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Workers often experience heat exposure before manual labour. This study investigated whether prior heat exposure diminished upper-body endurance work capacity and maximal isokinetic arm and leg strength in young men. Ten male participants completed two trials in a climatic chamber maintained at 25°C with 50% relative humidity. The two trials required them to complete a 30-min pre-exercise water-immersion at either 40°C (HOT) to provoke an approximately 1°C rise in rectal temperature (T

Objective To develop best-practice recommendations using thermal indices to determine work-to-rest ratios and facilitate further implementation of environmental monitoring for heat safety in secondary school athletics in the United States. Data Sources A narrative review of the current literature on environmental monitoring for heat safety during athletics was conducted by content experts. A list of action-oriented recommendations was established from the narrative review and further refined using the Delphi method. Conclusions Assessment of wet bulb globe temperature at the site of activity and throughout the duration of the event is recommended to assist clinicians and administrators in making appropriate decisions regarding the duration and frequency of activity and rest periods. Activity-modification guidelines should be predetermined and approved by stakeholders and should outline specific actions to be followed, such as the work-to-rest ratio, frequency and timing of hydration breaks, and adjustment of total exercise duration, equipment, and clothing. Furthermore, integration of exertional heat illness injury data with environmental condition characteristics is critical for the development of evidence-based heat safety guidelines for secondary school athletics. Athletic trainers play an essential role in conducting prospective injury data collection, recording onsite wet bulb globe temperature levels, and implementing recommendations to protect the health and safety of athletes.

Sustainable methods are required to reduce the risks of thermal strain and heat-related illness without exacerbating greenhouse gas emissions. We investigated the effects of sun-shade use on safe heat exposure limit on a sunny summer day using historical climate data in Japan. We simulated a heat-acclimatised person standing at rest (metabolic heat production, 70 W·m

Purpose The time-of-day variations in environmental heat stress have been known to affect thermoregulatory responses and the risk of exertional heat-related illness during outdoor exercise in the heat. However, such effect and risk are still needed to be examined during indoor sports/exercises. The current study investigated the diurnal relationships between thermoregulatory strain and environmental heat stress during regular judo training in a judo training facility without air conditioning on a clear day in the heat of summer. Methods Eight male high school judokas completed two 2.5-h indoor judo training sessions. The sessions were commenced at 09:00 h (AM) and 16:00 h (PM) on separate days. Results During the sessions, indoor and outdoor heat stress progressively increased in AM but decreased in PM, and indoor heat stress was less in AM than PM (mean ambient temperature: AM 32.7±0.4°C; PM 34.4±1.0°C, PPPP Conclusions This study indicates a greater thermoregulatory strain in the morning from 09:00 h than the late afternoon from 16:00 h during 2.5-h regular judo training in no air conditioning facility on a clear day in the heat of summer. This observation is associated with a progressive increase in indoor and outdoor heat stress in the morning, despite a less indoor heat stress in the morning than the afternoon.

This study investigated the combined effects of different levels of solar radiation and airflow on endurance exercise capacity and thermoregulatory responses during exercise-heat stress. Ten males cycled at 70% peak oxygen uptake until exhaustion in an environmental chamber (30°C, 50% relative humidity). Four combinations of solar radiation and airflow were tested (800 W⋅m−2 and 10 km⋅h−1 [High-Low], 800 W⋅m−2 and 25 km⋅h−1 [High-High], 0 W⋅m−2 and 10 km⋅h−1 [No-Low], and 0 W⋅m−2 and 25 km⋅h−1 [No-High]). Participants were exposed to solar radiation by a ceiling-mounted solar simulator (Metal halide lamps) and the headwind by two industrial fans. Time to exhaustion was shorter (p

High solar radiation has been recognised as a contributing factor to exertional heat-related illness in individuals exercising outdoors in the heat. Although solar radiation intensity has been known to have similar time-of-day variation as body temperature, the relationship between fluctuations in solar radiation associated with diurnal change in the angle of sunlight and thermoregulatory responses in individuals exercising outdoors in a hot environment remains largely unknown. The present study therefore investigated the time-of-day effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during moderate-intensity outdoor exercise in the heat of summer. Eight healthy, high school baseball players, heat-acclimatised male volunteers completed a 3-h outdoor baseball trainings under the clear sky in the heat. The trainings were commenced at 0900 h in AM trial and at 1600 h in PM trial each on a separate day. Solar radiation and solar elevation angle during exercise continued to increase in AM (672-1107 W/m2 and 44-69°) and decrease in PM (717-0 W/m2 and 34-0°) and were higher on AM than on PM (both P 0.05). Tympanic temperature measured by an infrared tympanic thermometer and mean skin temperature were higher in AM than PM at 120 and 180 min (P 0.05). The current study demonstrates a greater thermoregulatory strain in the morning than in the afternoon resulting from a higher body temperature and heart rate in relation to an increase in environmental heat stress with rising solar radiation and solar elevation angle during moderate-intensity outdoor exercise in the heat. This response is associated with a lesser net heat loss at the skin and a greater body heat gain from the sun in the morning compared with the afternoon.

