Physiological and perceptual responses to exercising in restrictive heat loss attire with use of an upper-body sauna suit in temperate and hot conditions

ABSTRACT

The aim of this experiment was to quantify physiological and perceptual responses to exercise with and without restrictive heat loss attire in hot and temperate conditions.

Ten moderately-trained individuals (mass; 69.44±7.50 kg, body fat; 19.7±7.6%) cycled for 30-mins (15-mins at 2 W.kg⁻¹ then 15-mins at 1 W.kg⁻¹) under four experimental conditions; temperate (TEMP, 22°C/45%), hot (HOT, 45°C/20%) and, temperate (TEMP_SUIT, 22°C/45%) and hot (HOT_SUIT, 45°C/20%) whilst wearing an upper-body “sauna suit”.

Core temperature changes were higher (P<0.05) in TEMP_SUIT (+1.7±0.4°C.hr⁻¹), HOT (+1.9±0.5°C.hr⁻¹) and HOT_SUIT (+2.3±0.5°C.hr⁻¹) than TEMP (+1.3±0.3°C.hr⁻¹). Skin temperature was higher (P<0.05) in HOT (36.53±0.93°C) and HOT_SUIT (37.68±0.68°C) than TEMP (33.50±1.77°C) and TEMP_SUIT (33.41±0.70°C). Sweat rate was greater (P<0.05) in TEMP_SUIT (0.89±0.24 L.hr⁻¹), HOT (1.14±0.48 L.hr⁻¹) and HOT_SUIT (1.51±0.52 L.hr⁻¹) than TEMP (0.56±0.27 L.hr⁻¹). Peak heart rate was higher (P<0.05) in TEMP_SUIT (155±23 b.min⁻¹), HOT (163±18 b.min⁻¹) and HOT_SUIT (171±18 b.min⁻¹) than TEMP (151±20 b.min⁻¹). Thermal sensation and perceived exertion were greater (P<0.05) in TEMP_SUIT (5.8±0.5 and 14±1), HOT (6.4±0.5 and 15±1) and HOT_SUIT (7.1±0.5 and 16±1) than TEMP (5.3±0.5 and 14±1).

Exercising in an upper-body sauna suit within temperate conditions induces a greater physiological strain and evokes larger sweat losses compared to exercising in the same conditions, without restricting heat loss. In hot conditions, wearing a sauna suit increases physiological and perceptual strain further, which may accelerate the stimuli for heat adaptation and improve HA efficiency.

Introduction

Heat acclimation (HA) is an intervention undertaken for athletic [1,2] and occupational purposes [3,4], completed in the days or weeks prior to competition or undertaking physical work in heat stress. HA improves the capacity of an individual to dissipate heat via augmented sweating [5], increases the capacity for heat storage by reducing body temperature [6], and reduces the negative thermal sensations [7]. The benefits of HA include enhanced endurance performance [8,9], improved thermal comfort [10,11] and a reduction in the likelihood of heat illness [12,13]. The HA phenotype is induced across numerous integrated physiological systems, such as cardiovascular [14], neuromuscular [15], and at a cellular/molecular level [16], which occurs following repeated exposures to potentiating stimuli for adaptation (e.g. core temperature ≥38.5°C [17], increased skin temperature [18] and elevated sweat rates [19]).

Whilst notable experimental work has highlighted the benefits of utilising passive HA (e.g. resting in hot-dry or hot-humid conditions) [20–22], or by implementing thermal exposures, such as hot water immersion (HWI) [23,24], or sauna post-exercise [25], the most common and potentially most potent HA methods require exercise-heat stress [2]. A variety of HA protocols have been published ranging in durations of 4–20 days, utilising prolonged exposures (30-120-mins) typically in hot-dry or hot-humid conditions (∼40°C, 40% relative humidity [RH]) [26]. Accordingly, two fundamental criteria are required during HA to stress the body, which is achieved by raising and sustaining an elevated core (T_re) and skin temperature (T_skin), and promoting profuse sweating, which concomitantly provide a multitude of prominent physiological and perceptual adaptations [3].

