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

Compromised heat loss leads to a delayed ice slurry induced reduction in heat storage

Compromised heat loss due to limited convection and evaporation can increase thermal strain. We aimed to determine the effectiveness of ice slurry ingestion to reduce thermal strain following hyperthermia in a state of compromised heat loss. Twelve healthy males (age: 25 ± 4y) underwent hot water immersion to elevate rectal temperature (Trec) by 1.82 ± 0.08°C on four occasions. In the subsequent 60-min of seated recovery, participants ingested either 6.8 g·kg-1 of ice slurry (-0.6°C) or control drink (37°C) in ambient conditions (21 ± 1°C, 39 ± 10% relative humidity), wearing either t-shirt and shorts (2 trials: ICE and CON) or a whole-body sweat suit (2 trials: ICE-SS and CON-SS). Trec and mean skin temperature (Tsk) were recorded and a two-compartment thermometry model of heat storage was calculated. Heat storage was lower in ICE compared with CON at 20-40min (p ≤ 0.044, d ≥ 0.88) and for ICE-SS compared with CON-SS at 40-60 min (p ≤ 0.012, d ≥ 0.93). Trec was lower in ICE compared with CON from 30-60min (p ≤ 0.034, d ≥ 0.65), with a trend for a reduced Trec in ICE-SS compared with CON-SS at 40min (p = 0.079, d = 0.60). A greater Tsk was found in ICE-SS and CON-SS compared with ICE and CON (p < 0.001, d ≥ 3.37). A trend for a lower Tsk for ICE compared with CON was found at 20-40min (p ≤ 0.099, d ≥ 0.53), no differences were found for ICE-SS vs CON-SS (p ≥ 0.554, d ≤ 0.43). Ice slurry ingestion can effectively reduce heat storage when heat loss through convection and evaporation is compromised, relevant to those wearing personal protective equipment or those with compromised sweat loss. Compromised heat loss delays the reduction in heat storage, possibly related to ice slurry ingestion not lowering Tsk.

Single Session Intermittent Heat Exposure With More Frequent and Shorter Cooling Breaks Facilitates Greater Training Intensity and Elicits Physiological Responses Comparable to Continuous Heat Exposure

PURPOSE

To investigate the influence of shorter, more frequent rest breaks with per-cooling as an alternative heat-acclimation session on physiological, perceptual, and self-paced maximal cycling performance, compared with continuous heat exposure.

METHODS

Thirteen participants completed 1 continuous and 3 intermittent-heat-exposure (IHE) maximal self-paced cycling protocols in a random order in heat (36 °C, 80% relative humidity): 1 × 60-minute exercise (CON), 3 × 20-minute exercise with 7.5-minute rest between sets (IHE-20), 4 × 15-minute exercise with 5-minute rest between sets (IHE-15), and 6 × 10-minute exercise with 3-minute rest between sets (IHE-10). Mixed-method per-cooling (crushed-ice ingestion and cooling vest) was applied during rest periods of all IHE protocols.

RESULTS

Total distance completed was greater in IHE-10, IHE-15, and IHE-20 than in CON (+11%, +9%, and +8%, respectively), with no difference observed between IHE protocols. Total time spent above 38.5 °C core temperature was longer in CON compared with IHE-15 and IHE-20 (+62% and +78%, respectively) but similar to IHE-10 (+5%). Furthermore, a longer time above 38.5 °C core temperature occurred in IHE-10 versus IHE-15 and IHE-20 (+54% and +69%, respectively). Sweat loss did not differ between conditions.

CONCLUSION

IHE with per-cooling may be a viable alternative heat-acclimation protocol in situations where training quality takes precedence over thermal stimulus or when both factors hold equal priority.

The impact of extreme heat on mass-gathering sporting events: Implications for Australia and other countries

OBJECTIVES

As temperatures increase across the globe due to climate change, human exposure to extreme heat is a public health challenge. During sporting events, athletes, officials, spectators, and staff are at risk of heat stress and resulting illness. The objective of this review was to explore the impact of heat on the health outcomes of these groups and the wider health system and discuss implications for outdoor mass-gathering sporting events in Australia.

DESIGN

A systematic review was undertaken to identify literature published from 2010 to 2023.

METHODS

Seven databases were searched: Web of Science, SportDiscus, Scopus, Medline, CINAHL, Emcare, and PsychInfo, for relevant key search terms such as heatwave, heat stress, extreme heat, stadium, arena, sports facilit*, sport, athletic, and Olympic. An inductive thematic analysis was undertaken. Articles were quality checked using Joanna Briggs Institute critical appraisal tools and data were extracted, tabulated, and synthesized.

RESULTS

Forty papers were included in the final analysis: 17 quantitative, and 23 descriptive and qualitative (including reviews). Health outcomes explored across the literature included exertional heat illness, exertional heat stroke, hyperthermia, and general heat related illness. Six recommendation themes emerged: planning, mitigation strategies, medical, policy, research, and education.

CONCLUSIONS

The impact of heat on health outcomes during sporting events is significant, and should be considered by individuals, coaches, officials, and organizers before, during, and after mass-gathering sporting events. These findings can inform evidence-based preparedness strategies to protect the health of those attending and competing in mass-gathering sporting events now and into the future.

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.

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.

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

BACKGROUND

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

HYPOTHESIS

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

STUDY DESIGN

Interventional study.

LEVEL OF EVIDENCE

Level 3.

METHODS

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

RESULTS

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

CONCLUSION

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

CLINICAL RELEVANCE

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

Twenty four-hour passive heat and cold exposures did not modify energy intake and appetite but strongly modify food reward

Effects of acute thermal exposures on appetite appear hypothetical in reason of very heterogeneous methodologies. The aim of this study was therefore to clearly define the effects of passive 24-h cold (16°C) and heat (32°C) exposures on appetitive responses compared with a thermoneutral condition (24°C). Twenty-three healthy, young and active male participants realised three sessions (from 13.00) in a laboratory conceived like an apartment dressed with the same outfit (Clo = 1). Three meals composed of three or four cold or warm dishes were served ad libitum to assess energy intake (EI). Leeds Food Preference Questionnaires were used before each meal to assess food reward. Subjective appetite was regularly assessed, and levels of appetitive hormones (acylated ghrelin, glucagon-like peptite-1, leptin and peptide YY) were assessed before and after the last meal (lunch). Contrary to the literature, total EI was not modified by cold or heat exposure (P = 0·120). Accordingly, hunger scores (P = 0·554) were not altered. Levels of acylated ghrelin and leptin were marginally higher during the 16 (P = 0·032) and 32°C (P < 0·023) sessions, respectively. Interestingly, implicit wanting for cold and low-fat foods at 32°C and for warm and high-fat foods at 16°C were increased during the whole exposure (P < 0·024). Moreover, cold entrées were more consumed at 32°C (P < 0·062) and warm main dishes more consumed at 16°C (P < 0·025). Thus, passive cold and hot exposures had limited effects on appetite, and it seems that offering some choice based on food temperature may help individuals to express their specific food preferences and maintain EI.

