Consensus recommendations on training and competing in the heat

Proposing adjustments to heat safety thresholds for junior high and high school sports clubs in Japan

With higher temperatures expected in the future due to global climate change, addressing health risks such as heat illness is increasingly important. In Japan, thousands of heat illness cases occur annually during school sports club activities. The risk may vary by sport, location, and region, but how heat safety thresholds (HSTs) should be adjusted to provide effective guidelines remains uncertain. Thus, we conducted a case-crossover study using data of heat illness cases and wet-bulb globe temperature (WBGT) throughout Japan to evaluate the heat illness risk for the current HSTs and propose adjustments. A significant relationship was found between heat illness incidence and WBGT at the time of the incident, as well as the average WBGT one and two days prior. The risk significantly varies with factors such as club, region, location, year, month, and the average WBGT in summer. Therefore, we recommend lowering the current HSTs by one category (3 °C) in the following cases: (1) clubs at high risk (baseball, softball, soccer/futsal, tennis, track and field, kyudo, and other with sustained exercise or thick uniforms); (2) from April to June; (3) in cooler regions (Hokkaido, Tohoku, Hokuriku, or where the average WBGT in summer≦18℃); (4) for outdoor activities; (5) when heat rapidly increases without adequate heat acclimatization. These findings may inform educators, students, sports authorities, and policymakers in adjusting HSTs to reduce the incidence of heat illness, thereby ensuring safer environments for school sports activities.

Hydration Considerations to Improve the Physical Performance and Health of Firefighters

Background/Objectives: Firefighters are exposed to a high level of stress as they often perform physically challenging work in hazardous environments while responsible for rescuing and keeping those around them safe. To add to this stress, they are also required to work in heavy, unbreathable personal protective equipment which promotes dehydration. These occupational demands paired with dehydration may lead to increased core temperatures, cardiac strain, and overall risk for sudden cardiac events. Thus, it is important to include hydration assessments and determine fluid needs when firefighters are on shift to ensure their personal safety as well as the safety of those around them by optimizing physical performance by maintaining adequate hydration. Therefore, the purpose of this review is to identify markers of hydration, classifications of hydration status, current hydration recommendations, and hydration interventions that may contribute to the overall clarity of hydration protocols that may optimize performance and health of firefighters. In addition, the impact of common medications, exercise training, and health conditions on hydration status related to firefighters will be discussed. Methods: A comprehensive literature search was conducted to discuss the purpose statements. Results: Hydration recommendations for firefighters include (1) assessing hydration status with multiple measurements including body mass, urine specific gravity and thirst sensation, and (2) following general hydration recommendations on rest days and exercise hydration protocols during firefighting activities which may be altered according to hydration status measurements. Conclusion: Randomized controlled trials in firefighters are needed to determine the impact of maintaining adequate hydration on health markers.

Efficacy of the FIFA cooling break heat policy during an intermittent treadmill football simulation in hot conditions in trained males

OBJECTIVE

To evaluate the efficacy of the Fédération Internationale de Football Association (FIFA) cooling break policy against alternative cooling configurations in attenuating thermal strain during simulated football in the heat.

METHODS

12 males (age: 27±6 years, V̇O2peak: 61±7 mL/kg/min) completed five 90 min intermittent treadmill football match simulations in 40°C and 41% relative humidity (32°C wet-bulb globe temperature) with different cooling configurations: regular match without cooling breaks (REG), 3 min breaks without cooling (BRKno-cool), 3 min breaks with cooling (BRKcool: current FIFA policy; chilled fluid ingestion and ice towel across neck and shoulders), 5 min extended half-time without cooling breaks (ExtHTonly) and 3 min cooling breaks with 5 min ExtHT (ExtHTcool). Rectal temperature (Tre), heart rate, whole-body sweat rate (WBSR) and rating of perceived exertion (RPE) were recorded. Data are presented as mean (95% CIs).

RESULTS

Final Tre was lower in BRKno-cool (0.20°C (0.01, 0.39), p=0.038), BRKcool (0.39°C (0.21, 0.57), p<0.001) and ExtHTcool (0.40°C (0.22, 0.58), p<0.001) than REG (39.1°C (38.8, 39.3)). Mean Tre was lower in ExtHTcool (38.2°C (38.0, 38.4)) than BRKcool (38.3°C (38.1, 38.5), p=0.018), BRKno-cool and ExtHTonly (38.4°C (38.2, 38.6), p<0.001) and REG (38.5°C (38.3, 38.7), p<0.001). Mean heart rate was lower during BRKcool (6 beats/min (4, 7), p<0.001) and ExtHTcool (7 beats/min (6, 8), p<0.001) compared with REG. WBSR was comparable across trials (p≥0.07) and RPE was attenuated during BRKcool (0.4 (0.1, 0.7), p=0.004) and ExtHTcool (0.5 (0.2, 0.7), p=0.002), compared with REG.

CONCLUSION

BRKcool and ExtHTcool attenuated thermal, cardiovascular and perceptual strain during a simulated football match in the heat. Additional strategies may be required in field settings or under harsher conditions.

The effect of 8-day oral taurine supplementation on thermoregulation during low-intensity exercise at fixed heat production in hot conditions of incremental humidity

PURPOSE

To determine the effect of taurine supplementation on sweating and core temperature responses, including the transition from compensable to uncompensable heat stress, during prolonged low-intensity exercise of a fixed-heat production (~ 200W/m2) in hot conditions (37.5 °C), at both fixed and incremental vapour-pressure.

METHODS

Fifteen females (n = 3) and males (n = 12; 27 ± 5 years, 78 ± 9 kg, V ˙ O2max 50.3 ± 7.8 mL/kg/min), completed a treadmill walking protocol (~ 200W/m2 heat production [Ḣprod]) in the heat (37.5 ± 0.1 °C) at fixed-(16-mmHg) and ramped-humidity (∆1.5-mmHg/5-min) following 1 week of oral taurine supplementation (50 mg/kg/bm) or placebo, in a double-blind, randomised, cross-over design. Participants were assessed for whole-body sweat loss (WBSL), local sweat rate (LSR), sweat gland activation (SGA), core temperature (Tcore), breakpoint of compensability (Pcrit) and calorimetric heat transfer components. Plasma volume and plasma taurine concentrations were established through pre- and post-trial blood samples.

RESULTS

Taurine supplementation increased WBSL by 26.6% and 5.1% (p = 0.035), LSR by 15.5% and 7.8% (p = 0.013), SGA (1 × 1 cm) by 32.2% and 29.9% (p < 0.001) and SGA (3 × 3 cm) by 22.1% and 17.1% (p = 0.015) during the fixed- and ramped-humidity exercise periods, respectively. Evaporative heat loss was enhanced by 27% (p = 0.010), heat-storage reduced by 72% (p = 0.024) and Pcrit was greater in taurine vs placebo (25.0-mmHg vs 21.7-mmHg; p = 0.002).

CONCLUSION

Taurine supplementation increased sweating responses during fixed Ḣprod in hot conditions, prior to substantial heat strain and before the breakpoint of compensability, demonstrating improved thermoregulatory capacity. The enhanced evaporative cooling and reduced heat-storage delayed the subsequent upward inflection in Tcore-represented by a greater Pcrit-and offers a potential dietary supplementation strategy to support thermoregulation.