Most HA interventions are completed within controlled environmental chambers, however, the duration of HA training, allied with logistical and financial issues, may limit the prescription of current laboratory HA protocols [1] and disrupt training quality prior to competition. These are likely reasons for only 15% of athletes undertaking a recognised HA intervention prior to the IAAF World Championships, in spite of hot climates being forecast [27]. A proposed method for inducing (should access to an environmental chamber not be possible), or enhancing the efficiency (by accelerating the attainment of necessary physiological stimuli for adaptation) of HA is the wearing of clothing/garments which restrict heat loss during exercise [28]. Wearing garments that inhibit evaporative heat loss such as vinyl suits, or by overdressing in regular clothing, attenuates the rate of heat dissipation in temperate training environments [28-31]. Inhibited heat loss results in a greater rate of heat storage during exercise and ultimately, elevations in physiological strain [29]. The elevation in T_re is a key stimuli for increasing T_skin and sweat output, with associated elevations in skin blood flow reducing blood pressure, elevating cardiovascular strain (e.g. heart rate), the initiation of cellular signalling and inducing thermal discomfort [32]. Greater physiological strain theoretically provides the necessary mechanisms to induce heat adaptation and may present an alternative strategy to be undertaken during normal training as opposed to that within simulated hot environments (e.g. within a chamber) or additional sessions (e.g. post-exercise HWI). Furthermore, the use of restrictive heat loss attire during typical HA within environmental chambers may provide an ergogenic benefit without the necessity of increasing exercise intensity, time and, or volume during important training periods (e.g. tapering), although investigation is required.

The primary aims of this experiment were to determine whether the wearing of an upper-body sauna suit during exercise in temperate conditions (22°C, 45% RH) would; 1) increase key potentiating stimuli for heat adaptation in comparison to exercising in regular clothing, and 2) whether these increases in temperate conditions were equivalent to exercise performed in a hot-dry environment replicating typical HA conditions (40°C, 20% RH). This study also aimed to 3) determine whether wearing an upper-body sauna suit in hot-dry conditions would enhance key potentiating stimuli required during heat acclimation. It was hypothesised that wearing a sauna suit in temperate conditions would elicit physiological strain equivalent to exercising in regular clothing in hot conditions, and that wearing the vinyl suit in hot conditions would provide a more rapid attainment of the necessary physiological strain to induce heat adaptation.

Methods

Participants

Ten moderately trained participants (6 males and 4 females; mean ± standard deviation [SD] age: 25 ± 3 years, mass: 69.44 ± 7.50 kg, stature: 175 ± 9 cm, body surface area 1.84 ± 0.13 m², and body fat: 19.7 ± 7.6%) volunteered, after providing written informed consent. Participants had not exercised in hot conditions (>25°C) for >3 months, nor were they regular sauna, steam or hot bath users. Each participant abstained from strenuous exercise, caffeine and alcohol 24-hrs prior to each session. Food intake was restricted 2-hrs prior to exercise, normal diets were maintained throughout the study. Participants arrived euhydrated, as indicated by urine osmolality (U_osm) <700mOsm·kg⁻¹ and specific gravity (U_sg) <1.020 [33]. All female participants were taking oral contraceptive pills, beginning experimentation on day 2 of the pill phase, which occurred during the early-follicular phase (e.g. 3–5 days after the onset of menstruation) of their self-reported menstrual cycle, as verified by a questionnaire [34,35]. The study was conducted in accordance with the Institution's ethics and governance committee, and Declaration of Helsinki [36]. Exercise was terminated if T_re ≥39.7°C (zero incidence).

Experimental design

A randomised, repeated-measures design was adopted, with each participant visiting the laboratory on four occasions, 72-hrs apart to minimise any acclimation effect from repeated heat exposures. During each visit, following instrumentation and 15-mins passive rest in temperate laboratory conditions, participants cycled for 30-mins, replicating the onset of a typical isothermic HA protocol [37], within four conditions; temperate (TEMP: 22°C, 45% RH), temperate whilst wearing an upper-body sauna suit (TEMP_SUIT: 22°C, 45% RH), hot (HOT: 45°C, 20% RH) and hot whilst wearing an upper-body sauna suit (HOT_SUIT: 45°C, 20% RH). In addition to shorts, socks and shoes (plus sports bra for females), only an upper-body suit was worn to mitigate against excessive heat gain, whilst still ensuring that sites for maximal relative sweating and thus evaporation, remained restricted (i.e. back, forearm, axilla, chest, abdomen and buttocks) [38].

Exercise protocol

Each trial was completed inside a controlled environmental chamber, (WatFlow, TISS, Hampshire, UK). Participants cycled (Monark, 620 Ergomedic, Vansbro, Sweden) at a power output prescribed relative to body mass (2 W.kg⁻¹ for 15-mins, then 1 W.kg⁻¹ for the following 15-mins [37], as opposed to intensities relative to maximal oxygen uptake (V̇O_2max), thus removing the requirement to undertake a V̇O_2max test. During the TEMP_SUIT and HOT_SUIT trials, each participant wore a commercially available, upper-body vinyl sauna suit (Everlast, London, UK), to restrict evaporative heat loss throughout the 30-mins of exercise. Fluid ingestion was not permitted during the trials. Physiological and perceptual measures were recorded every 5-mins during the exercise protocol.