Improvements in Orthostatic Tolerance with Exercise Are Augmented by Heat Acclimation: A Randomized Controlled Trial

INTRODUCTION

Heat adaptation is protective against heat illness; however, its role in heat syncope, due to reflex mechanisms, has not been conclusively established. The aim of this study was to evaluate if heat acclimation (HA) was protective against heat syncope and to ascertain underlying physiological mechanisms.

METHODS

Twenty (15 males, 5 females) endurance-trained athletes were randomized to either 8 d of mixed active and passive HA (HEAT) or climatically temperate exercise (CONTROL). Before, and after, the interventions participants underwent a head up tilt (HUT) with graded lower body negative pressure (LBNP), in a thermal chamber (32.0 ± 0.3°C), continued until presyncope with measurement of cardiovascular parameters. Heat stress tests (HST) were performed to determine physiological and perceptual measures of HA.

RESULTS

There was a significant increase in orthostatic tolerance (OT), as measured by HUT/LBNP, in the HEAT group (preintervention; 28 ± 9 min, postintervention; 40 ± 7 min) compared with CONTROL (preintervention; 30 ± 8 mins, postintervention; 33 ± 5 min) ( P = 0.01). Heat acclimation resulted in a significantly reduced peak and mean rectal and skin temperature ( P < 0.01), peak heat rate ( P < 0.003), thermal comfort ( P < 0.04), and rating of perceived exertion ( P < 0.02) during HST. There was a significantly increased plasma volume (PV) in the HEAT group in comparison to CONTROL ( P = 0.03).

CONCLUSIONS

Heat acclimation causes improvements in OT and is likely to be beneficial in patients with heat exacerbated reflex syncope. Heat acclimation-mediated PV expansion is a potential physiological mechanism underlying improved OT.

Precooling via immersion in CO2-enriched water at 25°C decreased core body temperature but did not improve 10-km cycling time trial in the heat

This study compared the effects of precooling via whole-body immersion in 25°C CO2-enriched water (CO2WI), 25°C unenriched water (WI) or no cooling (CON) on 10-km cycling time trial (TT) performance. After 30 min of precooling (CO2WI, CON, WI) in a randomized, crossover manner, 11 male cyclists/triathletes completed 30-min submaximal cycling (65%VO2peak), followed by 10-km TT in the heat (35°C, 65% relative humidity). Average power output and performance time during TT were similar between conditions (p = 0.387 to 0.833). Decreases in core temperature (Tcore) were greater in CO2WI (-0.54 ± 0.25°C) than in CON (-0.32 ± 0.09°C) and WI (-0.29 ± 0.20°C, p = 0.011 to 0.022). Lower Tcore in CO2WI versus CON was observed at 15th min of exercise (p = 0.050). Skin temperature was lower in CO2WI and WI than in CON during the exercise (p < 0.001 to 0.031). Only CO2WI (1029 ± 305 mL) decreased whole-body sweat loss compared with CON (1304 ± 246 mL, p = 0.029). Muscle oxygenation by near-infrared spectroscopy (NIRS), thermal sensation, and thermal comfort were lower in CO2WI and WI versus CON only during precooling (p < 0.001 to 0.041). NIRS-derived blood volume was significantly lower in CO2WI and WI versus CON during exercise (p < 0.001 to 0.022). Heart rate (p = 0.998) and rating of perceived exertion (p = 0.924) did not differ between conditions throughout the experiment. These results suggested that CO2WI maybe more effective than WI for enhanced core body cooling and minimized sweat losses.

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.

Enhancing physiological recovery and subsequent exercise performance in the heat using a phase-change material cooling blanket

This study aimed to assess the effect of a phase-change material (PCM) cooling blanket for cooling between exercise bouts on recovery of physiological parameters and subsequent exercise performance in the heat. Eighteen male volunteers were recruited to participate in human trials involving two exhaustive treadmill running bouts (Bout1 for 3 km and Bout2 for 1.5 km) in a climate chamber (temperature = 33 °C; relative humidity = 40%). Participants were randomly subjected to one of two cooling conditions for a 10-min period between exercise bouts: CON: natural cooling; 10-min PCM: with a PCM cooling blanket for 10 min. Several physiological parameters including mean skin temperature (Tskin), oral temperature (Toral), core temperature (Tcore), heart rate (HR), mean arterial pressure (MAP), respiratory rate (RR), peripheral capillary oxygen saturation (SpO2), average running speed and rating of perceived exertion (RPE) scale score were analyzed. The results showed that compared to the CON group, participants in the 10-min PCM group had a significant lower Tskin, Tcore, HR and RR at post-cooling, as well as greater reductions in mean skin temperature (ΔTskin) and core temperature (ΔTcore) from post-Bout1 to post-cooling. Additionally, the 10-min PCM group exhibited significantly lower peak Tcore, peak HR and RPE scale score during Bout2, while the average running speed during Bout2 was significantly higher. The present study suggests that cooling with a PCM cooling blanket can enhance physiological recovery and subsequent exercise performance in the heat.

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

INTRODUCTION

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

METHOD

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

RESULTS

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

CONCLUSION

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

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.

Blood Lactate Responses of Male and Female Players Across an International Rugby Sevens Tournament

PURPOSE

This study investigated within- and between-matches blood lactate (La-) responses across an international Rugby Sevens tournament (5 matches over 2 d) in male and female players.

METHODS

Earlobe blood samples were taken from 25 professional players around matches: before warm-up (PRE), immediately upon finishing match participation (POST), and 30 minutes postmatch (30 min).

RESULTS

POST [La-] (mean [SD], range) for males was 10.3 (3.2; 2.9-20.2) mmol·L-1 and for females was 9.1 (2.3; 3.4-14.6) mmol·L-1. Linear mixed-effects models revealed a decrease in POST [La-] after match 5, compared to match 1. Increased PRE [La-] was found before match 2 (+0.8 [0.6-1.1] mmol·L-1), match 3 (+0.8 [0.5-1.1] mmol·L-1), and match 5 (+0.6 [0.4-0.9] mmol·L-1) compared to match 1 (all P < .001). The [La-] remained elevated at 30 min, compared to PRE (+1.7 [1.4-2.0] mmol·L-1, P < .001), with ∼20% of values persisting >4 mmol·L-1. Higher POST was observed in males compared to females (+1.6 [0.1-3.2] mmol·L-1, P = .042); however, no differences between sexes were found across 30 min or PRE [La-]. No [La-] differences between positions (backs and forwards) were identified.

CONCLUSIONS

Lactate concentrations above 10 mmol·L-1 are required to effectively simulate the anaerobic demands of international Rugby Sevens matches. Practitioners are advised to individualize anaerobic training prescription due to the substantial variability observed within positional groups. Additionally, improving athletes' metabolic recovery capacity through training, nutrition, and recovery interventions may enhance physical preparation for subsequent matches within a day, where incomplete lactate clearance was observed.