Nutritional habits of professional team sport athletes: An insight into the carbohydrate, fluid, and caffeine habits of English Premier League football players during match play

To better understand the in-match fuelling practices of elite football players and compare against current guidelines, we quantified the carbohydrate, fluid, and caffeine intake of players from an English Premier League club (n = 22) during 90 min of competitive match-play. Mean carbohydrate intake across match-play was 17 ± 11 g.h-1 with players demonstrating a preference towards CHO-containing fluids (58%) when compared with semi-solids (38%) and solids (14%), respectively. CHO intake was significantly lower than reported by players (17 ± 11 vs 24.8 ± 11 g.h-1, p < 0.001) during initial consultation. Fluid was ingested at a rate of 0.45 ± 0.14 L.h-1, with 54, 40 and 6% of ingested fluid coming from water, carbohydrate, and electrolyte-only solutions, respectively. The majority of players (91%) met the UEFA guidelines for fluid consumption. Of the players who consumed caffeine across match-play (55%) the average dose was 233 ± 148 mg (2.8 ± 1.1 mg.kg-1 body mass [BM]), which meets the UEFA consensus guidelines for caffeine intake. Caffeine capsules (42%) and caffeine containing fluids (30%) were the preferred format prior to the warm-up whilst caffeine gum was exclusively used prior to kick-off and during the half-time period (100%). We conclude that 81% of the total playing squad failed to meet the current UEFA CHO intake recommendations of 30-60 g.h-1, which may be attributed to the preference towards fluid-based CHOs as the chosen format of delivery.

Health status and heat preparation at a UCI World Tour multistage cycling race

OBJECTIVES

To assess health status and heat preparation of cyclists at the 2019 Tour Down Under and determine the alignment of heat mitigation strategies with current recommendations.

DESIGN

Cohort study.

METHODS

Twenty-three (17 % participation rate) male World Tour cyclists from five teams and 10 countries completed a pre-competition questionnaire evaluating exertional heat illness (EHI) history, pre-race health status, and heat mitigation and recovery strategies use. Associations between arrival days pre-competition, years as professional, nationality, team, history of EHI symptoms and diagnosis on heat mitigation and recovery strategy utilisation were assessed.

RESULTS

65 % of cyclists reported previously experiencing one or more EHI symptom (cramping: 48 %) and 22 % a diagnosis of heat stroke. In the 10 days preceding the race, 26 % experienced one or more illness symptoms. 65 % trained in the heat (acclimatisation 8-25 days; acclimation: 3-7 days), which was associated with team (P = 0.047, ϕc = 0.61), nationality (P = 0.009, ϕc = 0.86) and EHI symptoms history (P = 0.058, ϕ = 0.43). All cyclists had a hydration plan, with links to team (0.5-1.0 L·h-1, P = 0.043, ϕc = 0.68) and EHI symptom history (1.0-1.5 L·h-1, P = 0.048, ϕ = 0.476). Most had pre-cooling (87 %) and mid-cooling (83 %) strategies, most commonly cold beverages (75 %) and neck collars (78 %), respectively. All cyclists planned on using at least one recovery strategy (massage: 87 %).

CONCLUSIONS

Our data indicate good alignment with current recommendations for competing in the heat, particularly for hydration, cooling and recovery strategies. Whilst the proportion of cyclists engaging in heat acclimation/acclimatisation is encouraging, greater awareness on adapting and implementing heat training is required.

Influence of acute heat mitigation strategies on core temperature, heart rate and aerobic performance in females: A systematic literature review

This review examined the effect of acute heat mitigation strategies on physiological strain and exercise performance in females exercising in the heat. Three databases were searched for original research with an acute heat mitigation (intervention) and control strategy in active females and reporting core temperature, heart rate and/or aerobic exercise performance/capacity with ≥ 24°C wet bulb globe temperature. Hedges' g effect sizes were calculated to evaluate outcomes. Thirteen studies (n = 118) were included. Most studies that applied an acute heat mitigation strategy to females did not reduce thermal (9/10) or cardiovascular (6/6) strain or improve exercise performance/capacity (8/10). The most effective strategies for attenuating thermal strain were pre-cooling with ice-slurry (effect size = -2.2 [95% CI, -3.2, -1.1]) and ice-vests (-1.9 [-2.7, -1.1]), and pre- and per-cooling with an ice-vest (-1.8 [-2.9, -0.7]). Only pre-cooling with an ice-vest improved running performance (-1.8 [-2.9, -0.7]; ~0.43 min) whilst sodium hyperhydration improved cycling capacity at 70% V O2peak (0.8 [0.0, 1.6]; ~20.1 min). There is currently limited research on acute heat mitigation strategies in females, so the evidence for the efficacy is scarce. Some studies show beneficial effects with ice-slurry, ice-vests and sodium hyperhydration, which can guide future research to support female exercise performance in the heat.

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.

Different humidity environments do not affect the subsequent exercise ability of college football players after aerobic high-intensity interval training

Previous studies have explored the effect of differing heat and relative humidity (RH) environments on the performance of multiple anaerobic high-intensity interval training (HIIT). Still, its impact on physiological responses and performance following aerobic HIIT has not been well studied. This study examined the effects of differing RH environments on physiological responses and performance in college football players following HIIT. Twelve college football completed HIIT under four different environmental conditions: (1) 25 °C/20% RH (Control group); (2) 35 °C/20% RH (H20 group); (3) 35 °C/40% RH (H40 group); (4) 35 °C/80% RH (H80 group). The heart rate (HR), mean arterial pressure (MAP), lactate, tympanic temperature (TT), skin temperature (TS), thermal sensation (TS), and rating of perceived exertion (RPE) were recorded continuously throughout the exercise. The heart rate variability (HRV): including root mean squared differences of the standard deviation (RMSSD)、standard deviation differences of the standard deviation (SDNN)、high frequency (HF), low frequency (LF), squat jump height (SJH), cycling time to exhaustion (TTE), and sweat rate (SR) were monitored pre-exercise and post-exercise. The HR, MAP, lactate, TT, Ts, TS, and RPE in the 4 groups showed a trend of rapid increase, then decreased gradually. There was no significant difference in HR, MAP, TT, or RPE between the 4 groups at the same time point (p > 0.05), in addition to this, when compared to the C group, the lactate, Ts, TS in the other 3 groups significant differences were observed at the corresponding time points (p < 0.05). The RMSSD, SDNN, HF, and LF levels in the 4 groups before exercise were not significantly different. The RMSSD and HF in the H40 and H80 groups were significantly decreased and other HRV indicators showed no significant difference after exercise. In sports performance measurement, the SJH and TTE were significantly decreased, but there was no significant difference in the 4 groups. The SR was no significant difference in the 4 groups after exercise. In conclusion, heat and humidity environments elicited generally greater physiological effects compared with the normal environment but did not affect sports performance in college football players.