Physiological measures

On the first visit, skinfold thickness was calculated using calipers (Harpenden, Burgess Hill, UK) and a four site skin fold calculation [39], later body fat (%) was calculated from body density [40]. Stature and nude body mass (NBM) were measured using a stadiometer (Detecto Scale Company, Missouri, USA) and weighing scales (Adam Equipment Inc., Connecticut, USA [to the nearest 0.01 kg [±0.2%]]), respectively, with these data used to estimate body surface area (BSA) [41].

Hydration status was measured prior to each experimental session using a Pocket Pal-Osmo meter (U_osm: Vitech Scientific Ltd., West Sussex, UK) and light refractometer (U_sg: Atago Co., Tokyo, Japan). T_re was continuously monitored using a single-use probe (Henleys Medical, Hertfordshire, UK) self-inserted 10 cm past the anal sphincter. T_skin was measured using telemetry thermistors (U-Type and Gen II transmitter, Eltek, UK) attached to the right-hand side of the body at the pectoralis major muscle belly (T_chest), lateral head of triceps brachii (T_arm), rectus femoris muscle belly (T_thigh) and lateral head of the gastrocnemius (T_calf). Mean T_skin [42] and T_re:T_skin gradient were retrospectively calculated [43]. Heart rate (HR) was continuously monitored using a Polar 810i strap (Polar, Electro Oy, Kempele, Finland). Sweat rate was estimated by the difference in towel-dried NBM pre and post-exercise, corrected for time, urine output (zero incidence), but not metabolic or respiration losses, which were assumed negligible and similar between trials [44].

Perceptual measures

Ratings of perceived exertion (RPE [45]) from 6 (no exertion) to 20 (maximal exertion), thermal comfort (TC [46]) from 0 (comfortable) to 4 (very uncomfortable), and thermal sensation scale (TSS [47]) from 0 (unbearably cold) to 8 (unbearably hot), were assessed every 5-mins during exercise.

Statistical analyses

Data are reported as mean ± SD, and were assessed for normality and sphericity prior to further statistical analyses (SPSS, IBM version 22.0). Baseline measures of NBM, U_osm and U_sg and calculated sweat rate during each trial were analysed using a 1-way ANOVA. Individual sites of peak T_chest, T_arm, T_thigh and T_calf were also analsyed using a 1-way ANOVA between the four experimental conditions (TEMP, TEMP_SUIT, HOT, HOT_SUIT) as participants only wore an upper-body sauna suit covering only 60% of BSA. All other dependent variables were analysed using a 2-way repeated-measures ANOVA between the four experimental conditions (TEMP, TEMP_SUIT, HOT, HOT_SUIT) and seven time points (0-mins, 5-mins, 10-mins, 15-mins, 20-mins, 25-mins, 30-mins), with Bonferroni correction applied during post-hoc analysis. Whilst a 3-way ANOVA separating environmental conditions (TEMP and HOT) and clothing (suit and no suit) would theoretically present an additional level of analysis, this would not permit all comparisons (i.e. between HOT and TEMP_SUIT), thus it was not utilised. This statistical approach also befits the research question whereby each experimental condition may be a strategy in its own right. Statistical significance was accepted as P<0.05. Effect sizes were estimated and meaningful differences evaluated for peak data using Cohen's d, with interpretation of data as; small = 0.2, moderate = 0.5, and large = 0.8 [48]. A-priori interpretation boundaries for meaningful physiological changes (Δ) were; ΔT_re >0.20°C, ΔHR >5 b.min⁻¹ and Δsweat rate >0.20 L.h⁻¹, and >1 in scale scores for perceptual measures [11]. Pearson's product moment correlation coefficients were used to identify relationships between trials for peak physiological and perceptual measures.

Results

The physiological and perceptual measures during each trial are displayed within Table 1, whereas the differences between trials and their associated effect size are displayed within Table 2. Prior to commencing testing, no differences (P>0.05) were observed between conditions for U_osm (F = 0.3), U_sg (F = 0.7) or NBM (F = 0.6) (Table 1).

Table 1.

Mean ± SD physiological and perceptual measures during each trial.

