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Thermoregulatory Responses to Acute Exercise‐Heat Stress and Heat Acclimation

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... Skin temperature was measured using skin thermistors (Type EUS-U-V5-V2, Grant Instruments Ltd, Cambridge, UK) placed on four, left-sided sites: chest, bicep, thigh, and calf, secured using micropore tape. Mean skin temperature ( T sk ) and mean body temperature ( T b ) were calculated as follows: T sk~0 :3T chest z0:3T bicep z0:2T thigh z0:2T calf (Ramanathan, 1964) T b~0 :9T r z0:1 T sk (Sawka et al., 1996) Temperature data were recorded at 1-minute intervals on a portable data logger (2020 series data logger, Grant Instruments Ltd, Cambridge, UK). ...
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Purpose: Effectiveness of short-term heat acclimation (STHA), over 5-days (permissive dehydration), on intermittent heat stress test (HST) with males. Methods: Ten, moderately-trained, males (mean [SD]; age 25.6 [8.9] y; stature 180.7 [5.6] cm; body mass 83.2 [10.8] kg; and 45.3 [6.5] mL.kg-1.min-1) participated. The HST was 9 x 5min (45-min) of intermittent exercise based on professional soccer players. One week apart, HST1 vs HST (11.0°C; 50%RH), as a reliability trial and HST3 in 31.0°C; 50%RH were completed. Then 90 min dehydration, STHA (no fluid intake), for 5 consecutive days (39.5oC; 60%RH), using controlled-hyperthermia (~rectal temperature [Tre] 38.5oC). The HST4 within one week after STHA. Blood plasma constituents: percent plasma volume (%PV), aldosterone, total protein, albumin, electrolytes, cortisol and HSP70. Data analysis reported as mean differences with 95% confidence intervals (95%CI) and Cohen’s d effect size. Results: Post STHA, there was a decrease of -0.20 Tre at 45-min in the HST (95%CI -0.40 to -0.05°C; P=0.03; d =-0.56); mean skin temperature (-0.80; -1.30 to -0.30°C; P=0.007; d =-1.46) and mean body temperature (-0.30; -0.50 to -0.10°C, P=0.01; d =-0.75). Cardiac frequency reduced (-3: -5 to -1 b.min-1; P=0.01; d =-0.20) and %PV increased (7.3: 0.9 to 13.7%; P=0.03; d=0.59). Mean Peak Power (MPO) increased (Pd =0.63). Conclusions: Short-term heat acclimation (5-days) with dehydration, using controlled-hyperthermia technique, is effective for physiological adaptations during intermittent exercise in the heat, with moderately trained males.
... °C) temperature since the beginning of KMA observation in Seoul. Earlier studies reported that heat acclimation begins on the initial day of exposure and is complete after 7-10 days or 2 weeks (Eichna et al. 1945;Sawka et al. 1996;Pandolf 1998). The prior experience of the subjects to exceptional heatwave events led to their heat acclimation in 2018. ...
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People perceive thermal sensation differently despite the same temperature value of thermal comfort index depending on various factors such as climate, culture, and physiological characteristics. The use of the thermal comfort index without optimization may lead to biases in assessment of thermal stress and sensation. This study aims to derive the perceived temperature (PT) ranges of thermal sensation levels related to heat stress for Koreans. The experiments were designed using a controlled environmental chamber to derive the PT ranges and were performed with subjects who are residents of Seoul, South Korea. The experiments were carried out in the summers of 2017 and 2018, and the thermal sensation votes were surveyed from 19 subjects whose mean age, height, weight, and body mass index were 22.5 years, 171 cm, 72 kg, and 23 kg⋅m ⁻² , respectively. The derived PT ranges for Koreans led to a better performance than the reference PT ranges for Germans based on the results of validation. The thresholds of ‘Warm,’ ‘Hot,’ and ‘Very hot’ thermal sensation classes for Koreans were 28 °C, 36 °C, and 43 °C, respectively: higher than those for Germans. The results indicate that Koreans may have higher heat resistance or lower heat sensitivity than Germans.