The Importance of Maintaining and Improving a Healthy Gut Microbiota in Athletes as a Preventive Strategy to Improve Heat Tolerance and Acclimatization

Exposure to passive heat (acclimation) and exercise under hot conditions (acclimatization), known as heat acclimation (HA), are methods that athletes include in their routines to promote faster recovery and enhance physiological adaptations and performance under hot conditions. Despite the potential positive effects of HA on health and physical performance in the heat, these stimuli can negatively affect gut health, impairing its functionality and contributing to gut dysbiosis. Blood redistribution to active muscles and peripheral vascularization exist during exercise and HA stimulus, promoting intestinal ischemia. Gastrointestinal ischemia can impair intestinal permeability and aggravate systemic endotoxemia in athletes during exercise. Systemic endotoxemia elevates the immune system as an inflammatory responses in athletes, impairing their adaptive capacity to exercise and their HA tolerance. Better gut microbiota health could benefit exercise performance and heat tolerance in athletes. This article suggests that: (1) the intestinal modifications induced by heat stress (HS), leading to dysbiosis and altered intestinal permeability in athletes, can decrease health, and (2) a previously acquired microbial dysbiosis and/or leaky gut condition in the athlete can negatively exacerbate the systemic effects of HA. Maintaining or improving the healthy gut microbiota in athletes can positively regulate the intestinal permeability, reduce endotoxemic levels, and control the systemic inflammatory response. In conclusion, strategies based on positive daily habits (nutrition, probiotics, hydration, chronoregulation, etc.) and preventing microbial dysbiosis can minimize the potentially undesired effects of applying HA, favoring thermotolerance and performance enhancement in athletes.

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

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

Combining Heat and Altitude Training to Enhance Temperate, Sea-Level Performance

BACKGROUND

Repeated exposure to heat (ie, plasma volume expansion) or altitude (ie, increase in total hemoglobin mass), in conjunction with exercise, induces hematological adaptations that enhance endurance performance in each respective environment. Recently, combining heat and altitude training has become increasingly common for athletes preparing to compete in temperate, sea-level conditions.

PURPOSE

To review the physiological adaptations to training interventions combining thermal and hypoxic stimuli and summarize the implications for temperate, sea-level performance. Current Evidence: To date, research on combining heat and hypoxia has employed 2 main approaches: simultaneously combining the stressors during training or concurrently training in the heat and sleeping at altitude, sometimes with additional training in hypoxia. When environmental stimuli are combined in a training session, improvements in aerobic fitness and time-trial performance in temperate, sea-level conditions are generally similar in magnitude to those observed with heat, or altitude, training alone. Similarly, training in the heat and sleeping at altitude does not appear to provide any additional hematological or nonhematological benefits for temperate; sea-level performance relative to training in hot, hypoxic, or control conditions.

CONCLUSIONS

Current research regarding combined heat and altitude interventions does not seem to indicate that it enhances temperate, sea-level performance to a greater extent than "traditional" (heat or hypoxia alone) training approaches. A major challenge in implementing combined-stressor approaches lies in the uncertainty surrounding the prescription of dosing regimens (ie, exercise and environmental stress). The potential benefits of conducting heat and altitude exposure sequentially (ie, one after the other) warrants further investigation.

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.

Auditing the Representation of Females Versus Males in Heat Adaptation Research

The aim of this audit was to quantify female representation in research on heat adaptation. Using a standardized audit tool, the PubMed database was searched for heat adaptation literature from inception to February 2023. Studies were included if they investigated heat adaptation among female and male adults (≥18-50 years) who were free from noncommunicable diseases, with heat adaptation the primary or secondary outcome of interest. The number and sex of participants, athletic caliber, menstrual status, research theme, journal impact factor, Altmetric score, Field-Weighted Citation Impact, and type of heat exposure were extracted. A total of 477 studies were identified in this audit, including 7,707 participants with ∼13% of these being female. Most studies investigated male-only cohorts (∼74%, n = 5,672 males), with ∼5% (n = 360 females) including female-only cohorts. Of the 126 studies that included females, only 10% provided some evidence of appropriate methodological control to account for ovarian hormone status, with no study meeting best-practice recommendations. Of the included female participants, 40% were able to be classified to an athletic caliber, with 67% of these being allocated to Tier 2 (i.e., trained/developmental) or below. Exercise heat acclimation was the dominant method of heat exposure (437 interventions), with 21 studies investigating sex differences in exercise heat acclimation interventions. We recommend that future research on heat adaptation in female participants use methodological approaches that consider the potential impact of sexual dimorphism on study outcomes to provide evidence-based guidelines for female athletes preparing for exercise or competition in hot conditions.

Ad libitum ice slurry ingestion and half-marathon performance in a hot environment: A study comparing the effects of the amount and moment of ingestion between ice slurry and water at 37 °C

Ice slurry ingestion during prolonged exercises may improve performance in hot environments; however, the ideal amount and timing of ingestion are still uncertain. We determined whether ad libitum ice slurry ingestion influences physiological and perceptual variables and half-marathon performance while comparing the effects of the amount and moment of ingestion between ice slurry and water at 37 °C. Ten trained participants (28 ± 2 years; mean and SD) were required to run two half marathons while consuming either ice slurry (-1 °C; Ad-1) or water (37 °C; 37 CE) ad libitum. They then performed two other half marathons where, during one, they were required to ingest an amount of water equivalent to the amount consumed during the Ad-1 trial (Pro37), and in the other, to ingest ice slurry in the amount consumed during the 37 CE trial (Pro-1). During the half marathons, dry-bulb temperature and relative humidity were controlled at 33.1 ± 0.3 °C and 60 ± 3%, respectively. Ad-1 ingestion (349.6 ± 58.5 g) was 45% less than 37 CE ingestion (635.5 ± 135.8 g). Physical performance, heart rate, perceived exertion, body temperatures, and thermal perception were not influenced by the temperature or amount of beverage ingestion. However, a secondary analysis suggested that lower beverage ingestion was associated with improved performance (Ad-1 + Pro37 vs. 37 CE + Pro-1: -4.0 min, Cohen's d = 0.39), with a significant relationship between lower beverage ingestion and faster running time (b = 0.02, t = 4.01, p < 0.001). In conclusion, ice slurry ingestion does not affect performance or physiological or perceptual variables during a half marathon in a hot environment. Preliminary evidence suggests that lower beverage ingestion (ice slurry or warm water) is associated with improved performance compared to higher ingestion.

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

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

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.

Small Performance Effects of a Practical Mixed-Methods Cooling Strategy in Elite Team Sport Athletes

Purpose: The ingestion of ice slurry and application of ice towels can elicit favorable physiological, perceptual, and performance benefits when used individually; however, the combined use and effectiveness of these practical cooling strategies have not been assessed using a sport-specific performance test, based on actual match demands, in an elite team sport context. Methods: Ten non-heat acclimated elite male rugby sevens athletes undertook two cycling heat response tests (HRT) designed to be specific to the demands of rugby sevens in hot conditions (35°C, 80% rH). In a crossover design, the HRTs were conducted with (COOLING) and without (HOT) the combined use of internal (ice slushy ingestion) and external (application of ice towels to the head, neck, and face) pre- and per-cooling strategies. Physiological, perceptual, and performance variables were monitored throughout each HRT. Results: COOLING resulted in reductions in mean tympanic temperature (-0.4 ± 0.2°C; d = 1.18); mean heart rate (-5 ± 8 bpm; d = 0.53); thermal discomfort (-0.5 ± 0.9 AU; d = 0.48); and thirst sensation (-1.0 ± 1.1 AU; d = 0.61) during the HRT. COOLING also resulted in a small increase in 4-min time trial power output (by 7 ± 33 W, ~3%; d = 0.35) compared to HOT. Discussion: A combination of internal and external pre- and per-cooling strategies can result in a range of small physiological, perceptual, and performance benefits during a rugby sevens specific HRT, compared to undertaking no cooling. Practitioners should include such strategies when performing in hot conditions.