	TEMP	TEMPSUIT	HOT	HOTSUIT
Rest measures
Tre (°C)	37.20 ± 0.36	37.17 ± 0.35	37.21 ± 0.42	37.18 ± 0.41
Tskin (°C)	30.72 ± 1.44	29.99 ± 0.54	29.42 ± 0.90	30.32 ± 0.82
HR (b.min−1)	70 ± 9	69 ± 12	71 ± 16	70 ± 11
NBM (kg)	69.45 ± 7.47	69.61 ± 7.50	69.28 ± 7.70	69.43 ± 7.40
Uosm (mOsm·kg−1)	350 ± 178	365 ± 197	371 ± 215	354 ± 197
Usg	1.011 ± 0.002	1.014 ± 0.005	1.012 ± 0.003	1.010 ± 0.004
Exercise measures
Peak Tre (°C)	37.84 ± 0.34bcd	38.02 ± 0.32ad	38.11 ± 0.31ad	38.33 ± 0.32abc
ΔTre (°C.hr−1)	1.3 ± 0.3bcd	1.7 ± 0.4ad	1.9 ± 0.5ad	2.3 ± 0.5abc
Peak Tskin (°C)	33.06 ± 0.97cd	33.41 ± 0.70cd	36.53 ± 0.93abd	37.68 ± 0.68abc
Mean Tre:Tsk (°C)	5.3 ± 1.9cd	5.5 ± 0.5cd	2.8 ± 1.1ab	1.7 ± 0.8ab
Peak HR (b.min−1)	151 ± 20 cd	155 ± 23d	163 ± 18ad	171 ± 18abc
Sweat rate (L.hr−1)	0.56 ± 0.27bcd	0.89 ± 0.24ad	1.14 ± 0.48ad	1.51 ± 0.52abc
Peak RPE	14 ± 1cd	14 ± 1d	15 ± 1a	16 ± 1ab
Peak TSS	5.3 ± 0.5cd	5.8 ± 0.5cd	6.4± 0.5abd	7.1 ± 0.5abc
Peak TC	1 ± 1	2 ± 1	2 ± 1	3 ± 1abc

Note; ^adifference vs. TEMP (P<0.05), ^bdifference vs. TEMP_SUIT (P<0.05), ^cdifference vs. HOT (P<0.05), ^ddifference vs. HOT_SUIT (P<0.05)

Table 2.

Mean ± SD differences between trials (effect size and correlation coefficient).

	TEMP vs. TEMPSUIT	TEMP vs. HOT	TEMP vs. HOTSUIT	TEMPSUITvs. HOT	TEMPSUITvs. HOTSUIT	HOT vs. HOTSUIT
Peak Tre (°C)	0.18 ± 0.11* (d = 0.5, r = 0.95)	0.27 ± 0.13*† (d = 0.8, r = 0.93)	0.50 ± 0.09*† (d = 1.5, r = 0.97)	0.09 ± 0.19 (d = 0.3, r = 0.84)	0.31 ± 0.14*† (d = 1.0, r = 0.92)	0.23 ± 0.11*† (d = 0.7, r = 0.95)
ΔTre (°C.hr−1)	0.41 ± 0.27*† (d = 0.4, r = 0.75)	0.53 ± 0.36*† (d = 1.5, r = 0.69)	1.02 ± 0.39*† (d = 2.5, r = 0.73)	0.12 ± 0.39 (d = 0.4, r = 0.63)	0.61 ± 0.36*† (d = 1.3, r = 0.77)	0.49 ± 0.25*† (d = 0.8, r = 0.90)
Peak Tskin (°C)	0.31 ± 1.06 (d = 0.4, r = 0.26)	3.44 ± 1.22 (d = 3.7, r = 0.22)	4.58 ± 1.25* (d = 5.6, r = 0.12)	3.13± 0.84 (d = 3.8, r = 0.53)	4.27 ± 0.90* (d = 6.2, r = 0.40)	1.14 ± 0.78* (d = 1.4, r = 0.66)
Mean Tre:Tsk (°C)	0.14 ± 0.99 (d = 0.2, r = 0.81)	2.80 ± 1.11 (d = 1.7, r = 0.61)	3.92 ± 1.41* (d = 2.7, r = 0.20)	2.66 ± 0.84* (d = 3.4, r = 0.68)	3.77 ± 0.86* (d = 5.8, r = 0.18)	1.11 ± 0.82 (d = 1.2, r = 0.68)
Peak HR (b.min−1)	4 ± 8 (d = 0.2, r = 0.94)	12 ± 12† (d = 0.6, r = 0.83)	20 ± 11*† (d = 1.1, r = 0.84)	8 ± 17† (d = 0.4, r = 0.71)	16 ± 14*† (d = 0.8, r = 0.80)	8 ± 9*† (d = 0.4, r = 0.90)
Sweat rate (L.hr−1)	0.33 ± 0.27*† (d = 1.2, r = 0.46)	0.58 ± 0.43*† (d = 1.3, r = 0.47)	0.95 ± 0.51*† (d = 2.3, r = 0.32)	0.26 ± 0.31† (d = 0.6, r = 0.86)	0.62 ± 0.37*† (d = 1.7, r = 0.80)	0.37 ± 0.37*† (d = 0.8, r = 0.76)
Peak RPE	1 ± 1† (d = 0.0, r = 0.84)	1 ± 1† (d = 0.0, r = 0.57)	2 ± 1*† (d = 2.0, r = 0.80)	1 ± 2† (d = 1.0, r = 0.15)	1 ± 1*† (d = 2.0, r = 0.53)	1 ± 1† (d = 1.0, r = 0.75)
Peak TSS	0.5 ± 0.5 (d = 1.0, r = 0.61)	1.1 ± 0.8† (d = 2.2, r = 0.13)	1.8 ± 0.7*† (d = 3.6, r = 0.04)	0.6 ± 0.5* (d = 1.2, r = 0.55)	1.3 ± 0.5*† (d = 2.6, r = 0.54)	1.3 ± 0.5*† (d = 1.4, r = 0.54)
Peak TC	1 ± 1† (d = 1.0, r = 0.54)	1 ± 1† (d = 1.0, r = 0.68)	1 ± 1† (d = 2.0, r = 0.68)	0 ± 1 (d = 0.0, r = 0.73)	0 ± 1 (d = 1.0, r = 0.73)	0 ± 1 (d = 1.0, r = 0.73)

