2 3 Thermal perceptions and skin temperatures during continuous and intermittent ventilation of the...

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European Journal of AppliedPhysiology ISSN 1439-6319Volume 113Number 11 Eur J Appl Physiol (2013) 113:2723-2735DOI 10.1007/s00421-013-2697-5

Thermal perceptions and skin temperaturesduring continuous and intermittentventilation of the torso throughout andafter exercise in the heat

Sarah L. Davey, Martin J. Barwood &Michael J. Tipton

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ORIGINAL ARTICLE

Thermal perceptions and skin temperatures during continuousand intermittent ventilation of the torso throughoutand after exercise in the heat

Sarah L. Davey • Martin J. Barwood •

Michael J. Tipton

Received: 5 March 2013 / Accepted: 15 July 2013 / Published online: 23 August 2013

� Crown Copyright 2013

Abstract

Objective This study tested the hypothesis that intermit-

tent cooling in air-perfused vests (APV) will not only

maintain thermal balance but, due to cyclical activations

of cutaneous thermoreceptors, also enhance thermal

perceptions.

Method Ten physically active males completed four

conditions where they exercised (walking: 5 km h-1, 2 %

gradient) in a hot environment (*34.0 �C, 50 % RH) for

72 min, followed by a 33-min period of rest. They wore an

APV throughout. The four conditions differed in respect to

the profile of ambient air that was perfused through the

APV: continuous perfusion (CP); intermittent perfusion of

6 min ON/OFF periods (IPonoff); a steady increase and

decrease in flow rate in equal increments (IPramp); and an

initial step-increase in the flow rate followed by an incre-

mental decrease to zero flow rate (IPtriang). Whole body and

torso thermal comfort (TC, TTC), whole body and torso

temperature sensation (TS, TTS), whole body and torso

skin temperature ( �Tsk, �Tsktorso), local relative humidity

(RHtorso) and rectal temperature (Tre) were measured.

Results There were no significant differences in Tre,

absolute whole body and local �Tsk, TC, TTC and TS

between the cooling profiles. However, TTS was cooler in

CP and IPramp than IPonoff and IPtriang. Even though inter-

mittent cooling did not significantly enhance thermal per-

ceptions in CP, a trend existed for TC (P = 0.063) to

become less favourable over time.

Conclusion To reduce the power consumption and extend

the battery life of an APV, it is recommended that an

intermittent cooling profile should be adopted.

Keywords Thermal comfort � Temperature

sensation � Skin temperature � Hyperthermia �Protective clothing � Cooling garments � Exercise � Heat

Introduction

Occupational groups such as the military, construction

workers, miners, firefighters, and sportsmen can be

required to wear personal protective clothing (PPC), which

is typically thick, multi-layered and impermeable to

moisture; all factors that limit heat transfer from the body.

Therefore, in many situations, wearing PPC in hot envi-

ronments, or during high intensity physical activity, can

cause uncompensable heat strain that results in the body

being unable to maintain thermal balance (Cheung et al.

2000). It is well documented that, in humans, a 1–3 �C rise

in deep body temperature can induce fatigue (Nybo and

Nielsen 2001) and reduce cognitive ability (Hancock et al.

2007). This can have a detrimental effect on physical

performance (Walters et al. 2000) and the ability to per-

form work-related tasks (Sawka et al. 1992). Personal

cooling garments (PCGs), such as liquid or air-perfused

garments, have been developed to combat the negative

effects of wearing PPC. These garments remove stored heat

either through conduction, convection or evaporation,

thereby helping to maintain thermal balance (McLellan

Communicated by Narihiko Kondo.

S. L. Davey (&)

Environmental Ergonomics Research Centre, Design School,

Loughborough University, Loughborough LE11 3TU, UK

e-mail: [email protected]

M. J. Barwood � M. J. Tipton

Department of Sport and Exercise Science,

Portsmouth University, Portsmouth, UK

e-mail: [email protected]

123

Eur J Appl Physiol (2013) 113:2723–2735

DOI 10.1007/s00421-013-2697-5

Author's personal copy

et al. 1999; Speckman et al. 1988) and thermal comfort

(Bomalaski et al. 1995; McLellan et al. 1999).

