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and Rehabilitation (CHPER),
1 Title: Once- and twice-daily heat acclimation confer similar heat adaptations, inflammatory responses
2 and exercise tolerance improvements.
3 Running Title: Once- vs. twice-daily heat acclimation
4 Authors:
5 1A.G.B Willmott, 1M. Hayes, 1,2C.A James, 1J. Dekerle, 1,3O.R Gibson and 1N.S Maxwell
6 Address for Authors:
7 1Environmental Extremes Laboratory, University of Brighton, Eastbourne, UK, 2Institut Sukan Negara
8 (National Sports Institute), National Sports Complex, Kuala Lumpur, Malaysia, 3Centre for Human
9 Performance, Exercise Brunel University London, Uxbridge, UK
10 Details for the
11 Ashley Willmott – A.Willmott2@brighton.ac.uk
12 Word Count:
13 Abstract Word Count:
14 Tables: 5
15 Figures: 3
Corresponding Author:
16 Abstract
17 This experiment aimed to investigate the efficacy of twice-daily, non-consecutive heat acclimation
18 (TDHA) in comparison to once-daily heat acclimation (ODHA) and work matched once- or twice-daily
19 temperate exercise (ODTEMP, TDTEMP) for inducing heat adaptations, improved exercise tolerance,
20 and cytokine (immune) responses. Forty males, matched biophysically and for aerobic capacity, were
21 assigned to ODHA, TDHA, ODTEMP or TDTEMP. Participants completed a cycling graded exercise
22 test, heat acclimation state test and a time to task failure (TTTF) at 80% peak power output in temperate
23 (TTTFTEMP: 22°C/40% RH) and hot conditions (TTTFHOT: 38°C/20% RH), before and after 10-sessions
24 (60-min of cycling at ~2W.kg-1) in 45°C/20% RH (ODHA and TDHA) or 22°C/40% RH (ODTEMP
25 or TDTEMP). Plasma IL-6, TNF-α and cortisol were measured pre- and post-sessions 1, 5 and 10.
26 ODHA and TDHA induced equivalent heat adaptations (P<0.05) (resting rectal temperature [-
27 0.28±0.22, -0.28±0.19°C], heart rate [-10±3, -10±4 b.min-1] and plasma volume expansion [+10.1±5.6,
28 +8.5±3.1%]) and improved heat acclimation state (sweat setpoint [-0.22±0.18, -0.22±0.14°C] and gain
29 [+0.14±0.10, +0.15±0.07g.sec-1.°C-1]). TTTFHOT increased (P<0.001) following ODHA (+25±4%) and
30 TDHA (+24±10%), but not ODTEMP (+5±14%) or TDTEMP (+5±17%). TTTFTEMP did not improve
31 (P>0.05) following ODHA (+14±4%), TDHA (14±8%), ODTEMP (9±10%) or TDTEMP (8±13%).
32 Acute (P<0.05) but no chronic (P>0.05) increases were observed in IL-6, TNF-α or cortisol during
33 ODHA and TDHA, or ODTEMP and TDTEMP. Once- and twice-daily heat acclimation conferred
34 similar magnitudes of heat adaptation and exercise tolerance improvements, without differentially
35 altering immune function, thus non-consecutive TDHA provides an effective, logistically flexible
36 method of HA, benefitting individuals preparing for exercise-heat stress.
37 New and Noteworthy
38 • Greater heat adaptations and enhanced exercise performance in the heat were induced by 10-
39 sessions of consecutive once-daily and non-consecutive twice-daily heat acclimation, compared
40 with equivalent temperate exercise, without adverse inflammatory or stress responses.
41 • No difference in the magnitude of adaptation and enhanced exercise performance were observed
42 between either non-consecutive twice-daily, or consecutive once-daily heat acclimation when
43 protocols were matched for volume and intensity.
44 • Non-consecutive twice-daily heat acclimation provides an alternate method to consecutive once-
45 daily heat acclimation to induce heat adaptation without requiring consecutive day training.
46 Glossary of terms 47 Blood lactate concentration – [La]b
48 Body surface area – BSA
49 Body mass index – BMI
50 Change – ∆
51 Cycling graded exercise test – GXT
52 Gross mechanical efficiency – GME
53 Heart rate – HR
54 Heat acclimation – HA
55 Heat acclimation state test – HAST
56 Interleuken-6 – IL-6
57 Lactate threshold – LT
58 Long- term heat acclimation – LTHA
59 Medium- term heat acclimation – MTHA
60 Metabolic heat production – Ḣprod
61 Once-daily heat acclimation – ODHA
62 Onset of blood lactate accumulation – OBLA
63 Peak oxygen uptake – V̇O2
64 Peak power output – PPO
65 Plasma volume – PV
66 Rating of perceived exertion – RPE
67 Rectal temperature – Tre
68 Relative humidity – RH
69 Respiratory exchange ratio – RER
70 Short-term heat acclimation – STHA
71 Sodium concentration – [Na+]
72 Thermal comfort – TC
73 Thermal sensation – TSS
74 Time to task failure – TTTF
75 Time to task failure in heat stress – TTTFHOT
76 Time to task failure in temperate conditions – TTTFTEMP
77 Tumour necrosis factor-alpha – TNF-α
78 Twice-daily heat acclimation – TDHA
79 Urine colour – Ucol
80 Urine osmolality – Uosm
81 Urine specific gravity – Usg
82 Ventilation – V̇E
83 Volume of oxygen uptake – V̇O2
84 Whole-body sweat loss – WBSL
85
86 Introduction
87 Heat acclimation (HA) is an important preparation strategy preceding exercise-heat stress (64, 70) to
88 alleviate physiological strain (61), attenuate heat related illness (HRI) (94), improve thermal perception
89 (33) and exercise tolerance in hot (56), and possibly temperate conditions (50). A variety of HA
90 strategies currently exist, predominantly differentiated by exercise-heat stress volume, and/or intensity
91 (18, 80). In this regard, HA may be applied within sporting and occupational settings (e.g. military),
92 with current recommendations advocating the use of repeated, consecutive once-daily exertional heat
93 exposures for 60-100-min, utilising an isothermic protocol (70). In spite of multiple manipulations of
94 volume/intensity, the optimal frequency for HA remains largely unknown (83). Current
95 recommendations for once-daily exposures are implied more readily than non-consecutive [e.g. 10-
96 sessions in 21-days (32)] and twice-daily exposures [e.g. 100-min vs. 2x50-min (49)], due to the
97 consistency of potentiating stimuli for adaptation e.g. daily elevations in rectal [Tre] and skin
98 temperature alongside profuse sweating, which are required to evoke a multitude of physiological and
99 perceptual adaptations (74). From a practical perspective, implementing consecutive-day protocols is
100 challenging given access to hot-humid conditions is not ubiquitous, and the need for daily exposures is
101 likely to interrupt sport/occupational-specific training, competition tapering and, or travel/recovery
102 schedules. Medium- (MTHA: 10-14-days) and long-term (LTHA: >14-days) protocols which maximise
103 adaptations exacerbate these challenges, a factor which may provide some explanation as to why, in
104 spite of clear recommendations from the scientific community, only ~15% of athletes undertook HA
105 prior to competition in heat stress (66).
