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We examined whether daily hot water immersion (HWI) after exercise in temperate conditions induces heat acclimation and improves endurance performance in temperate and hot conditions. Seventeen non-heat-acclimatized males performed a 6-day intervention involving a daily treadmill run for 40 min at 65% V̇O2max in temperate conditions (18 °C) followed immediately by either HWI (N = 10; 40 °C) or thermoneutral (CON, N = 7; 34 °C) immersion for 40 min. Before and after the 6-day intervention, participants performed a treadmill run for 40 min at 65% V̇O2max followed by a 5-km treadmill time trial (TT) in temperate (18 °C, 40% humidity) and hot (33 °C, 40% humidity) conditions. HWI induced heat acclimation demonstrated by lower resting rectal temperature (Tre , mean, -0.27 °C, P < 0.01), and final Tre during submaximal exercise in 18 °C (-0.28 °C, P < 0.01) and 33 °C (-0.36 °C, P < 0.01). Skin temperature, Tre at sweating onset and RPE were lower during submaximal exercise in 18 °C and 33 °C after 6 days in HWI (P < 0.05). Physiological strain and thermal sensation were also lower during submaximal exercise in 33 °C after 6 days in HWI (P < 0.05). HWI improved TT performance in 33 °C (4.9%, P < 0.01) but not in 18 °C. Thermoregulatory measures and performance did not change in CON. Hot water immersion after exercise on 6 days presents a simple, practical, and effective heat acclimation strategy to improve endurance performance in the heat.
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Post-exercise hot water immersion induces heat acclimation and improves endurance
exercise performance in the heat
Michael J. Zurawlew, Neil P. Walsh, Matthew B. Fortes and Claire Potter
College of Health and Behavioural Sciences, Bangor University, Bangor, Gwynedd, LL57
2PZ, UK.
Corresponding Author:
Prof. Neil P. Walsh FACSM
College of Health and Behavioural Sciences,
Bangor University,
Bangor,
LL57 2PZ,
UK.
Email: n.walsh@bangor.ac.uk
Telephone: + 44 1248 383480
Running Head: Heat acclimation by post-exercise hot bath
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Abstract
We examined whether daily hot-water-immersion (HWI) after exercise in temperate
conditions induces heat-acclimation and improves endurance performance in temperate and
hot conditions. Seventeen non-heat-acclimatized males performed a 6-day intervention
involving a daily treadmill-run for 40min at 65% V
̇O2max in temperate conditions (18°C)
followed immediately by either HWI (N=10; 40°C) or thermoneutral (CON, N=7; 34°C)
immersion for 40min. Before and after the 6-day intervention, participants performed a
treadmill run for 40min at 65% V
̇O2max followed by a 5 km treadmill-time-trial (TT) in
temperate (18°C, 40% humidity) and hot (33°C, 40% humidity) conditions. HWI induced
heat-acclimation demonstrated by lower resting rectal temperature (Tre, mean, -0.27°C,
P<0.01), and final Tre during submaximal exercise in 18°C (-0.28°C, P<0.01) and 33°C (-
0.36°C, P<0.01). Skin temperature, Tre at sweating onset and RPE were lower during
submaximal exercise in 18°C and 33°C after 6-days in HWI (P<0.05). Physiological strain
and thermal sensation were also lower during submaximal exercise in 33°C after 6-days in
HWI (P<0.05). HWI improved TT performance in 33°C (4.9%, P<0.01) but not in 18°C.
Thermoregulatory measures and performance did not change in CON. Hot-water-immersion
after exercise on 6-days presents a simple, practical and effective heat-acclimation strategy to
improve endurance performance in the heat.
Keywords: Thermoregulation; Hyperthermia; Performance; Running; Heat illness; Hot bath
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Introduction
Athletes, military personnel and firefighters are often required to perform in the heat which
increases physiological demands and places substantial strain on heat loss mechanisms
(Cheung et al., 2000). To reduce the risk of exertional-heat-illness (EHI) and improve
exercise capability in the heat these individuals often prepare by completing an exercise heat
acclimation protocol. The adaptive responses to exercise heat acclimation have been widely
documented (Taylor, 2014) and include, but are not limited to, earlier onset of cutaneous
vasodilatation and sweating, increases in sweating rate, reduced cardiovascular strain and, in-
turn, reduced core temperature and physiological strain, and improved endurance capacity
during exercise in the heat. Some evidence supports the notion that the ergogenic benefit of
exercise heat acclimation extends to endurance performance in cool conditions (Lorenzo et
al., 2010) but recent evidence presents a mixed picture (Neal et al., 2015).
Conventional exercise heat acclimation protocols typically consist of a daily bout of exercise
heat stress over a five to sixteen day period where core temperature is increased for 1-2 hours
in an artificial hot environment (Nadel et al., 1974; Garrett et al., 2009). Exercise intensity
during heat acclimation programs has either been fixed, self-regulated or manipulated to
control a precise level of hyperthermia (Fox et al., 1963; Taylor, 2014). Owing to the ensuing
adaptations, the training stimulus typically decreases during conventional exercise heat
acclimation programs (Taylor, 2014). The controlled hyperthermia technique has the
advantage that the adaptive stimulus is kept constant (core temperature clamped at 38.5°C)
(Fox et al., 1963), thus optimizing adaptation and, via thermal clamping, affording greater
insight into underlying mechanisms (Taylor, 2014). However, there are practical
disadvantages using these exercise heat acclimation protocols. These protocols can be costly
and impractical for non-acclimated individuals residing in cooler climates as their completion
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requires access to an environmental chamber or temporary relocation to a hotter climate to
complete the protocol. The controlled hyperthermia technique also requires measurement of
core temperature and control of exercise intensity to maintain core temperature at 38.5°C.
One novel, as yet unexplored, approach to these practical limitations may be to have non-
heat-acclimated individuals undertake hot-water immersion (HWI) immediately after daily
exercise performed in temperate conditions. Related support comes from studies showing
thermoregulatory adaptations (Fox et al., 1963; Beaudin et al., 2009) and performance
benefits (Scoon et al., 2007) of passive heat acclimation using controlled hyperthermia in a
vapor barrier suit or sauna, but these methods are somewhat limited in terms of their
practicality and no measures of thermoregulation were reported during exercise-heat-stress
(Scoon et al., 2007; Beaudin et al., 2009). Support also comes from studies in non-heat-
acclimated individuals showing that repeated HWI over a 10-14 day period decreased core
temperature at rest before and during HWI (Brebner et al., 1961; Brazaitis and Skurvydas,
2010) and during light exercise in the heat (Brebner et al., 1961; Bonner et al., 1976). More
practical alternatives for heat acclimation would be welcome as these studies used a thermal
clamp (Bonner et al., 1976) and an unpleasant 44°C HWI protocol (Brazaitis and Skurvydas,
2010). Extending beyond the obvious practical advantages, as combined elevations in core
temperature and skin temperature are important to achieve full heat acclimation (Fox et al.,
1964; Regan et al., 1996) there is a reasoned physiological argument for why HWI (elevated
skin temperature) immediately after daily exercise in temperate conditions (elevated core
temperature) might elicit favorable heat acclimation responses.
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To date, no study has investigated whether a daily HWI following exercise in temperate
conditions induces heat acclimation and improvements in endurance exercise performance in
hot and temperate conditions. To this end, we hypothesized that a short-term (6-day) protocol
involving a 40 min HWI each day after sub-maximal treadmill running in a temperate
laboratory would induce heat acclimation and performance improvements during a 5 km
treadmill time-trial in both temperate (18°C) and hot (33°C) conditions.
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Methods
Participants
Seventeen physically active, non-heat acclimatized males who had not been exposed to hot
environmental conditions in the past 3 months and completed two or more hours of
endurance exercise per week were recruited to participate in the study. Participants were
randomly assigned to either a 6-day hot water immersion (HWI: N = 10; mean ± SD, age: 23
± 3 years; body mass: 69.5 ± 6.9 kg; V
̇O2max 60.5 ± 6.8 mL/kg/min) or control (CON: N = 7;
age: 23 ± 3 years; body mass: 72.1 ± 5.8 kg; V
̇O2max 60.1 ± 8.9 mL/kg/min) intervention. A
6-day intervention was completed to align with other short-term heat acclimation protocols
(Aoyagi et al., 1995; Cotter et al., 1997). There were no differences in the characteristics of
participants in HWI and CON. The study received local ethical approval and was conducted
in accordance with the Declaration of Helsinki (2013). All participants provided full written
informed consent, were healthy, non-smokers, free from any known cardiovascular or
metabolic diseases and were not taking any medication.
Study design
Prior to (days -3 to -1), and following (days +1 to +4) the 6-day HWI or CON intervention,
experimental trials were completed in temperate (18.0 ± 0.1 °C, 42.5 ± 3.6 % RH) and hot
conditions (33.0 ± 0.3 °C, 40.2 ± 0.7 % RH; Fig. 1). Experimental trials included a 40 min
submaximal run followed by 60 min rest, then a 5 km treadmill time trial (TT). On the day of,
and in the 24 hours prior to experimental trials (Fig. 1) no alcohol, any form of diuretics,
caffeine or tobacco were consumed and no exercise, other than that prescribed, was
undertaken. During the intervention (days 1 to 6) participants were required to consume their
normal diet and fluid intake, including caffeine and alcohol (≤ 3 units per day) and to reduce
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their regular training by the volume of endurance exercise completed during the intervention
in the laboratory.