... Consequently, this study also appraised the human health implications of urban thermal characteristics on East London (South Africa) residents using RS and GIS technology over a period of 30 years. Many attempts (Sawka et al., 2011; have been made to investigate the stress posed by the wide range of heat index and climate or to investigate the physiological strain and form in a single index. To create a universal heat stress index is difficult due to different climate zone and locations. ...
... Of specific interest in this population, muscle work results in body heat, with more than 70% of metabolic energy converted to heat during physical activity. [31][32][33][34][35] The demand of job tasks performed on the fireline is likely further influenced by the weight of equipment carried (18.7±3.7 kg for all resource types; Table 1) and hot temperatures associated with wildfire conditions. Supported by the HR and T c values observed in this study, regulating the duration of high-intensity activities, management of work:rest guidelines during operations, and promoting aerobic fitness in this population should be emphasized. ...
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Introduction The purpose of this study was to evaluate heat stress occurring in wildfire management activities with variable environmental conditions. Methods Direct observation and real-time wireless physiological monitoring allowed for weather and physiological metrics, including heart rate, core temperature (Tc), skin temperature, and physiological strain index (PSI), of male (n=193) and female (n=28) wildland firefighters (WLFFs) to be recorded during wildfire management activities. Accelerometry data were used to categorize intensity level of activity. Results Ambient temperature and relative humidity values were used to compute the heat index (HI; n=3891 h) and divided into quartiles (Q1: 13.3–25.1°C; Q2: 25.2–26.4°C; Q3: 26.5–28.9°C; Q4: 29.0–49.1°C). Activity levels remained relatively constant across all HI quartiles. The percentage of time spent performing moderate/vigorous activities was lowest during the hotter Q4 (Q1: 3%; Q2: 2%; Q3: 2%; Q4: 1%). Heart rate, Tc, PSI, and skin temperature associations with HI varied by resource type. Sixty-one percent of WLFFs (n=134) experienced a Tc ≥38.0°C, and 50% of WLFFs (n=111) experienced a PSI ≥6.0. Conclusions Heat stress was prevalent as WLFFs performed job tasks of varying intensities in all ambient conditions. Spontaneous bouts of arduous labor, duration of work shifts, and other occupation characteristics present the possibility for substantial durations of hyperthermia, although no heat-related injuries occurred in this study. Despite chronic exposure to rugged sloped terrain, load carriage, and environmental conditions, self-regulation and individual attention to managing work:rest appears to be the primary management strategy in mitigating excessive accumulation of body heat in this occupation.
... 16 The adaptations that develop during heat acclimation or acclimatization include plasma volume expansion and better maintenance of fluid balance, lower exercising core temperature, enhanced sweating and skin blood-flow responses, improved cardiovascular stability, and less reliance on carbohydrate metabolism during exercise in the heat at a given workload. [16][17][18] A complete list of physiological adaptations afforded by heat acclimatization is available in the Figure. These adaptations allow for improved submaximal exercise performance and increased maximal aerobic uptake (VO 2 max), 19,20 attenuation of perceived fatigue within training sessions, 21 and enhanced thermal comfort in the heat. ...
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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.
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Moderate-intensity exercise is generally recommended for improving sleep, whereas, high-intensity exercise (HIE) prior to bedtime is often discouraged. We conducted a systematic review and meta-analysis to determine if acute or regular (chronic) HIE performed before bedtime disrupts nighttime sleep of healthy adult, good sleepers compared with a no-exercise control. Six databases (PubMed, EMBASE, Scopus, Web of Science, CENTRAL, and PsycINFO) were searched from inception to 31st May, 2021. Studies were experimental trials published in English language, objectively (polysomnography, actigraphy) and/or subjectively assessed sleep after evening HIE in sedentary and physically fit, good sleepers (aged 18–50 y old). The revised Cochrane risk of bias tool for randomized trials was used to assess risk of bias in the included studies. The random-effects model was used for the meta-analyses. We included 15 acute evening HIE studies in the meta-analysis with a total of 194 participants. Acute evening HIE ending 0.5–4 h before bedtime decreased rapid eye movement sleep (−2.34%; p = 0.002) compared with a no-exercise control. No other significant sleep changes occurred. A regular evening HIE did not disrupt nighttime sleep. Overall, acute evening HIE performed 2–4 h before bedtime does not disrupt nighttime sleep of healthy, young and middle-aged adults. PROSPERO, protocol registration number: CRD42020218299.