106 We have previously shown that four HA sessions i.e. a short-term HA (STHA) intervention (89),
107 administered over two consecutive days (i.e. twice-daily HA [TDHA]), demonstrated comparable
108 adaptations to four consecutive once-daily HA (ODHA) sessions. However the magnitudes of
109 adaptation using STHA are typically smaller than MTHA/LTHA interventions, thus the need to
110 examine the efficacy of a twice-daily approach over longer periods exists. Furthermore given challenges
111 associated with consecutive day interventions, completing TDHA intermittently (e.g. over non-
112 consecutive days), over MTHA/LTHA timescales, may be desirable given an improved ability to
113 integrate HA into complex training and travel schedules, potentially reducing disruption. For example,
114 by administering the same number of HA sessions (i.e. the same dose) non-consectutively, athletes may
115 be afforded recovery days during HA or have the ability to perform specific training on non-HA days.
116 Whilst hypothetically beneficial, investigations are needed to assess the efficacy of this strategy,
117 particularly given different markers of heat adaptation have differing timecourses for induction (67)
118 and the associations between adaptation and performance enhancement are not ubiquitously reported.
119 Previous research findings are equivocal, with sub-optimal adaptations reported during non-consecutive
120 versus daily HA (32), attributable to heat decay (88) and insufficient physiological stimulus (4).
121 Consequently, refining non-consecutive protocols so that the timescale, protocol and dose are in line
122 with best practice recommendations i.e. using an isothermic model of ~10-sessions over 10-14-days
123 (70) and thus, ensuring twice-daily methods implement appropriate potentiating stimuli, may ameliorate
124 current limitations and provide an alternative strategy for practitioners who pursue HA benefits but
125 prioritise training quality and recovery schedules.
126 Whilst acute exercise-heat stress is unlikely to impair immune function (84, 85), few studies have
127 investigated immunological biomarkers during HA despite the potential for immunological
128 perturbations to culminate in exacerbated inflammatory (e.g. interleukin-6 [IL-6] and tumour necrosis
129 factor-alpha [TNF-α]) and stress responses (e.g. cortisol) (14, 92), potentially increasing HRI
130 susceptibility (48) and diminishing the application and efficacy of HA (34, 69). Investigation of
131 inflammatory responses to once-daily isothermic HA reported few negative findings (14), however the
132 immune response to our proposed twice-daily model of matched volume (dose), but altered frequency,
133 remains unknown and maladaptation may be a concern. Therefore, investigation is required given the
134 repeated exercise-induced hyperthermia, coupled with shorter recovery time during the ‘heat days’ of
135 TDHA that may result in an overload of physiological strain, inducing residual stress between sessions
136 (72).
137 This study investigated the efficacy of short- (i.e. 5-sessions) and medium-term (i.e. 10-sessions) HA,
138 using non-consecutive TDHA and consecutive ODHA protocols, and compared these to temperate
139 exercise groups (i.e. once-daily: ODTEMP and non-consecutive twice-daily: TDTEMP) as frequency
140 and duration matched exercise controls. Secondly, this study investigated exercise tolerance through
141 the determinants of aerobic performance, and subsequent performance in both hot and temperate
142 conditions between interventions. Finally, this study also investigated the inflammatory and stress
143 responses during interventions to determine whether a compromised immune function was an artefact
144 of the twice-daily protocol. It was hypothesised that as the dose of HA was the same, TDHA would
145 induce the same physiological and ergogenic benefits as ODHA, with both TDHA and ODHA superior
146 to ODTEMP and TDTEMP. Given the alteration in frequency of the HA dose, it was hypothesised that
147 the reduced duration between TDHA sessions would lead to undesirable inflammatory/stress responses
148 in comparison to ODHA.
149 Methods
150 Participants and ethical approval
151 Forty moderately-trained [performance level 3 (62)] males provided informed consent to participate in
152 the experiment, which was approved by the Univeristy of Brighton Institution’s Research Ethics and
153 Governance Committee and conducted in accordance with Declaration of Helsinki (2013). Participants
154 were matched for biophysical characteristics and aerobic capacity and assigned to; consecutive ODHA,
155 non-consecutive TDHA, consecutive ODTEMP or non-consecutive TDTEMP. No differences in
156 participant characteristics were observed (P>0.05 [Table 1]),
157 ***Add Table 1 near here***
158 Experimental design
159 Prior to group allocation, participants completed four tests comprising; cycling graded exercise test
160 (GXT), heat acclimation state test (HAST) and time to task failure test in hot (TTTFHOT) and temperate
161 conditions (TTTFTEMP), in a semi-randomised order, 48-hr apart with the GXT completed first.
162 Interventions consisted of, 60-min exercise sessions performed in hot (45°C, 20% RH) or temperate
163 conditions (22°C, 40% RH) over a 12-day period. Post-tests were repeated in the same order 48-hr apart
164 (Figure 1). This study was completed during November-February, with trials occurring at the same time
165 of day to minimise the effect of circadian variation on exercise tolerance (21) and thermoregulation
166 (86). Participants avoided alcohol and caffeine 12-hr before experimentation, arrived in a euhydrated
167 state (73) and replicated food intake the day of the each exercise trial (2).
168 ***Add Figure 1 near here***
169 Determinants of aerobic performance - Graded exercise test (GXT)
170 Height (Detecto Scale Company, USA) and body mass (Adam Equipment Inc., USA) were measured,
171 enabling the estimation of body surface area (BSA) (5). Skinfold thickness was measured (Harpenden,
172 Baty International, UK) across four sites (22) to estimate body fat (%) (76). The GXT was completed
173 on an electronically-braked stationary ergometer (SRM High performance model, Germany) within
174 temperate conditions (22°C, 40% RH). Power output was initially set at 80 W and increased by 24 W
175 every stage (3-min), with cadence kept at 80 rev.min-1. Capillary blood lactate concentration ([La]b)
176 was sampled within the final 30-s of each stage and analysed immediately (2300 Plus, YSI, USA).
177 Breath-by-breath metabolic gas data were continuously collected (Metalyzer 3B, Cortex, Germany).
178 Lactate threshold (LT) was determined by an increase (>1 mmol.L-1) in [La]b above resting level (15)
179 and the test was terminated when the onset of blood lactate accumulation (OBLA) occurred (>4 mmol.L-
180 1) (93). Gross mechanical efficiency (GME) was calculated from steady-state oxygen consumption and
181 respiratory exchange ratio (RER <1.0) values collected during the final 30-s of each stage of the LT test
182 (25). Following 15-min rest, participants performed a second test with an initial power output 48 W
183 below OBLA that was increased by 20 W.min-1 until volitional exhaustion (39). Peak oxygen uptake
184 (V̇O2peak) and power output (PPO) were determined as the highest average V̇O2 and power output during
185 the final 30-s of each stage. Following 15-min rest, participants were familiarized to the TTTF at 80%
186 of their PPO.