*** Fig. 1 near here ***
Preliminary measurements and familiarization
A continuous incremental exercise test on a motorized treadmill (HP Cosmos Mercury 4.0,
Nussdorf-Traunstein, Germany) was used to assess V
̇O2max in temperate conditions (19 °C,
42 % RH), as described (Fortes et al., 2013). Using the interpolation of the running speed
V
̇O2 relationship, the running speed that elicited 65 % V
̇O2max was then determined and
verified 30 min later. This individualized running speed was used for both the submaximal
exercise during experimental trials and in the daily exercise throughout the 6-day
intervention. Following the speed verification, participants rested in the laboratory for 60
min. During this time they were familiarized with the speed controls of the treadmill within
the environmental chamber (Delta Environmental Systems, Chester, UK) and with all
instrumentation and procedures used in the experimental trials. Participants then entered the
environmental chamber (18 °C, 40 % RH) and completed a maximal effort 5 km treadmill TT
at self-selected intensities. One familiarization was deemed sufficient to mitigate against
learning effects (Laursen et al., 2007). The chamber was silent and the only information the
participant received was the distance covered displayed on a screen in front of them.
Experimental trials
Participants completed a diet diary in the 24 h prior to their first experimental trial (Fig. 1)
and were asked to replicate this prior to further experimental trials. On the day of each
experimental trial, participants arrived at the laboratory at 0730 h fasted. They were provided
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with a standardized breakfast (0.03 MJ/kg) and a bolus of water equivalent to 7 mL/kg of
body mass. At 0800 h on days -1 and +2 a venous blood sample was taken without stasis
following a seated rest. Urine samples were collected on all experimental trials and analyzed
for urine specific gravity (USG) using a handheld refractometer (Atago Uricon-Ne
refractometer, NSG Precision cells, New York, USA). A pre-exercise nude body mass
(NBM) was taken using a digital platform scale (Model 705; Seca, Hamburg, Germany) and
then the participant was instrumented for the exercise protocol. At 0845 h, dressed in T-shirt,
running shorts, socks and shoes participants rested for 15 min in the laboratory (18 °C) to
establish baseline measures.
Submaximal exercise. At 0900 h dressed in running shorts, socks and shoes the participant
entered the environmental chamber that was maintained at either 18 °C, 40 % RH or 33 ºC,
40 % RH and completed a 40 min 65 % V
̇O2max treadmill run (1 % gradient; (Jones and
Doust, 1996). During this time no fluids were consumed. Rectal temperature (Tre), skin
temperatures (Tsk) and heart rate (HR) were monitored continuously and rating of perceived
exertion (RPE) (Borg, 1970) and thermal sensation (TS) (Hollies and Goldman, 1977) were
recorded every 5 min. V
̇O2, and respiratory exchange ratio (RER) were assessed from 60 s
expired gas samples collected by Douglas bag method at 9-10, 19-20, 29-30 and 39-40 min of
exercise. Local forearm sweat rate was measured every 20 s for the first 15 min of exercise to
assess the onset of sweating. Immediately following exercise a finger prick blood sample was
taken and assessed for blood lactate concentration. A NBM was then taken to estimate whole
body sweat losses, and the participant sat quietly in the laboratory in temperate conditions (18
°C) dressed in T-shirt, running shorts, socks and shoes for 60 min. A single bolus of water (5
mL/kg body mass) was consumed within the first 20 min of this rest period.
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5 km treadmill time trial. The TT was completed immediately following a NBM after the
60 min rest period. The participant re-entered the environmental chamber dressed in running
shorts, socks and shoes and completed the TT run on a motorized treadmill (1 % gradient) at
self-selected speeds. Participants were instructed to run the 5 km TT as quickly as possible.
No feedback other than the distance covered was provided. No fluids were consumed during
the TT. Tre and HR were measured continuously, and on completion a NBM was recorded to
estimate whole body sweat losses. The participant was then provided with water equivalent to
sweat losses and was free to leave the laboratory.
Intervention
The 6-day HWI and CON interventions were completed over consecutive days (days 1 to 6;
Fig. 1). Participants in HWI and CON completed the same submaximal exercise protocol on
each of these days in temperate conditions (18 °C) and a 40 min water immersion (HWI; 40
°C and CON; 34 °C) following its cessation. The CON intervention controlled for any
training and/or hydrostatic effects on subsequent thermoregulatory measures and endurance
performance.
Submaximal exercise. On each day participants reported to the laboratory between 0600 h
and 1000 h. A pre-exercise NBM (after voiding) was taken and after fitting a rectal thermistor
and HR monitor participants rested in the laboratory for 15 min to establish baseline
measures. Participants then ran for 40 min on a motorized treadmill at 65 % V
̇O2max in a
temperate environment (18 °C) dressed in running shorts, socks and shoes. A bolus of water
(5 mL/kg of body mass) was consumed in the first 20 min of exercise, to replicate normal
training procedures, and Tre and HR were monitored continuously. At the cessation of
exercise participants undertook the water immersion (2-3 min transition).
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Water immersion. Following transition, participants were immersed to the neck in a water
bath dressed in shorts. Those completing HWI were immersed in 39.9 ± 0.3 °C water while a
thermoneutral water temperature of 34.1 ± 0.4 °C was used for CON. The water temperature
was maintained during immersions by adding hot or cold water and allowing water to drain to
maintain immersion to neck level, where necessary. The 34 °C water temperature on CON
was chosen as pilot testing showed that Tre returned to baseline after exercise at a similar rate
to sitting in temperate laboratory conditions (18 °C), and thus would not provide any
additional cooling effect. During immersion, no fluids were consumed and Tre and HR were
monitored continuously. Immersion ended after 40 min unless the participant removed
themselves due to discomfort in HWI. Following immersion participants sat in the laboratory
for 15 min without fluids and a NBM was taken and adjusted for fluid intake during the
submaximal exercise in order to estimate whole body sweat loss. Participants were free to
leave the laboratory when Tre ≤ 38.5 °C.
Measurement and instrumentation
Body temperatures. Tre was measured using a flexible, sterile, disposable rectal thermistor
(Henleys Medical Supplies Ltd., Herts, UK) inserted 10 cm beyond the rectal sphincter and
recorded using a data logger (YSI model 4000A, YSI, Dayton, USA). Skin temperatures from
four sites on the right side of the body (on the chest at a midpoint between the acromion
process and the nipple, the lateral mid-bicep, the anterior mid-thigh, and lateral calf) were
measured using insulated thermistors (Grant EUS-U, Cambridge, UK) and recorded using a
portable data logger (Grant SQ2020, Cambridge, UK). Mean Tsk was calculated using a four-
site weighted equation (Ramanathan, 1964).
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Sweating responses. Changes in dry NBM were used to estimate whole body sweating rate
during all intervention days and experimental trials. Local forearm sweating rate was
measured by dew point hygrometry during the submaximal run of experimental trials as
described (Fortes et al., 2013).
Physiological strain. Physiological strain index (PhSI) was calculated using Tre and HR data
collected every 5 min throughout submaximal exercise during experimental trials, as
described (Tikuisis et al., 2002). This index describes physiological strain on a 0 (no strain) to
10 (very high strain) scale.
Blood sample collection and analysis
Venous blood samples were collected from an antecubital vein into an EDTA vacutainer
(BD, Oxford, UK) and aliquots of whole blood were used for the immediate determination of
hemoglobin in duplicate (Hemocue, Sheffield, UK) and hematocrit in triplicate (capillary
tube method). Plasma volume (day -1) was estimated from body mass, as described (Sawka et
al., 1992). The change in plasma volume (day +2) was estimated by correcting the initial
plasma volume for the percentage change in plasma volume as described (Dill and Costill,
1974).
Statistical analysis
Data in the results are presented as mean ± standard deviation (SD), or mean and 90 %
confidence interval of the change for one-tailed tests where stated, and statistical significance
was accepted at P < 0.05. The meaningfulness of the within-subject differences was also
calculated using Cohen’s d effect size with effect sizes greater than 0.2, 0.5 and 0.8
representing small, medium and large effects. Two sample size calculations (G*Power 3.1.2)
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were performed using mean data taken from a 5-day heat acclimation study (Garrett et al.,
2009) and a 5 km treadmill TT reliability study (Laursen et al., 2007). For a one-tailed t-test
with alpha level set at 0.05 and power set at 0.8 a sample size of 9 participants was calculated
to detect a meaningful heat acclimation induced difference in final exercising Tre (0.3 °C). To
detect a meaningful improvement in 5 km treadmill TT performance (set at 3 %) it was
estimated that a sample size of 7 participants was needed. To ensure adequate power for both
key variables, and allowing for dropout, a sample size of 10 participants was used for HWI.
All data were checked for normality and sphericity, and analyzed using t-tests. RER and V
̇O2
were analyzed using one-way repeated measures ANOVA’s with Greenhouse Geisser
correction to the degrees of freedom where necessary. Tukeys HSD or Bonferroni-adjusted
paired t-test post hoc tests were used where appropriate. Sweating threshold was calculated
by plotting individual relationships between local forearm sweating rate and Tre, as described
(Cheuvront et al., 2009). To assess cumulative hyperthermia, area under the curve (AUC)
analysis was performed on the daily Tre (time Tre was >38.5 °C) in HWI (Cheuvront et al.,
2008). Pearson’s correlations were performed to determine the strength of the relationship
between the AUC and the change in hallmark heat acclimation variables e.g. change in
resting Tre. All data was analyzed using SPSS version 20 (IBM Corporation, NY, USA), or
GraphPad Prism Version 5.02 (GraphPad Software Inc. La Jolla, USA).