Effects of ice slurry ingestion on body temperature and softball pitching performance in a hot environment: a randomized crossover trial

BACKGROUND

Although softball players are often required to play in hot environments, scarce evidence is available regarding the effects of ice slurry ingestion on body temperature and pitching performance in softball pitchers in a hot environment. Thus, this study investigated the effects of ice slurry ingestion before and between innings on body temperature and softball pitching performance in a hot environment.

METHODS

In a randomized crossover design, seven heat-acclimatized amateur softball pitchers (four males and three females) completed simulated softball games consisting of 15 best-effort pitches per inning for seven innings with between-pitch rest intervals of 20 s. Participants were assigned to either a control trial (CON: ingestion of 5.0 g·kg^-1 of cool fluid [9.8 ± 2.2 °C] before simulated softball games and 1.25 g·kg^-1 of cool fluid between inning intervals) or an ice trial (ICE: ingestion of ice slurry [- 1.2 ± 0.1 °C] based on the same timings and doses as the CON). Participants performed both trials in an outdoor ground during the summer season (30.8 ± 2.7 °C, 57.0 ± 7.9% relative humidity).

RESULTS

Ice slurry ingestion before the simulated softball game (pre-cooling) resulted in a greater reduction in rectal temperature compared with cool fluid ingestion (p = 0.021, d = 0.68). No significant differences were observed between the trials in rectal temperature changes during the simulated softball game (p > 0.05). Compared to the CON, heart rate during the game was significantly decreased (p < 0.001, d = 0.43), and handgrip strength during the game was significantly increased (p = 0.001, d = 1.16) in the ICE. Ratings of perceived exertion, thermal comfort, and thermal sensation were improved in the ICE compared to those in the CON (p < 0.05). Ball velocity and pitching accuracy were not affected by ICE.

CONCLUSIONS

Ice slurry ingestion before and between innings reduced thermal, cardiovascular, and perceptual strain. However, it did not affect softball pitching performance compared to cool fluid ingestion.

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.

Validation of upper thermal thresholds for outdoor sports using thermal physiology modelling

Thermal safety guidelines with upper thresholds aim to protect athletes' health, yet evidence-based sport-specific thresholds remain unestablished. Experimenting with athletes in severely hot conditions raises ethical concerns, so we used a thermo-physiological model to validate the thresholds of guidelines for outdoor sports. First, the reproducibility of the joint system thermoregulation model (JOS-3) of core temperature has been validated for 18 sports experiments (n = 213) and 11 general exercise experiments (n = 121) using the Bland - Altman analysis. Then, core temperatures were predicted using the JOS-3 in conditions corresponding to the upper thresholds, and if the 90th-99.7th percentile core temperature value (corresponding to 0.3%-10% of the participants) exceeded 40°C, the thresholds were judged as potentially hazardous. Finally, we proposed revisions for sports with potentially hazardous thresholds. As a result, the JOS-3 could simulate core temperature increases in most experiments (27/29) for six sports and general exercises with an accuracy of 0.5°C. The current upper thresholds for marathons, triathlons, and football are potentially hazardous. Suggested revisions, based on specified percentiles, include: Football: revise from wet bulb globe temperature (WBGT) 32°C to 29-31°C or not revise. Marathon: revise from WBGT 28°C to 24-27°C. Triathlon: revise from WBGT 32.2°C to 23-26°C. If conducting sports events under the revised upper thresholds proves difficult, taking measures for a possible high incidence of heat illness becomes crucial, such as placing additional medical resources, assisting heat acclimatization and cooling strategies for participants, and rule changes such as shorter match times and increased breaks.

Critical illness aspects of heatstroke: A hot topic

Heatstroke represents the most severe end of the heat illness spectrum, and is increasingly seen in those undergoing exercise or exertion ('exertional heatstroke') and those exposed to high ambient temperatures, for example in heatwaves ('classical heatstroke'). Both forms may be associated with significant thermal injury, leading to organ dysfunction and the need for admission to an intensive care unit. The process may be exacerbated by translocation of bacteria or endotoxin through an intestinal wall rendered more permeable by the hyperthermia. This narrative review highlights the importance of early diagnosis, rapid cooling and effective management of complications. It discusses the incidence, clinical features and treatment of heatstroke, and discusses the possible role of intestinal permeability and advances in follow-up and recovery of this condition. Optimum treatment involves an integrated input from prehospital, emergency department and critical care teams, along with follow-up by rehabilitation teams and, if appropriate, sports or clinical physiologists.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Do environmental temperatures and altitudes affect physical outputs of elite football athletes in match conditions? A systematic review of the 'real world' studies

Players involved in the various football codes compete throughout the calendar year around the world. Therefore, environmental stressors such as temperature and altitude should be considered in preparation for, and during, matches. We aimed to systematically review the observational and quasi-experimental studies that have been specifically designed to quantify the effects of temperature (hot or cold) high altitude on in-match physical performance indicators. A search of electronic databases (Web of Science, Scopus, SPORTDiscus, PubMed/MEDLINE) was conducted, with 19,424 papers identified as relevant. Following sifting in relation to the eligibility criteria, 12 papers were deemed directly relevant. The reviewed studies scored 6-9 (on a 0-9 scale) for quality assessment using a previously used scale. The major outcome variables relevant to the current review were total distance (m), high-speed running (m) and high-speed runs (count) measured during matches. Standardized effect sizes (ES) were heterogeneous across studies for total distance (ES: -0.96 to -0.14) and high-speed running (ES: -0.69 to 0.12) for >1000 m vs sea-level, time spent at the given altitude being a putative factor for this heterogeneity. Heat had mainly detrimental effects on performance, but ES were, again, heterogeneous across studies (ES: -1.25 to 0.26), dependent on temperature. Given the small number of studies that involved mostly male athletes, and large heterogeneity across studies, more research needs be conducted on physical performance in these environmental conditions, with attention paid to standardizing outcomes and broadening the approaches of studies to guide future decision-making in professional sporting environments.

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

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

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

Introduction

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

Methods

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

Results

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

Conclusion

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

Psychophysiological Responses to a Preseason Training Camp in High-Level Youth Soccer Players

PURPOSE

This study aimed to examine the responsiveness of commonly used measurement instruments to a short training camp by examining the time course of psychophysiological responses in high-level youth soccer players.

METHODS

Monitoring was carried out in 14 U15 male soccer players of 1 professional youth academy. Players provided data 3 days prior to (D - 3), during (D2-D4), and 1 (D + 1) and 4 days (D + 4) after the camp: 4 items for the Short Recovery and Stress Scale (SRSS), a countermovement jump (CMJ), and a submaximal run to assess exercise heart rate and heart-rate recovery. Training load during the camp followed an alternating low-high pattern, with lower training loads on D1 and D3 and higher training loads on D2 and D4.