Otani, H, Goto, T, Goto, H, Hosokawa, Y, and Shirato, M. Solar radiation exposure has diurnal effects on thermoregulatory responses during high-intensity exercise in the heat outdoors. J Strength Cond Res 33(10): 2608-2615, 2019-This study investigated the diurnal effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during high-intensity exercise in the heat outdoors. Ten male high school soccer players completed two 2-hour soccer training sessions under a clear sky in the heat of summer. These sessions were commenced at 0900 hours (AM) and 1600 hours (PM) on separate days. Solar radiation and elevation angle were higher in AM (820-1,070 W·m and 45-69°) than PM (620-110 W·m and 34-10°: both p < 0.001). Neither ambient temperature (AM 29-32° C; PM 31-31° C) nor wet-bulb globe temperature was different between trials. Although mean skin temperature was not different between trials, infrared tympanic temperature was higher at the end of exercise in AM than PM (p < 0.001). Heart rate (p < 0.01) and body heat gain from the sun (p < 0.001) were greater during exercise in AM than PM. Dry heat loss was smaller, but evaporative heat loss was greater in AM than PM (both p < 0.001). Thermal sensation and rating of perceived exertion were similar between trials, but GPS measurements showed a less total distance and distance covered by walking, jogging, and running in AM than PM (p < 0.01). This study demonstrates a greater thermoregulatory strain in AM than PM during 2-hour high-intensity soccer training in the heat under a clear sky. This observation is accompanied by a progressive increase in environmental heat stress with rising solar radiation and elevation angle in AM and a greater body heat gain from the sun in AM compared with PM.

We investigated the validity of infrared tympanic temperature (IR-Tty) during exercise in the heat with variations in solar radiation. Eight healthy males completed stationary cycling trials at 70% peak oxygen uptake until exhaustion in an environmental chamber maintained at 30°C with 50% relative humidity. Three solar radiation conditions, 0, 250 and 500 W/m2, were tested using a ceiling-mounted solar simulator (metal-halide lamps) over a 3 × 2 m irradiated area. IR-Tty and rectal temperature (Tre) were similar before and during exercise in each trial (P > 0.05). Spearman’s rank correlation coefficient (rs) demonstrated very strong (250 W/m2, rs = 0.87) and strong (0 W/m2, rs = 0.73; 500 W/m2, rs = 0.78) correlations between IR-Tty and Tre in all trials (P

High radiant heat load reduces endurance exercise performance in the heat indoors, but this remains unconfirmed in outdoor exercise. The current study investigated the effects of variations in solar radiation on self-regulated exercise intensity and thermoregulatory responses in the heat outdoors at a fixed rating of perceived exertion (RPE). Ten male participants completed 45-min cycling exercise in hot outdoor environments (about 31 °C) at a freely chosen resistance and cadence at an RPE of 13 (somewhat hard). Participants were blinded to resistance, pedal cadence, distance and elapsed time and exercised at three sunlight exposure conditions: clear sky (mean ± SD: 1072 ± 91 W·m−2; HIGH); thin cloud (592 ± 32 W·m−2; MID); and thick cloud (306 ± 52 W·m−2; LOW). Power output (HIGH 96 ± 22 W; MID 103 ± 20 W; LOW 108 ± 20 W) and resistance were lower in HIGH than MID and LOW (P

This study examined the effects of variations in air velocity on time to exhaustion and thermoregulatory and perceptual responses to exercise in a hot environment. Eight male volunteers completed stationary cycle exercise trials at 70% peak oxygen uptake until exhaustion in an environmental chamber maintained at 30 °C and 50% relative humidity. Four air velocity conditions, 30, 20, 10, and 0 km/h, were tested, and the headwind was directed at the frontal aspect of the subject by 2 industrial fans, with blade diameters of 1 m and 0.5 m, set in series and positioned 3 m from the subject’s chest. Mean ± SD time to exhaustion was 90 ± 17, 73 ± 16, 58 ± 13, and 41 ± 10 min in 30-, 20-, 10-, and 0-km/h trials, respectively, and was different between all trials (P < 0.05). There were progressive elevations in the rate of core temperature rise, mean skin temperature, and perceived thermal sensation as airflow decreases (P < 0.05). Core temperature, heart rate, cutaneous vascular conductance, and perceived exertion were higher and evaporative heat loss was lower without airflow than at any given airflow (P < 0.05). Dry heat loss and plasma volume were similar between trials (P > 0.05). The present study demonstrated a progressive reduction in time to exhaustion as air velocity decreases. This response is associated with a faster rate of core temperature rise and a higher skin temperature and perceived thermal stress with decreasing airflow. Moreover, airflow greater than 10 km/h (2.8 m/s) might contribute to enhancing endurance exercise capacity and reducing thermoregulatory, cardiovascular, and perceptual strain during exercise in a hot environment.