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Results
Intervention
All participants in HWI and CON completed the 6-day intervention. Tre increased on average
1.13 ± 0.24 °C during 40 min of daily submaximal exercise. Tre increased a further 1.01 ±
0.31 °C during HWI and returned to the pre-exercise resting level during CON immersion (-
1.10 ± 0.26 °C). Total AUC (Tre >38.5 °C during submaximal exercise and immersion) for
the 6-day HWI intervention was 156 ± 83 °C/min and for CON was 2 ± 4 °C/min. Total AUC
in HWI was greater on day 3 compared with day 1 (P = 0.05) but was not different on days 4
to 6 compared with day 1; indicating no significant reduction in the total AUC. No daily
differences for total AUC were observed in CON. Heat acclimation was demonstrated in
HWI by an increase in whole body sweat rate by day 4 (P = 0.02) and an increase in
immersion time by day 3 (P = 0.04; Table 1). By day 5, 9 out of 10 participants completed
the full 40 min immersion in HWI: one participant was removed due to reaching the Tre
safety limit (Tre 39.5 °C). On all other occasions when the 40 min immersion was not
completed participants removed themselves due to discomfort (Table 1). In CON, all
participants completed all 40 min immersions and whole body sweat rate was unchanged
from day 1 (0.39 ± 0.08 L/h).
*** Table 1 near here ***
Experimental trials
Resting responses. Resting Tre was lower following 6-days in HWI in 9 out of 10 participants
with a mean change in resting Tre of -0.27 °C (CI: -0.16 to -0.39 °C, P = 0.001, d = 0.75; Fig.
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2A). There was no change in resting Tre in CON (Fig. 2A). A moderate negative correlation (r
= -0.39) was observed between the total AUC for the 6-day HWI intervention and the
decrease in resting Tre. USG was not different between experimental trials and there was a
modest increase in plasma volume from day -1 to day +2 in HWI (3 ± 5 %, P = 0.05), with no
change in CON (1 ± 3 %, P = 0.31).
*** Fig. 2 near here ***
Submaximal exercise responses. After the 6-day HWI intervention, end Tre during
submaximal exercise was lower in 9 of 10 participants in 18 °C and 10 of 10 participants in
33 °C (Fig. 2B) where change in end Tre was -0.28 °C (CI: -0.16 to -0.40 °C, P = 0.001, d =
0.78) in 18 °C and -0.36 °C (CI: -0.24 to -0.49 °C, : P = 0.0001, d = 0.70) in 33 °C. A modest
negative correlation (r = -0.45) was observed between total AUC for the 6-day HWI
intervention and the decrease in end submaximal exercise Tre in 33 °C. CON demonstrated no
change in end exercise Tre in either 18 °C or 33 °C (Fig. 2B). HWI decreased end exercise Tsk
(18 °C: P = 0.001, d = 0.86; 33 °C: P = 0.001, d = 0.60; Fig. 3C) and decreased Tre at the
onset of sweating in both 18 °C (P = 0.001, d = 0.86; Fig. 3A) and 33 °C (P = 0.02, d = 0.57).
End exercise RPE (18 °C: P = 0.01, d = 0.74; 33 °C: P = 0.04, d = 0.72; Fig. 3E) and HR
were lowered in 18 °C and 33 °C after 6-days in HWI (18 °C: -7, CI: -2 to -11 bpm; P = 0.02,
d = 0.52; 33 °C: -6, CI: -2 to -10 bpm; P = 0.01, d = 0.40) and PhSI (P = 0.01, d = 0.87; Fig.
3D) and TS were lower in 33 °C (P = 0.01, d = 0.70; Fig. 3F). HWI had no effect on V
̇O2 and
RER in 18 °C or 33 °C. There was no effect of CON on any of the above variables (Fig. 3A-
F).
*** Fig. 3 near here ***
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5 km treadmill time trial performance. Endurance exercise performance, assessed via a 5
km treadmill TT, was not altered in CON in either 18 °C (PRE: 1208 ± 191 s and POST:
1216 ± 167 s) or 33 °C (PRE: 1321 ± 219 s and POST: 1299 ± 207 s) indicating no training
effect. One HWI participant did not complete the PRE 33 °C TT and another HWI
participant’s TT data was excluded from analysis due to obvious lack of effort on the POST
18 °C TT (mean % HR max was 82 % compared with 91 % for the group and 96 % for his
PRE 18 °C TT). Endurance exercise performance was impaired in 33 °C compared with 18
°C before the intervention (P = 0.03, d = 0.40; Fig. 4A). The 6-day HWI intervention did not
alter TT performance in 18 °C but improved TT performance in 33 °C (P = 0.01, d = 0.42;
Fig. 4A and B). The 4.9 % improvement in TT performance in 33 °C in HWI restored
performance to the level observed in 18 °C conditions (Fig. 4A). After the 6-day HWI
intervention end TT Tre was lower in 33 °C (-0.17 °C; CI:-0.04 to -0.30; P = 0.02, d = 0.49).
There were no other PRE to POST differences in Tre in HWI or CON during the TT.
*** Fig. 4 near here ***
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Discussion
These novel findings suggest that heat acclimation can be achieved by HWI after exercise in
temperate conditions on 6-days and, as such, presents a practical strategy to improve heat
dissipation and endurance performance in the heat. There are two principal findings in the
present study that support this recommendation. Firstly, we observed clear evidence of heat
acclimation after the HWI intervention demonstrated by lower resting Tre (-0.27 °C) and
lower end submaximal exercise Tre in both 18 °C (-0.28 °C) and 33 °C (-0.36 °C). Hallmark
heat acclimation responses also included a lower set point for sweating onset and reductions
in Tsk, PhSI, RPE and TS during submaximal exercise in the heat after 6-days in HWI.
Adaptations were evident sooner than day 6 of HWI; for example, whole body sweat rate was
increased by day 4 of the intervention. Secondly, the HWI intervention improved 5 km
treadmill TT performance 4.9 % in 33 °C restoring performance to the level achieved in 18
°C. Strengths of this study include control of the time of day for the intervention and
experimental trials and the inclusion of a control group. Including CON provides confidence
that the adaptations shown in HWI were attributed to bathing in hot water after exercise,
since the daily exercise and thermoneutral water immersions completed by CON did not
affect thermoregulatory or performance outcomes. We recognize that the addition of a
traditional exercise heat acclimation group would have enabled for comparisons with the
HWI intervention. Additionally, though we observed a modest expansion of plasma volume
in HWI we recognize the weakness of estimating this using hemoglobin and hematocrit and
recommend tracer techniques be used to verify this finding.
Current recommendations state that heat acclimation should comprise repeated bouts of
exercise in the heat over 1-2 weeks (Racinais et al., 2015). Here the findings suggest that
HWI after exercise in temperate conditions on 6-days presents an alternative heat acclimation
17
strategy that overcomes some of the practical limitations of current heat acclimation
strategies. Hallmarks of successful heat acclimation include a decrease in resting and
exercising core temperature and an improved exercise capacity in the heat (Nadel et al.,
1974). The utility of short-term exercise-heat acclimation protocols lasting 4-6 days has been
investigated (Sunderland et al., 2008; Garrett et al., 2012), since most adaptations occur
within the first 6 days of heat acclimation (Armstrong and Maresh, 1991) and because a
shorter heat acclimation protocol is considered to integrate better into an athletes
training/tapering program. In line with other short-term (Cotter et al., 1997), and traditional,
longer term exercise-heat acclimation protocols (Armstrong and Kenney, 1993), our 6-day
post-exercise HWI intervention, decreased exercise Tre (Fig. 2B), the Tre at the onset of
sweating (Fig. 3A), Tsk (Fig. 3C), PhSI (Fig. 3D) and improved exercise performance in the
heat (Fig. 4A). Furthermore, the thermoregulatory benefits of HWI we observed during
submaximal exercise in the heat were also apparent in temperate (18 °C) conditions (Fig.
2B). There appear to be additional acclimation advantages of the post-exercise HWI
intervention because, unlike some short-term exercise-heat acclimation studies (Sunderland et
al., 2008; Garrett et al., 2009; Garrett et al., 2012), we also demonstrate a reduction in resting
Tre (-0.27 °C). The responsible mechanism requires elucidation but likely includes increased
resting skin blood flow and sweating sensitivity (Taylor, 2014), and/or a potential decrease in
thermoregulatory set point (Aoyagi et al., 1997); although this concept is controversial
(Romanovsky, 2007). The magnitude of adaptation demonstrated in HWI in the current
study, where the total AUC for Tre >38.5 °C was not different on day 6 vs. day 1, compares
very favorably with short-term exercise-heat acclimation studies (Sunderland et al., 2008;
Garrett et al., 2009; Garrett et al., 2012), including those using controlled hyperthermia (Tre
38.5 °C) that maintain a constant adaptation impulse during daily exercise-heat stress (Garrett
et al., 2012; Taylor, 2014). Work by Fox et al. and later by Regan et al. demonstrated that
18
whilst heat acclimation is dependent upon the degree of core temperature elevation the
elevation of skin temperature is important for full heat acclimation (Fox et al., 1964; Regan et
al., 1996); therefore, indicating the importance of the external thermal stress and a likely role
for raised Tsk in the observed adaptations in HWI. Peripheral adaptations to local HWI, with
and without a rise in core temperature increased local sweating responses (Fox et al., 1964),
later coined “sweat gland training” (Avellini et al., 1982), but the increase in local sweating
was more dramatic when both core temperature and skin temperature were elevated (Fox et
al., 1964). Thus the combined elevation of Tre (~39.3 °C after each HWI) and Tsk during daily
HWI in 40 °C (where Tsk equilibrates with water temperature) after exercise likely accounts
for the additional observed benefits shown compared with short-term exercise-heat
acclimation.