RESULTS

Changes in SRSS physical performance capability, emotional balance, overall recovery, muscular stress, and overall stress were small to moderate on D3 and moderate to large on D + 1, while changes were trivial on D + 4. Some CMJ parameters related to the eccentric phase were slightly improved on D3, and these parameters were slightly impaired on D4. Changes in CMJ parameters were trivial on D + 1 and D + 4. After a moderate decrease in exercise heart rate on D3, there was a small decrease on D + 4 and a moderate increase in heart-rate recovery.

CONCLUSION

Measurement instruments such as the SRSS and submaximal runs can be used to monitor acute psychophysiological responses to load, while the CMJ may provide little insight during periods of intensified training load.

The Role of Sports Clothing in Thermoregulation, Comfort, and Performance During Exercise in the Heat: A Narrative Review

The aims of this review are to (1) summarise the current research of sports clothing as it relates to thermoregulation, comfort, and performance during exercise in the heat, (2) identify methodological limitations and gaps in the knowledge base of sports clothing, and (3) provide recommendations for exercise testing protocols to accurately assess the impact of sports clothing in athletic populations during exercise in the heat. Sports clothing consists of lightweight and breathable fabrics, surface treatments, and various designs which aim to enhance sweat evaporation and comfort during exercise in the heat. Sports clothing comprised of natural, synthetic, and chemically treated fabrics has been investigated during exercise of varying durations (15–120 min), intensities (20–70% VO₂ max) and types (fixed intensity, incremental, self-paced), and in an array of climatic conditions (18–40 °C, 20–60% relative humidity). To date, few studies have identified significant differences in thermo-physiological, perceptual, and performance measures between natural and synthetic fabrics or compared the effect of chemical treatments to their non-treated equivalent on such measures during exercise. Collectively, previous wearer trials have failed to replicate the upper limit of training and competition demands when assessing sports clothing in endurance-trained individuals who regularly train and compete in hot and humid climates. Clothing comfort has also been evaluated using simple scales which fail to capture intricate detail pertaining to psychological and sensorial parameters. The incorporation of protocols using hot and humid climates (≥ 30 °C, ≥ 70% relative humidity) and longer exercise durations (> 45 min) is warranted. Future research should also consider exploring the effect of sports clothing on thermal, physiological, perceptual, and performance measures between males and females, and assessing clothing comfort using a multi-dimensional approach.

Effects of mixed-cooling strategies on executive functions in simulated tennis in hot and humid conditions

This study aimed to investigate the effects of mixed-cooling strategies, which combines external (cooling vest + neck cooled collar) and internal cooling (cold sports drink ingestion) on measures of executive function during simulated tennis in hot/humid conditions. In a counterbalanced design (randomised order), eight males undertook two trials [one with the mixed-cooling strategy, (MCOOL condition) and another without (CON condition)] in a climate chamber (36.5°C, 50% relative humidity). All subjects completed an intermittent treadmill protocol simulating a three-set tennis match with a 90-second break during odd-numbered games and 120-second breaks between sets, in accordance with the activity profile and International Tennis Federation rules. The mixed-cooling strategies were adopted before test and break time during the simulated tennis match. Stroop task, 2-back task, More-odd shifting task, gastrointestinal temperature (Tgi), skin temperature, blood lactic acid (BLA), heart rate, urine specific gravity (USG), sweat rate (SR), thermal sensation (TS) and perceived exertion (RPE) were measured. Results showed that the mean exercise time was longer in the MCOOL condition than in the CON condition. The SR was greater in CON trial compared with that in MCOOL trial. Results of two-way analysis of variance with repeated measures revealed that time×condition interactions were significant in BLA, Stroop response time, and switch cost of the more-odd shifting task. There were main effects of condition for Tgi, HR, TS, RPE, BLA, Stroop response time, and switch cost of the more-odd shifting task. In a hot/wet environment, pre- and intermittent mixed-cooling strategies can significantly improve exercise time and measures of executive function of tennis players in a simulated tennis match.

Performance effects of internal pre- and per-cooling across different exercise and environmental conditions: A systematic review

Exercise in a hot and humid environment may endanger athlete's health and affect physical performance. This systematic review aimed to examine whether internal administration of ice, cold beverages or menthol solutions may be beneficial for physical performance when exercising in different environmental conditions and sports backgrounds. A systematic search was performed in PubMed, Web of Science, Scopus and SPORTDiscus databases, from inception to April 2022, to identify studies meeting the following inclusion criteria: healthy male and female physically active individuals or athletes (aged ≥18 years); an intervention consisting in the internal administration (i.e., ingestion or mouth rinse) of ice slush, ice slurry or crushed ice and/or cold beverages and/or menthol solutions before and/or during exercise; a randomized crossover design with a control or placebo condition; the report of at least one physical performance outcome; and to be written in English. Our search retrieved 2,714 articles in total; after selection, 43 studies were considered, including 472 participants, 408 men and 64 women, aged 18-42 years, with a VO2max ranging from 46.2 to 67.2 mL⋅kg-1⋅min-1. Average ambient temperature and relative humidity during the exercise tasks were 32.4 ± 3.5°C (ranging from 22°C to 38°C) and 50.8 ± 13.4% (varying from 20.0% to 80.0%), respectively. Across the 43 studies, 7 exclusively included a menthol solution mouth rinse, 30 exclusively involved ice slurry/ice slush/crushed ice/cold beverages intake, and 6 examined both the effect of thermal and non-thermal internal techniques in the same protocol. Rinsing a menthol solution (0.01%) improved physical performance during continuous endurance exercise in the heat. Conversely, the ingestion of ice or cold beverages did not seem to consistently increase performance, being more likely to improve performance in continuous endurance trials, especially when consumed during exercises. Co-administration of menthol with or within ice beverages seems to exert a synergistic effect by improving physical performance. Even in environmental conditions that are not extreme, internal cooling strategies may have an ergogenic effect. Further studies exploring both intermittent and outdoor exercise protocols, involving elite male and female athletes and performed under not extreme environmental conditions are warranted. Systematic review registration: [https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021268197], identifier [CRD42021268197].

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).

Case Report: Countermeasures Against Heat and Coronavirus for Japanese Athletes at the Tokyo 2020 Olympics and Paralympic Games

The Tokyo 2020 Olympics and Paralympic Games were held in the hottest environment in the history of the games. Additionally, the worldwide coronavirus disease 2019 (COVID-19) pandemic necessitated daily polymerase chain reaction (PCR) testing during the games, wearing a mask became mandatory publicly, and it was an unheard and unique Olympic with no spectators. Heat acclimation, hydration, and body cooling are essential for safe and high-performance activities in hot environments. In 2015, the Japan Institute of Sports Sciences launched the “Heat Countermeasure Project” to conduct experiments and practical research on heat countermeasures and investigate issues related to heat countermeasures in each athletic event. The results obtained were proposed to various Japan national sports teams, and support for heat countermeasures for the Tokyo 2020 games was promoted in consultation with national federations. Furthermore, due to the COVID-19 pandemic, infectious disease countermeasures for the Tokyo 2020 Games during support were a must. Moreover, athletes, coaches, and team staff could not avoid implementing heat countermeasures while adopting measures against infectious diseases. This study aimed to clarify the issues faced with heat countermeasures and report on heat acclimation training and cooling support efforts, considering measures against infectious diseases.

