Seasonal Heat Acclimatisation in Healthy Adults:A Systematic Review

BACKGROUND: Physiological heat adaptations can be induced following various protocols that use either artificially controlled (i.e. acclimation) or naturally occurring (i.e. acclimatisation) environments. During the summer months in seasonal climates, adequate exposure to outdoor environmental heat stress should lead to transient seasonal heat acclimatisation. OBJECTIVES: The aim of the systematic review was to assess the available literature and characterise seasonal heat acclimatisation during the summer months and identify key factors that influence the magnitude of adaptation. ELIGIBILITY CRITERIA: English language, full-text articles that assessed seasonal heat acclimatisation on the same sample of healthy adults a minimum of 3 months apart were included. DATA SOURCES: Studies were identified using first- and second-order search terms in the databases MEDLINE, SPORTDiscus, CINAHL Plus with Full Text, Scopus and Cochrane, with the last search taking place on 15 July 2021. RISK OF BIAS: Studies were independently assessed by two authors for the risk of bias using a modified version of the McMaster critical review form. DATA EXTRACTION: Data for the following outcome variables were extracted: participant age, sex, body mass, height, body fat percentage, maximal oxygen uptake, time spent exercising outdoors (i.e. intensity, duration, environmental conditions), heat response test (i.e. protocol, time between tests), core temperature, skin temperature, heart rate, whole-body sweat loss, whole-body and local sweat rate, sweat sodium concentration, skin blood flow and plasma volume changes. RESULTS: Twenty-nine studies were included in this systematic review, including 561 participants across eight countries with a mean summer daytime wet-bulb globe temperature (WBGT) of 24.9 °C (range: 19.5–29.8 °C). Two studies reported a reduction in resting core temperature (0.16 °C; p < 0.05), 11 reported an increased sweat rate (range: 0.03–0.53 L·h(−1); p < 0.05), two observed a reduced heart rate during a heat response test (range: 3–8 beats·min(−1); p < 0.05), and six noted a reduced sweat sodium concentration (range: − 22 to − 59%; p < 0.05) following summer. The adaptations were associated with a mean summer WBGT of 25.2 °C (range: 19.6–28.7 °C). LIMITATIONS: The available studies primarily focussed on healthy male adults and demonstrated large differences in the reporting of factors that influence the development of seasonal heat acclimatisation, namely, exposure time and duration, exercise task and environmental conditions. CONCLUSIONS: Seasonal heat acclimatisation is induced across various climates in healthy adults. The magnitude of adaptation is dependent on a combination of environmental and physical activity characteristics. Providing environmental conditions are conducive to adaptation, the duration and intensity of outdoor physical activity, along with the timing of exposures, can influence seasonal heat acclimatisation. Future research should ensure the documentation of these factors to allow for a better characterisation of seasonal heat acclimatisation. PROSPERO REGISTRATION: CRD42020201883. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s40279-022-01677-0

High-intensity interval training

This is a draft of a chapter that has been accepted for publication by Oxford University Press in the forthcoming book Paediatric Exercise Science and Medicine edited by N. Armstrong and W. van Mechelen published in 2017.High-intensity interval training (HIIT) is characterised by brief, intermittent bursts of near- or maximal-intensity exercise, interspersed by periods of active or passive recovery. The limited available evidence suggests that HIIT is an efficacious training method for young athletes. The effect of HIIT on cardiorespiratory fitness (CRF), endurance performance, explosive strength and sport-specific performance has been examined in a range of young athletic populations from various sports. Furthermore, promising preliminary findings suggest that HIIT may confer further benefits to a range of health outcome measures including fasting insulin, lipoproteins, systolic blood pressure and endothelial function; obese youth may benefit particularly from this type of training. Improved cardiorespiratory fitness has been observed consistently after HIIT in athletic and non-athletic populations. Larger studies, extended over longer periods, that include valid measures of exercise compliance, tolerance and enjoyment are required to further delineate the priority that could be afforded to this type of training

Electric fans: A potential stay-at-home cooling strategy during the COVID-19 pandemic this summer?