The majority of studies investigating the effect of heat acclimation on endurance performance
have used time-to-exhaustion protocols, e.g. V
̇O2max ramp protocols (Sawka et al., 1985;
Garrett et al., 2009) or fixed intensity tests (Nielsen et al., 1997; Scoon et al., 2007). Whilst
such tests have shown heat acclimation improvements of ~14 to 32 % (Nielsen et al., 1997;
Scoon et al., 2007; Garrett et al., 2009), this magnitude of improvement needs to be
considered in the context of the variability of time-to-exhaustion protocols, reported to be as
high as 27 % (Jeukendrup et al., 1996). Only a handful of studies have assessed the effect of
heat acclimation on self-paced TT performance and these used daily exercise in the heat to
induce heat acclimation (Lorenzo et al., 2010; Garrett et al., 2012). To our knowledge, this is
the first study to investigate the effects of a daily post-exercise HWI intervention on TT
performance and here we demonstrate an improvement in 5 km TT performance of 4.9 % in
the heat, where performance was restored to the level achieved in 18 °C conditions (Fig. 4A).
The magnitude of performance improvement exceeds the CV (~2 %) for the 5 km TT
19
(Laursen et al., 2007), and thus, we contend, represents a meaningful performance
improvement (Fig. 4B) attributable to the heat acclimation adaptations observed. The reduced
heat strain after 6-days in HWI is also likely to benefit more prolonged endurance exercise
performance in the heat, by blunting the rise in core temperature; though this requires
investigation. The potential benefits of heat acclimation on endurance performance in cooler
conditions received little attention until one study showed that 10 daily bouts of exercise-heat
stress improved cycling TT performance by 6 % in 13 °C conditions (Lorenzo et al., 2010).
Considering the decrease in thermal strain during submaximal exercise in 18 °C after 6-days
in HWI (Fig. 2B) we might have anticipated, but did not observe, an improvement in 5 km
TT performance in 18 °C. It is conceivable, but requires investigation, that the HWI
intervention might improve endurance performance in temperate conditions that presents a
greater thermal burden such as a 10 km TT (final Tre during 18 °C 5 km TT was only 38.6
°C).
In conclusion, hot-water-immersion immediately after exercise in temperate conditions on six
consecutive days reduced heat strain during submaximal exercise in both temperate and hot
conditions, and improved 5 km treadmill TT performance in the heat. For those residing and
training in temperate conditions, incorporating a hot bath into the post-exercise washing
routine on six consecutive days represents a simple, practical, economical and effective heat
acclimation strategy to improve endurance performance in the heat.
Perspectives
This heat-acclimation intervention overcomes a number of practical limitations with current
exercise-heat-acclimation protocols. For example, access to a hot environment is not
required, neither is precise control of exercising Tre, but also because a post-exercise hot bath
20
does not interfere with daily training and might be incorporated into post-exercise washing
routines. Analogous to “live-high train-low” (Stray-Gundersen et al., 2001) we contend these
findings support the concept, ‘train-cool bathe-hot’. Although this alternative heat-
acclimation strategy conflicts with current athlete practice which includes post-exercise
cryotherapy, the benefits of cryotherapy to improve recovery have been questioned (Leeder et
al., 2012). The benefits of HWI are likely greater when core temperature is elevated
following exercise, but future research that is mindful of the prior exercise-heat strain, safety
and real-world limitations is required to verify this and establish whether the intervention can
be optimized for military/occupational or athlete scenarios. For example, the intervention
might be manipulated (e.g. reducing the water temperature, duration and/or frequency of
exposures) for the military/occupational scenario where the aim is to improve tolerance and
safety (reduce EHI risk) to a standard heat challenge in large groups (one-size-fits-all). For
athletes wishing to optimize performance in the heat, the intervention could be manipulated
to ensure constant physiological strain during exposures. Future studies are also required to
investigate the decay of heat-acclimation following this intervention, in males and females,
and to assess the purported benefits for cellular training adaptations (Tamura et al., 2014) and
immunity (Walsh et al., 2011).
Acknowledgements
We would like to thank the following people for their valuable assistance with data
collection: Tom Ibbitson, David Harding, Liam Renton, Jonathan Donoghue, Lauren Casling,
Benjamin Price, Thomas Storer, Jason Edwards and Kevin Williams. We are also indebted to
the participants for their time and co-operation.
Conflicts of interest: The authors of the study declare that they have no conflicts of interest.
21
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Figure Legends
FIGURE 1. Schematic of study design.
FIGURE 2. Influence of a 6-day post-exercise hot water immersion (HWI) or control (CON)
intervention on resting rectal core temperature (Tre, A) and end exercise Tre (B) assessed
following 40 min running at 65 % V
̇O2max in 18 °C (40 % RH) and 33 °C (40 % RH). Bars
show mean at PRE and POST. Lines represent individual participants. ** P < 0.01, PRE
greater than POST.
FIGURE 3. Influence of a 6-day post-exercise hot water immersion (HWI) or control (CON)
intervention on rectal core temperature at sweating onset (Tre, A), whole body sweat rate
(WBSR, B) and end exercise responses for mean skin temperature (Tsk, C), physiological
strain index (PhSI, D), RPE (E) and thermal sensation (F) following 40 min running at 65 %
V
̇O2max in 18 °C (40 % RH) and 33 °C (40 % RH). Bars show mean at PRE and POST and
SD. * P < 0.05 and ** P < 0.01, PRE greater than POST.
FIGURE 4. Influence of a 6-day post-exercise hot water immersion (HWI) intervention on 5
km treadmill TT performance (A) and % change in 5 km treadmill TT performance (B) in 18
°C (40 % RH) and 33 °C (40 % RH). Shown are mean and SD (A) and mean and 90 % CI of
the difference (B). * P < 0.05 and ** P < 0.01.
25
Figure 1.
26
Figure 2.
37.0
38.0
39.0
40.0
HWI CON
18 C 33 C 18 C 33 C
End Tre
(°C)
35.5
36.0
36.5
37.0
37.5
38.0
HWI CON
Resting Tre
(°C)
PRE
POST
**
** **
A
B
27
Figure 3.
0.0
0.5
1.0
1.5
HWI CON
18 C 33 C 18 C 33 C
WBSR (L/h)
36.5
36.7
36.9
37.1
37.3
37.5
HWI CON
18 C 33 C 18 C 33 C
Tre
(°C) at sweating onset
27.0
29.0
31.0
33.0
35.0
37.0
HWI CON
18 C 33 C 18 C 33 C
End Tsk (°C)
0
1
2
3
4
5
HWI CON
18 C 33 C 18 C 33 C
End PhSI
10
15
20
HWI CON
18 C 33 C 18 C 33 C
End RPE
7
8
9
10
11
12
13
HWI CON
18 C 33 C 18 C 33 C
End thermal sensation
A
B
D
E
PRE
POST
PRE
POST
CF
** **
**
* *
*
**
*
28
Figure 4.
1000
1100
1200
1300
1400
18 C 33 C
Time to complete 5 km (s)
-2
0
2
4
6
8
18 C 33 C
TT performance (%)
A
B
**
PRE
POST
*
29
HWI intervention day
1
2
3
4
5
6
Submaximal exercise
Change in Tre (° C)
1.11 ± 0.25
1.15 ± 0.23
1.15 ± 0.26
1.22 ± 0.17
1.12 ± 0.19
1.17 ± 0.23
Heart rate (beats/min)
142 ± 13
142 ± 15
142 ± 14
140 ± 12
139* ± 12
140 ± 11
Hot water immersion
Change in Tre (° C)
0.95 ± 0.27
0.94 ± 0.33
1.04 ± 0.40
0.99 ± 0.31
1.08 ± 0.28
1.09 ± 0.30
Immersion time (min:s)
32:50 ± 07:14
35:18 ± 06:43
38:00* ± 03:30
39:21* ± 01:25
39:36* ± 01:16
39:45* ± 00:47
Participants completing 40 min immersion (n)
4
6
7
8
9
9
Submaximal exercise and hot water immersion
Sweat rate (L/h)
0.89 ± 0.30
0.98 ± 0.33
1.03 ± 0.36
1.08* ± 0.30
1.08** ± 0.26
1.14** ± 0.31
*P < 0.05, **P < 0.01 vs. day 1.
... with individual exposures of 1 to 2 h in temperatures ≥35.0 o C ( Daanen et al., 2018;Stanley et al., 2015;Zurawlew et al., 2015). It has been proposed that heat exposures must be of sufficient duration and thermal strain to increase Tc, Tsk and sweat rate, which appear to be the main drivers for adaptation in AB athletes, above a set threshold (Casadio et al., 2017;Taylor et al., 2014;Tyler et al., 2016;Zurawlew et al., 2015). ...