Article
Purpose Since construction workers often need to carry various types of loads in their daily routine, they are at risk of sustaining musculoskeletal injuries. Additionally, carrying a load during walking may disturb their walking balance and lead to fall injuries among construction workers. Different load carrying techniques may also cause different extents of physical exertion. Therefore, the purpose of this paper is to examine the effects of different load-carrying techniques on gait parameters, dynamic balance, and physiological parameters in asymptomatic individuals on both stable and unstable surfaces. Design/methodology/approach Fifteen asymptomatic male participants (mean age: 31.5 ± 2.6 years) walked along an 8-m walkway on flat and foam surfaces with and without a load thrice using three different techniques (e.g. load carriage on the head, on the dominant shoulder, and in both hands). Temporal gait parameters (e.g. gait speed, cadence, and double support time), gait symmetry (e.g. step time, stance time, and swing time symmetry), and dynamic balance parameters [e.g. anteroposterior and mediolateral center of pressure (CoP) displacement, and CoP velocity] were evaluated. Additionally, the heart rate (HR) and electrodermal activity (EDA) was assessed to estimate physiological parameters. Findings The gait speed was significantly higher when the load was carried in both hands compared to other techniques (Hand load, 1.02 ms vs Head load, 0.82 ms vs Shoulder load, 0.78 ms). Stride frequency was significantly decreased during load carrying on the head than the load in both hands (46.5 vs 51.7 strides/m). Step, stance, and swing time symmetry were significantly poorer during load carrying on the shoulder than the load in both hands (Step time symmetry ration, 1.10 vs 1.04; Stance time symmetry ratio, 1.11 vs 1.05; Swing time symmetry ratio, 1.11 vs 1.04). The anteroposterior (Shoulder load, 17.47 mm vs Head load, 21.10 mm vs Hand load, −5.10 mm) and mediolateral CoP displacements (Shoulder load, −0.57 mm vs Head load, −1.53 mm vs Hand load, −3.37 ms) significantly increased during load carrying on the shoulder or head compared to a load in both hands. The HR (Head load, 85.2 beats/m vs Shoulder load, 77.5 beats/m vs No load, 69.5 beats/m) and EDA (Hand load, 14.0 µS vs Head load, 14.3 µS vs Shoulder load, 14.1 µS vs No load, 9.0 µS) were significantly larger during load carrying than no load. Research limitations/implications The findings suggest that carrying loads in both hands yields better gait symmetry and dynamic balance than carrying loads on the dominant shoulder or head. Construction managers/instructors should recommend construction workers to carry loads in both hands to improve their gait symmetry and dynamic balance and to lower their risk of falls. Practical implications The potential changes in gait and balance parameters during various load carrying methods will aid the assessment of fall risk in construction workers during loaded walking. Wearable insole sensors that monitor gait and balance in real-time would enable safety managers to identify workers who are at risk of falling during load carriage due to various reasons (e.g. physical exertion, improper carrying techniques, fatigue). Such technology can also empower them to take the necessary steps to prevent falls. Originality/value This is the first study to use wearable insole sensors and a photoplethysmography device to assess the impacts of various load carrying approaches on gait parameters, dynamic balance, and physiological measures (i.e. HR and EDA) while walking on stable and unstable terrains.