Current public health guidance designed to protect individuals against extreme heat and the ongoing COVID-19 pandemic is seemingly discordant, yet during the northern hemisphere summer, we are faced with the imminent threat of their simultaneous existence. Here we examine the environmental limits of electric fan-use in the context of the United States summer as a potential stay-at-home cooling strategy that aligns with existing efforts to mitigate the spread of SARS-COV-2

Quantifying the impact of heat on human physical work capacity; part II:the observed interaction of air velocity with temperature, humidity, sweat rate, and clothing is not captured by most heat stress indices

Increasing air movement can alleviate or exacerbate occupational heat strain, but the impact is not well defined across a wide range of hot environments, with different clothing levels. Therefore, we combined a large empirical study with a physical model of human heat transfer to determine the climates where increased air movement (with electric fans) provides effective body cooling. The model allowed us to generate practical advice using a high-resolution matrix of temperature and humidity. The empirical study involved a total of 300 1-h work trials in a variety of environments (35, 40, 45, and 50 °C, with 20 up to 80% relative humidity) with and without simulated wind (3.5 vs 0.2 m∙s-1), and wearing either minimal clothing or a full body work coverall. Our data provides compelling evidence that the impact of fans is strongly determined by air temperature and humidity. When air temperature is ≥ 35 °C, fans are ineffective and potentially harmful when relative humidity is below 50%. Our simulated data also show the climates where high wind/fans are beneficial or harmful, considering heat acclimation, age, and wind speed. Using unified weather indices, the impact of air movement is well captured by the universal thermal climate index, but not by wet-bulb globe temperature and aspirated wet-bulb temperature. Overall, the data from this study can inform new guidance for major public and occupational health agencies, potentially maintaining health and productivity in a warming climate.</p

Supplementary Information Files for: Body mapping of sweating patterns of pre-pubertal children during intermittent exercise in a warm environment

Supplementary Information Files for: Body mapping of sweating patterns of pre-pubertal children during intermittent exercise in a warm environmentPurposeTo determine sweating responses of pre-pubertal children during intermittent exercise in a warm environment and create whole-body maps of regional sweat rate (RSRs) distribution across the body.MethodsThirteen pre-pubertal children; six girls and seven boys (8.1 ± 0.8 years) took part. Sweat was collected using the technical absorbent method in the last 5 min of a 30-min intermittent exercise protocol performed at 30 ℃, 40% relative humidity and 2 m·s−1 frontal wind.ResultsMean gross sweat loss (GSL) was 126 ± 47 g·m−2·h−1 and metabolic heat production was 278 ± 50 W·m2. The lower anterior torso area had the lowest RSR with a median (IQR) sweat rate (SR) of 40 (32) g·m−2·h−1. The highest was the forehead with a median SR of 255 (163) g·m−2·h−1. Normalised sweat maps (the ratio of each region’s SR to the mean SR for all measured pad regions) showed girls displayed lower ratio values at the anterior and posterior torso, and higher ratios at the hands, feet and forehead compared to boys. Absolute SRs were similar at hands and feet, but girls sweated less in most other areas, even after correction for metabolic rate.ConclusionPre-pubertal children have different RSRs across the body, also showing sex differences in sweat distribution. Distributions differ from adults. Hands and feet RSR remain stable, but SR across other body areas increase with maturation. These data can increase specificity of models of human thermoregulation, improve the measurement accuracy of child-sized thermal manikins, and aid companies during product design and communication</div

Quantifying the impact of heat on human physical work capacity; part IV:interactions between work duration and heat stress severity

High workplace temperatures negatively impact physical work capacity (PWC). Although PWC loss models with heat based on 1-h exposures are available, it is unclear if further adjustments are required to accommodate repeated work/rest cycles over the course of a full work shift. Therefore, we examined the impact of heat stress exposure on human PWC during a simulated work shift consisting of six 1-h work-rest cycles. Nine healthy males completed six 50-min work bouts, separated by 10-min rest intervals and an extended lunch break, on four separate occasions: once in a cool environment (15 °C/50% RH) and in three different air temperature and relative humidity combinations (moderate, 35 °C/50% RH; hot, 40 °C/50% RH; and very hot, 40 °C/70%). To mimic moderate to heavy workload, work was performed on a treadmill at a fixed heart rate of 130 beats·min-1. During each work bout, PWC was quantified as the kilojoules expended above resting levels. Over the shift, work output per cycle decreased, even in the cool climate, with the biggest decrement after the lunch break and meal consumption. Expressing PWC relative to that achieved in the cool environment for the same work duration, there was an additional 5(± 4)%, 7(± 6)%, and 16(± 7)% decrease in PWC when work was performed across a full work shift for the moderate, hot, and very hot condition respectively, compared with 1-h projections. Empirical models to predict PWC based on the level of heat stress (Wet-Bulb Globe Temperature, Universal Thermal Climate Index, Psychrometric Wet-Bulb Temperature, Humidex, and Heat Index) and the number of work cycles performed are presented.</p

An advanced empirical model for quantifying the impact of heat and climate change on human physical work capacity