... with individual exposures of 1 to 2 h in temperatures ≥35.0 o C ( Daanen et al., 2018;Stanley et al., 2015;Zurawlew et al., 2015). It has been proposed that heat exposures must be of sufficient duration and thermal strain to increase Tc, Tsk and sweat rate, which appear to be the main drivers for adaptation in AB athletes, above a set threshold (Casadio et al., 2017;Taylor et al., 2014;Tyler et al., 2016;Zurawlew et al., 2015). It has commonly been reported that a significant proportion of HA adaptations occur within the first week of chronic heat exposures (Garrett et al., 2011). ...
... Utilising a HR associated with a percentage of V ̇ O2peak, AnLT or maximum HR in temperate conditions would provide a constant cardiovascular stimulus as athletes acclimate (Périard et al., 2015). Support for this proposed regimen comes from evidence of unchanged HR during isothermic HA ( Garrett et al., 2012;Magalhães et al., 2010;Zurawlew et al., 2015). ...
Thesis
Full-text available
Paratriathlon is a multi-impairment, endurance sport which made its Paralympic Games debut in 2016. Athletes’ impairments typically include but, are not limited to, spinal cord injury; cerebral palsy, or other neurological disorders; amputations or visual impairments. However, despite athletes displaying impairments that present several considerations for coaches and practitioners, there has been very little research in the sport. Specifically, there is little understanding of how athletes’ impairments may impact their physiological response to acute or chronic changes in training load. Similarly, it is not known how consequences of athletes’ impairments affect thermoregulation and the ability to adapt to the heat. Thus, this thesis aimed to elucidate these unknown areas whilst bridging the knowledge gap to research in able-bodied triathlon. The first two studies of this thesis investigated paratriathletes’ response to changes in training load, longitudinally (Chapter four) and more acutely (Chapter five). Specifically, Chapter four noted paratriathletes’ mucosal immune function, represented by salivary secretory immunoglobulin A, displayed an inverse relationship with weekly training duration, but not measures of training load. Furthermore, upper respiratory illness incidence was not related to mucosal immunity. In Chapter five, it was shown that paratriathletes are resilient to large changes in training load in the form of a two-week intensified training period. A 14-d overseas training camp did not negatively affect hormonal, immunological or wellness measures whilst self-perceived sleep, stress and recovery parameters were improved. One explanation is that the camp environment minimised external life stresses and coaches’ careful management of training load reduced the likelihood of overreaching. In Chapter six, the thermoregulatory strain of paratriathlon competition in the heat was characterised. It was shown, via the use of ingestible sensors, that paratriathletes’ core temperature reached levels significantly higher than previous research in able-bodied triathletes. Furthermore, trends for category-specific responses are presented, namely between those in PTWC and PTVI, highlighting the differences between impairment groups. Selfreported heat illness symptomatology was also greater than previous research in able-bodied athletes. Acknowledging the thermal strain paratriathletes face during competition in hot environments, Chapter seven sought to present the effectiveness of an ecologically valid preparatory heat acclimation strategy. Utilising a mixed active and passive intervention, controlling the relative intensity of exercise by heart rate, it was shown that paratriathletes are capable of partial heat acclimation through thermoregulatory adaptations. However, the breadth of adaptations was less than able-bodied triathletes. These were the first studies of paratriathletes’ physiological and thermoregulatory response to training load and competition in the heat. It was shown that paratriathletes of a high training level are robust to acute changes in training load whilst training load had no relationship with mucosal immunity, despite a high illness incidence. However, paratriathletes are at heightened risk of thermoregulatory strain when competing in the heat, as shown by high core temperatures and self-reported heat illness symptomatology. Nonetheless, strategies can be utilised to induce thermoregulatory adaptations in this cohort. This provides valuable information for coaches and practitioners working with paratriathletes as they seek to minimise training time-loss and ameliorate the strain of competition in the heat.
... Additional clothing would also reduce evaporative cooling and create a localised heat storage response; this is of greater concern to cyclists than runners due to faster training velocities, resultant wind speed and therefore potential rate of evaporative cooling (Shimazaki, Yoshida and Yamamoto, 2015). Post-exercise hot water immersion (Zurawlew et al., 2016;Zurawlew, Mee and Walsh, 2018) at 40ºC for ≤40 min after moderate intensity exercise has also been shown to induce thermoregulatory adaptations after as little as six days exposure. Immersion in hot water extends the time at an elevated core temperature following exercise, effectively lengthening and intensifying the thermal load placed upon an athlete following a session. ...
... Further, this suggests that menthol mouth swilling has the potential to be incorporated alongside other nutritional practices that may not alter TS such as CHO intake during or following exercise. Such findings may be of use to athletes undertaking heat acclimation training, whereby the heat stimulus may be applied actively i.e. during exercise Willmott et al., 2016;Stevens, 2018), passively via hot-water immersion (Zurawlew et al., 2016;Zurawlew, Mee and Walsh, 2018), or a sauna during recovery from exercise (Stanley et al., 2014). ...
... Menthol mouth swilling may also be a useful addition to adjunct heat acclimation strategies such as hot water immersion (Zurawlew et al., 2016;Zurawlew, Mee and Walsh, 2018), or the wearing of additional clothing (Stevens, 2018), where the treatment elicits either a heat maintenance (hot water immersion) or heat storage (additional clothing) response. In these instances, as is often reported during exercise, menthol may serve to ameliorate thermal comfort and sensation, again extending the time for which an athlete is exposed to the adaptive stimulus. ...
Thesis
Full-text available
Heat challenges multiple physiological systems, and its effects are heavily felt by endurance athletes due to the duration and intensity that must be sustained in competition and training sessions. Runners may demonstrate impaired thermoregulatory responses or opportunities due to lower rates of convective cooling and fewer opportunities to provide cooling interventions during exercise than other endurance athletes e.g. cyclists. Cooling strategies may be employed before or during exercise to minimise the effects of heat exposure, and their effects have been studied for at least three Olympic cycles. Hence, the optimisation of timing and method of delivery of cooling provision, with the addition of any novel strategies, would be of benefit to the contemporary sport and exercise science practitioner. Initially this thesis sought to better understand the effects of cooling strategies upon time trial performance in endurance sports with a systematic review and meta-analysis. The efficacy of strategies was assessed with respect to intervention timing (pre or per-cooling or both) and method of delivery (oral or topical or both). Cooling strategies were found to provide small but ecologically relevant improvements in time trial performance, especially when administered during the exercise bout to the oral cavity; the addition of menthol was seen to possibly enhance ergogenic effects. Hence, a second systematic review regarding external or internal application of menthol was conducted and found that menthol demonstrated improvements in performance when applied internally, most likely due to altered thermal and ventilatory responses. A range in menthol concentrations and dilution methods was noted in the literature, establishing a clear need for a randomised trial to ascertain menthol concentration preference. Following appropriate dilution, 0.1% menthol was determined to be preferred; colour preference was also established to maximise the perceptual cooling effect of menthol solution. This solution was then used (without colour to ensure blindness) in subsequent investigations. At rest this solution was shown to improve perceptions of thermal comfort, thermal sensation and thirst, when compared to carbohydrate and water swilling. Two exercise trials were conducted: the first examined the effects of menthol mouth swilling upon physiological and perceptual markers over four minute intervals at a range of pertinent running speeds (14-20km.h-1), and following 1km time trial performance. Effects on time trial performance were unclear, as were the effects in physiological parameters. Thermal comfort however was improved, with menthol mouth swilling counterintuitively increasing thermal sensation and thirst in the heat (35ºC), but ameliorating these factors in the cold (15ºC). Secondly, at a fixed rating of perceived exertion, corresponding to 2mmol.L-1 blood lactate, runners demonstrated a lower oxygen consumption following menthol mouth swilling for the latter two thirds of a 30-minute training session than compared to no swill or ice swilling. No changes in ventilation were shown, and the perceptual responses at a group level were unclear – suggesting that whilst menthol may improve the oxygen cost of running at a fixed rating of perceived exertion, this does not correspond to improvements in thermoregulatory perception in this sample. Qualitative responses regarding the swill from the athletes involved in the exercise studies were collated and menthol was considered an enjoyable and useful strategy by the athletes. Further research is required to assess if these hedonic and utilitarian perspectives are rated as highly in more ecologically valid environments; the athletes indicated this would be the case. The findings presented in this thesis demonstrate that a light blue or light green 0.1% menthol mouth rinse is preferred and can alleviate thermal sensation and thirst, and improve thermal comfort at rest in the heat. During exercise in a small sample of trained distance runners, menthol mouth swilling may alleviate perceptual symptoms of heat exposure without necessarily improving performance, dependent upon the running speeds chosen. Furthermore, menthol mouth swilling is considered a pleasant and potentially ergogenic strategy by athletes who have used it, suggesting that even in the absence of performance or physiological enhancements that exceed the typical coefficient of variation in performance, menthol mouth swilling is a viable nutritional support strategy for trained distance runners, when exercising in the heat.
... In response to these challenges, alternative HA strategies have been investigated, including post-exercise sauna bathing (Scoon et al., 2007) and post-exercise hot-water immersion [HWI; (Zurawlew et al., 2016)]. A total of 16 original investigations have been performed on the topic to-date (Heathcote et al., 2018); the majority demonstrating beneficial hallmark physiological adaptations of heat acclimation (including lowered resting and exercising Tc and heart rate, and increased plasma volume), and importantly, these adaptations were demonstrated in both recreationally active and endurance-trained individuals (Zurawlew et al., 2018a). ...