187 Aerobic performance - Time to task failure (TTTF)
188 TTTFTEMP (22°C, 40% RH) and TTTFHOT (38°C, 40% RH) were completed at 80% of PPO (51) on a
189 modified cycle ergometer (SRM crankset and wireless PowerControl meter on a Monark 874E,
190 Sweden). Following a standardised warm-up (2-min seated rest, 5-min at 90% of LT, 3-min rest and
191 then 3-min of unloaded pedalling at 80 rev.min-1), power output was increased to 80% PPO. HR, Tre
192 and metabolic gas data were collected every minute and RPE was recorded at task failure (i.e. when
193 cadence failed <77 rev.min-1 for >3-s following a warning). Power output, HR and time were obscured
194 with only cadence displayed.
195 Heat acclimation state test (HAST)
196 HASTs were completed in hot-dry conditions (45°C, 15% RH) within an environmental chamber (TISS,
197 UK) on a cycle ergometer (Monark 620, Sweden). HASTs simulated Havenith and Middendorp (1986)
198 protocol, but prescribed exercise intensities at given rates of Ḣprod relative to body mass (3.0, 4.5 and
199 6.0 W.kg-1) (91). Heat acclimation state was identified via sweat setpoint and sweat gain measures (37).
200 Metabolic energy expenditure was estimated from known values of V̇O2 and RER below LT during the
201 GXT (58). Ḣprod was subsequently calculated and associated exercise intensities prescribed (Cramer and
202 Jay 2014) during the HAST, which were re-calculated post-intervention.
203 Heat acclimation and temperate exercise protocols
204 Participants completed ten 60-min exercise sessions over 12-days. Once-a-day groups (ODHA,
205 ODTEMP) exercised on days 1-5 and 8-12 at 08:00-hr, whereas twice-daily groups (TDHA, TDTEMP)
206 exercised twice on days 1, 3, 8 and 10 at 08:00-hr and 16:00-hr, and then once on days 5 and 12 at
207 08:00-hr (Figure 1). Exercise commenced at 2.3 W.kg-1 (~65% V̇O2peak) for 15-min at 80 rev.min-1, in
208 line with recommended guidelines to rapidly attain the desired change in core temperature (31, 41).
209 Power output was subsequently altered depending on changes in Tre (∆Tre) and perceived effort (55), to
210 target a Tre of ≥38.5°C for the remainder of the session (80) (see Table 2 for actual training data). To
211 amplify ∆Tre, upper-body sauna suits (Everlast, London, UK) (90) were worn during the initial 15-min
212 of exercise. This method has been applied prior to HA (53) to increase physiological strain without
213 increasing exercise intensity or volume (19, 90). Physiological and perceptual measures were recorded
214 at rest and every 5-min during exercise for all 10 sessions. During sessions 1, 5 and 10, fluid ingestion
215 was prohibited for accurate estimation of sweat loss. Participants were permitted to drink ab libitum
216 during the remaining sessions (55). Euhydration was determined on arrival to each session by collection
217 of mid flow urine; colour <3 (Ucol), osmolality <700 mOsmol.kg-1 (Uosm) (Osmocheck, Vitech Scientific
218 Ltd., Japan) and specific gravity <1.020 (Usg) (hand refractometer, Atago, Japan) (73). HR was
219 manually recorded (Polar Electro, Oy, Finland) and Tre was continuously monitored using a thermistor
220 probe (Henleys Medical Supplies, UK) self-inserted 10 cm past the anal sphincter. Whole-body sweat
221 loss (WBSL) was estimated for each session from towel dried nude body mass differences pre- to post-
222 exercise. Sweat samples (~2 mL) were collected in a Tegaderm+Pad (3MTM, USA) placed on the
223 midpoint of the trapezius before being analysed for sodium concentration ([Na+]) using a Sweat-ChekTM
224 (Eli Tech Group, Wescor Inc., USA) for sessions 1, 5 and 10.
225 Phlebotomy and biochemistry
226 Following 10-min of seated rest immediately before and after sessions 1, 5 and 10, fingertip capillary
227 blood (~200 µL) was sampled for haemoglobin (HemoCue, Ltd., Sweden) and haematocrit (Hawksley
228 and Sons Ltd., England) to estimate ΔPV (20). A 10 mL venepuncture sample was also collected from
229 the antecubital fossa, transferred into two 5 mL tubes (EDTA Sarstedt, Akteingesellscaft and Co,
230 Germany), centrifuged (Eppendorf 5702 R Centrifuge, UK) for 10-min at 5000 rev.min-1, and then
231 plasma stored at -86°C. Upon analysis, commercially available ELISA kits were used to measure IL-6
232 and TNF-α (Ready Set Go!®, eBioscience, Affymetrix Inc., USA) and cortisol (Sigma-Aldrich, USA)
233 in duplicate and corrected for ∆PV.
234 Perceptual measures
235 RPE (6) from 6 (No exertion) to 20 (Maximal Exertion), thermal sensation scale [TSS (82) from 0 (Very
236 Very Cold), 4 (Neutral) to 8 (Very Very Hot)] and thermal comfort [TC (95) from 0 (Very Comfortable)
237 to 5 (Very Uncomfortable)], were collected during exercise sessions every 5-min following
238 familiarisation.
239 Data and statistical analyses
240 All data are reported as mean ± SD, with statistical significance set at P<0.05. Data were assessed and
241 conformed to normality and sphericity prior to further statistical analysis. Within-group differences for
242 pre-intervention data sets were analysed using a one-way ANOVA. To assess intervention efficacy,
243 physiological, performance and perceptual data were analysed using a three-way mixed design ANOVA
244 (Time*Condition*Frequency), for time (pre- to post-intervention), condition (HA and TEMP) and
245 frequency (once- and twice-daily exercise). Following a significant F-value, follow up Bonferroni-
246 corrected post-hoc comparisons were used. Predefined analytical limits to highlight meaningful heat
247 adaptations were; ∆Tre >0.20°C, ∆HR >5 b.min-1 ∆WBSL >200 mL, ∆PV >5% and >1 in perceptual
248 scales (RPE, TSS and TC) (92). Typical error of measurement (TEM) were used to determine
249 meaningful differences for sweat setpoint (0.21°C), sweat gain (0.09 g.sec-1.°C-1), TTTF test (15%),
250 V̇O2peak test (4.8%), IL-6 (2 pg.mL-1), TNF-α (1 pg.mL-1) and cortisol (57 nmol.L-1). Isotime data (i.e.
251 task failure time-point pre-intervention compared to the corresponding time-point post-intervention)
252 was also analysed.
