EFFECTIVENESS OF AN INTERMITTENT EXERCISE-HEAT EXPOSURE PROTOCOL TO SUSTAIN HEAT ACCLIMATION ADAPTATIONS

We examined if the change in heart rate variability (HRV) during passive heat exposure is modified by hot water heat acclimation (HA). Sixteen healthy adults (28 ± 5 years, 5 females/11 males) underwent heat exposure in a water-perfused suit, pre and post 7 days of HA (60 minutes at rectal temperature ≥38.6°C). During passive heat exposure, heart rate (HR), the standard deviation of NN intervals (SDNN), the square root of the mean squared differences of successive NN intervals (RMSSD) and the power in the high frequency range (HF) were measured. No difference in HR (P=0.22), SDNN (P=0.87), RMSSD (P=0.79) and HF (P=0.23) was observed at baseline. The increase in HR (pre-HA: 43 ± 10, post-HA: 42 ± 9 bpm, P=0.57) and the decrease of SDNN (pre-HA: -54.1 ± 41.0, post-HA: -52.2 ± 36.8 ms, P=0.85), RMSSD (pre-HA: -70.8 ± 49.5, post-HA: -72.7 ± 50.4 ms, P=0.91) and HF (pre-HA: -28.0 ± 14.5, post-HA: -23.2 ± 17.1%, P=0.27) were not different between experimental visits at fixed increases in esophageal temperature. These results suggest that 7 consecutive days of hot water HA does not modify the change in HRV indices during passive heat exposure. Novelty bullets: - It remains unclear if heat acclimation alters the change in heart rate variability that occurs during passive heat exposure. - At matched levels of thermal strain, 7 consecutive days of hot water immersion did not modulate the change in indices of heart rate variability during passive heat exposure.

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Passive heat acclimation improves skeletal muscle contractility in humans

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00431.2016 ◽

2017 ◽

Vol 312 (1) ◽

pp. R101-R107 ◽

Cited By ~ 19

Author(s):

S. Racinais ◽

M. G. Wilson ◽

J. D. Périard

Keyword(s):

Skeletal Muscle ◽

Contractile Function ◽

Sweat Rate ◽

Heat Exposure ◽

Heat Acclimation ◽

Voluntary Activation ◽

Ambient Conditions ◽

Muscle Contractility ◽

C 50 ◽

Time Required

The aim of this study was to investigate the effect of repeated passive heat exposure (i.e., acclimation) on muscle contractility in humans. Fourteen nonheat-acclimated males completed two trials including electrically evoked twitches and voluntary contractions in thermoneutral conditions [Cool: 24°C, 40% relative humidity (RH)] and hot ambient conditions in the hyperthermic state (Hot: 44–50°C, 50% RH) on consecutive days in a counterbalanced order. Rectal temperature was ~36.5°C in Cool and was maintained at ~39°C throughout Hot. Both trials were repeated after 11 days of passive heat acclimation (1 h per day, 48–50°C, 50% RH). Heat acclimation decreased core temperature in Cool (−0.2°C, P < 0.05), increased the time required to reach 39°C in Hot (+9 min, P < 0.05) and increased sweat rate in Hot (+0.7 liter/h, P < 0.05). Moreover, passive heat acclimation improved skeletal muscle contractility as evidenced by an increase in evoked peak twitch amplitude both in Cool (20.5 ± 3.6 vs. 22.0 ± 4.0 N·m) and Hot (20.5 ± 4.7 vs. 22.0 ± 4.0 N·m) (+9%, P < 0.05). Maximal voluntary torque production was also increased both in Cool (145 ± 42 vs. 161 ± 36 N·m) and Hot (125 ± 36 vs. 145 ± 30 N·m) (+17%, P < 0.05), despite voluntary activation remaining unchanged. Furthermore, the slope of the relative torque/electromyographic linear relationship was improved postacclimation ( P < 0.05). These adjustments demonstrate that passive heat acclimation improves skeletal muscle contractile function during electrically evoked and voluntary muscle contractions of different intensities both in Cool and Hot. These results suggest that repeated heat exposure may have important implications to passively maintain or even improve muscle function in a variety of performance and clinical settings.

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Intermittent exercise-heat exposures and intense physical activity sustain heat acclimation adaptations

Journal of Science and Medicine in Sport ◽

10.1016/j.jsams.2018.06.009 ◽

2019 ◽

Vol 22 (1) ◽

pp. 117-122 ◽

Cited By ~ 11

Author(s):

J. Luke Pryor ◽

Riana R. Pryor ◽

Lesley W. Vandermark ◽

Elizabeth L. Adams ◽

Rachel M. VanScoy ◽

...