... A total of 16 original investigations have been performed on the topic to-date (Heathcote et al., 2018); the majority demonstrating beneficial hallmark physiological adaptations of heat acclimation (including lowered resting and exercising Tc and heart rate, and increased plasma volume), and importantly, these adaptations were demonstrated in both recreationally active and endurance-trained individuals (Zurawlew et al., 2018a). Further, the use of post-exercise sauna (12 × 30 min exposures) improved running time to exhaustion by 32% in competitive runners/ triathletes (Scoon et al., 2007) and post-exercise HWI (6 × 40 min exposures) improved 5 km running performance time in the heat by 4.9% in recreationally active individuals (Zurawlew et al., 2016). ...
... Post-exercise HWI therefore presents a practical HA strategy for athletes residing in cooler climates, compared to expensive alternatives requiring artificial heat chambers and/or relocation. Passive heating has typically been applied immediately after exercise training when used for HA purposes (Scoon et al., 2007;Stanley et al., 2015;Zurawlew et al., 2016), with exercise conducted in laboratory settings, enabling easy access to heating facilities. Practically however, the ability to commence HWI immediately after exercise could be challenging for athletes who lack such facilities near training locations. ...
Article
Full-text available
Hot-water immersion following exercise in a temperate environment can elicit heat acclimation in endurance-trained individuals. However, a delay between exercise cessation and immersion is likely a common occurrence in practice. Precisely how such a delay potentially alters hot-water immersion mediated acute physiological responses (e.g., total heat-load) remains unexplored. Such data would aid in optimizing prescription of post-exercise hot-water immersion in cool environments, relative to heat acclimation goals. Twelve male recreational runners (mean ± SD; age: 38 ± 13 years, height: 180 ± 7 cm, body mass: 81 ± 13.7 kg, body fat: 13.9 ± 3.5%) completed three separate 40-min treadmill runs (18°C), followed by either a 10 min (10M), 1 h (1H), or 8 h (8H) delay, prior to a 30-min hot-water immersion (39°C), with a randomized crossover design. Core and skin temperatures, heart rate, sweat, and perceptual responses were measured across the trials. Mean core temperature during immersion was significantly lower in 1H (37.39 ± 0.30°C) compared to 10M (37.83 ± 0.24°C; p = 0.0032) and 8H (37.74 ± 0.19°C; p = 0.0140). Mean skin temperature was significantly higher in 8H (32.70 ± 0.41°C) compared to 10M (31.93 ± 0.60°C; p = 0.0042) at the end of the hot-water immersion. Mean and maximal heart rates were also higher during immersion in 10M compared to 1H and 8H (p < 0.05), despite no significant differences in the sweat or perceptual responses. The shortest delay between exercise and immersion (10M) provoked the greatest heat-load during immersion. However, performing the hot-water immersion in the afternoon (8H), which coincided with peak circadian body temperature, provided a larger heat-load stimulus than the 1 h delay (1H).
... 17 Sauna bathing (12 × 30-min exposures at 90°C) after exercise in temperate conditions increased running time to exhaustion by ∼32% 18 and whole-body postexercise HWI (6 × 40 min exposures at 40°C) improved 5-km running performance time by ∼5%. 19 A range of heat adaptations was also demonstrated following repeated postexercise HWI. As such, postexercise passive heating appears to be an effective HA strategy when training occurs in a temperate environment. ...
... The 40-minute duration was chosen to reflect the duration used in a previous study. 19 A shorter duration was implemented in the first week as the preceding training occurred in a hot environment as opposed to a temperate environment as per the previously mentioned previous study. The immersion was performed in the Australian Institute of Sport Recovery Center, which allowed the entire HWI group to perform immersion in the same pool simultaneously. ...
Article
Purpose: Hot-water immersion (HWI) after training in temperate conditions has been shown to induce thermophysiological adaptations and improve endurance performance in the heat; however, the potential additive effects of HWI and training in hot outdoor conditions remain unknown. Therefore, this study aimed to determine the effect of repeated postexercise HWI in athletes training in a hot environment. Methods: A total of 13 (9 female) elite/preelite racewalkers completed a 15-day training program in outdoor heat (mean afternoon high temperature = 34.6°C). Athletes were divided into 2 matched groups that completed either HWI (40°C for 30-40 min) or seated rest in 21°C (CON), following 8 training sessions. Pre-post testing included a 30-minute fixed-intensity walk in heat, laboratory incremental walk to exhaustion, and 10,000-m outdoor time trial. Results: Training frequency and volume were similar between groups (P = .54). Core temperature was significantly higher during immersion in HWI (38.5 [0.3]) than CON (37.8°C [0.2°C]; P < .001). There were no differences between groups in resting or exercise rectal temperature or heart rate, skin temperature, sweat rate, or the speed at lactate threshold 2, maximal O2 uptake, or 10,000-m performance (P > .05). There were significant (P < .05) pre-post differences for both groups in submaximal exercising heart rate (∼11 beats·min-1), sweat rate (0.34-0.55 L·h-1) and thermal comfort (1.2-1.5 arbitrary units), and 10,000-m racewalking performance time (∼3 min). Conclusions: Both groups demonstrated significant improvement in markers of heat adaptation and performance; however, the addition of HWI did not provide further enhancements. Improvements in adaptation appeared to be maximized by the training program in hot conditions.
... Though plasma volume was not directly measured in the present study, it could be expected to have happened based on the high correlation of 0.93 between increased plasma volume and elevations in sweat rate [22]. The advantages of donning the skin-heating suit after exercise on HA responses are in accordance with previous studies: it was suggested that post-exercise hot water immersion [23] or sauna bathing [13,24] were more effective HA strategies than exercise alone. It is also worthy to note that this strategy was more effective in alleviating perceptual thermal strain not only during the passive heating but also during exercise. ...
... The decreases of T re and heart rate during the early phase of suit donning in HA EXE+SUIT are less likely to be solely caused by the skin-heating, because in HA SUIT such reduction in T re and heart rate appeared at the end of the heat acclimation protocol. Therefore, it is more likely that as previously suggested [15,23], the post-exercise passive heating further induced actively induced HA responses by maintaining greater thermal impulses after exercise using the suit. This synergic effect of exercise and suit donning seems to have facilitated improvement in heat loss mechanism leading to decreased heat storage [33] as well as in reduction of cardiovascular strain, whose mechanisms are known to be associated with (1) plasma volume expansion, (2) blood volume redistribution, (3) enhanced skin cooling, (4) increase in venous tone, and (5) less stimulated sympathetic nervous activity [17,34]. ...
Article
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Background: While active heat acclimation strategies have been robustly explored, not many studies highlighted passive heat acclimation strategies. Particularly, little evidence demonstrated advantages of utilizing a water-perfused suit as a passive heating strategy. This study aimed to explore heat adaptive changes in physiological and perceptual responses during 10-day heat acclimation training using a water-perfused suit. Methods: Nineteen young males were divided into three experimental groups: exercise condition (N = 6, HAEXE, 1-h exercise at 6 km h-1 followed by 1-h rest in a sitting position), exercise and passive heating condition (N = 6, HAEXE+SUIT, 1-h exercise at 6 km h-1 followed 1-h passive heating in a sitting position), and passive heating condition (N = 7, HASUIT, 2-h passive heating in a sitting position). All heating programs were conducted for 10 consecutive days in a climatic chamber maintained at 33 °C with 60% relative humidity. The passive heating was conducted using a newly developed water-perfused suit with 44 °C water. Results: Greater whole-body sweat rate and alleviated perceptual strain were found in HASUIT and HAEXE+SUIT after 5 and/or 10 days (P < 0.05) but not in the exercise-only condition (HAEXE). Lower rectal temperature and heart rate were found in all conditions after the training (P < 0.05). Heat adaptive changes appeared earlier in HASUIT except for sweat responses. Conclusions: For heat acclimation in hot humid environments, passive and post-exercise heat acclimation training using the suit (water inflow temperature 44 °C) were more effective than the mild exercise (1-h walking at 6 km h-1). This form of passive heating (HASUIT) may be an especially effective strategy for the elderly and the disabled who are not able to exercise in hot environments.
... As part of the process of making weight in weight category sports, this is a practically-meaningful amount of weight loss and speaks to the importance of water temperature in the hot bath process, but should be kept within safe limits, which remain to be defined. For illustration, water temperatures rarely exceeded 40ºC across all participants and baths, and previous immersion studies have typically used temperatures of ~38/39ºC[14][15][16][22][23][24][25], but water temperatures of ~41ºC acutely[18], and ~40ºC repeated daily for six days[26], have also been employed without adverse effects being reported.Despite the greater body mass loss with the higher water temperature in the present study, consistent with our previous work, there was a greater loss of body mass by the 24 h of restriction of carbohydrate, fibre, and fluid intake (FWB, -2.54 ± 0.93%; SWB -2.45 ± 1.11%), than from either bathing protocol (FWB, -2.00 ± 0.71%; SWB, -1.97 ± 0.91%). The loss of body mass with 24 h of such restriction is attributed to dehydration, short-duration glycogen depletion, and emptying of the intestinal contents[2], and like the present study is typically ~2-3% of body mass[2,14,27]. ...