253 Results
254 Heat adaptations
255 During both ODHA and TDHA interventions, resting Tre, resting HR, and sweat [Na+] were reduced,
256 while WBSL and PV were increased within session 5 (STHA) and 10 (MTHA) (P<0.05) compared to
257 session 1 (Table 2). The highest recorded perceptual measures (i.e. peak RPE, TSS and TC) were also
258 lower (P<0.05) from session 1-5 (STHA), and 1-10 (MTHA) during ODHA and TDHA. These
259 physiological and perceptual adaptations were greater following session 10 (MTHA) compared to
260 session 5 (STHA) (P<0.05). Adaptations did not differ between HA groups (all P>0.05), but larger
261 magnitudes in adaptations were observed compared to both TEMP interventions (P<0.05) (Table 2).
262 There were no differences (P>0.05) between groups for exercise time, intensity or work completed
263 during the HA or TEMP sessions. However, as expected physiological strain (i.e. time >38.5°C and
264 ∆Tre) was larger (P<0.05) during HA compared to TEMP (Table 3). Exercise time and work completed
265 during exercise sessions were greater (P<0.05) between session 1-10 (MTHA) and session 1-5 (STHA)
266 for each group (Table 3).
267 Post-intervention HASTs demonstrated reductions in sweat setpoint, HRpeak and TCpeak, and
268 improvements in sweat gain and WBSL (P<0.05) for ODHA and TDHA groups, with greater
269 improvements compared to TEMP (P<0.05), yet no differences were found between HA protocols
270 (P>0.05) (Table 2).
271 ***Add Table 2 and 3 near here***
272 Exercise tolerance
273 Determinants of aerobic performance - GXT
274 A main effect was found for power output at LT and V̇O2peak (P<0.05), with a greater (P<0.05)
275 improvement following HA (ODHA and TDHA), compared to TEMP (ODTEMP and TDTEMP; Table
276 4). No Time*Condition*Frequency interaction (P>0.05) was found for any GXT data. No
277 improvements (P>0.05) were found in PPO or GME.
278 Aerobic performance - TTTF
279 Pre-intervention TTTFHOT was shorter (all P<0.001) compared to TTTFTEMP for all groups, with no
280 between-group differences (P>0.05).
281 TTTFHOT improved (P<0.001) following ODHA and TDHA, but not ODTEMP or TDTEMP (P>0.05),
282 whereas TTTFTEMP did not improve (P>0.05) following any intervention (Table 4). Following TDHA
283 and ODHA only, Tre and HR were lower at isotime (P<0.05) during TTTFHOT and TTTFTEMP (Table 5).
284 ***Add Table 4 and 5, and Figure 2 near here***
285 Biomarkers
286 Increased plasma [IL-6], [TNF-α] and [cortisol] (P<0.05) were observed from pre- to post-session 1, 5
287 and 10 during both HA and TEMP protocols (Figure 3). Inflammatory and stress responses were greater
288 for HA compared to TEMP with larger mean: ∆IL-6 values following session 1, 5 and 10 (P<0.001);
289 ∆TNF-α following session 1 and 10, but not 5 when comparing HA to ODTEMP only (P<0.05); and
290 ∆cortisol following session 5 for ODHA vs. TEMP, and following session 5 and 10 for TDHA vs.
291 TEMP (Figure 3). No differences in inflammatory or stress responses were observed between the HA
292 protocols at any time point (P<0.05). Interestingly, there was no evidence of chronic effects over the
293 course of HA or TEMP (P>0.05), however there was a trend (P<0.10) for the ∆IL-6 and ∆cortisol to be
294 lower and ∆TNF-α to be higher for session 10 when compared to the other sessions for ODHA and
295 TDHA only.
296 ***Add Figure 3 near here***
297 Discussion
298 In agreement with our hypothesis, ODHA and TDHA induced comparable heat adaptations to one
299 another, thus demonstrating an improved heat acclimation state compared to ODTEMP and TDTEMP.
300 Improvements in power at LT and V̇O2peak were found following HA, in addition to both ODHA and
301 TDHA enhancing performance (TTTF) in hot, but not temperate conditions, an improvement that was
302 not observed by either TEMP group. Inflammatory responses increased acutely following single
303 sessions in all groups, with larger responses during HA vs TEMP. However, contrary to out hypothesis,
304 no difference was observed between ODHA and TDHA groups. These data highlight that non-
305 consecutive TDHA presents no difference to ODHA, inducing similar heat adaptation and
306 improvements in exercise tolerance during heat stress, without compromising immune status. These
307 findings suggest the dose of HA (e.g. matched weekly exposure and intensity) is most important for the
308 mechanisms which underpin adaptation, as opposed to the structure of HA (e.g. frequency [once- or
309 twice-daily] and timing [morning or afternoon]).
310 Heat adaptations
311 HA efficacy was confirmed by the acquisition of key physiological heat adaptations including
312 reductions in resting Tre (-0.3°C) and HR (-10 b.min-1), [Na+] retention (-14 to -27 mmol.L-1) and,
313 increased WBSL (+398 to +533 mL) and PV expansion (+8.5 to +10.1) (Table 2). Proportional
314 improvements were also observed following just 5-sessions (i.e. STHA). Reductions in RPE (-2) and
315 TSS (-0.7 to -0.9), and an improved TC (-1), also demonstrate positive perceptual improvements
316 following 10-sessions of both ODHA and TDHA. Collectively, these adaptations are in line with a
317 recent meta-analysis on HA (83) and, whilst direct comparisons across studies are difficult due to
318 differences in HA exercise protocols, MTHA studies (i.e. once-daily) do report equivalent magnitudes
319 of adaptation to the present study [e.g. resting Tre: -0.17°C, and HR: -5 b.min-1, [Na+] retention: -
320 22mmol.L-1, WBSL: +29% and PV expansion: +4.3% (83)]. Moreover, ODHA and TDHA induced
321 adaptation superior to our predefined analytical limits (∆Tre >0.20°C, ∆HR >5 b.min-1, ∆WBSL >200
322 mL, ∆PV >5% and >1 in perceptual scales [RPE, TSS and TC] (92)) highlighting meaningful heat
323 adaptations, a critical factor when assessing intervention strategies.
324 Both HA strategies improved heat acclimation state, as indicated by a lower sweat setpoint (-0.22°C)
325 and a larger sweat gain (+0.14 to +0.15 g.sec-1.°C-1) during the post-intervention HAST (Table 2).Whilst
326 no reductions in ∆Tre, Trepeak, RPEpeak or TSSpeak occurred following HA, this can be explained by the re-
327 prescription of exercise intensities, thus, controlling for Ḣprod post-intervention and providing
328 confidence in our adaptations. The unchanged Tre but larger WBSL (both +35%) shows
329 thermosensitivity is enhanced via increased sweat gain for ODHA (+48%) and TDHA (+49%). Though
330 theese changes are superior to the meta-analysis findings (+25% (83)) and TEM (0.09 g.sec-1.°C-1 (91))
331 the authors accept that an oesophageal core temperature and real-time local sweat rate measurements
332 would offer superior assessment of these data given a more rapid response in comparison to our rectal
333 thermistor (81). Parallel reductions in resting Tre and sweat setpoint, following ODHA (-0.28 and -
334 0.22°C) and TDHA (-0.28 and -0.22°C), respectively, agree with meta-analysis findings (-0.28 (83))
335 and are larger than the TEM (0.21°C (92)). MTHA (e.g. 10-sessions) induced greater magnitudes of
336 physiological and perceptual heat adaptation compared to STHA (e.g. 5-sessions [Table 2 and 3]).
