Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress

1998 ◽  
Vol 84 (5) ◽  
pp. 1731-1739 ◽  
Author(s):  
Stephen S. Cheung ◽  
Tom M. McLellan
Keyword(s):  
Heart Rate ◽  
Heat Stress ◽  
Aerobic Fitness ◽  
Protective Clothing ◽  
Sweat Rate ◽  
Heat Acclimation ◽  
Short Term ◽  
Chemical Protective Clothing ◽  
Uncompensable Heat Stress ◽  
C 30

—The purpose of the present study was to determine the separate and combined effects of aerobic fitness, short-term heat acclimation, and hypohydration on tolerance during light exercise while wearing nuclear, biological, and chemical protective clothing in the heat (40°C, 30% relative humidity). Men who were moderately fit [(MF); <50 ml ⋅ kg−1 ⋅ min−1maximal O2 consumption; n = 7] and highly fit [(HF); >55 ml ⋅ kg−1 ⋅ min−1maximal O2 consumption; n = 8] were tested while they were euhydrated or hypohydrated by ∼2.5% of body mass through exercise and fluid restriction the day preceding the trials. Tests were conducted before and after 2 wk of daily heat acclimation (1-h treadmill exercise at 40°C, 30% relative humidity, while wearing the nuclear, biological, and chemical protective clothing). Heat acclimation increased sweat rate and decreased skin temperature and rectal temperature (Tre) in HF subjects but had no effect on tolerance time (TT). MF subjects increased sweat rate but did not alter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased Tre and heart rate and decreased the respiratory exchange ratio and the TT regardless of acclimation state. Overall, the rate of rise of skin temperature was less, while ΔTre, the rate of rise of Tre, and the TT were greater in HF than in MF subjects. It was concluded that exercise-heat tolerance in this uncompensable heat-stress environment is not influenced by short-term heat acclimation but is significantly improved by long-term aerobic fitness.

Heat Strain in Protective Clothing Following Hot-Wet or Hot-Dry Heat Acclimation

1996 ◽  
Vol 21 (2) ◽  
pp. 90-108 ◽  
Author(s):  
Tom M. McLellan ◽  
Yukitoshi Aoyagi
Keyword(s):  
Heart Rate ◽  
Skin Temperature ◽  
Rectal Temperature ◽  
Protective Clothing ◽  
Intermittent Exercise ◽  
Sweat Rate ◽  
Heat Acclimation ◽  
Heat Strain ◽  
Mean Skin Temperature ◽  
Time Required

The purpose of the present study was to compare the heat strain while wearing nuclear, biological, and chemical (NBC) protective clothing following a hot-wet (HW) or hot-dry (HD) heat acclimation protocol. Twenty-two males were assigned to groups HW (n = 7), HD (n = 8), or control (C, n = 7). Subjects were evaluated during continuous treadmill walking while wearing lightweight combat clothing and during intermittent exercise while wearing the NBC protective clothing. While wearing Combat clothing, greater decreases in rectal temperature (Tre), mean skin temperature [Formula: see text], and heart rate were observed for both acclimation groups. For the NBC clothing trials, lower Tre, [Formula: see text], and heart rates were observed only for group HW. The time required for Tre to increase 1.0 °C and 1.5 °C was significantly delayed for groups HW and HD. Sweat evaporation increased for HW, whereas no change was found for HD. The most significant changes in Tre, [Formula: see text], and heart rate while wearing the NBC protective clothing occur following heat acclimation that involves wearing the clothing during exercise. Key words: rectal temperature, mean skin temperature, heart rate, sweat rate

scholarly journals Thermoregulatory adaptations with progressive heat acclimation are predominantly evident in uncompensable, but not compensable, conditions

2019 ◽  
Vol 127 (4) ◽  
pp. 1095-1106 ◽  
Author(s):  
Nicholas Ravanelli ◽  
Geoff Coombs ◽  
Pascal Imbeault ◽  
Ollie Jay
Keyword(s):  
Heat Stress ◽  
Core Temperature ◽  
Heat Dissipation ◽  
Sweat Rate ◽  
Heat Acclimation ◽  
Whole Body ◽  
Evaporative Heat Loss ◽  
Physiological Limits ◽  
Physiological Capacity ◽  
Uncompensable Heat Stress