Article
Hot water immersion is used by athletes in weight category sports to produce rapid weight loss (RWL) by means of passive fluid loss, and often is performed with the addition of Epsom salts (magnesium sulphate). This study investigated the magnitude of body mass losses during hot water immersion with or without the addition of salt, with the temperature commencing at 37.8°C and being self-adjusted by participants to their maximum tolerable temperature. In a crossover design, eight male MMA athletes (29.4 ± 5.3 y; 1.83 ± 0.05 m; 85.0 ± 4.9 kg) performed a 20 min whole-body immersion followed by a 40 min wrap in a warm room, twice in sequence per visit. During one visit, only fresh water was used (FWB), and in the other visit, magnesium sulphate (1.6% wt/vol) was added to the bath (SWB). Prior to each visit, 24 h of carbohydrate, fibre and fluid restriction was undertaken. Water temperatures at the end of the first and second baths were ~39.0°C and ~39.5°C, respectively. Body mass losses induced by the hot bath protocols were 1.71 ± 0.70 kg and 1.66 ± 0.78 kg for FWB and SWB, respectively (P = 0.867 between trials, d = 0.07), and equivalent to ~2.0% body mass. Body mass lost during the entire RWL protocol was 4.5 ± 0.7%. Under the conditions employed, the magnitude of body mass lost in SWB was similar to FWB. Augmenting passive fluid loss during hot water immersion with the addition of salt may require a higher salt concentration than that presently utilised.
... Several strategies to induce HA in a safe and practicable manner have been explored, including sauna bathing (7), postexercise hot water immersion (8), and isothermic/controlled hyperthermia HA (4). The latter approach utilizes alterations in exogenous (i.e., ambient) or endogenous (i.e., metabolic) heat stress to maintain the thermal stimulus for adaptation; a core temperature ≥38.5°C (4,(9)(10)(11). ...
Article
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Purpose: To characterize the adaptive responses to heat acclimation (HA) with controlled heart rate (HR) and determine whether hydration strategy alters adaptations. The influence of HA on V[Combining Dot Above]O2max in cool conditions and self-paced exercise in the heat was also determined. Methods: Eight males (V[Combining Dot Above]O2max: 55±7 mlkgmin) completed two 10-day interventions in a counterbalanced cross-over design. Fluid intakes differed between interventions to either maintain euhydration (HA-EUH) or elicit similar daily body mass deficits (2.85±0.26%; HA-DEH). HA consisted of 90 min of cycling in 40°C and 40% RH. Initial workload (172±22 W) was adjusted over the last 75 min to maintain exercising HR equivalent to 65% V[Combining Dot Above]O2max. A V[Combining Dot Above]O2max test in cool conditions and 30 min time-trial in hot-humid conditions were completed before and after HA. Results: HR at the end of the initial 15 min workload was 10±5 beatsmin lower on day 10 in both interventions (P<0.001). The workload necessary to maintain exercising HR (145±7 beatsmin) increased throughout HA-EUH (25±10 W, P=0.001) and HA-DEH (16±18 W, P=0.02). There was a main effect of HA on sweat rate (P=0.014), which tended to increase with HA-EUH (0.19±0.18 Lh, P=0.06) but not HA-DEH (P=0.12). Skin temperature decreased during HA-EUH (0.6 ± 0.5°C, P=0.03), but not HA-DEH (P=0.30). There was a main effect of HA on V[Combining Dot Above]O2max (~3 mlkgmin, P=0.02); however, neither intervention independently increased V[Combining Dot Above]O2max (both P=0.08). Time-trial performance increased following HA-EUH (19±16 W, P=0.02), but not HA-DEH (P=0.21). Conclusions: Controlled HR exercise in the heat induces several HA adaptations, which may be optimized by maintaining euhydration. HA-EUH also improves self-paced exercise performance in the heat. However, HA does not appear to significantly increase V[Combining Dot Above]O2max in cool conditions.
Article
Applying heat training interventions in a team sports setting remains challenging. This study investigated the effects of integrating short-term, repeat sprint heat training with passive heat exposure on running performance and general conditioning in team sport players. Thirty male club-level Australian Football players were assigned randomly to: Passive + Active Heat (PAH; n=10), Active Heat (AH; n=10) or Control (CON; n=10) to complete 6 × 40 min high-intensity cycling training sessions over 12 days in 35°C (PAH and AH) or 18°C (CON), 50% RH in parallel with mid-season sports-specific training and games. Players in PAH were exposed to 20 min pre-exercise passive heat. Physiological adaptation and running capacity were assessed via a treadmill submaximal heat stress test followed by a time-to-exhaustion run in 35°C, 50% RH. Running capacity increased by 26 ± 8% PAH (0.88, ±0.23; standardised mean, ± 90% confidence limits), 29 ± 12% AH (1.23, ±0.45) and 10 ± 11% CON (0.45, ±0.48) compared with baseline. Both PAH (0.52, ±0.42; standardised mean, ± 90% confidence limits) and AH (0.35, ±0.57) conditions yielded a greater improvement in running capacity than CON. Physiological and perceptual measures remained relatively unchanged between baseline and post-intervention heat stress tests, within and between conditions. Where thermal adaptation is not a direct priority, short-term, repeat effort high-intensity cycling in hot conditions combined with sports-specific training can further enhance running performance in team sport players. Six heat exposures across 12-days should improve running performance while minimising lower limb load and cumulative fatigue for team sports players.
Article
PurposeThis study investigated whether regular precooling would help to maintain day-to-day training intensity and improve 20-km cycling time trial (TT) performed in the heat. Twenty males cycled for 10 day × 60 min at perceived exertion equivalent to 15 in the heat (35 °C, 50% relative humidity), preceded by no cooling (CON, n = 10) or 30-min water immersion at 22 °C (PRECOOL, n = 10).Methods19 participants (n = 9 and 10 for CON and PRECOOL, respectively) completed heat stress tests (25-min at 60% \({\dot{\text{V}}\text{O}}_{{{\text{2peak}}}}\) and 20-km TT) before and after heat acclimation.ResultsChanges in mean power output (∆MPO, P = 0.024) and heart rate (∆HR, P = 0.029) during heat acclimation were lower for CON (∆MPO − 2.6 ± 8.1%, ∆HR − 7 ± 7 bpm), compared with PRECOOL (∆MPO + 2.9 ± 6.6%, ∆HR − 1 ± 8 bpm). HR during constant-paced cycling was decreased from the pre-acclimation test in both groups (P < 0.001). Only PRECOOL demonstrated lower rectal temperature (Tre) during constant-paced cycling (P = 0.002) and lower Tre threshold for sweating (P = 0.042). However, skin perfusion and total sweat output did not change in either CON or PRECOOL (all P > 0.05). MPO (P = 0.016) and finish time (P = 0.013) for the 20-km TT were improved in PRECOOL but did not change in CON (P = 0.052 for MPO, P = 0.140 for finish time).Conclusion Precooling maintains day-to-day training intensity and does not appear to attenuate adaptation to training in the heat.
Chapter
The thermal adaptation process of migrants is a complex system considering psychological, physiological and behavioral layers, also accompany space-time and development to change. This study explored constraints affecting adaptation process, especially in behavioral level. A long-term tracking field study has been conducted in Xi’an, including Indoor thermal environment parameters, subjective questionnaires, and behavioral adjustment methods. The migrant groups from severe cold area, hot-summer and cold-winter area and hot-summer and warm-winter area were recruited. The results showed the behavior adjustment modes of migrants were significantly influenced by the thermal experience of native residence. Migrants with better thermal experience showed the diversity of behavior adjustment. In addition, behavioral acclimation was relatively a slow process compared with thermal perception. The results could provide reference for studying dynamic process of migrants’ thermal adaptation and contribute toward indoor climate design for migrants.
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Exercising in the heat induces thermoregulatory and other physiological strain that can lead to impairments in endurance exercise capacity. The purpose of this consensus statement is to provide up-to-date recommendations to optimise performance during sporting activities undertaken in hot ambient conditions. The most important intervention one can adopt to reduce physiological strain and optimise performance is to heat acclimatise. Heat acclimatisation should comprise repeated exercise-heat exposures over 1-2 weeks. In addition, athletes should initiate competition and training in a euhydrated state and minimise dehydration during exercise. Following the development of commercial cooling systems (eg, cooling-vest), athletes can implement cooling strategies to facilitate heat loss or increase heat storage capacity before training or competing in the heat. Moreover, event organisers should plan for large shaded areas, along with cooling and rehydration facilities, and schedule events in accordance with minimising the health risks of athletes, especially in mass participation events and during the first hot days of the year. Following the recent examples of the 2008 Olympics and the 2014 FIFA World Cup, sport governing bodies should consider allowing additional (or longer) recovery periods between and during events, for hydration and body cooling opportunities, when competitions are held in the heat.
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Full-text available
Exercising in the heat induces thermoregulatory and other physiological strain that can lead to impairments in endurance exercise capacity. The purpose of this consensus statement is to provide up-to-date recommendations to optimise performance during sporting activities undertaken in hot ambient conditions. The most important intervention one can adopt to reduce physiological strain and optimise performance is to heat acclimatise. Heat acclimatisation should comprise repeated exercise-heat exposures over 1-2 weeks. In addition, athletes should initiate competition and training in a euhydrated state and minimise dehydration during exercise. Following the development of commercial cooling systems (eg, cooling-vest), athletes can implement cooling strategies to facilitate heat loss or increase heat storage capacity before training or competing in the heat. Moreover, event organisers should plan for large shaded areas, along with cooling and rehydration facilities, and schedule events in accordance with minimising the health risks of athletes, especially in mass participation events and during the first hot days of the year. Following the recent examples of the 2008 Olympics and the 2014 FIFA World Cup, sport governing bodies should consider allowing additional (or longer) recovery periods between and during events, for hydration and body cooling opportunities, when competitions are held in the heat.