Keyword(s):

Physical Activity ◽

Intermittent Exercise ◽

Heat Acclimation ◽

Intense Physical Activity

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IS INTERMITTENT HEAT EXPOSURE AS EFFECTIVE AS DAILY HEAT EXPOSURE FOR HEAT ACCLIMATION? 563

Medicine & Science in Sports & Exercise ◽

10.1097/00005768-199705001-00562 ◽

1997 ◽

Vol 29 (Supplement) ◽

pp. 97

Author(s):

N. Gill ◽

G. Sleivert

Keyword(s):

Heat Exposure ◽

Heat Acclimation

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Passive heat acclimation does not modulate processing speed and executive functions during cognitive tasks performed at fixed levels of thermal strain

Applied Physiology Nutrition and Metabolism ◽

10.1139/apnm-2021-0243 ◽

2021 ◽

Author(s):

Hadiatou Barry ◽

Philippe Gendron ◽

Christine Gagnon ◽

Louis Bherer ◽

Daniel Gagnon

Keyword(s):

Reaction Time ◽

Executive Functions ◽

Cognitive Performance ◽

Core Temperature ◽

Processing Speed ◽

Heat Exposure ◽

Thermal Strain ◽

Heat Acclimation ◽

Cognitive Tasks ◽

Time Control

Purpose: This study evaluated if passive controlled hyperthermia heat acclimation (HA) modulates cognitive performance during passive heat stress. Methods: Eight healthy adults (25 ± 4 years) underwent 7 consecutive days of hot water immersion (core temperature ≥38.6°C) and a 7-day time-control period. On days 1 and 7 of HA, participants performed a digital Stroop test at baseline, when core temperature reached 38.6°C, and after 60 minutes at a core temperature ≥38.6°C to evaluate reaction time during tasks targeting processing speed (reading and counting) and executive functions (inhibition and switching). On days 1 and 7 of the time-control intervention, participants performed the Stroop test with equivalent amounts of time separating each task as for HA. Results: During day 1 of HA, reaction time was quicker during the reading (-44 ms [-71, -17], P<0.01) and counting (-39 ms [-76, -2], P=0.04) tasks when rectal temperature reached 38.6°C, but after a further 60 minutes of heat exposure, reaction time only remained quicker during the reading task (-56 ms [-83, -29], P<0.01). Changes in reaction time during heat exposure were unaffected by subsequent HA (interaction, all P≥0.09). Conclusion: Seven days of HA does not modulate processing speed and executive functions during passive heat exposure. Novelty: - Whether heat acclimation (HA) to improve cognitive performance during heat exposure remains understudied. - We tested the hypothesis that HA modulates reaction time during cognitive tasks performed at matched levels of thermal strain. - Despite classical signs of HA, reaction time during heat exposure was unaffected by HA.

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Effects of Heat Acclimatization, Heat Acclimation, and Intermittent Exercise Heat Training on Time-Trial Performance

Sports Health A Multidisciplinary Approach ◽

10.1177/19417381211050643 ◽

2021 ◽

pp. 194173812110506

Author(s):

Yasuki Sekiguchi ◽

Courteney L. Benjamin ◽

Ciara N. Manning ◽

Jeb F. Struder ◽

Lawrence E. Armstrong ◽

...

Keyword(s):

Intermittent Exercise ◽

Statistical Significance ◽

Time Trial ◽

Heat Acclimation ◽

Peak Oxygen Consumption ◽

Heat Acclimatization ◽

Level Of Evidence ◽

Time Trial Performance ◽

Level 3 ◽

Trial Performance

Background: The purpose of this study was to investigate effects of heat acclimatization (HAz) followed by heat acclimation (HA), and intermittent heat training (IHT) on time-trial performance. Hypothesis: Time-trial performance will improve after HA and will further improve with twice a week of IHT. Study Design: Interventional study. Level of Evidence: Level 3. Methods: A total of 26 male athletes (mean ± SD; age, 35 ± 12 years; body mass, 72.8 ± 8.9 kg; peak oxygen consumption [VO2peak], 57.3 ± 6.7 mL·kg−1·min−1) completed five 4-km time trials (baseline, post-HAz, post-HA, post-IHT4, post-IHT8) in the heat (ambient temperature, 35.4°C ± 0.3°C; relative humidity, 46.7% ± 1.2%) on a motorized treadmill. After baseline time trial, participants performed HAz (109 ± 10 days) followed by post-HAz time trial. Then, participants completed 5 days of HA, which involved exercising to induce hyperthermia (38.50°C-39.75°C) for 60 minutes. Participants were then divided into 3 groups and completed IHT either twice per week (IHTMAX), once per week (IHTMIN), or not at all (IHTCON) over an 8-week period. The exercise used for the IHT matched the HA. Four-kilometer time trials were performed after 4 weeks (post-IHT4) and 8 weeks of IHT (post-IHT8). Results: Time trial was faster in post-HA (17.98 ± 2.51 minutes) compared with baseline (18.61 ± 3.06 minutes; P = 0.037) and post-HAz (18.66 ± 3.12 minutes; P = 0.023). Percentage change in time trial was faster in IHTMAX (−3.9% ± 5.2%) compared with IHTCON (11.5% ± 16.9%) ( P = 0.020) and approached statistical significance with large effect (effect size = 0.96) compared with IHTMIN (1.6% ± 6.2%; P = 0.059) at post-IHT8. Additionally, IHTMAX (−2.2% ± 4.2%) was faster than IHTCON (3.6% ± 6.9%) ( P = 0.05) at post-IHT4. Conclusion: These results indicate that HA after HAz induces additional improvement in time-trial performance. IHT twice per week shows improvement after 8 weeks, while once per week maintains performance for 8 weeks. No IHT results in a loss of adaptations after 4 weeks and even greater losses after 8 weeks. Clinical Relevance: HA after HAz improves time-trial performance, twice a week of IHT improves performance further, and once a week of IHT maintains performance for at least 8 weeks.