Occupational heat stress directly hampers physical work capacity (PWC), with large economic consequences for industries and regions vulnerable to global warming. Accurately quantifying PWC is essential for forecasting impacts of different climate change scenarios, but the current state of knowledge is limited, leading to potential underestimations in mild heat, and overestimations in extreme heat. We therefore developed advanced empirical equations for PWC based on 338 work sessions in climatic chambers (low air movement, no solar radiation) spanning mild to extreme heat stress. Equations for PWC are available based on air temperature and humidity, for a suite of heat stress assessment metrics, and mean skin temperature. Our models are highly sensitive to mild heat and to our knowledge are the first to include empirical data across the full range of warm and hot environments possible with future climate change across the world. Using wet bulb globe temperature (WBGT) as an example, we noted 10% reductions in PWC at mild heat stress (WBGT = 18°C) and reductions of 78% in the most extreme conditions (WBGT = 40°C). Of the different heat stress indices available, the heat index was the best predictor of group level PWC (R2 = 0.96) but can only be applied in shaded conditions. The skin temperature, but not internal/core temperature, was a strong predictor of PWC (R2 = 0.88), thermal sensation (R2 = 0.84), and thermal comfort (R2 = 0.73). The models presented apply to occupational workloads and can be used in climate projection models to predict economic and social consequences of climate change.</p

Quantifying the impact of heat on human physical work capacity; part III:the impact of solar radiation varies with air temperature, humidity, and clothing coverage

Heat stress decreases human physical work capacity (PWC), but the extent to which solar radiation (SOLAR) compounds this response is not well understood. This study empirically quantified how SOLAR impacts PWC in the heat, considering wide, but controlled, variations in air temperature, humidity, and clothing coverage. We also provide correction equations so PWC can be quantified outdoors using heat stress indices that do not ordinarily account for SOLAR (including the Heat Stress Index, Humidex, and Wet-Bulb Temperature). Fourteen young adult males (7 donning a work coverall, 7 with shorts and trainers) walked for 1 h at a fixed heart rate of 130 beats∙min-1, in seven combinations of air temperature (25 to 45°C) and relative humidity (20 or 80%), with and without SOLAR (800 W/m2 from solar lamps). Cumulative energy expenditure in the heat, relative to the work achieved in a cool reference condition, was used to determine PWC%. Skin temperature was the primary determinant of PWC in the heat. In dry climates with exposed skin (0.3 Clo), SOLAR caused PWC to decrease exponentially with rising air temperature, whereas work coveralls (0.9 Clo) negated this effect. In humid conditions, the SOLAR-induced reduction in PWC was consistent and linear across all levels of air temperature and clothing conditions. Wet-Bulb Globe Temperature and the Universal Thermal Climate Index represented SOLAR correctly and did not require a correction factor. For the Heat Stress Index, Humidex, and Wet-Bulb Temperature, correction factors are provided enabling forecasting of heat effects on work productivity.</p

Update of clothing database for existing and new Western clothing ensembles, including effects of posture, body and air movement : Report on manikin measurements for ASHRAE 1760-TRP.

ASHRAE Research Project RP-1760 aimed to update thedatabase of western clothing as used in ANSI/ASHRAE Stan-dard 55-2013, Thermal Environmental Conditions for HumanOccupancy (ASHRAE 2013a), ISO Standard 7730-2005,Ergonomics of the Thermal Environment—Analytical Deter-mination and Interpretation of Thermal Comfort Using Calcu-lation of the PMV and PPD Indices and Local ThermalComfort Criteria (ISO 2005), and ISO Standard 9920-2009,Ergonomics of the Thermal Environment—Estimation ofThermal Insulation and Water Vapor Resistance of a ClothingEnsemble (ISO 2009). The previous database, established inthe 1970s and 1980s, relied mostly on single-zone manikinsand did not provide detail on air and body movement effects oninsulation. Insulation values of up to 70 clothing ensembles(31 male, 39 female) were measured in a static standingposture at 0.2, 0.4, and 1 m/s–1 (0.66, 1.31, and 3.28 ft/s–1) airspeed, walking in 0.2 and 1.0 m.s–1 air speed, and in a sittingposture in 0.2 m.s–1 air speed. Measurements were conductedin three laboratories on three manikins with up to 34 individ-ually controlled zones. In addition, vapor resistance was deter-mined in 31 ensembles. This new database provides total,intrinsic and air insulation values, vapor resistance, and cloth-ing area factors as well as correction factors to estimate thewind and movement effect on clothing insulation. Further-more, the model to estimate the clothing area factor from insu-lation values of the ensembles was updated reflecting changesin commonly worn clothing styles