Sleep and global warming: How will we sleep when the Earth is hotter?

Societal concern about climate change and global warming has grown worldwide along with the concomitant awareness that health will be impacted deeply. Among living beings, humans have quite large capacities for adaptation to varied temperature conditions. Despite their tropical origin, they live under all Earth climates, such as polar, temperate, altitude, arid, and tropical climates, using a wide range of behavioral and physiological adaptive responses. We address the adaptive abilities of human sleep-wake regulation and its interplay with thermoregulation under different natural climates. Sleep represents one-third of our living time and is also a major determinant of morbidity and mortality; shortening sleep duration increases mortality and multimorbidity. In addition, major advances in sleep neurology have occurred in the last decades. Some have been extensively reviewed, notably comparative sleep physiology among animals, allowing one to hypothesize about the functions of the different sleep states, as well as their relation to cognitive neuroscience or body biorhythms. However, the question of the sleep adaptive capacity of humans to global warming has barely been addressed. We examine "normal" sleep and thermoregulation in young adults residing in temperate conditions. We then review the sleep and thermoregulatory reactions under various climatic conditions, demonstrating the role of sleep changes as potent adaptive responses to living under natural hot climatic conditions. As a result, we show that humans are well-equipped to adapt to severe climates.

Impact of Preparticipation Hypohydration on Cognitive Performance and Concussion-like Symptoms in Recreational Athletes

BACKGROUND

Sports-related concussion is a relevant risk of contact sports, with several million cases per year worldwide. Prompt identification is crucial to prevent complications and late effects but may be impeded by an overlap with dehydration-associated impairment of cognitive function. Researchers have extensively studied the effects of pronounced dehydration in endurance sports, especially in the heat. However, little is known about the effects of isolated and mild dehydration.

METHODS

Healthy recreational athletes underwent a standardized fluid deprivation test. Hypohydration was assessed by bioelectrical impedance analysis (BIA) and laboratory testing of electrolytes and retention parameters. Participants underwent cardiopulmonary exercise testing (CPET) with a cycle ramp protocol. Each participant served as their own control undergoing CPET in a hypohydrated [HYH] and a euhydrated [EUH] state. Effects were assessed using a shortened version of Sport Concussion Assessment Tool 3 (SCAT3).

RESULTS

Fluid deprivation caused a mild (2%) reduction in body water, resulting in a calculated body mass loss of 0.8% without alterations of electrolytes, serum-osmolality, or hematocrit. Athletes reported significantly more (1.8 ± 2.2 vs. 0.4 ± 0.7; p < 0.01) and more severe (4.4 ± 6.2 vs. 1.0 ± 1.9; p < 0.01) concussion-like symptoms in a hypohydrated state. Balance was worse in HYH by trend with a significant difference for tandem stance (1.1 ± 1.3 vs. 0.6 ± 1.1; p = 0.02). No relevant differences were presented for items of memory and concentration.

CONCLUSIONS

Mild dehydration caused relevant alterations of concussion-like symptoms and balance in healthy recreational athletes in the absence of endurance exercise or heat. Further research is needed to clarify the real-life relevance of these findings and to strengthen the differential diagnosis of concussion.

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.

Impact of Preparticipating Hypohydration on Cardiopulmonary Exercise Capacity in Ambitious Recreational Athletes

BACKGROUND

Heat induces a thermoregulatory strain that impairs cardiopulmonary exercise capacity. The aim of the current study is to elucidate the effect of isolated dehydration on cardiopulmonary exercise capacity in a model of preparticipating hypohydration.

METHODS

Healthy recreational athletes underwent a standardised fluid deprivation test. Hypohydration was assessed by bioelectrical impedance analysis (BIA) and laboratory testing of electrolytes and retention parameters in the blood and urine. The participants underwent cardiopulmonary exercise testing (CPET) with a cycle ramp protocol. Each participant served as their own control undergoing CPET in a hypohydrated [HYH] and euhydrated [EUH] state.

RESULTS

Fluid deprivation caused a mild (2%) but significant reduction of body water (38.6 [36.6; 40.7] vs. 39.4 [37.4; 41.5] %; p < 0.01) and an increase of urine osmolality (767 [694; 839] vs. 537 [445; 629] mosm/kg; p < 0.01). Hypohydration was without alterations of electrolytes, serum osmolality or hematocrit. The oxygen uptake was significantly lower after hypohydration (-4.8%; p = 0.02 at ventilatory threshold1; -2.0%; p < 0.01 at maximum power), with a corresponding decrease of minute ventilation (-4% at ventilatory threshold1; p = 0.01, -3.3% at maximum power; p < 0.01). The power output was lower in hypohydration (-6.8%; p < 0.01 at ventilatory threshold1; -2.2%; p = 0.01 at maximum power).

CONCLUSION

Isolated hypohydration causes impairment of workload as well as peak oxygen uptake in recreational athletes. Our findings might indicate an important role of hypohydration in the heat-induced reduction of exercise capacity.

Beating the heat: military training and operations in the era of global warming

Global climate change has resulted in an increase in the number and intensity of environmental heat waves, both in areas traditionally associated with hot temperatures and in areas where heat waves did not previously occur. For military communities around the world, these changes pose progressively increasing risks of heat-related illnesses and interference with training sessions. This is a significant and persistent "noncombat threat" to both training and operational activities of military personnel. In addition to these important health and safety concerns, there are broader implications in terms of the ability of worldwide security forces to effectively do their job (particularly in areas that historically already have high ambient temperatures). In the present review, we attempt to quantify the impact of climate change on various aspects of military training and performance. We also summarize ongoing research efforts designed to minimize and/or prevent heat injuries and illness. In terms of future approaches, we propose the need to "think outside the box" for a more effective training/schedule paradigm. One approach may be to investigate potential impacts of a reversal of sleep-wake cycles during basic training during the hot months of the year, to minimize the usual increase in heat-related injuries, and to enhance the capacity for physical training and combat performance. Regardless of which approaches are taken, a central feature of successful present and future interventions will be that they are rigorously tested using integrative physiological approaches.

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.

Elite North American soccer performance in thermally challenging environments: An explorative approach to tracking outcomes

AIMS

The physiologic challenges related to performances in hot conditions calls for dedicated consideration when planning athlete training, although complete amelioration of the effects of heat may not be possible. We aimed to quantify within-subject correlations between different measures of environmental temperature and performance changes over multiple elite soccer competitions.

METHODS

Thirty-seven elite male soccer players (age: 26 ± 3.4 years, height: 171 ± 2 cm, body mass: 78 ± 7.1 kg) competed in North America over four seasons (range: 3 to 98 matches). Players wore global positioning system devices during games and reported differential-RPE immediately post game. Temperatures at kick-off, week average temperature, the difference between game-day and week average (DiffTemp), and heat index at kick-off were obtained. Within-player correlations were calculated using general linear models to quantify associations between fluctuations in temperature measures and physical and perceived outputs.