Untrained subjects ( n = 6 ) performed three cycle exercise trials at 80% maximal O2 uptake until exhaustion in a climatic chamber (32 °C, 60% relative humidity) under the following three conditions: (1) 2% hypohydration, (2) 1% hypohydration, and (3) euhydration. Exercise tolerance time averaged 7.7±1.5, 14.0±2.4, and 20.9±3.7 min, respectively, under these three conditions. Endurance capacity in untrained individuals is substantially impaired even at low levels of hypohydration during heavy exercise in a compensable hot environment, and this impairment is associated with hypohydration-induced higher initial body temperatures that leads to earlier attainment of a critical core temperature.

Long-term cold exposure (10 °C for 11weeks) led to hyperplasia of brown adipose tissue (BAT) in rats. After deacclimation (25 °C for 2 weeks, after 10 °C for 9 weeks), BAT retained almost the same mass as in cold acclimation, and lipid content and droplet size decreased slightly, although the droplets fused with each other. The increased amounts of UCP1 and GLUT4 in BAT in the cold were maintained, and the levels of plasma insulin, leptin and triacylglycerol that decreased in the cold were increased to greater levels than the control after deacclimation. UCP3 in skeletal muscle in the cold decreased, in contrast to that in BAT, and returned to the control level. Predominant use of lipid for non-shivering thermogenesis was preserved for cold after deacclimation.

Male obese and lean Zucker rats, aged 7 weeks, were maintained at 25 °C for 11 weeks, at 10 °C for 11weeks or at 25 °C for 2 weeks following exposure to 10 °C for 9 weeks. The increase in food intake during cold exposure was much smaller in obese rats than in lean rats. The suppressive effect on body mass increment by cold acclimation was more profound in obese rats. In obese rats, the concentration of plasma triglycerides and insulin was lower during cold acclimation, and improvement in carbohydrate and lipid metabolism due to cold exposure was maintained during deacclimation.

This study examined the influence of relative humidity on endurance exercise performance in a warm environment. Eight male volunteers performed four cycle exercise trials at 70% maximum oxygen uptake until volitional exhaustion in an environmental chamber maintained at 30.2 ± 0.2°C. Volunteers were tested under four relative humidity (rh) conditions: 24, 40, 60 and 80%. Core and weighted mean skin temperature, heart rate, skin blood flow, and cutaneous vascular conductance were recorded at rest and at regular intervals during exercise. Mean ± SD time to exhaustion was 68 ± 19, 60 ± 17, 54 ± 17, and 46 ± 14 min at 24, 40, 60, and 80% rh, respectively (P < 0.001); exercise time was significantly less at 60% (P = 0.013) and 80% (P = 0.005) rh than recorded at 24% rh. There were no differences in core temperature (P = 0.480) and heart rate (P = 0.097) between trials. Core temperature at exhaustion was 39.0 ± 0.3°C at 24, 40, and 60% rh and 39.1 ± 0.3°C at 80% rh (P = 0.159). Mean skin temperature at the point of exhaustion was higher at 80% rh than at 24% rh (P < 0.001). Total sweat loss was similar between trials (P = 0.345), but sweating rate was higher at 60 and 80% rh than at 24% rh (P < 0.001). The results suggest that exercise capacity at moderate intensity in a warm environment is progressively impaired as the relative humidity increases.

We examined the agreement between the tympanic membrane temperature (Tty) measured by a contact tympanic thermometer (Contact-Tty) and the Tty measured by a noncontact tympanic thermometer (Infrared-Tty). In addition, we also evaluated the usefulness of an assessment of core body temperature using a noncontact tympanic thermometer during prolonged exercise in the heat. Seven healthy male subjects cycled for the same four experimental trials at 50% peak oxygen uptake for 90 min in the heat (32°C ambient temperature, 50% relative humidity and 26.6°C wet bulb globe temeprature). The correlation coefficient between both temperatures was strong, 0.89 ( p