The Type of Per-Cooling Strategies Currently Employed by Competitive and Professional Cyclists-Triathletes During Training and Competition Are Condition (Dry vs. Humid) Dependant

Purpose

To investigate cooling strategies employed by athletes (cyclists-triathletes) during training and competition in hot and dry (HD) and hot and humid (HH) conditions.

Methods

Thirty-five athletes completed an online questionnaire on the type, timing, and justification of cooling strategies employed during past training and/or competitions in HD and HH conditions. In addition, 3 athletes also completed a one-to-one follow-up interview.

Results

Comparisons between strategies employed in all conditions were based on N = 14 (40%). Cold-water pouring was the most employed (N = 4; 21%) strategy during training and/or competing in hot conditions. The timing of the strategies employed was based on pitstops only (N = 7; 50%). The justification for strategies employed was based on trial and error (N = 9, 42.85%: N = 10, 47.61%). All athletes rated strategies employed as 1 (“not effective for minimising performance impairments and heat-related illnesses”). Comparisons between HD and HH were based on N = 21 (60%), who employed different strategies based on condition. Cold-water ingestion was the most employed (N = 9, 43%) strategy in HD, whereas a combination of cold-water ingestion and pouring was the most employed (N = 9, 43%) strategy in HH. The timing of strategies employed in the HD split was pre-planned by distance but was modified based on how athletes felt during (N = 8, 38%), and pre-planned by distance and pit stops (N = 8, 38%). The timing of strategies employed in HH was pre-planned based on distance and how athletes felt during (N = 9, 42%). About 57% (N = 12) of the 60% (N = 21) perceived effectiveness in HD and HH as 3 (“Sometimes effective and sometimes not effective”), whereas 43% (N = 9) of the 60% (N = 21) perceived effectiveness in HD and HH as 4 (“Effective for minimising performance impairments”).

Conclusion

Cold-water ingestion is the preferred strategy by athletes in HD compared to a combination of cold-water ingestion and pouring in HH conditions. All strategies were pre-planned and trialled based on distance and how athletes felt during training and/or competition. These strategies were perceived as effective for minimising performance impairments, but not heat-related illnesses. Future studies should evaluate the effectiveness of these cooling strategies on performance and thermoregulatory responses in HD and HH conditions.

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.

Increased air temperature decreases high-speed, but not total distance, in international field hockey

This study investigated the effect of heat stress on locomotor activity within international field hockey at team, positional and playing-quarter levels. Analysis was conducted on 71 matches played by the Malaysia national men’s team against 24 opponents. Fixtures were assigned to match conditions, based on air temperature [COOL (14 ± 3°C), WARM (24 ± 1°C), HOT (27 ± 1°C), or VHOT (32 ± 2°C), p < 0.001]. Relationships between locomotor metrics and air temperature (AIR), absolute and relative humidity, and wet bulb globe temperature (WBGT) were investigated further using correlation and regression analyses. Increased AIR and WBGT revealed similar correlations (p < 0.01) with intensity metrics; high-speed running (AIR r = −0.51, WBGT r = −0.45), average speed (AIR r = −0.48, WBGT r = −0.46), decelerations (AIR r = −0.41, WBGT r = −0.41), sprinting efforts (AIR r = −0.40, WBGT r = −0.36), and sprinting distance (AIR r = −0.37, WBGT r = −0.29). In comparison to COOL, HOT, and VHOT matches demonstrated reduced high-speed running intensity (−14–17%; p < 0.001), average speed (−5-6%; p < 0.001), sprinting efforts (−17%; p = 0.010) and decelerations per min (−12%; p = 0.008). Interactions were found between match conditions and playing quarter for average speed (+4-7%; p = 0.002) and sprinting distance (+16-36%; p < 0.001), both of which were higher in the fourth quarter in COOL versus WARM, HOT and VHOT. There was an interaction for “low-speed” (p < 0.001), but not for “high-speed” running (p = 0.076) demonstrating the modulating effect of air temperature (particularly >25°C) on pacing within international hockey. These are the first data demonstrating the effect of air temperature on locomotor activity within international men’s hockey, notably that increased air temperature impairs high-intensity activities by 5–15%. Higher air temperatures compromise high-speed running distances between matches in hockey.

Impact of thermal sensation on exercise performance in the heat: a Thermo Tokyo sub-study

PURPOSE

Thermal perception, including thermal sensation (TS), influences exercise performance in the heat. TS is a widely used measure and we examined the impact of initial TS (iTS) on performance loss during exercise in simulated Tokyo environmental conditions among elite athletes.

METHODS

105 Elite outdoor athletes (endurance, skill, power and mixed trained) participated in this crossover study. Participants performed a standardized exercise test in control (15.8 ± 1.2 °C, 55 ± 6% relative humidity (RH)) and simulated Tokyo (31.6 ± 1.0 °C, 74 ± 5% RH) conditions to determine performance loss. TS was assessed ± 5 min prior to exercise (iTS) and every 5 min during the incremental exercise test (TS). Based on iTS in the Tokyo condition, participants were allocated to a neutral (iTS = 0, n = 11), slightly warm (iTS = 1, n = 50), or warm-to-hot (iTS = 2/3, n = 44) subgroup.

RESULTS

For the whole cohort iTS was 1 [1-2] and TS increased to 3 [3-3] at the end of exercise in the Tokyo condition. Average performance loss was 26.0 ± 10.7% in the Tokyo versus control condition. The slightly warm subgroup had less performance loss (22.3 ± 11.3%) compared to the warm-to-hot subgroup (29.4 ± 8.5%, p = 0.003), whereas the neutral subgroup did not respond different (28.8 ± 11.0%, p = 0.18) from the slightly warm subgroup.

CONCLUSION

iTS impacted the magnitude of performance loss among elite athletes exercising in hot and humid conditions. Athletes with a warm-to-hot iTS had more performance loss compared to counterparts with a slightly warm iTS, indicating that pre-cooling strategies and/or heat acclimation may be of additional importance for athletes in the warm-to-hot iTS group to mitigate the impact of heat stress.