RESULTS

Correlations between total distance and the various temperature measures were trivial to small (range: -0.08 to 0.13, p=<0.001-0.02). Small negative correlations were found between all temperature measures except DiffTemp and high-speed running (HSR) (range: -0.17 to -0.14, p=<0.001). Most correlations between differential-RPE and temperature measures were trivial to small and not significant (r = 0.06 to 0.18 p = 0.03-0.92) although breathlessness-RPE and heat index showed a small significant association (P = 0.018).

CONCLUSION

Decrements in HSR appear to be associated with increased environmental temperature, however, these associations are small in magnitude.

Thermoregulatory, Cardiovascular and Perceptual Responses of Spectators of a Simulated Football Match in Hot and Humid Environmental Conditions

Major sporting events are often scheduled in thermally challenging environments. The heat stress may impact athletes but also spectators. We examined the thermal, cardiovascular, and perceptual responses of spectators watching a football match in a simulated hot and humid environment. A total of 48 participants (43 ± 9 years; n = 27 participants <50 years and n = 21 participants ≥50 years, n = 21) watched a 90 min football match in addition to a 15 min baseline and 15 min halftime break, seated in an environmental chamber (Tair = 31.9 ± 0.4 °C; RH = 76 ± 4%). Gastrointestinal temperature (Tgi), skin temperature (Tskin), and heart rate (HR) were measured continuously throughout the match. Mean arterial pressure (MAP) and perceptual parameters (i.e., thermal sensation and thermal comfort) were scored every 15 min. Tri (37.3 ± 0.4 °C to 37.4 ± 0.3 °C, p = 0.11), HR (76 ± 15 bpm to 77 ± 14 bpm, p = 0.96) and MAP (97 ± 10 mm Hg to 97 ± 10 mm Hg, p = 0.67) did not change throughout the match. In contrast, an increase in Tskin (32.9 ± 0.8 °C to 35.4 ± 0.3 °C, p < 0.001) was found. Further, 81% of participants reported thermal discomfort and 87% a (slightly) warm thermal sensation at the end of the match. Moreover, the thermal or cardiovascular responses were not affected by age (all p-values > 0.05). Heat stress induced by watching a football match in simulated hot and humid conditions does not result in substantial thermal or cardiovascular strain, whereas a significant perceptual strain was observed.

Tracking sleep in the field: sleep quality and sleep behaviours of elite track and field athletes during preparation and competition

Elite athletes are susceptible to inadequate sleep, which may peak during competition and be exacerbated by poor sleep behaviours. This study sought to characterise and compare the sleep quality and sleep behaviours of elite track and field athletes during preparation and major competitions. Forty elite international track and field athletes (50% female, aged 25.1 ± 3.9 years) completed the Athlete Sleep Screening Questionnaire and the Athlete Sleep Behaviour Questionnaire on three separate occasions: during habitual training, during a pre-meet training camp and during a major international competition. Overall, 62.5% of athletes reported at least mild sleep difficulty during competition. Athletes reported higher sleep difficulty and poorer sleep behaviour during major competitions and the pre-meet training camp compared to habitual training (P = .001-.025). No significant differences were observed between the training camp and major competition. Global sleep behaviour scores were underpinned by unique characteristics at each timepoint. Sleep behaviour (R² = .330, P = .017), injury status (R² = .253, P = .003) and major championship experience (R² = .113, P = .034) were associated with sleep difficulty during competition. Sleep quality and behaviours vary according to stage of the track and field season, providing a foundation for targeted intervention.

Risk factors for, and prediction of, exertional heat illness in Thoroughbred racehorses at British racecourses

The development of exertional heat illness (EHI) is a health, welfare and performance concern for racehorses. However, there has been limited multivariable assessment of the possible risk factors for EHI in racehorses, despite such information being vital for regulators to effectively manage the condition. Consequently, this study aimed to identify the risk factors associated with the occurrence of EHI in Thoroughbred racehorses and assess the ability of the risk factor model to predict the occurrence of EHI in racehorses to assist in early identification. Runners at British racecourses recorded in the British Horseracing Authority database between 1st July 2010 and 30th April 2018 were used to model the probability that a horse would present with EHI as a function of a suite of environmental, horse level and race level factors. EHI was reported in 0.1% of runners. Race distance, wet bulb globe temperature, preceding 5-day temperature average, occurrence of a previous EHI incident, going, year and race off time were identified as risk factors for EHI. The model performed better than chance in classifying incidents with a mean area under the receiver operating characteristic curve score of 0.884 (SD = 0.02) but had a large number of false positives. The results provide vital evidence for industry on the need to provide appropriate cool down facilities, identify horses that have repeated EHI incidents for early intervention, and collect new data streams such as on course wet bulb globe temperature measurements. The results are especially relevant as the sport is operating in a changing climate and must mitigate against more extreme and longer spells of hot weather.

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.

Reliability of Biomarkers of Physiological Stress at Rest and Post-exertional Heat Stress

The purpose of this study was to assess the reliability of blood biomarkers that can signify exercise-induced heat stress in hot conditions. Fourteen males completed two heat stress tests separated by 5-7 days. Venous blood was drawn pre- and post- heat stress for the concentration of normetanephrine, metanephrine, serum osmolality, copeptin, kidney-injury molecule 1, and neutrophil gelatinase-associated lipocalin. No biomarker, except copeptin, displayed systematic trial order bias (p≥0.05). Normetanephrine, copeptin and neutrophil gelatinase-associated lipocalin presented acceptable reliability (CV range: 0.9-14.3%), while greater variability was present in metanephrine, osmolality and kidney-injury molecule 1 (CV range: 28.6-43.2%). Normetanephrine exhibited the largest increase (p<0.001) in response to heat stress (trial 1=1048±461 pmol. L^-1; trial 2=1067±408 pmol. L^-1), whilst kidney-injury molecule 1 presented trivial changes (trial 1=-4±20 ng. L^-1; trial 2=2 ± 16 ng. L^-1, p>0.05). Normetanephrine, copeptin, and neutrophil gelatinase-associated lipocalin demonstrated good reliability and sensitivity to an acute bout of heat stress. These biomarkers may be suitable for application in laboratory and field research to understand the efficacy of interventions that can attenuate the risk of thermal injury whilst exercising in the heat.

Heat Acclimation Knowledge among Recreational Runners

Heat acclimation (HA) is the foremost method of preventing exertional heat illness during exercise in hot and humid environments. However, the prevalence of HA training and its associated knowledge is not currently known in recreational running populations. The purpose of this study was to determine the knowledge of recreational runners toward HA. A survey consisting of 38 questions that required approximately 10-15 min to complete was disseminated to running clubs throughout the Southeastern United States. Questions were designed to collect data on participant demographics, yearly training habits, and HA knowledge. Recreational runners (N = 125) demonstrated a lack of knowledge toward proper HA training and its associated benefits. Participants largely received HA advice from their peers (31.2%) and reported no professional guidance in their training (79.2%). Finally, participants' beliefs toward proper HA training differed among training groups with moderate and high groups perceiving greater frequency, miles/wk, and min/wk as appropriate for HA compared to the low group (p ≤ 0.05). Due to the warmer temperatures and higher relative humidity experienced in the southeastern, southwestern, and mid-Atlantic locations of the United States and throughout certain regions of the European Union, governing bodies in sport and exercise science should develop more educational initiatives to convey the importance and advantages of HA, especially when runners are training for major marathons that are typically held in the late spring and early fall seasons.