337 Though not in agreement with all experimental data (27, 28), these findings agree with consensus
338 recommendations that longer-term HA (e.g. ≥10-days) is preferable to induce greater physiological heat
339 adaptations (67, 70) achieved in this study through the maintained physiological strain imposed using
340 isothermic prescription (80). These data provide supporting evidence that medium- to long-term HA
341 could be prescribed immediately before, or potentially several weeks before major athletic competition
342 or military deployment in heat stress (18) to induce greater initial adaptations, as opposed to solely
343 implementing STHA during the final training microcycle. This notion, alongside the decay of these
344 aforementioned adatations (18), should be experimentally examined as this strategy would allow
345 alternate approaches (e.g. intermittent ‘top up’ exposures in the days preceding exposure) to be
346 implementated to maintain the enhanced heat acclimation state (8).
347 Seminal work by Lind and Bass (49) demonstrated the benefits of continuous, once-daily HA (i.e. 100-
348 min sessions), as opposed to longer and shorter intermittent times (e.g. twice-daily, 2x50-min), which
349 contributed to duration recommendations for optimal heat adaptations (70). Our data indicate no
350 advantage but more importantly, no disadvantage of non-consecutive TDHA over consecutive ODHA,
351 agreeing with our previous STHA investigation (89). Further to this, as outlined above, these
352 observations are true even when the session duration is 60 min (this study), as opposed to 90 – 100 min
353 which has been previously described as preferable (70). Our novel findings are in contrast to others
354 which have not demonstrated efficacy of TDHA (32), but may be explained by a) the use of an
355 isothermic model, b) the matching of exercise-heat dose (e.g. duration, intensity and total number of
356 exposures) to induce equivalent heat adaptations and improved exercise tolerance, and/or c) more
357 significant heat strain i.e. maximising time spent at the targeted Tre. This non-consecutive twice-daily
358 structure is likely to be appealing to coaches and practitioners with upcoming competitions in
359 challenging, hot conditions (e.g. Tokyo 2020 Olympic and Paralympic Games) for whom scheduling
360 HA around sport-specific training, competition tapering, rest and travel is challenging. This study is the
361 first to demonstrate equivalent heat adaptations following both TDHA and ODHA, with greater
362 adaptations for longer interventions (i.e. 5- vs. 10-days) suggesting the dose of HA (i.e. attaining key
363 physiological responses to a greater extent) is the primary factor that underpins adaptation.
364 Exercise tolerance
365 Determinants of aerobic performance
366 Our study provides a holistic overview of the changes in exercise tolerance following non-consecutive
367 TDHA, in comparison to consecutive ODHA and matched TEMP interventions. V̇O2peak improved
368 following HA (ODHA: +4.6%; TDHA: +3.7%), with this change greater than TEMP changes
369 (ODTEMP: +2.6%; TDTEMP: +1.4%). This is likely due to hypervolemia following HA and potential
370 increments in cardiac output (50) however it must be acknowledged that participants were not elite
371 athletes whom as a cohort may be less responsive to this mechanism (59). Nonetheless, previous studies
372 report ergogenic benefits of HA on V̇O2peak and PPO in temperate conditions (23, 24, 50, 71, 75, 79)
373 whilst others present no changes (43, 44, 55). Power at LT also improved significantly following HA,
374 in agreement with previous findings (9, 10, 42, 45, 50, 55, 68, 71) however improvements following
375 ODHA (+7±10 W) and TDHA (+7±8 W) were of. A lower magnitude than those reported in well-
376 trained cyclists in 13°C (+12-15 W (50)) and 22°C (+16 W [(55)] and +15 W [(71)]). Furthermore,
377 GME did not change following interventions, in agreement with previous LTHA (43). Whilst the
378 ergogenic benefits of HA remain disputed between research groups, potentially as a result of insufficient
379 potentiating stimuli or inter-individual differences (13), our data are the first to demonstrate that
380 implementing a non-consecutive twice-daily intervention does not induce differential ergogenic effects
381 to that of a matched dose once-daily protocol, for the determinants of aerobic performance (e.g. V̇O2peak
382 and power at LT) in temperate conditions.
383 Aerobic performance - Time to task failure Determinants of aerobic performance
384 TTTFHOT improved following ODHA (+25%) and TDHA (+24%), but not ODTEMP and TDTEMP
385 (both +5%), agreeing with previous reports following MTHA (+67% [(56)], +17% [(57)] and +24%
386 [(17)]), which appear to exceed STHA (+14% [(26)] and +7% [(11)]) likely due to greater physiological
387 adaptation. Evidence for TTTFHOT improvements likely reflecting the magnitude HA adaptations (e.g.
388 PV expansion improving cardiac output (50, 56), leading to increased V̇O2peak and power at LT, resulting
389 in a lessened physiological strain (67), is indicated by a lower mean Tre (-0.26°C) and HR (-8 b.min-1)
390 at isotime (Table 5). Consequently, non-consecutive TDHA appears equally effective as ODHA for
391 improving aerobic performance (e.g. extending exercise tolerance time) in sub-elite athletes within the
392 severe-intensity domain under heat stress. It is likely adaptations contributed to the improved TTTFTEMP
393 following ODHA and TDHA (both +14%). However, these data describe that HA (irrespective of once-
394 or twice-daily frequency) provided only moderate ergogenic benefits for performance in temperate
395 conditions, opposing significant time trial improvements in 13°C (50) and 22°C (71) but agreeing with
396 recent studies which suggest these physiological constructs are not limitng (43, 44, 55). Nonetheless,
397 this is the first study to collectively assess TTTF in both hot and temperate conditions which whilst
398 demonstrating some inter-individual differences (Figure 3), describes ODHA and TDHA as providing
399 ergogenic benefits for enhanced performance in hot conditions with data in temperate conditions
400 encouraging, albeit not unequivocal (54, 60).
401 Inflammatory and stress responses
402 Agreeing with previous literature, larger ∆IL-6 and ∆TNF-α were observed during HA compared to
403 TEMP (Figure 2) (3, 34, 35, 46, 47, 63, 77). The larger responses observed for ∆cortisol for session 5
404 and 10, but not 1 (1, 7, 12, 36, 40, 63, 77) during HA are a response to increased physiological strain
405 due to the heat stress for the same absolute exercise intensity (e.g. higher Tre, ∆Tre and HR) (77). Our
406 changes in IL-6 (+55%), TNF-α (+45%) and cortisol (+34%) during HA were comparable in ODHA
407 and TDHA, and are less than, or comparable to, responses published elsewhere (IL-6: +20-2000%,
408 TNF-α: +15-65% and cortisol: +20-70%) (1, 3, 7, 14, 34, 35, 40, 46, 63, 77). Our findings also agree
409 with reported transient ∆IL-6 during MTHA (14, 34) alongside induced heat adaptations (71) and no
410 evidence of chronic inflammatory effects or signs of exaggerated ∆TNF-α (e.g. possible endotoxemia)
411 (34). The absence of augmented ∆cortisol as HA progresses, conforms to previous literature describing
412 the sensitivity of this biomarker to various stressors (1, 14, 26, 78, 87). In summary, our data indicates
413 no chronic inflammatory effects or stress responses during ODHA and, for the first time during non-
414 consecutive TDHA, which is likely due to the equivalent acquisition in physiological heat adaptation.