Note; *significant difference between trials (P<0.05), ^†difference above the a-priori pre-defined limits.

Physiological measures

T_re demonstrated a difference between conditions (F = 14.5, P<0.001) and over time (F = 146.5, P<0.001). Observation of an interaction effect (F = 13.1, P<0.001) and post-hoc analyses identified that from 20-mins onwards HOT_SUIT was greater than TEMP and TEMP_SUIT, and from 25-mins onwards HOT_SUIT was greater than TEMP, TEMP_SUIT and HOT. HOT was also greater than TEMP from 25-mins onwards. No difference was observed between TEMP_SUIT and HOT at any time (P>0.05).

T_skin demonstrated a difference between conditions (F = 36.6, P<0.001) and over time (F = 93.4, P<0.001). Observation of an interaction effect (F = 11.6, P<0.001) and post-hoc analyses identified that from 5-mins onwards HOT and HOT_SUIT were greater than TEMP and TEMP_SUIT. From 15-mins onwards, HOT_SUIT was also greater than HOT. Peak T_chest (F = 41.7, P<0.001), T_arm (F = 7.3, P<0.001), T_thigh (F = 15.3, P<0.001) and T_calf (F = 60.3, P<0.001) differed between conditions. Post-hoc analyses identified a higher (P<0.05) T_chest in; HOT_SUIT compared to TEMP, TEMP_SUIT and HOT, in HOT compared to TEMP, and, in TEMP_SUIT compared to TEMP. T_arm was higher in HOT and HOT_SUIT compared to TEMP. No differences (P>0.05) were observed between TEMP_SUIT and HOT for T_chest or T_arm. T_thigh and T_calf were higher (P>0.05) in HOT and HOT_SUIT compared to TEMP and TEMP_SUIT.

The T_re:T_skin gradient demonstrated a difference between conditions (F = 72.6, P<0.001) and over time (F = 76.0, P<0.001). Observation of an interaction effect (F = 13.9, P<0.001) and post-hoc analyses observed that from 5-mins onwards HOT and HOT_SUIT were greater than TEMP and TEMP_SUIT. From 25-mins onwards HOT_SUIT was also greater than HOT. At 30-mins, HOT and HOT_SUIT were no longer different.

HR demonstrated a difference between conditions (F = 19.5, P<0.001) and over time (F = 491.2, P<0.001). Observation of an interaction effect (F = 11.7, P<0.001) and post-hoc analyses identified that from 5-mins onwards HOT_SUIT was greater than TEMP_SUIT and from 10-mins onwards HOT_SUIT was greater than TEMP and TEMP_SUIT. From 20-mins onwards HOT was greater than TEMP_SUIT and at 30-mins HOT_SUIT was greater than HOT.

Sweat rate demonstrated a difference between conditions (F = 10.3, P<0.001). A greater (P<0.05) sweat rate was observed within TEMP_SUIT, HOT and HOT_SUIT compared to TEMP.

Perceptual measures

RPE demonstrated a difference between conditions (F = 17.9, P<0.001) and over time (F = 93.4, P<0.001). Observation of an interaction effect (F = 11.6, P<0.001) and post-hoc analyses identified that from 5-mins onwards HOT_SUIT was greater than TEMP. From 15-mins HOT was also greater than TEMP. No difference was observed between TEMP and TEMP_SUIT, or TEMP_SUIT and HOT.