Hyperthermia before exercise has been demonstrated to attenuate subsequent endurance capacity in the heat (Gonzalez-Alonso et al., JAP 1999). Mental fatigue before exercise has also been reported to impair endurance performance in a temperate environment (Marcora et al., JAP 2009). However, a combined effect of hyperthermia and mental fatigue before exercise on endurance capacity has not been evaluated in the heat. Therefore, this study examined the effects of hyperthermia and mental fatigue before exercise on subsequent endurance capacity and cognitive function during heavy exercise in the heat. Eight male volunteers completed four cycling trials at 80% VO2max until exhaustion in a climatic chamber (30°C, 50% RH). Participants cycled after: a 90 min seated rest (CON), a 90 min demanding cognitive task to induce mental fatigue (MF), a 30 min water immersion (40°C) to induce hyperthermia following a 60 min seated rest (HT), or a 90 min demanding cognitive task with a 30 min water immersion (40°C) during the last 30 min of task (MF + HT). A demanding cognitive task before exercise and the cognitive function test after exhaustion involved the completion of several computer-based tests which included the Stroop, Sternburg and RVIP tests. Rectal temperature at the start of exercise was higher in HT and MF + HT than in CON and MF (p

This study examined whether the prevention of body mass loss during exercise in the heat by fluid ingestion would attenuate the decline in renal concentrating ability. Seven untrained subjects performed 105 min of intermittent exercise (15 min of exercise alternating with 3 min of rest) on a cycle ergometer at 70% VO 2max (32°C and 60% RH). Subjects were tested under four conditions: (1) no fluid ingestion (NF), (2) ad libitum fluid ingestion (AF), (3) fluid ingestion equal to 1% of body mass (1% FL), (4) fluid ingestion equal to 2% of body mass (2% FL). Fluid was ingested immediately before exercise and at 15, 33, 51, 69 and 87 min of exercise in AF, 1% FL and 2% FL. Urine and blood samples were taken before and after exercise. During NF, AF, 1% FL and 2% FL, body mass loss was 2.2±0.1%, 1.1±0.2%, 1.1±0.1%, 0.1±0.1%, respectively. Total sweat loss was approximately 2% of body mass in all trials. Urine flow rate during exercise was 0.3±0.0, 0.4±0.1, 0.4±0.1, 0.5±0.1 ml/min during NF, AF, 1% FL and 2% FL, respectively, and was lower (p

This study examined the effect of the volume of fluid ingested on urine concentrating ability during prolonged heavy exercise in a hot environment at low levels of dehydration. Seven healthy males performed 105 min of intermittent cycle exercise at 70% maximum oxygen uptake (32°C, 60% relative humidity) while receiving no fluid ingestion (NF), voluntary fluid ingestion (VF), partial fluid ingestion equivalent to one-half of body mass loss (PF), and full fluid ingestion equivalent to body mass loss (FF). Fluid (5°C, 3.4% carbohydrate, 10.5 mmol·L(-1) sodium) was ingested just before commencing exercise and at 15, 33, 51, 69, and 87 min of exercise, and the total amount of fluid ingested in PF and FF was divided into six equal volumes. During exercise, body mass loss was 2.2 ± 0.2, 1.1 ± 0.5, 1.1 ± 0.2, and 0.1 ± 0.2% in NF, VF, PF, and FF, respectively, whereas total sweat loss was about 2% of body mass in each trial. Subjects in VF ingested 719 ± 240 ml of fluid during exercise; the volume of fluid ingested was 1.1 ± 0.4% of body mass. Creatinine clearance was significantly higher and free water clearance was significantly lower in FF than in NF during exercise. Urine flow rate during exercise decreased significantly in NF. There were significant decreases in creatinine and osmolar clearance and was a significant increase in free water clearance during exercise in NF and VF. Creatinine clearance decreased significantly and free water clearance increased significantly during exercise in PF. There was no statistical change in urinary indices of renal function during exercise in FF. The findings suggest that full fluid ingestion equivalent to body mass loss has attenuated the decline in urine concentrating ability during prolonged heavy exercise in a hot environment at low levels of dehydration. Key pointsDuring prolonged heavy exercise in a hot environment at low levels of dehydration, fluid ingestion equivalent to body mass loss results in no changes in urinary indices of renal function.Fluid ingestion equivalent to body mass loss can attenuate the decline in urine concentrating ability during exercise.Ad libitum or voluntary fluid ingestion is ineffective in reducing the decline in urine concentrating ability during exercise.

Both aerobic and anaerobic exercise performances have a diurnal variation. As commonly reported in previous studies, circadian rhythm in exercise performance is low in the morning and peaks in the evening. It has been demonstrated that hyperthermia before exercise attenuates subsequent exercise performance in the heat. However, combined effects of passive hyperthermia and the time-of-day on both aerobic and anaerobic exercise capacity and cognitive function in the heat have not been evaluated. Therefore, the aim of this study was to examine the effects of passive hyperthermia and circadian rhythm on aerobic and anaerobic exercise performances and cognitive function after exercise in the heat.