Commercially available carbohydrate drink with menthol fails to improve thermal perception or cycling exercise capacity in males

The purpose of this double-blinded, crossover randomized and counterbalanced study was to compare the effects of ingesting a tepid commercially available carbohydrate-menthol-containing sports drink (menthol) and an isocaloric carbohydrate-containing sports drink (placebo) on thermal perception and cycling endurance capacity "in a simulated home virtual cycling environment". It was hypothesized that the addition of menthol would improve indicators of thermal perception and improve endurance exercise capacity. Twelve healthy, endurance-trained males (age 29 ± 5 years, height 181 ± 6 cm, body mass 79 ± 2 kg and V̇O₂max 57.3 ± 6.4 mL kg^-1 min^-1) completed two experimental trials on a stationary bicycle without external air flow. Each trial consisted of (1) cycling for 60 min at 90% of the first ventilatory threshold while receiving a fixed amount of menthol or placebo every 10 min followed immediately by (2) cycling until volitional exhaustion (TTE) at 105% of the intensity corresponding to the respiratory compensation point. TTE did not differ between both conditions (541 ± 177 and 566 ± 150 s for menthol and placebo; p > 0.05) and neither did ratings of perceived thermal comfort or thermal sensation (p > 0.05). Also, the rectal temperature at the end of TTE was comparable between menthol and placebo trials (38.7 ± 0.2°C and 38.7 ± 0.3°C, respectively; p > 0.05). The present results demonstrate that the addition of menthol to commercially available sports drink does not improve thermal comfort or endurance exercise capacity during ∼65 min of intense virtual cycling.

Thermoregulatory responses in persons with lower-limb amputation during upper-limb endurance exercise in a hot and humid environment

BACKGROUND

Persons with an amputation may have an increased heat strain due to reduced surface area. However, there is limited evidence on the thermoregulatory responses in persons with lower-limb amputation (LLA). Although a previous study reported no difference in their rectal temperatures (Tres) in a hot environment, suggesting compensatory sweating of the intact limb, we examined the thermoregulatory responses of such persons in a hot and humid environment.

OBJECTIVE

To compare the thermoregulatory responses-through changes in Tre, sweat, and oxygen uptake (O2)-between persons with LLA and able-bodied (AB) individuals, in hot and humid environments.

STUDY DESIGN

A nonrandomized control trial.

METHODS

Nine AB men (AB group) and nine persons with LLA group performed the arm ergometer exercise at 60% peak power output intensity for 60 min in a hot and humid environment, and they were tested before and after performing. The O2, Tre and skin temperature, and total body sweating, and local sweating during exercise were measured and compared between the groups.

RESULTS

The changes in O2 and Tre after the endurance exercise did not differ between the groups (ΔTre: AB group, 1.1°C ± 0.5°C; LLA group, 1.2°C ±0.3 °C; P = 0.65), whereas the amount of local sweating of the chest (group effect, P < 0.01 by two-way analysis of variance [group × time], the group effect size was medium, η2 = 0.10) and dehydration rate (AB group, 1.5% ± 0.5%; LLA group, 2.1% ± 0.5%; P = 0.03) were higher in the LLA than in the AB group.

CONCLUSIONS

We compared the thermoregulatory responses of persons with LLA with those of AB individuals in hot and humid environments. Core body temperatures of persons with LLAs during endurance exercise were not different from those of AB men even in hot and humid environments. We found compensatory increases in the sweat rate of the chest and increased dehydration rate in persons with LLA. More sweat potentially means that athletes with LLA need to drink more fulids.

Performance and thermoregulation of Dutch Olympic and Paralympic athletes exercising in the heat: Rationale and design of the Thermo Tokyo study: The journal Temperature toolbox

The environmental conditions during the Tokyo Olympic and Paralympic Games are expected to be challenging, which increases the risk for participating athletes to develop heat-related illnesses and experience performance loss. To allow safe and optimal exercise performance of Dutch elite athletes, the Thermo Tokyo study aimed to determine thermoregulatory responses and performance loss among elite athletes during exercise in the heat, and to identify personal, sports-related, and environmental factors that contribute to the magnitude of these outcomes. For this purpose, Dutch Olympic and Paralympic athletes performed two personalized incremental exercise tests in simulated control (15°C, relative humidity (RH) 50%) and Tokyo (32°C, RH 75%) conditions, during which exercise performance and (thermo)physiological parameters were obtained. Thereafter, athletes were invited for an additional visit to conduct anthropometric, dual-energy X-ray absorptiometry (DXA), and 3D scan measurements. Collected data also served as input for a thermophysiological computer simulation model to estimate the impact of a wider range of environmental conditions on thermoregulatory responses. Findings of this study can be used to inform elite athletes and their coaches on how heat impacts their individual (thermo)physiological responses and, based on these data, advise which personalized countermeasures (i.e. heat acclimation, cooling interventions, rehydration plan) can be taken to allow safe and maximal performance in the challenging environmental conditions of the Tokyo 2020 Olympic and Paralympic Games.

The Effect of Medium-Term Sauna-Based Heat Acclimation (MPHA) on Thermophysiological and Plasma Volume Responses to Exercise Performed under Temperate Conditions in Elite Cross-Country Skiers

The influence of a series of ten sauna baths (MPHA) on thermophysiological and selected hematological responses in 14 elite cross-country skiers to a submaximal endurance exercise test performed under thermoneutral environmental conditions was studied. Thermal and physiological variables were measured before and after the exercise test, whereas selected hematological indices were studied before, immediately after, and during recovery after a run, before (T1) and after sauna baths (T2). MPHA did not influence the baseline internal, body, and skin temperatures. There was a decrease in the resting heart rate (HR: p = 0.001) and physiological strain (PSI: p = 0.052) after MPHA and a significant effect of MPHA on systolic blood pressure (p = 0.03), hematological indices, and an exercise effect but no combined effect of treatments and exercise on the tested variables. A positive correlation was reported between PSI and total protein (%ΔTP) in T2 and a negative between plasma volume (%ΔPV) and mean red cellular volume (%ΔMCV) in T1 and T2 in response to exercise and a positive one during recovery. This may suggest that MPHA has a weak influence on body temperatures but causes a moderate decrease in PSI and modifications of plasma volume restoration in response to exercise under temperate conditions in elite athletes.

Roundtable on Preseason Heat Safety in Secondary School Athletics: Heat Acclimatization

OBJECTIVE

To provide best-practice recommendations for developing and implementing heat-acclimatization strategies in secondary school athletics.

DATA SOURCES

An extensive literature review on topics related to heat acclimatization and heat acclimation was conducted by a group of content experts. Using the Delphi method, action-oriented recommendations were developed.

CONCLUSIONS

A period of heat acclimatization consisting of ≥14 consecutive days should be implemented at the start of fall preseason training or practices for all secondary school athletes to mitigate the risk of exertional heat illness. The heat-acclimatization guidelines should outline specific actions for secondary school athletics personnel to use, including the duration of training, the number of training sessions permitted per day, and adequate rest periods in a cool environment. Further, these guidelines should include sport-specific and athlete-specific recommendations, such as phasing in protective equipment and reintroducing heat acclimatization after periods of inactivity. Heat-acclimatization guidelines should be clearly detailed in the secondary school's policy and procedures manual and disseminated to all stakeholders. Heat-acclimatization guidelines, when used in conjunction with current best practices surrounding the prevention, management, and care of secondary school student-athletes with exertional heat stroke, will optimize their health and safety.