TSS demonstrated a difference between conditions (F = 53.9, P<0.001) and over time (F = 105.0, P<0.001). Observation of an interaction effect (F = 11.9, P<0.001) and post-hoc analyses identified that from 5-mins onwards HOT and HOT_SUIT were greater than TEMP and TEMP_SUIT. TEMP_SUIT was greater than TEMP from 10-mins onwards, and from 20-mins HOT_SUIT was greater than HOT. At 30-mins, TEMP and TEMP_SUIT were not different.

TC demonstrated a difference between conditions (F = 10.4, P<0.001) and over time (F = 43.1, P<0.001). Observation of an interaction effect (F = 7.0, P<0.001) and post-hoc analyses identified that from 20-mins HOT_SUIT was greater than TEMP and TEMP_SUIT and from 25-mins HOT_SUIT was greater than TEMP, TEMP_SUIT and HOT.

Mean and individual data for ΔT_re, end T_skin, peak HR and sweat rate for each condition are displayed within Figure 3.

Figure 3.

Mean ± SD (black marker and line) and individual responses (grey marker and lines) to TEMP, TEMPSUIT, HOT and HOTSUIT conditions at the end of exercise for ? core temperature (1), end skin temperature (2), peak heart rate (3) and sweat rate (4). Letters indicate statistical difference (P<0.05) whereby; a: vs. TEMP, b: vs. TEMPSUIT, c: vs. HOT, d: vs. HOTSUIT.

Discussion

Overview

A raised and maintained T_re, T_skin and an increased sweat rate have been identified as the primary stimuli for inducing heat adaptation [3,17]. Our data highlights that, when commencing equivalent exercise from a similar fluid balance (U_osm, U_sg and NBM) and physiological state (T_re, T_skin and HR), wearing an upper-body vinyl ‘sauna’ suit can increase the magnitude of change in an individual's T_re and resultant sweat rate (Table 1, Figures 1 and 3). The increased T_re is greater than that during temperate exercise and is similar to that observed within hot conditions. Furthermore, the increase in physiological strain is enhanced by combining hot conditions and a vinyl suit, purporting a potential for increased efficiency during HA without the requirement to increase exercise intensity nor volume to achieve the same physiological strain.

Figure 1.

Mean physiological measurements, core temperature (Top), skin temperature (Middle) and heart rate (Bottom) during 30-mins of exercise in TEMP, TEMP_SUIT, HOT and HOT_SUIT conditions. SD has been removed for clarity. Letters indicate statistical difference (P<0.05) whereby; a: TEMP vs. TEMP_SUIT, b: TEMP vs. HOT, c: TEMP vs. HOT_SUIT, d: TEMP, TEMP_SUIT vs. HOT, HOT_SUIT, e: TEMP, TEMP_SUIT vs. HOT_SUIT, f: TEMP, TEMP_SUIT, HOT vs. HOT_SUIT, g: TEMP_SUIT vs. HOT_SUIT, h: HOT vs. HOT_SUIT.

A primary aim of this experiment was to determine whether wearing an upper-body vinyl suit in temperate conditions would elicit equivalent physiological responses to potentiate heat adaptation as training in a hot environment. Whilst there were no differences (P>0.05) between TEMP_SUIT and HOT for peak T_re, HR or sweat rate (Table 2), it is clear the HOT trial provided a larger physiological strain than temperate exercise without restricted heat loss (TEMP) (Figure 1 and 3). Further, no perceptual differences (P>0.05) were observed between TEMP_SUIT and HOT for RPE or TC, with the only differences being T_skin, T_re:T_skin gradient and TSS (P<0.05 [Table 1 and 2]). The elevated T_skin between TEMP_SUIT and HOT (∼3°C) likely reflected the higher ambient air temperature in the chamber, compared to the air temperature in the microclimate under the suit [35], which only covered the upper-body (∼60% of the BSA). Whilst much of the body was insulated by the upper-body sauna suit, the legs still represent ∼40% of the BSA [42,49] with an approximate 1.0 mg.cm⁻².min⁻¹ cutaneous water loss potential [38], which may have limited the benefit of TEMP_SUIT. The elevation in TSS between TEMP_SUIT and HOT (∼0.5 A.U.) is likely a result of the difference in T_skin [50].