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Core temperature and sweating onset in humans acclimated to heat given at a fixed daily time

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.1999.276.4.r1095 ◽

1999 ◽

Vol 276 (4) ◽

pp. R1095-R1101 ◽

Cited By ~ 17

Author(s):

Osamu Shido ◽

Naotoshi Sugimoto ◽

Minoru Tanabe ◽

Sotaro Sakurada

Keyword(s):

Relative Humidity ◽

Core Temperature ◽

Heat Exposure ◽

Heat Acclimation ◽

Fixed Time ◽

Temperature Level ◽

Before And After ◽

Thermoregulatory Function ◽

Daily Time ◽

Made In

The thermoregulatory functions of rats acclimated to heat given daily at a fixed time are altered, especially during the period in which they were previously exposed to heat. In this study, we investigated the existence of similar phenomena in humans. Volunteers were exposed to an ambient temperature (Ta) of 46°C and a relative humidity of 20% for 4 h (1400–1800) for 9–10 consecutive days. In the first experiment, the rectal temperatures (Tre) of six subjects were measured over 24 h at a Ta of 27°C with and without heat acclimation. Heat acclimation significantly lowered Tre only between 1400 and 1800. In the second experiment, six subjects rested in a chair at a Ta of 28°C and a relative humidity of 40% with both legs immersed in warm water (42°C) for 30 min. The Tre and sweating rates at the forearm and chest were measured. Measurements were made in the morning (0900–1100) and afternoon (1500–1700) on the same day before and after heat acclimation. Heat acclimation shortened the sweating latency and decreased the threshold Tre for sweating. However, these changes were significant only in the afternoon. The results suggest that repeated heat exposure in humans, limited to a fixed time daily, alters the core temperature level and thermoregulatory function, especially during the period in which the subjects had previously been exposed to heat.

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Norepinephrine turnover in heart and spleen of 7-, 22-, and 34 degree C-acclimated hamsters

American Journal of Physiology-Legacy Content ◽

10.1152/ajplegacy.1976.230.3.564 ◽

1976 ◽

Vol 230 (3) ◽

pp. 564-568 ◽

Cited By ~ 14

Author(s):

SB Jones ◽

XJ Musacchia

Keyword(s):

Sympathetic Nerve Activity ◽

Heat Exposure ◽

Heat Acclimation ◽

Competitive Inhibitor ◽

Turnover Time ◽

Turnover Rates ◽

Nerve Activity ◽

Spleen Tissue ◽

Tissue Levels ◽

Norepinephrine Turnover

Norepinephrine (NE) turnover rates were measured in hamster (Mesocricetus auratus) heart and spleen tissues after 7-10 wk exposure to 7, 22, and 34 degrees C. The competitive inhibitor of NE synthesis, alpha-methyl-p-tyrosine methyl ester, was injected (200 mg/kg, ip) into acclimated animals. At sequential periods after drug treatment, hamsters were sacrificied by cervical transection, and tissues were removed and frozen. Rate constants, turnover time, and turnover rates were determined from regression analysis of NE tissue decay. Heart NE turnover was highest in cold-acclimated and lowest in heat-exposed animals (.111 and .047 mug/g per h, respectively), control values being intermediate (.081 mug/g per h). NE turnover is inversely related to tissue levels in the myocardium of temperature-acclimated hamsters. Spleen NE turnover was lowest with heat exposure, but spleen tissue levels of NE with heat or cold exposure were not different from control (22 degrees C) measurements. Heat-acclimated hamsters are slightly hyperthermic (Tre, 37.24 +/- .18 C; P, .032) compared to 22 degrees C-maintained controls (Tre, 36.85 +/- .07), but body temperature were unchanged with 7 degrees C exposure. The implication of altered NE synthesis is that sympathetic nerve activity is decreased with heat acclimation and increased with cold acclimation.

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