: Power output remained stable in the control group from Pre to Mid (effect size: −0.10 [0.26]) and increased from Mid to Post (0.18 [0.22]). The HA-L group demonstrated a progressive increase in performance from Pre to Mid (0.62 [0.33]) and from Mid to Post (0.53 [0.30]), alongside typical physiological signs of HA (reduced core temperature and heart rate and increased body-mass loss). While the HA-H group presented similar adaptations, increased perceived fatigue and decreased performance at Mid (−0.35 [0.26]) were evidenced and reversed at Post (0.50 [0.20]). No difference in power output was reported at Post between the HA-H and control groups. Conclusion: HA-H can quickly induce functional overreaching in nonacclimatized endurance athletes. As it was associated with a weak subsequent performance supercompensation, coaches and athletes should pay particular attention to training monitoring during a final preparation in the heat and reduce training intensity when early signs of functional overreaching are identified.

Exertional heat illness incidence and on-site medical team preparedness in warm weather

To investigate the influence of estimated wet bulb globe temperature (WBGT) and the International Institute of Race Medicine (IIRM) activity modification guidelines on the incidence of exertional heat stroke (EHS) and heat exhaustion (HEx) and the ability of an on-site medical team to treat those afflicted. Medical records of EHS and HEx patients over a 17-year period from the New Balance Falmouth Road Race were examined. Climatologic data from nearby weather stations were obtained to calculate WBGT with the Australian Bureau of Meteorology (WBGT_A) and Liljegren (WBGT_L) models. Incidence rate (IR) of EHS, HEx, and combined total of EHS and HEx (COM) were calculated, and linear regression analyses were performed to assess the relationship between IR and WBGT_A or WBGT_L. One-way ANOVA was performed to compare differences in EHS, HEx, and COM incidence to four alert levels in the IIRM guidelines. Incidence of EHS, HEx, and COM was 2.12, 0.98, and 3.10 cases per 1000 finishers. WBGT_A explained 48, 4, and 46% of the variance in EHS, HEx, and COM IR; WBGT_L explained 63, 13, and 69% of the variance in EHS, HEx, and COM IR. Main effect of WBGT_A and WBGT_L on the alert levels were observed in EHS and COM IR (p < 0.05). The cumulative number of EHS patients treated did not exceed the number of cold water immersion tubs available to treat them. EHS IR increased as WBGT and IIRM alert level increased, indicating the need for appropriate risk mitigation strategies and on-site medical treatment.

Practical Cooling Strategies During Continuous Exercise in Hot Environments: A Systematic Review and Meta-Analysis

Background

Performing exercise in thermally stressful environments impairs exercise capacity and performance. Cooling during exercise has the potential to attenuate detrimental increases in body temperature and improve exercise capacity and performance.

Objective

The objective of this review was to assess the effectiveness of practical cooling strategies applied during continuous exercise in hot environments on body temperature, heart rate, whole body sweat production, rating of perceived exertion (RPE), thermal perception and exercise performance.

Methods

Electronic database searches of MEDLINE, SPORTDiscus, Scopus and Physiotherapy Evidence Database (PEDro) were conducted using medical subject headings, indexing terms and keywords. Studies were eligible if participants were defined as ‘healthy’, the exercise task was conducted in an environment ≥25 °C, it used a cooling strategy that would be practical for athletes to use during competition, cooling was applied during a self-paced or fixed-intensity trial, participants exercised continuously, and the study was a randomised controlled trial with the comparator either a thermoneutral equivalent or no cooling. Data for experimental and comparator groups were meta-analysed and expressed as a standardised mean difference and 95 % confidence interval.

Results

Fourteen studies including 135 participants met the eligibility criteria. Confidence intervals for meta-analysed data included beneficial and detrimental effects for cooling during exercise on core temperature, mean skin temperature, heart rate and sweat production during fixed-intensity exercise. Cooling benefited RPE and thermal perception during fixed-intensity exercise and improved self-paced exercise performance.

Conclusion

Cooling during fixed-intensity exercise, particularly before a self-paced exercise trial, improves endurance performance in hot environments by benefiting RPE and thermal perception, but does not appear to attenuate increases in body temperature.

Association Between Exercise-Induced Hyperthermia and Intestinal Permeability: A Systematic Review

BACKGROUND

Prolonged and strenuous physical exercise increases intestinal permeability, allowing luminal endotoxins to translocate through the intestinal barrier and reach the bloodstream. When recognized by the immune system, these endotoxins trigger a systemic inflammatory response that may affect physical performance and, in severe cases, induce heat stroke. However, it remains to be elucidated whether there is a relationship between the magnitude of exercise-induced hyperthermia and changes in intestinal permeability.

OBJECTIVE

In this systematic review, we evaluated whether an exercise-induced increase in core body temperature (T _Core) is associated with an exercise-induced increase in intestinal permeability.

METHODS

The present systematic review screened the MEDLINE/PubMed and Web of Science databases in September 2016, without any date restrictions. Sixteen studies that were performed in healthy participants, presented original data, and measured both the exercise-induced changes in T _Core and intestinal permeability were selected. These studies assessed intestinal permeability through the measurement of sugar levels in the urine and measurement of intestinal fatty acid binding protein or lipopolysaccharide levels in the blood.

RESULTS

Exercise increased both T _Core and intestinal permeability in most of the 16 studies. In addition, a positive and strong correlation was observed between the two parameters (r = 0.793; p < 0.001), and a T _Core exceeding 39 °C was always associated with augmented permeability.

CONCLUSION

The magnitude of exercise-induced hyperthermia is directly associated with the increase in intestinal permeability.