In a similar experiment, eight trained athletes ran at 50% of their V̇O_2max in hot conditions (40°C, 30% RH) in normal clothing for training, and in cool conditions (15°C, 50% RH) whilst wearing excess clothing [29]. Both approaches increased physiological strain index ([PSI] +5.8 and +4.5, respectively), though mean PSI was higher in hot conditions compared with overdressing in cool conditions (6.0 ± 1.0 vs. 5.2 ± 1.1, respectively). Unfortunately, based upon available data it is not possible to determine whether T_re or HR were lower or whether one measurement had a greater influence on the calculation of PSI. Nonetheless, these exploratory data are supportive of our HOT vs. TEMP_SUIT comparison, with the authors suggesting that by adequately overdressing, athletes may be able to mimic heat stress and potentially obtain the benefits of heat adaptation in a cooler environment [29]. In line with the present experiment, recent data have reported the effects of wearing additional clothing (shorts, top, winter cycle jacket and gloves) vs. regular clothing (shorts and top only), with a greater increase in physiological strain when completing an 80-mins standardised cycling training session outdoors in temperate conditions (∼17°C, ∼82% RH) [30]. In spite of differences in training session duration which likely explain the magnitude of difference (ΔT_re) in comparison to our data, Stevens et al. [30] noted a similar pattern of physiological differences to that of our comparisons between TEMP and TEMP_SUIT with elevated mean T_re (theirs +0.4°C, ours +0.2°C) and sweat rate (both ∼+0.3 L.hr⁻¹) alongside similar magnitudes of HR increase (theirs +3 b.min⁻¹, ours +4 b.min⁻¹), albeit without statistical difference in our data (Table 2). These exploratory data point to a potential benefit of “overdressing” and restricting heat loss, to elicit greater potentiating physiological stimuli, which may promote heat adaptation via attenuating evaporative cooling (Figure 3). Consequently, it is proposed that training in an upper-body sauna suit within temperate conditions (TEMP_SUIT) confers a larger physiological strain compared to training without a sauna suit (TEMP). Furthermore, TEMP_SUIT elicits similar increases in T_re, HR and sweat rate to that of training without a sauna suit in hot conditions (HOT).

Previous acute HA experiments utilising cycle ergometry in conditions of ∼40°C and 50% RH have identified that workloads of a comparable nature to this experiment (e.g. ∼1.4-2.0 W.kg⁻¹) elicit rates of heat storage, that increase T_re by 1.0-1.9°C.hr⁻¹ in a range of participants including trained athletes and recreationally active individuals, and both males [7,8,11,51] and females [34,52]. These data, which are similar to the current experiment (Table 1 and 2, Figures 1 and 3) give confidence that “overdressing” by utilising upper-body vinyl suits, or perhaps additional layers of regular clothing can elicit comparable physiological responses to experiments where heat adaptation has occurred. Additionally, the participants only wore an upper-body sauna suit; both alterations may have lessened the effectiveness of the TEMP_SUIT in comparison to the HOT trial. Whilst the influence of restricted heat loss clothing during exercise in occupational contexts have been described [53,54], the use of sauna suits during training for performance has been less robustly investigated. Emerging data has reported the benefit of wearing a sauna suit during a 6-week training programme in temperate conditions (30-min sessions, 5 days per week) [31]. Physiological adaptations included; reductions in resting HR (−4 b.min⁻¹), systolic (−2 mmHg) and diastolic blood pressure (−3 mmHg), and, an improved anaerobic threshold (+5.6% of V̇O_2max) and V̇O_2max (+4.3 mL.kg⁻¹.min⁻¹) [31]. These enhancements appear congruous with the magnitudes of adaptation associated with HA [26], albeit they were achieved over a considerably longer period than most HA protocols and did not include a comparable control group. Nonetheless, it might be proposed that the same dose of HA over more frequent (daily or twice-daily), longer exposures (∼90-mins) condensed into a shorter training period (7-14 days) and increasing the surface area of restricted heat loss (i.e. full-body sauna suit) would be effective at inducing the HA phenotype and thus requires further investigation. Likewise, the potential to shorten or complete training sessions across different locations will allow team/groups or individuals to implement heat alleviating strategies (e.g. HA), when previously restricted to attend warm-weather training camps or use heat chambers, due to logistical or monetary limitations.