Taking the plunge: When is best for hot water immersion to complement exercise in heat and hypoxia

This investigation assessed the psycho-physiological and performance effects of hot water immersion (HWI) implemented either before or after a repeated-sprint training in hypoxia (RSH) session conducted in the heat. Ten participants completed three RSH trials (3 × 10 × 5-s sprints), conducted at 40°C and simulated altitude of 3000 m. A 30-min monitoring period preceded and followed all exercise sessions. In PRE, the pre-exercise period was HWI, and the post-exercise period was seated rest in temperate conditions. This combination was reversed in POST. In CON, participants were seated in temperate conditions for both periods. Compared to CON, PRE elicited a reduction in power output during each repeated-sprint set (14.8-16.2%, all p < 0.001), and a significantly higher core temperature (Tc) during the pre-exercise period and throughout the exercise session (p < 0.001 and p = 0.025, respectively). In POST, power output and Tc until the end of exercise were similar to CON, with Tc higher at the conclusion of the post-exercise period (p < 0.001). Time across the entire protocol spent ≥38.5°C Tc was significantly longer in PRE (48.1 ± 22.5 min) than POST (31.0 ± 11.3 min, p = 0.05) and CON (15.8 ± 16.3 min, p < 0.001). Employing HWI following RSH conducted in the heat provides effective outcomes regarding physiological strain and cycling performance when compared to pre-exercise or no HWI.

Exertional heat stroke in sport and the military: epidemiology and mitigation

NEW FINDINGS

What is the topic of this review? Exertional heat stroke epidemiology in sport and military settings, along with common risk factors and strategies and policies designed to mitigate its occurrence. What advances does it highlight? Individual susceptibility to exertional heat stroke risk is dependent on the interaction of intrinsic and extrinsic factors. Heat policies in sport should assess environmental conditions, as well as the characteristics of the athlete, clothing/equipment worn and activity level of the sport. Exertional heat stroke risk reduction in the military should account for factors specific to training and personnel.

ABSTRACT

Exertional heat illness occurs along a continuum, developing from the relatively mild condition of muscle cramps, to heat exhaustion, and in some cases to the life-threatening condition of heat stroke. The development of exertional heat stroke (EHS) is associated with an increase in core temperature stemming from inadequate heat dissipation to offset the rate of metabolically generated heat. Susceptibility to EHS is linked to the interaction of several factors including environmental conditions, individual characteristics, health conditions, medication and drug use, behavioural responses, and sport/organisational requirements. Two settings in which EHS is commonly observed are competitive sport and the military. In sport, the exact prevalence of EHS is unclear due to inconsistent exertional heat illness terminology, diagnostic criteria and data reporting. In contrast, exertional heat illness surveillance in the military is facilitated by standardised case definitions, a requirement to report all heat illness cases and a centralised medical record repository. To mitigate EHS risk, several strategies can be implemented by athletes and military personnel, including heat acclimation, ensuring adequate hydration, cold-water immersion and mandated work-to-rest ratios. Organisations may also consider developing sport or military task-specific heat stress policies that account for the evaporative heat loss requirement of participants, relative to the evaporative capacity of the environment. This review examines the epidemiology of EHS along with the strategies and policies designed to reduce its occurrence in sport and military settings. We highlight the nuances of identifying individuals at risk of EHS and summarise the benefits and shortcomings of various mitigation strategies.

Practical application of a mixed active and passive heat acclimation protocol in elite male Olympic team sport athletes

To investigate effectiveness and retention of heat acclimation (HA) integrated within an elite rugby sevens team training program, 12 elite male rugby sevens athletes undertook 10-days of mixed active/passive HA across two-weeks of normal training. Physiological and performance variables were assessed using a sport specific, repeated high-intensity heat-response test Pre-HA; after five days (Mid-HA); after 10 days (Post-HA); and 16-days post-HA (Decay). Resting, submaximal, and end-exercise core temperature were lower at Mid-HA (≤ -0.26 °C; d ≥-0.47), Post-HA (≤ -0.30 °C; d ≥-0.72), and Decay (≤ -0.29 °C; d ≥-0.56), compared to Pre-HA. Sweat rate was greater Post-HA compared to Pre-HA (0.3 ± 0.3 L·hr-1; d =0.63). Submaximal HR was lower at Mid (-9 ±4 bpm; d =-0.68) and Post-HA (-11 ± 4 bpm; d =-0.90) compared to Pre-HA. Mean and peak 6-s power output improved Mid-HA (83 ± 52 W; 112 ± 67 W; d ≥0.47) and Post-HA (125 ± 62 W; 172 ± 85 W; d ≥0.72) compared to Pre-HA. Improvements in HR and performance persisted at Decay (d ≥0.66). The initial five days of mixed-methods HA elicited many typical HA adaptations, with an additional five days eliciting further thermoregulatory, sudomotor, and performance improvements. Adaptations were well-retained after 16-days of normal training, without any further heat stimulus. The trial was retrospectively registered with the Australian New Zealand Clinical Trials Registry (ACTRN12622000732785).

Heat Adaptation and Nutrition Practices: Athlete and Practitioner Knowledge and Use

PURPOSE

To survey elite athletes and practitioners to identify (1) knowledge and application of heat acclimation/acclimatization (HA) interventions, (2) barriers to HA application, and (3) nutritional practices supporting HA.

METHODS

Elite athletes (n = 55) and practitioners (n = 99) completed an online survey. Mann-Whitney U tests (effect size [ES; r]) assessed differences between ROLE (athletes vs practitioners) and CLIMATE (hot vs temperate). Logistic regression and Pearson chi-square (ES Phi [ϕ]) assessed relationships.

RESULTS

Practitioners were more likely to report measuring athletes' core temperature (training: practitioners 40% [athletes 15%]; P = .001, odds ratio = 4.0, 95% CI, 2%-9%; competition: practitioners 25% [athletes 9%]; P = .020, odds ratio = 3.4, 95% CI, 1%-10%). Practitioners (55% [15% athletes]) were more likely to perceive rectal as the gold standard core temperature measurement site (P = .013, ϕ = .49, medium ES). Temperate (57% [22% hot]) CLIMATE dwellers ranked active HA effectiveness higher (P < .001, r = .30, medium ES). Practitioners commonly identified athletes' preference (48%), accessibility, and cost (both 47%) as barriers to HA. Increasing carbohydrate intake when training in the heat was more likely recommended by practitioners (49%) than adopted by athletes (26%; P = .006, 95% CI, 0.1%-1%). Practitioners (56% [28% athletes]) were more likely to plan athletes' daily fluid strategies, adopting a preplanned approach (P = .001; 95% CI, 0.1%-1%).

CONCLUSIONS

Practitioners, and to a greater extent athletes, lacked self-reported key HA knowledge (eg, core temperature assessment/monitoring methods) yet demonstrated comparatively more appropriate nutritional practices (eg, hydration).