415 These novel data provide confidence that our TDHA protocol did not induce unexpected inflammatory
416 or stress responses which could compromise immune status in subsequent heat exposures to any greater
417 extent than ODHA. This further strengthens the argument for TDHA when ODHA is impractical.
418 Application
419 The similarity of the responses to non-consecutive TDHA and ODHA, may be of particular interest to
420 sporting and occupational organisations that require heat adaptations to lessen the physiological strain
421 and HRI risk, and improve exercise performance in heat stress. Non-consecutive TDHA provides an
422 alternate and flexible strategy, providing the potential to half the number of interrupted training days,
423 thus maximising an individual’s time to complete specific (e.g. non-heat) training or rest/recover
424 without compromising the magnitude of adaptation. Logistically, the non-consecutive TDHA is
425 appealing given the cost and time associated with athletes or workers travelling to specialist heat
426 training facilities in cool climates, may be reduced if multiple heat sessions can be completed on one
427 day. The transient nature of heat adaptations requires STHA during crucial preparation periods, where
428 training is predominantly sport-specific with volume often adjusted to optimise recovery, thus resulting
429 in training that opposes targetted physiological adaptations. It is unsurprising therefore, that repeated
430 steady-state exercise during consecutive day HA, do not appear to be widely embraced by competitive
431 athletes (65). Prescribing TDHA and specifically afternoon sessions, may also increase HA efficiency
432 as time spent at the desired isothermic Tre of >38.5°C was extended during afternoon compared to
433 morning sessions (+14 vs. +6 min), yet ∆Tre were lower (+1.3°C vs. +1.6°C), thus requiring less exercise
434 time to reach target temperatures due to circadian rhythm and higher resting Tre. Ultimately, shorter
435 duration HA (~60 min) that provides sufficient physiological strain to evoke meaningful phenotypic
436 adaptations irrespective of daily frequency and consecutive scheduling is desirable, with non-
437 consecutive TDHA providing greater flexibility than a consecutive day protocol.
438 Limitations and future direction
439 Despite our biomarker data indicating TDHA does not induce excessive inflammatory/immune
440 responses, our mechanistic insights are limited due to the number and timing of blood sampling.
441 Collecting additional biomarker measures and across more time-points during the recovery phase (e.g.
442 1-24-hr) would provide further insight into the inflammatory responses and potential maladaptive
443 influences on the magnitude and kinetics of heat adaptation. An extension of this work would also
444 examine intracellular heat shock proteins (46) and the relevant gene transcripts (27) to elucidate the
445 impact of TDHA vs. ODHA on attaining thermotolerance (46), and potential benefits across
446 environmental stressors (29, 30). We also highlight a need to investigate the precise effect of
447 consecutive and non-consecutive TDHA in females, who experience different thermoregulatory
448 adaptation kinetics to males (52). Moreover, the effect of HA duration should be considered (e.g. 60-
449 vs. 90/100-min sessions) given an extended heat dose may impact the kinetics and magnitude of both
450 adaptation and the inflammatory responses. A paucity of data still exists to effectively characterise the
451 rate of heat decay and re-induction of HA at a physiological and molecular level, which is critical for
452 the implementation of all HA protocols including TDHA. Finally, we highlight the need for
453 investigations regarding the feasibility and appropriateness of HA and other concurrent training (e.g.
454 interval or competition specific intensity sessions) for elite athletes.
455 Conclusion
456 This is the first study to investigate the efficacy of non-consecutive twice-daily HA compared to daily
457 HA for adaptations, biomarkers and exercise tolerance. Greater heat adaptations were induced by both
458 once- and twice-daily HA protocols, compared with equivalent temperate exercise, without adverse
459 effects on inflammatory or stress responses. Exercise tolerance in heat stress was improved following
460 both HA protocols, yet no effect was found for matched-volume TEMP, nor were improvements found
461 for exercise tolerance in temperate conditions for all interventions. The concomitant increase in power
462 at LT and V̇O2peak following HA, reaffirms the erogenicity of HA on aerobic performance within heat
463 stress, although our data do not provide supportive evidence for HA to enhance aerobic performance in
464 temperate conditions.
465 Acknowledgments
466 The authors would like to thank the all the participants who volunteered for this study.
467 Conflict of interest
468 The authors confirm there are no conflict of interest.
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706 Figure Legends
707 Figure 1. Schematic design of the study. Note. HAST, TTTFHOT and TTTFTEMP performed in 708
randomised order. GXT = graded exercise test, HAST = heat acclimation state test, TTTF = time to task 709
failure in hot (HOT) or temperate (TEMP) conditions, ODHA = once daily heat acclimation, TDHA = twice 710
daily, non-consecutive day heat acclimation, ODTEMP = once daily temperate training, TDTEMP = 711 twice
daily, non-consecutive day temperate training.
712 Figure 2. Mean ± SD Mean ± SD changes in the determinants of aerobic performance and aerobic 713
performance in hot and temperate conditions. *represents a significant (P<0.05) within-group 714
difference pre- to post-session. ∂ represents a significant (P<0.05) between-group difference with (HA 715 vs.
TEMP). Shapes denote individual participants within group.
716 Figure 3. Mean ± SD changes in cortisol, TNF-α and IL-6 for session 1, 5, 10. *represents a 717
significant (P<0.05) within-group difference pre- to post-session. †represents a significant 718
(P<0.05) between-group difference with ODTEMP. ‡represents a significant (P<0.05) between- 719
group difference with TDTEMP. Shapes denote individual participants within group.
720
721 Tables
722 Table 1. Mean ± standard deviation (SD) participant characteristics.
723 Group
(n = 40)
Age
(years)
Body
mass (kg)
Height
(m)
BMI
(kg.m2)
BSA
(m2)
Sum of
skinfolds
Body fat
(%)
(n = 10)
(n = 10)
(n = 10)
(n = 10)
BMI: body mass index, BSA: body surface area, ODHA: once-daily heat acclimation, TDHA:
twice-daily heat acclimation, ODTEMP: once-daily temperate exercise and TDTEMP: twice-
daily temperate exercise.