A further aim of this study was to identify whether wearing an upper-body vinyl suit would enhance key potentiating stimuli in hot-dry conditions replicating a traditional HA session. As evidenced by the significantly greater physiological (Figure 1 and 3) and perceptual (Figure 2) responses to HOT_SUIT (Table 2), and proposed by others, wearing additional clothing appears to be a potential viable strategy which may be utilised throughout training to enhance heat tolerance [28], in addition to the previously identified benefit as a priming stimuli for sweat adaptations [35]. Therefore, restricting heat loss during HA may present a more efficient method for attaining sufficient physiological stimuli, without increasing absolute exercise intensity. Further, although speculative, wearing sauna suits may offer supplementary perceptual, sudomotor and circulatory (e.g. skin blood flow) adaptations, imposed by the benefits of hot-wet conditions [18,55], which are recommended at the latter stages of HA [2,56]. This may support individuals exercising in either hot-wet or hot-dry conditions [57,58], however, despite the benefits of superior sweat loss capacity, an earlier onset of dehydration and cardiovascular strain may occur [59], especially if evaporative requirement is inhibited [60], thus hydration guidelines must be followed [61]. It is also acknowledged that a possible time-lag using rectal thermometry may have reduced the differences between the ΔT_re across conditions, as opposed to using a more responsive index of core temperature (e.g. oesophageal [62]).

Figure 2.

Mean perceptual measurements, rating of perceived exertion (Top), thermal sensation (Middle) and thermal comfort (Bottom) during 30-mins of exercise in TEMP, TEMPSUIT, HOT and HOTSUIT conditions. SD has been removed for clarity. Letters indicate statistical difference (P<0.05) whereby; a: TEMP vs. TEMPSUIT, b: TEMP vs. HOT, c: TEMP vs. HOTSUIT, d: TEMP, TEMPSUIT vs. HOT, HOTSUIT, e: TEMP, TEMPSUIT vs. HOTSUIT, f: TEMP, TEMPSUIT, HOT vs. HOTSUIT, g: HOT vs. HOTSUIT.

Practical application and future direction

Our data indicate that clothing which restricts sweat evaporation may be worn during exercise to enhance heat adaptation stimuli, however further work is required to elucidate the full potential of this method in both acute and chronic interventions using intensities and durations of exercise consistent with existing heat acclimation protocols. This may demonstrate its efficacy across exercise modalities (e.g. running/walking, rowing and cycling), both indoors and outdoors for athletic or occupational populations (e.g. firefighters and military), and for those individuals for whom heat tolerance is compromised (e.g. elderly or clinical populations), but access to heat training facilities is limited. Additional research is required to determine whether repeated restriction of evaporation via excessive/specific clothing can induce the HA phenotype to the same extent as a traditional chamber based protocol. Given the problems with accessing chamber facilities for large cohorts (e.g. team-sports, occupational or military personnel), the opportunity to overdress and train in temperate conditions to induce heat adaptation prior to competing in a warmer climate is appealing, although, we highlight the need for monitoring body temperature and hydration status, whilst also calculating fluid loss and individualising rehydration strategies for individuals adopting this technique.

Refinements to the exercise protocol (i.e. increased exercise intensity) and increasing the area of restricted heat loss (i.e. full-body sauna suit) may provide equivalent responses to exercising in hot conditions. Restricting whole-body water loss using a full-body sauna suit, the subsequent evaporation from limbs undergoing mechanical work and elevated self-generated air flow [63] may facilitate faster increases in T_re, helping to expedite HA. A further practical advancement would be to determine whether wearing restrictive clothing expedites HA when exercising at higher intensities, and therefore greater heat production, than those implemented in the present study is undertaken. Finally, sauna suits may assist in storing the heat generated from exercise, helping to maintain a higher body temperature. If these garments can expedite the time to achieving an elevated body temperature, this may reduce the amount of physical work that must be completed in each training session to achieve recommended guidelines for the maintenance of an elevated T_re (i.e. >38.5°C [7,32]). Previously, another ‘passive’ technique of HWI post-exercise, has been shown to be effective method of raising and maintaining T_re, to elicit heat adaptation [24]. Therefore, for those undertaking HA, overdressing will help to quickly raise body temperature and we highlight the potential to follow physical exertion with HWI to minimise the exercise requirement of HA, although further research is required to support this.

Conclusion

Exercising within temperate conditions whilst wearing an upper-body sauna suit induces a greater physiological strain (i.e. core temperature) and evokes a larger sweat loss in comparison to exercising without a sauna suit in the same conditions. In hot conditions, wearing a sauna suit further enhances physiological and perceptual strain. Wearing a full-body sauna suit during repeated training in temperate conditions may be a viable alternative to HA undertaken in environmental chambers, when a greater exercise intensity is used, with further research warranted. The use of a sauna suit during exercise in a hot environment may accelerate the attainment of important potentiating stimuli for heat adaptation and reduce the physical work, making HA more efficient.

Disclosure of Potential Conflicts of interest

The authors confirm there are no conflicts of interest.

Acknowledgement

The authors would like to thank the participants for their time and commitment to this study.