Repeated-sprint training in heat and hypoxia: effect of exercise-to-rest ratio

AbstractThe aim of this study was to investigate acute performance and physiological responses to the manipulation of exercise-to-rest ratio (E:R) during repeated-sprint hypoxic training (RSH) in hot conditions. Twelve male team-sport players completed two experimental sessions at a simulated altitude of ∼3000 m (F_IO₂ 0.144), air temperature of 40°C and relative humidity of 50%. Exercise involved either 3×5×10-s (E:R_1:2) or 3×10×5-s (E:R_1:4) maximal cycling sprints interspersed with active recoveries at 120W (20-s between sprints, 2.5 and 5-min between sets for E:R_1:2 and E:R_1:4 respectively). Sessions were matched for overall sprint and total session duration (47.5-min). Peak and mean power output, and total work were greater in E:R_1:4 than E:R_1:2 (p < 0.05). Peak core temperature was significantly higher in E:R_1:4 than E:R_1:2 (38.44 ± 0.33 vs. 38.20 ± 0.35°C, p = 0.028). Muscle deoxygenation magnitude during sprints was greater in E:R_1:2 (28.2 ± 1.6 vs. 22.4 ± 4.6%, p < 0.001), while muscle reoxygenation did not differ between conditions (p > 0.05).These results indicate E:R_1:4 increased mechanical power output and core temperature compared to E:R_1:2. Both protocols had different effects on measures of muscle oxygenation, with E:R_1:2 generating greater muscle oxygen extraction and E:R_1:4 producing more muscle oxygenation flux, which are both important signals for peripheral adaptation. We conclude that the E:R manipulation during RSH in the heat might be used to target different physiological and performance outcomes, with these findings forming a strong base for future mechanistic investigation.

Addition of in‐Play Cooling Breaks During Intermittent Exercise while Wearing Lacrosse Uniforms in the Heat Attenuates Increases in Rectal Temperature

To reduce the risk of heat-related illness, appropriate body cooling strategies are needed during men’s lacrosse competitions in hot-humid environments. The current study investigated whether additional in-play cooling breaks would attenuate the core (rectal) body temperature rise during exercise designed to mimic the activity patterns of a men’s lacrosse match while wearing uniform in hot-humid environments. In randomized and counterbalanced order, ten physically active men completed two experimental trials comprising four quarters of 15-min intermittent cycling exercises separated by a 10-min simulated half-time break and two 2-min quarter breaks in a climate chamber (35°C, 50% relative humidity). The two trials included a regular simulated match condition (REG) and a water break time out condition (WBTO) that included additional 2-min breaks 7.5-min into each quarter. Rectal temperature was significantly lower (p=0.017) in the WBTO condition (38.23±0.23°C) compared with the REG condition (38.50±0.46°C) at the end of the 4th quarter. In addition, ratings of perceived exertion, thermal sensation, heart rate and the physiological strain index were significantly lower (all p<0.05) in the WBTO condition compared with the REG condition. The current results indicated that, even when wearing lacrosse uniforms, the implementation of WBTO attenuated the core body temperature elevation only in the latter stages of an intermittent cycling exercise protocol designed to mimic the exercise pattern of a men’s lacrosse match. Therefore, WBTO may provide a feasible and effective cooling strategy for lacrosse players to reduce the risk of heat-illness.

Road to Tokyo 2020 Olympic Games: Training Characteristics of a World Class Male Triathlete

There is a growing interest in the scientific literature for reporting top-class endurance athletes training programs. This case study reports on the training program of a world-class male triathlete preparing to compete in the Tokyo 2020 Olympic Games. A macrocycle of 43 weeks is presented. The triathlete performed 14.74 ± 3.01 h of weekly endurance training volume. Training intensity distribution (TID) was 81.93% ± 6.74%/7.16% ± 2.03%/10.91% ± 6.90% for zones 1 (low intensity, VT2) respectively. Pyramidal TID model is observed during the initial stages of the periodization and Polarized TID model is observed at the end of the macrocycle. The triathlete's peak ⩒O2 was increased by 20% on cycling and by 14% on running. Peak power was increased by 3.13% on cycling test and peak speed by 9.71% on running test. Finally, the triathlete placed 12th in Olympic distance and 10th in Mixed Relay in Tokyo 2020 Olympic games.

Competing in Hot Conditions at the Tokyo Olympic Games: Preparation Strategies Used by Australian Race Walkers

Introduction

The Tokyo 2021 Olympic Games was anticipated to expose athletes to the most challenging climatic conditions experienced in the history of the modern Olympic Games. This study documents strategies executed by Australian endurance athletes during the team holding camp and Olympic Games experiences, including (1) baseline physiological data, training data, and heat acclimation/acclimatization practices; (2) pre- and in-race cooling and nutritional strategies, and (3) Olympic Games race performance data.

Methods

Six athletes (three males, three females; age 24 ± 4 years; VO_2max 63.2 ± 8.7 mL⋅kg^–1⋅min^–1; sum of 7 skinfolds 53.1 ± 23.4 mm) were observed prior to and during the team holding camp held in Cairns, QLD, Australia. Athletes completed 6–7 weeks of intermittent heat acclimation training, utilizing a combination of 2–4 passive and active acclimation sessions per week. Active acclimation was systematically increased via exposure time, exercise intensity, temperature, and humidity. In the team holding camp, athletes undertook a further 23 heat acclimatization training sessions over 18 days in a continuous fashion. Hyperhydration (using sodium and glycerol osmolytes), and internal and external pre-and in-race cooling methods were also utilized. A low energy availability intervention was implemented with two athletes, as a strategy to periodize ideal race body composition. Race performance data and environmental conditions from the 2021 Olympic Games were also documented.

Results

The highest values for aerobic capacity were 63.6 mL⋅kg^–1⋅min^–1 for female race walkers and 73.7 mL⋅kg^–1⋅min^–1 for males. Training volume for the six athletes was the highest in the second week of the team holding camp, and training intensity was lowest in the first week of the team holding camp. Performance outcomes included 6th place in the women’s 20 km event (1:30:39), which was within 2% of her 20 km personal best time, and 8th place in the men’s 50 km event (3:52:01), which was a personal best performance time.

Conclusion

Periodized training, heat acclimation/acclimatization, cooling and nutritional strategies study may have contributed to the race outcomes in Olympic Games held hot, humid conditions, for the race walkers within this observational study.

A 32-day long fieldwork in Antarctica improves heat tolerance during physical exercise

We evaluated the influence of a 32-day camping in Antarctica on physical performance and exercise-induced thermoregulatory responses. In Brazil, before and after the Antarctic camping, the volunteers performed an incremental exercise at temperate conditions and, two days later, an exercise heat stress protocol (45-min running at 60% of maximum aerobic speed, at 31°C and 60% of relative humidity). In Antarctica, core temperature was assessed on a day of fieldwork, and average values higher than 38.5°C were reported. At pre- and post-Antarctica, physiological (whole-body and local sweat rate, number of active sweat glands, sweat gland output, core and skin temperatures) and perceptual (thermal comfort and sensation) variables were measured. The Antarctic camping improved the participants' performance and induced heat-related adaptations, as evidenced by sweat redistribution (lower in the chest but higher in grouped data from the forehead, forearm, and thigh) and reduced skin temperatures in the forehead and chest during the exercise heat stress protocol. Notwithstanding the acclimatization, the participants did not report differences of the thermal sensation and comfort. In conclusion, staying in an Antarctic camp for 32 days improved physical performance and elicited physiological adaptations to heat due to the physical exertion-induced hyperthermia in the field.

Effect of Acetaminophen on Endurance Cycling Performance in Trained Triathletes in Hot and Humid Conditions

PURPOSE

The effect of acetaminophen (ACT, also known as paracetamol) on endurance performance in hot and humid conditions has been shown previously in recreationally active populations. The aim of this study was to determine the effect of ACT on physiological and perceptual variables during steady-state and time-trial cycling performance of trained triathletes in hot and humid conditions.