Article
Full-text available
Exercising in the heat induces thermoregulatory and other physiological strain that can lead to impairments in endurance exercise capacity. The purpose of this consensus statement is to provide up-to-date recommendations to optimize performance during sporting activities undertaken in hot ambient conditions. The most important intervention one can adopt to reduce physiological strain and optimize performance is to heat acclimatize. Heat acclimatization should comprise repeated exercise-heat exposures over 1-2 weeks. In addition, athletes should initiate competition and training in a euhydrated state and minimize dehydration during exercise. Following the development of commercial cooling systems (e.g., cooling vest), athletes can implement cooling strategies to facilitate heat loss or increase heat storage capacity before training or competing in the heat. Moreover, event organizers should plan for large shaded areas, along with cooling and rehydration facilities, and schedule events in accordance with minimizing the health risks of athletes, especially in mass participation events and during the first hot days of the year. Following the recent examples of the 2008 Olympics and the 2014 FIFA World Cup, sport governing bodies should consider allowing additional (or longer) recovery periods between and during events for hydration and body cooling opportunities when competitions are held in the heat. © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
Article
We examined the effect of short-term heat acclimation with permissive dehydration (STHADe) on heat acclimation (HA) and cycling performance in a temperate environment. Ten trained male cyclists [mean (SD) maximal oxygen uptake: 63.3(4.0) mL/kg/min; peak power output (PPO): 385(40) W; training: 10 (3) h/week] underwent a STHADe program consisting of 5 days of exercise (maximum 90 min/day) in a hot environment (40 °C, 50% RH) to elicit isothermic heat strain [rectal temperature 38.64(0.27) °C]. Participants abstained from fluids during, and 30 min after, HA sessions. Pre- and post-STHADe HA was evaluated during euhydrated fixed-intensity exercise (60 min) in hot conditions; the effect of STHADe on thermoregulation was also examined under temperate conditions (20 min fixed-intensity exercise; 22 °C, 60% RH). Temperate cycling performance was assessed by a graded exercise test (GXT) and 20-km time trial (TT). STHADe reduced thermal and cardiovascular strain in hot and temperate environments. Lactate threshold [Δ = 16 (17) W] and GXT PPO [Δ = 6 (7) W] were improved following STHADe (P < 0.05), but TT performance was not affected (P > 0.05), although there was a trend for a higher mean power (P = 0.06). In conclusion, STHADE can reduce thermal and cardiovascular strain under hot and temperate conditions and there is some evidence of ergogenic potential for temperate exercise, but longer HA regimens may be necessary for this to meaningfully influence performance. © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
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Physical training and heat acclimation are both commonly adopted tactics to improve performance and/or tolerance times when individuals must compete or work in the heat. Potential benefits include: (i) improved aerobic fitness and thus a greater cardiovascular reserve (probably seen mainly after training); (ii) a lower resting body temperature that allows greater heat storage (probably seen mainly after acclimation); (iii) a decreased energy cost of a given intensity of exercise (seen after acclimation and also as the learning component of training); (iv) an enhanced sweating response at a given percentage of maximal effort (probably developed by both treatments); (v) a slower increase in body temperature owing to (iii) and/or (iv) [seen after both treatments]; (vi) a reduced cardiovascular stress because of changes in the autonomic nervous system (probably realised mainly by training), expansion of blood volume (seen after both treatments) and/or a decreased peripheral pooling of blood (probably found after both treatments); and (vii) improved subjective tolerance reflecting a decrease in the relative intensity of a given activity (probably seen mainly after training), a reduction in the physiological strain (found after both treatments) and/or habituation to heat-exercise stress (probably developed by both treatments). Factors affecting improvements in physiological and psychological responses to a given set of conditions include: (i) the individual’s initial fitness and acclimatisation to heat; (ii) age, gender, hydration, sleep deprivation, circadian rhythms and in women the menstrual cycle; (iii) use of ergogenic aids such as fluid ingestion, carbohydrate and/or electrolyte replacement and blood doping; (iv) event or test conditions such as the mode of exercise, the severity of environmental heat stress and the type of clothing worn; and (v) treatment conditions such as the intensity, duration and frequency of exercise and/or heat exposure, the length of any rest intervals and cumulative depletion of body water and minerals.
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In many athletic and occupational settings, the wearing of protective clothing in warm or hot environments creates conditions of uncompensable heat stress where the body is unable to maintain a thermal steady state. Therefore, special precautions must be taken to minimise the threat of thermal injury. Assuming that manipulations known to reduce thermoregulatory strain during compensable heat stress would be equally effective in an uncompensable heat stress environment is not valid. In this review, we discuss the impact of hydration status, aerobic fitness, endurance training, heat acclimation, gender, menstrual cycle, oral contraceptive use, body composition and circadian rhythm on heat tolerance while wearing protective clothing in hot environments. The most effective countermeasure is ensuring that the individual is adequately hydrated both before and throughout the exercise or work session. In contrast, neither short term aerobic training or heat acclimation significantly improve exercise-heat tolerance during uncompensable heat stress. While short term aerobic training is relatively ineffective, long term improvements in physical fitness appear to provide some degree of protection. Individuals with higher proportions of body fat have a lower heat tolerance because of a reduced capacity to store heat. Women not using oral contraceptives are at a thermoregulatory disadvantage during the luteal phase of the menstrual cycle. The use of oral contraceptives eliminates any differences in heat tolerance throughout the menstrual cycle but tolerance is reduced during the quasi-follicular phase compared with non-users. Diurnal variations in resting core temperature do not appear to influence tolerance to uncompensable heat stress.
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A recent study demonstrated that heat stress induces mitochondrial biogenesis in C2C12 myotubes, thereby implying that heat stress may be an effective treatment to enhance endurance training-induced mitochondrial adaptations in skeletal muscle. However, whether heat stress actually induces mitochondrial adaptations in skeletal muscle in vivo is unclear. We herein report the novel findings that 1) whole-body heat stress produced by exposure of ICR mice to a hot environment (40 ºC, 30 min/day, 5 days/week, 3 weeks) induced mitochondrial adaptations such as increased mitochondrial enzyme activity (citrate synthase and 3-hydroxyacyl CoA dehydrogenase) and respiratory chain protein content (complexes I-V) in skeletal muscle in vivo and 2) post-exercise whole-body heat stress additively enhanced endurance training-induced mitochondrial adaptations (treadmill running, 25 m/min, 30 min/day, 5 days/week, 3 weeks). Moreover, to determine candidate mechanisms underlying mitochondrial adaptations, we investigated the acute effects of post-exercise whole-body heat stress on the phosphorylation status of cellular signaling cascades that subsequently induce mitochondrial gene transcription. We found that whole-body heat stress boosted the endurance exercise-induced phosphorylation of p38 MAPK; increased the phosphorylation status of p70S6K, a biomarker of mTORC1 activity; and unexpectedly dephosphorylated AMPK and its downstream target ACC in skeletal muscle. Our current observations suggest that heat stress can act as an effective post-exercise treatment. Heat stress treatment appeared to be clinically beneficial for people who have difficulty participating in sufficient exercise training, such as the elderly, injured athletes, and patients.
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In this overview, human morphological and functional adaptations during naturally and artificially induced heat adaptation are explored. Through discussions of adaptation theory and practice, a theoretical basis is constructed for evaluating heat adaptation. It will be argued that some adaptations are specific to the treatment used, while others are generalized. Regarding ethnic differences in heat tolerance, the case is put that reported differences in heat tolerance are not due to natural selection, but can be explained on the basis of variations in adaptation opportunity. These concepts are expanded to illustrate how traditional heat adaptation and acclimatization represent forms of habituation, and thermal clamping (controlled hyperthermia) is proposed as a superior model for mechanistic research. Indeed, this technique has led to questioning the perceived wisdom of body-fluid changes, such as the expansion and subsequent decay of plasma volume, and sudomotor function, including sweat habituation and redistribution. Throughout, this contribution was aimed at taking another step toward understanding the phenomenon of heat adaptation and stimulating future research. In this regard, research questions are posed concerning the influence that variations in morphological configuration may exert upon adaptation, the determinants of postexercise plasma volume recovery, and the physiological mechanisms that modify the cholinergic sensitivity of sweat glands, and changes in basal metabolic rate and body core temperature following adaptation. © 2014 American Physiological Society. Compr Physiol 4:325-365, 2014.
Conference Paper
A comprehensive review is presented of the present state of our knowledge in the science of clothing comfort. Information is given on thermal comfort factors and concepts, physiological aspects, use of acoustic testing to predict clothing comfort properties, comfort finishing (synthetic fabrics), cotton and cotton blends with comfort stretch, analysis of comfort's gestalt, psycho-physical and neurophysiological studies of somatic sensibility, psychological scaling, subjective fabric and garment comfort assessment techniques, interaction of clothing and environment, ventilation of clothing from body motion, body motion and convective/evaporative heat exchange, measurement and prediction for clothing designs, and air exchange in lightweight cloth coats.