Article
Humans sweat to cool down and dissipate heat in indoor and outdoor environments. The evaporation of liquid sweat absorbs body heat and reduces skin temperature. However, when the wearer sweats a lot or is fully wet by external liquids such as rain, even the best moisture management fabrics can be soaked and saturated, becoming unbreathable, heavy, and clingy, and imposing significant reduction in wearer’s comfort and performance. Herein, inspired by liquid collection and release processes on the desert beetle’s skin, we develop an innovative moisture management fabric, transferring excessive sweat from the skin side to the outer surface of the fabric and dissipating it in terms of droplets, while blocking and repelling the external liquids. The completely unidirectional liquid transport is achieved in the developed nature-inspired fabric, which is treated with global hydrophobicity and discrete localized wettability gradient by a sustainable plasma treatment method. Moreover, the proposed fabric shows improved thermal comfort with high breathability, reduced clinginess, and dry skin surface. Specifically, a highly directional water transport capability of 1647.9% is obtained using a Moisture Management Tester, while the water resistance from the external side is equivalent to a water column of 15 mm height. The water flow rate from the inner skin side to the environment is equal to 91.44 g/min∙m², which is much higher than that of the high sweating rate of an adult (i.e., 15g/min∙m²). Regulation of liquid movement in the proposed fabric will attract wide potential applications, including sportswear, worker uniform, casualwear, protective clothing, firefighter apparel, and military clothing. Maintaining moisture and thermal comfort while adapting to the expanded ambient temperature range of the air-conditioning system will also be desirable in building energy conservation.
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A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat, by examining the benefits of heat acclimation, cooling strategies and hyperhydration. Finally, contemporary controversies are summarized and future research directions provided.
Chapter
Across the globe, the negative impacts of extreme heat have become increasingly apparent to individuals, policymakers, public health scientists, physiologists, climatologists, and economists. Extreme heat exposures in occupational settings can have detrimental impacts on the whole of society. Understanding the full impact requires quantifying trade‐offs between what may be valued economically as part of society (work output) and human health. The heat load affecting one's body––a combination of environmental and personal factors––can significantly affect thermoregulation, particularly for those completing physically demanding work in environmentally stressful conditions. In such situations, an imbalance occurs between high metabolic heat production and the ability of the body to dissipate heat, as controlled by the environment. The spectrum of physiological impacts from heat strain implicates a worker's health, with new research connecting occupational heat stress to worker productivity and overall well‐being, both of which relate to long‐term economic resilience under a changing climate. This chapter provides an overview of common methods used to quantify heat stress and related health effects in occupational settings, presents insights into the impacts of climate change on health and productivity in specific regions of the world, and brings forth future perspectives and guidance for practical heat impact prevention approaches.
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This study examined evaporative and dry heat exchange during upper- and lower-body exercise. Four male subjects performed arm-crank or cycle exercise at the same O2 uptake level (approximately 1.6 l/min) in an environment facilitating dry heat exchange [radiative and convective (R + C)] [ambient temperature (Ta) = 18 degrees C, dew-point temperature (Tdp) = 14 degrees C] and an environment facilitating evaporative heat loss (Esk) (Ta = 35 degrees C, Tdp = 14 degrees C). (R + C) was determined from the torso with a net radiometer and from the limbs with heat flow discs, whereas Esk was determined from the torso and limbs by ventilated dew-point sensors. In both environments neither esophageal temperature nor mean skin temperature were different between exercise types (P greater than 0.05). Torso (R + C) was significantly (P less than 0.05) greater during arm-crank than during cycle exercise in both environments. Torso Esk, as well as arm (R + C), and arm Esk were not different (P greater than 0.05) between exercise types in each environment. Leg (R + C) was greater (P less than 0.05) during cycle than during arm-crank exercise in the 18 degrees C environment, whereas leg Esk was greater (P less than 0.05) during cycle than during arm-crank exercise in the 35 degrees C environment. These data indicate that to compensate for greater torso sensible heat loss during upper body exercise lower body exercise elicits additional (R + C) or Esk from the legs. The avenue for this compensatory sensible and insensible heat loss depends upon the differential heat transfer coefficients which influence tissue conductivity and mass transfer.