(mm)
ODHA 23±6 77.2±10.0 1.78±0.08 24.4±2.1 1.95±0.16 34.5±7.3 14.9±2.7
TDHA 25±7 75.3±9.5 1.79±0.04 23.4±2.5 1.94±0.13 33.4±9.9 14.3±3.7
ODTEMP 22±1 77.3±8.6 1.77±0.04 25.5±3.0 1.92±0.10 35.7±6.4 15.0±1.7
TDTEMP 22±1 75.2±7.8 1.78±0.07 23.8±1.5 1.93±0.14 33.8±7.5 14.6±2.9
O
724
Table 2. Mean ± SD changes (∆) in heat adaptations over days 1-5 (short-term) and days 1-10 (medium-term) and during the heat acclimation state pre-post intervention.
Group ODHA TDHA ODTEMP TDTEMP Session 1-5 1-10 1-5 1-10 1-5 1-10 1-5 1-10
Heat adaptations
∆Rest Tre(°C) -0.18±0.27* -0.28±0.22*†‡+ -0.22±0.17*†‡ -0.28±0.19*†‡+ +0.03±0.21 -0.10±0.16 -0.04±0.17 -0.11±0.18 ∆Rest HR (b.min-1) -5±1* -10±3*+ -5±5* -10±4*+ -1±1 -2±1 +1±3 -2±6 ∆PV (%) +6.3±4.0 +10.1±5.6*+ +5.4±4.0 +8.5±3.1*+ +0.5±2.8 +1.5±3.5 +1.5±3.4 +0.7±4.1 ∆WBSL (mL) +230±207* +533±261*†‡+ +178±142*†‡ +398±97*†‡+ +83±86 +81±97 +48±68 +90±118 ∆ [Na+] (mmol.L-1) -13±13*†‡ -27±19*+†‡+ -7±6 -14±5*+ -12±12 -24±20+ -6±12 -11±13+ ∆RPEpeak -1±1 -2±1*+ -1±1 -2±1*+ -1±1 -2±1*+ 0±2 0±2 ∆TSSpeak -0.3±0.4 -0.7±0.5*+ -0.5±0.5 -0.9±0.5*+ +0.4±0.5 +0.2±0.8 +0.1±0.9 +0.1±0.7 ∆TCpeak -1±1 -1±1* 0±1 -1±1*+ 0±0 0±0 0±0 0±0
Heat acclimation state (1-10)
∆Sweat setpoint (°C) -0.22±0.18*† -0.22±0.14*‡† -0.14±0.18* -0.11±0.10* ∆Sweat gain (g.sec-1.°C-1) +0.14±0.10* +0.15±0.07* +0.05±0.07 +0.06±0.06 ∆WBSL (mL) +262±180* +278±211* +68±118 +68±112 ∆Trepeak (°C) -0.25±0.11* -0.28±0.11* -0.15±0.27 -0.08±0.25 ∆HRpeak (b.min-1) -13±9* -14±10* -4±1 -2±6 ∆RPEpeak -3±2* -3±1* -3±2* -2±2* ∆TSSpeak -0.7±0.5 -0.6±0.7 -0.4±0.9 -0.3±0.4 ∆TCpeak -1±1*‡ -1±1*‡ -1±1* 0±1 *represents a significant (P<0.05) within-group difference, †represents a significant (P<0.05) between-group difference with ODTEMP, ‡represents a significant (P<0.05) between-group difference with TDTEMP, and +represents a significant difference (P<0.05) between 1-5 and 1-10 adaptations. ODHA: once-daily heat acclimation, TDHA: twice-daily heat acclimation, ODTEMP: once-daily temperate exercise, TDTEMP: twice-daily temperate exercise, Tre: rectal temperature, HR: heart rate, PV: plasma volume, WBSL: whole-bbody sweat loss, [Na+]: sodium concentration, RPE: rating of perceived exertion, TSS: thermal sensation and TC: thermal comfort.
725
For ev ew
O
Table 3. Mean ± SD exercise data for sessions 1-5 (short-term) and 1-10 (medium-term).
Group ODHA (44.4±1.2°C, 21.1±2.4 % RH)†‡
TDHA (44.3±1.3°C, 22.2±3.9 % RH)†‡
ODTEMP (21.6±1.1°C, 40.9±4.2 % RH)
TDTEMP (21.8±1.0°C, 38.6±4.7 % RH)
Session 1-5 1-10 1-5 1-10 1-5 1-10 1-5 1-10
Time (min) 300±0 600±0*+ 300±0 600±0*+ 300±0 600±0*+ 300±0* 600±0*+
Total work (kJ) 2378±280 4838±573*+ 2338±211 4751±374*+ 2419±199 4834±405*+ 2361±254* 4778±440*+
Mean power (W.kg-1) 1.7±0.1 1.7±0.2 1.7±0.1 1.7±0.1 1.7±0.1 1.7±0.1 1.6±0.2 1.6±0.2
Mean power (% PPO) 49±4 49±3 47±5 47±5 47±5 47±5 46±6 46±6
Trepeak (°C) 38.45±0.33 38.39±0.36 38.52±0.24 38.44±0.21 38.39±0.32 38.27±0.26 38.15±0.22 38.16±0.30
∆Tre (°C) 1.29±0.21†‡∂ 1.52±0.23*†‡+∂ 1.48±0.22†‡∂ 1.68±0.28*†‡+∂ 0.70±0.17 0.69±0.18 0.78±0.17 0.90±0.19
Time >38.5°C (min) 62.8±61.2†‡∂ 118.6±118.1*†‡+∂ 50.2±30.3†‡∂ 90.9±49.5*†‡+∂ 21.3±24.5 23.5±17.0 21.5±24.7 23.5±18.8
HRpeak (b.min-1) 167±15 163±13 163±11 155±11+ 169±19 167±17 177±9 163±6+
Sweat loss (mL) 980±287 1513±504*†‡+ 1146±429†‡ 1545±375*†‡+ 655±95 736±139 616±150 733±150
∆PV (%) -7.9±4.0 -4.4±2.8+ -9.4±5.5 -4.6±3.3+ -3.2±2.6 -2.3±1.1 -2.2±1.4 -1.8±1.1
RPEpeak 15±1† 13±1*†‡+ 15±1† 14±1*†+ 17±1 16±2* 16±1 16±2
TSSpeak 6.8±0.4†‡ 6.1±0.6*+ 7.0±0.4†‡ 6.1±0.4*+ 5.8±0.5 5.9±0.7 6.2±0.7 6.1±0.5
TCpeak 4±1 2±1*+ 4±1 3±1*+ 3±0 3±1 3±0 3±0
*represents a (P<0.05) within-group difference, †represents a (P<0.05) between-group difference with ODTEMP, ‡represents a (P<0.05) between-group difference with TDTEMP, +represents a significant difference (P<0.05) between 1-5 and 1-10 adaptations and ∂represents a significant (P<0.05) between-intervention difference (e.g. HA vs. TEMP). ODHA: once-daily heat acclimation, TDHA: twice-daily heat acclimation, ODTEMP: once-daily temperate exercise, TDTEMP: twice-daily temperate exercise, Tre:
rectal temperature, HR: heart rate, PV: plasma volume, ∆: change, RPE: rating of perceived exertion, TSS: thermal sensation and TC: thermal comfort. 726
727
or R vi w l y
Determinants of Aerobic Performance
728
Table 4. Mean ± SD changes (∆) in exercise tolerance (determinants of aerobic performance and aerobic performance).