Head pre-cooling improves 5-km time-trial performance in male amateur runners in the heat

This study aimed to evaluate the effect of head pre-cooling on the 5-km time-trial performance of amateur runners in the heat. In a counterbalanced design, 15 male amateur runners (22.6 ± 3.5 y; VO₂ max in heat 42.3 ± 4.4 mLO₂ /kg/min) completed two 5-km time trials performed in the heat (35°C, 50% relative humidity). In one trial (HCOOL), participants underwent 20 min of head cooling in a temperate environment (23°C, 70% relative humidity) prior to exercise. In another trial (CON), exercise was preceded by 20 min of rest under the same temperature conditions. Exercise time was shorter in HCOOL (25 min and 36 s ± 3 min) compared to CON (27 ± 3 min; p = 0.02). Rectal temperature was reduced during the pre-exercise intervention in HCOOL (p < 0.001), but not in CON (p = 0.55). Relative changes in rectal temperature and mean head temperature were lower throughout HCOOL when compared with CON condition (p = 0.005 and p = 0.022, respectively). Mean skin temperature, heart rate, and rating of perceived exertion did not differ between HCOOL and CON conditions throughout exercise (p = 0.20, p = 0.52 and 0.31, respectively). Thermal comfort was lower in HCOOL condition in pre-exercise (p = 0.014) with no differences observed throughout exercise (p = 0.61). 5-km running performance in a hot environment was improved after a 20-min head cooling intervention, suggesting that this method may be practical as pre-cooling strategy and easily administered to both professional and amateur runners alike.

Effect of ice slurry ingestion on core temperature and blood pressure response after exercise in a hot environment

Delays in the restoration of thermoregulation after exercise in a hot environment has been associated with post-exercise hypotension. This study tested the hypothesis that simultaneous internal cooling and rehydration by ingesting ice slurry prevents the excessive decrease in mean arterial pressure (MAP) and promotes recovery of core and skin temperatures in male athletes. Seven male athletes participated in this randomized controlled trial with a crossover design. The participants ran on a treadmill at 75% of their maximal oxygen uptake in the heat (35 °C, 60% relative humidity), up to exhaustion. Immediately after exercise, participants ingested either 4 g⋅kg ^-1 body weight of ice slurry (0.5 °C, ICE) or a control beverage (28 °C, CON). The participants then recovered by sitting for 20 min. We measured participants' rectal temperature (T_re), skin temperature (T_sk), mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), total peripheral resistance (TPR), and physiological strain index (PSI) before exercise (Pre), after running to exhaustion (PEx), and at 0 (P0), 10 (P10), and 20 (P20) minutes after ice slurry or control beverage ingestion. MAP, CO, HR, TPR, or PSI did not change significantly during the recovery period. At P10 and P20, T_re and T_sk significantly decreased in the ICE group compared to the CON group (p < 0.05). These results suggested that ingestion of ice slurry, post-exercise, promoted core and skin temperature recovery but did not affect the central and peripheral cardiovascular responses during the acute recovery period.

Additional Clothing Increases Heat Load in Elite Female Rugby Sevens Players

PURPOSE

To determine whether elite female rugby sevens players are exposed to core temperatures (Tc) during training in the heat that replicate the temperate match demands previously reported and to investigate whether additional clothing worn during a hot training session meaningfully increases the heat load experienced.

METHODS

A randomized parallel-group study design was employed, with all players completing the same approximately 70-minute training session (27.5°C-34.8°C wet bulb globe temperature) and wearing a standardized training ensemble (synthetic rugby shorts and training tee [control (CON); n = 8]) or additional clothing (standardized training ensemble plus compression garments and full tracksuit [additional clothing (AC); n = 6]). Groupwise differences in Tc, sweat rate, GPS-measured external locomotive output, rating of perceived exertion, and perceptual thermal load were compared.

RESULTS

Mean (P = .006, ηp2=.88) and peak (P < .001, ηp2=.97) Tc were higher in AC compared with CON during the training session. There were no differences in external load (F4,9 = 0.155, P = .956, Wilks Λ = 0.935, ηp2=.06) or sweat rate (P = .054, Cohen d = 1.09). A higher rating of perceived exertion (P = .016, Cohen d = 1.49) was observed in AC compared with CON. No exertional-heat-illness symptomology was reported in either group.

CONCLUSIONS

Player Tc is similar between training performed in hot environments and match play in temperate conditions when involved for >6 minutes. Additional clothing is a viable and effective method to increase heat strain in female rugby sevens players without compromising training specificity or external locomotive capacity.

Is there meaningful influence from situational and environmental factors on the physical and technical activity of elite football players? Evidence from the data of 5 consecutive seasons of the German Bundesliga

The study aimed to identify the effects of situational (match location, match outcome and strength of team/opponent team) and environmental (ambient temperature, relative humidity, WBGT, ground and weather condition) factors on the physical and technical activity of elite football on individual playing positions. Physical and technical activity were collected from 779 football players competing in the German Bundesliga during 5 domestic seasons, from 2014/2015 to 2018/2019, totalling 1530 matches. The data on players' physical and technical activity was taken from the IMPIRE AG system. Based on the available data, 11 variables were selected to quantify the match activity profiles of players. The results showed that situational variables had major effects on the technical performance (especially number of passes performed) but minor effects on physical performance. In turn, among the analysed environmental factors, temperature is the most sensitive, which affects the Total Distance and Sprint Efforts of players in all five positions. This investigation demonstrated that, given that passing is a key technical activity in modern football, players and training staff should be particularly aware that passing maybe affected by situational variables. Professional players are able to react and adapt to various environmental conditions, modifying physical activity depending on the needs in German Bundesliga. These results could help coaches and analysts to better understand the influences of situational and environmental variables on individual playing positions during the evaluation of players' physical and technical performance.