This study assessed whether, notwithstanding lower resting absolute core temperatures, alterations in time-dependent changes in thermoregulatory responses following partial and complete heat acclimation (HA) are only evident during uncompensable heat stress. Eight untrained individuals underwent 8 wk of aerobic training (i.e., partial HA) followed by 6 days of HA in 38°C/65% relative humidity (RH) (i.e., complete HA). On separate days, esophageal temperature (Tes), arm (LSRarm), and back (LSRback) sweat rate, and whole body sweat rate (WBSR) were measured during a 45-min compensable (37°C/30% RH) and 60-min uncompensable (37°C/60% RH) heat stress trial pre-training (PRE-TRN), post-training (POST-TRN), and post–heat acclimation (POST-HA). For compensable heat stress trials, resting Tes was lower POST-TRN (36.74 ± 0.27°C, P = 0.05) and POST-HA (36.60 ± 0.27°C, P = 0.001) compared with PRE-TRN (36.99 ± 0.19°C); however, ΔTes was similar in all trials (PRE-TRN:0.40 ± 0.23°C; POST-TRN:0.42 ± 0.20°C; POST-HA:0.43 ± 0.12°C, P = 0.97). While LSRback was unaltered by HA ( P = 0.94), end-exercise LSRarm was higher POST-TRN (0.70 ± 0.14 mg·cm−2·min−1, P < 0.001) and POST-HA (0.75 ± 0.16 mg·cm−2·min−1, P < 0.001) compared with PRE-TRN (0.61 ± 0.15 mg·cm−2·min−1). Despite matched evaporative heat balance requirements, steady-state WBSR (31st–45th min) was greater POST-TRN (12.7 ± 1.0 g/min, P = 0.02) and POST-HA (12.9 ± 0.8 g/min, P = 0.004), compared with PRE-TRN (11.7 ± 0.9 g/min). For uncompensable heat stress trials, resting Tes was lower POST-TRN (36.77 ± 0.22°C, P = 0.05) and POST-HA (36.62 ± 0.15°C, P = 0.03) compared with PRE-TRN (36.86 ± 0.24°C). But ΔTes was smaller POST-TRN (0.77 ± 0.19°C, P = 0.05) and POST-HA (0.75 ± 0.15°C, P = 0.04) compared with PRE-TRN (1.10 ± 0.32°C). LSRback and LSRarm increased with HA ( P < 0.007), supporting the greater WBSR with HA (POST-TRN:14.4 ± 2.4 g/min, P < 0.001; POST-HA:16.8 ± 2.8 g/min, P < 0.001) compared with PRE-TRN (12.7 ± 3.2 g/min). In conclusion, the thermal benefits of HA are primarily evident when conditions challenge the physiological capacity to dissipate heat. NEW & NOTEWORTHY We demonstrate that neither partial nor complete heat acclimation alters the change in core temperature during compensable heat stress compared with an unacclimated state, despite a marginally greater whole body sweat rate. However, the greater local and whole body sweat rate with partial and complete heat acclimation reduced the rise in core temperature during 60 min of uncompensable heat stress compared with an unacclimated state, suggesting the improvements in heat dissipation associated with heat acclimation are best observed when the upper physiological limits for evaporative heat loss are challenged.

Cardiac function during heat stress: impact of short-term passive heat acclimation

2020 ◽  
Vol 319 (4) ◽  
pp. H753-H764
Author(s):  
Lukas D. Trachsel ◽  
Hadiatou Barry ◽  
Hugo Gravel ◽  
Parya Behzadi ◽  
Christine Henri ◽  
...  
Keyword(s):  
Heart Rate ◽  
Heat Stress ◽  
Cardiac Function ◽  
Systolic Function ◽  
Heat Exposure ◽  
Heat Acclimation ◽  
Passive Heating ◽  
Lower Heart Rate ◽  
Short Term ◽  
Cardiac Systolic Function

A lower heart rate during heat exposure is a classic marker of heat acclimation (HA). It remains unknown if improved cardiac function contributes to this response. A 7-day passive HA protocol did not alter cardiac systolic function during passive heating, whereas it improved some indexes of diastolic function in young adults. Nonetheless, heart rate during heating was unaffected by HA. These results suggest that passive HA induces limited adaptations in cardiac function during passive heating.

Short term heat acclimation reduces heat stress, but is not augmented by dehydration

2018 ◽  
Vol 78 ◽  
pp. 227-234 ◽  
Author(s):  
Michael W. Schleh ◽  
Brent C. Ruby ◽  
Charles L. Dumke
Keyword(s):  
Heat Stress ◽  
Heat Acclimation ◽  
Short Term

Effect of vapor pressure on physiologic strain and body heat storage

1963 ◽  
Vol 18 (4) ◽  
pp. 808-811 ◽  
Author(s):  
John F. Hall
Keyword(s):  
Heart Rate ◽  
Heat Stress ◽  
Vapor Pressure ◽  
Heat Storage ◽  
Sweat Rate ◽  
Human Subjects ◽  
Vapor Pressures ◽  
Physiologic Responses ◽  
Male Subjects ◽  
Body Heat