Group ODHA TDHA ODTEMP TDTEMP
Pre Post ∆ (% ∆) Pre Post ∆
(% ∆) Pre Post ∆ (% ∆) Pre Post ∆
(% ∆)
Power at LT (W) 159±20 166±26 +7±10∂
(+3.4±4.9) 163±30 170±28 +7±8∂
(+4.6±5.4) 157±21 160±23 +3±5 159±17 160±13 +1±6
(+1.9±2.8) (+1.1±4.1)
GME (%) 19.9±1.0 21.0±2.0 +1.0±2.2 20.5±1.7 20.8±1.4 +0.2±1.6 19.3±1.7 19.2±1.6 -0.1±1.5 19.7±1.9 19.7±2.0 +0.1±1.2
V̇O2peak (L.min-1) 3.76±0.46 3.95±0.52 +0.18±0.12∂
(+4.6±3.1) 3.74±0.50 3.89±0.45 +0.13±0.09∂
(+3.7±2.8) 3.73±0.43 3.83±0.45 +0.10±0.09 3.69±0.34 3.73±0.31 +0.05±0.07
(+2.6±2.5) (+1.4±2.0)
PPO (W) 291±39 304±48 +13±18 (+4.2±5.7) 296±50 308±46 +11±8
(+3.9±3.3) 288±27 291±31 +3±14 287±18 296±18 +6±11
(+1.6±4.1) (+2.3±4.1)
Aerobic Performance
TTTFTEMP (s) 519±151 588±153 +68±11 (+14±4) 553±74 631±82 +78±47
(+14±9) 510±102 553±106 +42±51 (+9±10) 532±116 579±161 +47±62
(+8±18)
TTTFHOT (s) 412±111 516±140* +104±31 (+25±4)* 450±85 558±117* +109±57
(+24±11)* 416±131 435±149 +19±58 (+5±14) 430±91 444±97 +15±77
(+5±17) *represents a significant (P<0.05) within-group difference and ∂represents a significant (P<0.05) between-intervention difference (e.g. HA vs. TEMP). ODHA: once-daily heat
acclimation, TDHA: twice-daily heat acclimation, ODTEMP: once-daily temperate exercise, TDTEMP: twice-daily temperate exercise, LT: lactate threshold, GME: gross mechanical efficiency, V̇O2peak: peak oxygen uptake, PPO: peak power output, TTTFTEMP: time to task failure in temperate condition and TTTFHOT: time to task failure in heat stress.
729
730
Re v ew
731
732
733
734
735
736
737
738
739
740
741
Table 5. Mean ± SD changes (∆) in physiological measures compared to pre-intervention time to task failure in temperate (TTTFTEMP) and hot conditions (TTTFHOT).
Group ODHA TDHA ODTEMP TDTEMP
TTTFTEMP
∆Tre (°C) ∆HR(b.min-1)
-0.21±0.12* -6±8*
-0.29±0.24* -6±4*
-0.14±0.16 +1±7
-0.14±0.28 -3±10
∆V̇O2 (L.min-1) -0.02±0.21 0.00±0.17 -0.03±0.32 -0.02±0.28 ∆RER -0.08±0.15 -0.01±0.10 -0.06±0.05 -0.07±0.08 ∆Ḣprod (W) -26±73 -28±72 +36±127 +12±170 ∆V̇E (L.min-1) -8.4±20.1 +5.2±16.0 +8.1±16.3 -6.2±21.4
TTTFHOT
∆Tre (°C) -0.26±0.26* -0.26±0.27* -0.14±0.28 -0.16±0.33 ∆HR(b.min-1) -6±5* -8±6* 0±6 -3±7 ∆V̇O2 (L.min-1) +0.01±0.29 -0.09±0.20 +0.03±0.22 -0.05±0.12 ∆RER -0.01±0.08 +0.02±0.06 -0.04±0.10 -0.07±0.08 ∆Ḣprod (W) -17±104 -11±99 +16±179 +20±209 ∆V̇E (L.min-1) -5.5±20.4 -2.3±16.0 +5.7±20.7 +1.9±16.0 *represents a significant (P<0.05) within-group difference. ODHA: once-daily heat acclimation, TDHA: twice-daily heat acclimation, ODTEMP: once-daily temperate exercise, TDTEMP: twice-daily temperate exercise, Tre: rectal temperature, HR: heart rate, V̇O2: oxygen uptake, RER: respiratory exchange ratio, Ḣprod: metabolic heat production, V̇E: ventilation, ∆: change, TTTFTEMP: time to task
failure in temperate condition and TTTFHOT: time to task failure in heat stress.
ew
Week 1 Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Preliminary and GXT
(22°C, 40% R.H.)
Rest
HAST
(45°C, 20% R.H.)
Rest
TTTFHOT
(38°C, 40% R.H.)
Rest
TTTFTEMP
(22°C, 40% R.H.)
Week 2 Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Morning (08:00)
ODHA, TDHA,
ODTEMP, TDTEMP
ODHA
ODTEMP
ODHA, TDHA,
ODTEMP, TDTEMP
ODHA
ODTEMP
ODHA, TDHA,
ODTEMP, TDTEMP
Rest
Rest Afternoon
(16:00)
TDHA,
TDTEMP
Rest
TDHA,
TDTEMP
Rest
Rest
Week 3 Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Morning (08:00)
ODHA, TDHA,
ODTEMP, TDTEMP
ODHA
ODTEMP
ODHA, TDHA,
ODTEMP, TDTEMP
ODHA
ODTEMP
ODHA, TDHA,
ODTEMP, TDTEMP
Rest
Rest Afternoon
(16:00)
TDHA,
TDTEMP
Rest
TDHA,
TDTEMP
Rest
Rest
Week 4 Monday Tuesday Wednesday Thursday Friday Saturday Sunday
GXT (22°C, 40% R.H.)
Rest
HAST
(45°C, 20% R.H.)
Rest
TTTFHOT
(38°C, 40% R.H.)
Rest
TTTFTEMP
(22°C, 40% R.H.)
For Review
Only
Figure 2. Mean ± SD Mean ± SD changes in the determinants of aerobic performance and aerobic performance in hot and temperate conditions. *represents a significant (P<0.05) within-group difference pre- to post-session. ∂ represents a significant (P<0.05) between-group difference with (HA vs. TEMP).
Shapes denote individual participants within group.
For Review
Only
Figure 3. Mean ± SD changes in cortisol, TNF-α and IL-6 for session 1, 5, 10. *represents a significant (P<0.05) within-group difference pre- to post-session. †represents a significant (P<0.05) between-group
difference with ODTEMP. ‡represents a significant (P<0.05) between-group difference with TDTEMP. Shapes denote individual participants within group.