METHODS

In a randomized, double-blind crossover design, 11 triathletes completed ∼60 minutes steady-state cycling at 63% peak power output followed by a time trial (7 kJ·kg body mass-1, ∼30 min) in hot and humid conditions (∼30°C, ∼69% relative humidity) 60 minutes after consuming either 20 mg·kg body mass-1 ACT or a color-matched placebo. Time-trial completion time, gastrointestinal temperature, skin temperature, thermal sensation, thermal comfort, rating of perceived exertion, and fluid balance were recorded throughout each session.

RESULTS

There was no difference in performance in the ACT trial compared with placebo (P = .086, d = 0.57), nor were there differences in gastrointestinal and skin temperature, thermal sensation and comfort, or fluid balance between trials.

CONCLUSION

In conclusion, there was no effect of ACT (20 mg·kg body mass-1) ingestion on physiology, perception, and performance of trained triathletes in hot and humid conditions, and existing precooling and percooling strategies appear to be more appropriate for endurance cycling performance in the heat.

Head, Face and Neck Cooling as Per-cooling (Cooling During Exercise) Modalities to Improve Exercise Performance in the Heat: A Narrative Review and Practical Applications

It is well known that uncompensable heat stress greatly impairs endurance and team sport-related performance because an increase in the core temperature directly induces a greater magnitude of the central fatigue in the heat than in thermal neutral environments. Numerous studies have been conducted in an attempt to discover reliable cooling strategies for improving endurance performance and repeated sprint ability while exercising in the heat. Whole-body pre-cooling has been shown to improve endurance performance in both dry and humid heat. Despite this, the reduction in thermal perceptions associated with pre-cooling gradually narrows during intense exercise. Hence, effective per-cooling strategies to improve athletic performance in the heat are required. Unfortunately, due to practical issues, adopting pre-cooling approaches as a per-cooling (cooling during exercise) modality to improve athletic performance is impractical. Thus, we sought to examine the impact of head, neck and face cooling on athletic performance in heat. According to current evidence, cooling the head, neck and face reduced local skin temperature in the areas where cooling was applied, resulting in improved local perceptual sensations. In the heat, neck cooling during exercise improves athletic performance in both endurance and team sports athletes. Furthermore, from a practical standpoint, neck cooling is preferred over head, face and combined head/face and neck cooling for both endurance and team sport athletes in the heat. Nonetheless, for all athletes who have access to water, face cooling is a recommended cooling strategy. There is a lack of research on the systematic selection of per-cooling modalities to improve athletic performance based on environmental conditions and the nature of sports. In addition, powerful but portable head, neck and face cooling systems are urgently needed to assist athletes in improving their performance in hot conditions.

Submaximal Fitness Tests in Team Sports: A Theoretical Framework for Evaluating Physiological State

Team-sports staff often administer non-exhaustive exercise assessments with a view to evaluating physiological state, to inform decision making on athlete management (e.g., future training or recovery). Submaximal fitness tests have become prominent in team-sports settings for observing responses to a standardized physical stimulus, likely because of their time-efficient nature, relative ease of administration, and physiological rationale. It is evident, however, that many variations of submaximal fitness test characteristics, response measures, and monitoring purposes exist. The aim of this scoping review is to provide a theoretical framework of submaximal fitness tests and a detailed summary of their use as proxy indicators of training effects in team sports. Using a review of the literature stemming from a systematic search strategy, we identified five distinct submaximal fitness test protocols characterized in their combinations of exercise regimen (continuous or intermittent) and the progression of exercise intensity (fixed, incremental, or variable). Heart rate-derived indices were the most studied outcome measures in submaximal fitness tests and included exercise (exercise heart rate) and recovery (heart rate recovery and vagal-related heart rate variability) responses. Despite the disparity between studies, these measures appear more relevant to detect positive chronic endurance-oriented training effects, whereas their role in detecting negative transient effects associated with variations in autonomic nervous system function is not yet clear. Subjective outcome measures such as ratings of perceived exertion were less common in team sports, but their potential utility when collected alongside objective measures (e.g., exercise heart rate) has been advocated. Mechanical outcome measures either included global positioning system-derived locomotor outputs such as distance covered, primarily during standardized training drills (e.g., small-sided games) to monitor exercise performance, or responses derived from inertial measurement units to make inferences about lower limb neuromuscular function. Whilst there is an emerging interest regarding the utility of these mechanical measures, their measurement properties and underpinning mechanisms are yet to be fully established. Here, we provide a deeper synthesis of the available literature, culminating with evidence-based practical recommendations and directions for future research.

Effects of Half-Time Cooling Using a Fan with Skin Wetting on Thermal Response During Intermittent Cycling Exercise in the Heat

The present study investigated the effects of half-time (HT) break cooling using a fan and damp sponge on physiological and perceptual responses during the 2 ^nd half of a repeated-sprint exercise in a hot environment. Eight physically active men performed a familiarization trial and two experimental trials of a 2×30-min intermittent cycling exercise protocol with a 15-min HT break in hot conditions (35°C, 50% relative humidity). Two experimental trials were conducted in random order: skin wetting with a fan (FAN _wet ) and no cooling (CON). During the 2 ^nd half, a repeated-sprint cycling exercise was performed: i. e., 5 s of maximal pedaling (body weight×0.075 kp) every minute, separated by 25 s of unloaded pedaling (80 rpm) and 30 s of rest. Rectal temperature, skin temperature (chest, forearm, thigh, and calf), heart rate, physiological strain index, rating of perceived exertion, thermal sensation, and comfort were significantly improved in the FAN _wet condition (P<0.05). There was no significant difference in the repeated-sprint cycling exercise performance between conditions. The results suggest that skin wetting with a fan during the HT break is a practical and effective cooling strategy for mitigating physiological and perceptual strain during the 2 ^nd half in hot conditions.

The relationship between physiological and performance variables during a hot/humid international rugby sevens tournament

To characterise physiological responses to competing in an international rugby sevens tournament played in hot/humid conditions, core temperature (T_c) and T_c predictors were collected from 11 elite men's rugby sevens athletes competing in the Oceania sevens tournament in Suva, Fiji. T_c, body mass change, sweat electrolytes, playing minutes, total running distance, high speed running distance (HSD), psychrometric wet bulb temperature and exertional heat illness symptoms were collected pre, during and post games. Linear mixed-models were used to assess the effect of T_c predictors on post-game T_c, along with differences in T_c across measurement periods. Compared to baseline on both tournament days, mean T_c was higher during all between game (recovery) measures (all d >1.30, p <0.01). On both tournament days, eight athletes reached a post-game T_c >39.0°C, with several athletes reaching >39.0°C during warm-ups. Mean post-game T_c was related to playing minutes, total running distance, HSD, and post warm-up T_c (all p < 0.01). The T_c during warm-ups and games regularly exceeded those demonstrated to be detrimental to repeated sprint performance (> 39°C). Warm-up T_c represents the easiest predictor of post-game T_c to control via time/intensity modulation and the use of appropriate pre- and per-cooling strategies. Practitioners should be prepared to modulate warm-ups and other heat preparation strategies based on likely environmental conditions during hot/humid tournaments.