Physiologic strain, in terms of body heat storage, and other physiologic responses were measured and compared in two series of heat stress experiments performed on human subjects exposed to different ambient vapor pressures. One group of 75 experiments conducted on 5 healthy nonacclimatized male subjects exposed 5 times each to 38, 54, and 71 C at 10 mm Hg vapor pressure was compared with a series of 81 experiments performed on 10 similar subjects exposed 1–6 times each to 38, 41, and 54 C at 20 mm Hg vapor pressure. Subjects were sitting and wore 1.0 clo insulation. The data show relation between body heat storage and a) the modified Craig index of physiologic strain; b) over-all sweat rate; c) evaporative rate; d) sweat-evaporative ratio; e) mean skin and rectal temperatures; and f) change of heart rate at the respective vapor pressure levels. Statistically significant correlation between sweat-evaporative ratio and over-all sweat rate with body heat storage is shown. Use of the over-all sweat response as a physiologic strain index is suggested. Submitted on August 14, 1962

Heat storage in horses during submaximal exercise before and after humid heat acclimation

2000 ◽  
Vol 89 (6) ◽  
pp. 2283-2293 ◽  
Author(s):  
Raymond J. Geor ◽  
Laura Jill McCutcheon ◽  
Gayle L. Ecker ◽  
Michael I. Lindinger
Keyword(s):  
Heat Stress ◽  
Heat Storage ◽  
Heat Acclimation ◽  
Exercise Duration ◽  
Moderate Intensity ◽  
Moderate Intensity Exercise ◽  
Exercise Tests ◽  
Before And After ◽  
Uncompensable Heat Stress ◽  
Globe Temperature

The effect of humid heat acclimation on thermoregulatory responses to humid and dry exercise-heat stress was studied in six exercise-trained Thoroughbred horses. Horses were heat acclimated by performing moderate-intensity exercise for 21 days in heat and humidity (HH) [34.2–35.7°C; 84–86% relative humidity (RH); wet bulb globe temperature (WBGT) index ∼32°C]. Horses completed exercise tests at 50% of peak O2 uptake until a pulmonary arterial temperature (Tpa) of 41.5°C was attained in cool dry (CD) (20–21.5°C; 45–50% RH; WBGT ∼16°C), hot dry (HD 0) [32–34°C room temperature (RT); 45–55% RH; WBGT ∼25°C], and HH conditions (HH 0), and during the second hour of HH on days 3, 7, 14, and 21, and in HD on the 18th day (HD 18) of heat acclimation. The ratios of required evaporative capacity to maximal evaporative capacity of the environment (Ereq/Emax) for CD, HD, and HH were ∼1.2, 1.6, and 2.5, respectively. Preexercise Tpa and rectal temperature were ∼0.5°C lower ( P < 0.05) on days 7, 14, and 21 compared with day 0. With exercise in HH, there was no effect of heat acclimation on the rate of rise in Tpa (and therefore exercise duration) nor the rate of heat storage. In contrast, exercise duration was longer, rate of rise in Tpa was significantly slower, and rate of heat storage was decreased on HD 18 compared with HD 0. It was concluded that, during uncompensable heat stress in horses, heat acclimation provided modest heat strain advantages when Ereq/Emax was ∼1.6, but at higher Ereq/Emax no advantages were observed.

Low doses of melatonin and diurnal effects on thermoregulation and tolerance to uncompensable heat stress

1999 ◽  
Vol 87 (1) ◽  
pp. 308-316 ◽  
Author(s):  
Tom M. McLellan ◽  
Greg A. Gannon ◽  
Jiri Zamecnik ◽  
Valerie Gil ◽  
Greg M. Brown
Keyword(s):  
Heat Stress ◽  
Protective Clothing ◽  
Low Dose ◽  
Heart Rate Response ◽  
Heat Storage ◽  
Stress Exposure ◽  
Double Blind ◽  
Early Afternoon ◽  
Hypothermic Effect ◽  
Uncompensable Heat Stress

This study examined whether the reported hypothermic effect of melatonin ingestion increased tolerance to exercise at 40°C, for trials conducted either in the morning or afternoon, while subjects were wearing protective clothing. Nine men performed four randomly ordered trials; two each in the morning (0930) and afternoon (1330) after the double-blind ingestion of either two placebo capsules or two 1-mg capsules of melatonin. Despite significant elevations in plasma melatonin to over 1,000 ng/ml 1 h after the ingestion of the first 1-mg dose, rectal temperature (Tre) was unchanged before or during the heat-stress exposure. Also, all other indexes of temperature regulation and the heart rate response during the uncompensable heat stress were unaffected by the ingestion of melatonin. Initial Tre was increased during the afternoon (37.1 ± 0.2°C), compared with the morning (36.8 ± 0.2°C) exposures, and these differences remained throughout the uncompensable heat stress, such that final Tre was also increased for the afternoon (39.2 ± 0.2°C) vs. the morning (39.0 ± 0.3°C) trials. Tolerance times and heat storage were not different among the exposures at ∼110 min and 16 kJ/kg, respectively. It was concluded that this low dose of melatonin had no impact on tolerance to uncompensable heat stress and that trials conducted in the early afternoon were associated with an increased Tre tolerated at exhaustion that offset the circadian influence on resting Tre and thus maintained tolerance times similar to those of trials conducted in the morning.

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