A Mixed-Method Approach of Pre-Cooling Enhances High-Intensity Running Performance in the Heat

We investigated whether single or combined methods of pre-cooling could affect high-intensity exercise performance in a hot environment. Seven male athletes were subjected to four experimental conditions for 30 min in a randomised order. The four experimental conditions were: 1) wearing a vest cooled to a temperature of 4 ℃ (Vest), 2) consuming a beverage cooled to a temperature of 4 ℃ (Beverage), 3) simultaneous usage of vest and consumption of beverage (Mix), and 4) the control trial without pre-cooling (CON). Following those experimental conditions, they exercised at a speed of 80% VO₂max until exhaustion in the heat (38.1 ± 0.6 ℃, 55.3 ± 0.3% RH). Heart rate (HR), rectal temperature (T_core), skin temperature (T_skin), sweat loss (SL), urine specific gravity (USG), levels of sodium (Na⁺) and potassium (K⁺), rating of perceived exertion (RPE), thermal sensation (TS), and levels of blood lactic acid ([Bla]) were monitored. Performance was improved using the mixed pre-cooling strategy (648.43 ± 77.53 s, p = 0.016) compared to CON (509.14 ± 54.57 s). T_core after pre-cooling was not different (Mix: 37.01 ± 0.27 ℃, Vest: 37.19 ± 0.33 ℃, Beverage: 37.03 ± 0.35 ℃) in all cooling conditions compared to those of CON (37.31 ±0.29 ℃). A similar T_core values was achieved at exhaustion in all trials (from 38.10 ℃ to 39.00 ℃). No difference in the level of USG was observed between the conditions. Our findings suggest that pre-cooling with a combination of cold vest usage and cold fluid intake can improve performance in the heat.

Hydration and cooling in elite athletes: relationship with performance, body mass loss and body temperatures during the Doha 2019 IAAF World Athletics Championships

Purpose

To characterise hydration, cooling, body mass loss, and core (T_core) and skin (T_sk) temperatures during World Athletics Championships in hot-humid conditions.

Methods

Marathon and race-walk (20 km and 50 km) athletes (n=83, 36 women) completed a pre-race questionnaire. Pre-race and post-race body weight (n=74), T_core (n=56) and T_sk (n=49; thermography) were measured.

Results

Most athletes (93%) had a pre-planned drinking strategy (electrolytes (83%), carbohydrates (81%)) while ice slurry was less common (11%; p<0.001). More men than women relied on electrolytes and carbohydrates (91%–93% vs 67%–72%, p≤0.029). Drinking strategies were based on personal experience (91%) rather than external sources (p<0.001). Most athletes (80%) planned pre-cooling (ice vests (53%), cold towels (45%), neck collars (21%) and ice slurry (21%)) and/or mid-cooling (93%; head/face dousing (65%) and cold water ingestion (52%)). Menthol usage was negligible (1%–2%). Pre-race T_core was lower in athletes using ice vests (37.5°C±0.4°C vs 37.8°C±0.3°C, p=0.024). T_core (pre-race 37.7°C±0.3°C, post-race 39.6°C±0.6°C) was independent of event, ranking or performance (p≥0.225). Pre-race T_sk was correlated with faster race completion (r=0.32, p=0.046) and was higher in non-finishers (did not finish (DNF); 33.8°C±0.9°C vs 32.6°C±1.4°C, p=0.017). Body mass loss was higher in men than women (−2.8±1.5% vs −1.3±1.6%, p<0.001), although not associated with performance.

Conclusion

Most athletes’ hydration strategies were pre-planned based on personal experience. Ice vests were the most adopted pre-cooling strategy and the only one minimising T_core, suggesting that event organisers should be cognisant of logistics (ie, freezers). Dehydration was moderate and unrelated to performance. Pre-race T_sk was related to performance and DNF, suggesting that T_sk modulation should be incorporated into pre-race strategies.

Cooling During Endurance Cycling in the Heat: Blunted Core Temperature but Not Inflammatory Responses

PURPOSE

This study assessed the effects of cooling during endurance cycling (percooling) on changes in core body temperature (Tcore), inflammatory, and metabolic responses.

METHODS

A total of 12 male cyclists (peak oxygen uptake 60 [4] mL·kg-1·min-1) completed a 60-minute constant workload trial (55% of peak power output and ambient temperature 30.4°C [0.6°C]) in a randomized order both with (ICE) and without (CON) an ice vest. An ingestible capsule was used to measure Tcore. Blood samples were collected immediately before and after each trial to determine concentrations of blood lactate, serum cortisol, interleukin-6, and reactive oxygen and nitrogen species.

RESULTS

Tcore increased statistically (P < .001) both in CON (7.0% [1.4%], effect size [ES] = 6.3) and ICE (5.1% [1.1%], ES = 5.7). The increase in CON was statistically larger compared with ICE (P = .006, ES = 1.4). Concentrations of blood lactate (CON: 163% [63%], ES = 1.3; ICE: 149% [91%], ES = 1.3), cortisol (CON: 138% [123%], ES = 1.7; ICE: 81% [102%], ES = 1.0), and interleukin-6 (CON: 661% [324%], ES = 2.1; ICE: 624% [368%], ES = 1.2) statistically increased in both conditions (P < .01) to a similar extent. In addition, reactive oxygen and nitrogen species statistically decreased in both conditions (CON: -19.2% [14.9%], P = .002, ES = 0.9; ICE: -15.1% [16.5%], P = .02, ES = 0.9). No correlations were found between the changes of Tcore and blood parameters across the conditions.

CONCLUSIONS

Despite attenuated Tcore, similar inflammatory and metabolic responses were observed. Our findings suggest percooling to be a promising strategy to attenuate thermal strain without compromising physiological function.

Comparison of two different cooling systems in alleviating thermal and physiological strain during prolonged exercise in the heat

This study compared the efficacy of an ice vest comprising of water (WATER) or a water-carbon (CARBON) emulsion on thermophysiological responses to strenuous exercise in the heat. Twelve male cyclists completed three 50-minute constant workload trials (55% of peak power output, ambient temperature 30.4 ± 0.6°C) with WATER, CARBON, and without ice vest (CONTROL), respectively. The increase in core body temperature (Tcore) was lower in WATER at 40 (-0.49 ± 0.34 °C) and 50 minutes (-0.48 ± 0.48 °C) and in CARBON at 30 (-0.41 ± 0.48 °C), 40 (-0.54 ± 0.51 °C), and 50 minutes (-0.67 ± 0.62 °C) as compared to CONTROL (p < 0.05, ES > 0.8). While heart rate and blood lactate kinetics did not differ between the conditions, statistical main effects in favour of both WATER and CARBON were found for thermal sensation (condition p < 0.001 and interaction p < 0.01) and rating of perceived exertion (condition p < 0.05). Per-cooling with CARBON and WATER similarly reduced Tcore but not physiological strain during prolonged exercise in the heat. Practitioner Summary: Exercise in the heat is characterised by increases in thermophysiological strain. Both per-cooling with a novel carbon-based and a conventional water-based ice vest were shown to reduce core temperature significantly. However, due to its lower mass, the carbon-based system may be recommended especially for weight-bearing sports.

Heat Versus Altitude Training for Endurance Performance at Sea Level

Environmental stressors, such as heat or altitude, elicit dissimilar physiological adaptations to endurance training programs. Whether these differences (i.e., increased hemoglobin mass vs plasma volume) differentially influence performance is debated. We review data in support of our novel hypothesis, which proposes altitude as the preferred environmental training stimulus for elite endurance athletes preparing to compete in temperate, sea-level climates (5°C-18°C).