In general, the flow of air or liquid delivered to the PCG

is continuous. To provide efficient cooling over long

periods of time the garments have to be connected to a

refrigeration and/or air conditioning system or, if person-

ally mounted, supplied with a heavy duty battery. This can

restrict the movement of the wearer and increase their

metabolic activity (Dorman and Havenith 2009). It is also

possible that continuous cooling over long periods can

diminish the thermal comfort provided by the PCG as a

level of thermal adaptation may occur within peripheral or

central thermoreceptors, or at a conscious level (de Dear

and Brager 2001; Zhang 2003). This adaptation is evi-

denced by the observation that thermal comfort levels can

be similar with, or without, a continuously cooled PCG

under PPC during moderate exercise in hot, dry conditions,

even though skin temperatures are cooler when a PCG is

worn (i.e. chest temperature *2 �C lower, back skin

temperature *1 �C lower) (Barwood et al. 2009).

It is well known that changes in thermal comfort are

experienced during rapid temperature changes as opposed

to constant temperatures, largely due to the dynamic

characteristics of cutaneous thermoreceptors (de Dear et al.

1993; Zhang 2003). It has also been suggested that, over

time, appropriately designed transient thermal environ-

ments can achieve higher levels of thermal comfort than

stable environments (Arens et al. 2006; de Dear 2011).

This suggests that the problem of unimproved thermal

comfort during continuous cooling in PCGs might be

overcome by providing dynamic cooling via intermittent

cooling profiles. The resulting fluctuations in skin tem-

perature may not only maintain thermal comfort, but pos-

sibly improve it over time whilst potentially increasing

battery life and the efficacy of a PCG.

To prevent situations where improvements in thermal

perceptions mask dangerous elevations in deep body tem-

perature, it is important that intermittent cooling profiles

also maintain thermal balance. Previous research has

shown that intermittent cooling cycles (ratio 1:1) of up to

10 min are as effective at reducing stored heat as contin-

uous cooling (Cadarette et al. 2006; Stephenson et al.

2007). In contrast, relatively few investigations have

evaluated the effects of intermittent cooling in PCGs on

thermal perceptions during continuous exercise. The results

from these few studies are conflicting; fluctuating skin

temperatures were found to either increase (Hexamer and

Werner 1997), make no difference to (Vernieuw et al.

2007), or decrease thermal comfort (Keatinge et al. 1986).

These contradictory findings may be the result of several

methodological limitations, e.g. low sample size (Hexamer

and Werner 1997) and a limited recording frequency

of perceptual measurements (Vernieuw et al. 2007).

Furthermore, these conclusions are drawn from the per-

formance of liquid-perfused garments which, from a

practical and logistical perspective, are of limited use in the

field because of their large bulk and high energy demands.

Therefore, the effect of intermittent cooling profiles on

thermal perceptions in air-perfused garments is relatively

unknown.

Intermittent cooling profiles can be provided in many

forms: ramp or step-changes (rapid changes), cyclical

(sinusoidal) or drifts (monotonic, steady changes). Several

investigations examining ventilation in buildings have

shown that the type of intermittent cooling can influence

thermal comfort (Arens et al. 1998; de Dear et al. 1993;

Tanabe and Kimura 1994; Zhou et al. 2006), with sinu-

soidal profiles often being found to be the preferred choice

(de Dear et al. 1993; Tanabe and Kimura 1994; Zhou et al.

2006). To our knowledge, the effects of different inter-

mittent cooling profiles used in an air-perfused garment

(APG) on thermal balance and thermal comfort have not

been investigated, despite the advantages of APGs over

their liquid-perfused equivalents (Shapiro et al. 1982;

Speckman et al. 1988). Therefore, the main aim of this

study was to examine the influence of cooling profiles in an

air-perfused vest (APV) to identify which profile is the

most effective at maintaining thermal balance and

enhancing thermal comfort. It was hypothesised that (1)

there will be no significant difference in thermal balance

between intermittent and continuous cooling, but (2) the

use of and (3) the type of intermittent cooling will signif-

icantly influence thermal perceptions.

Method

Participants

Ten physically active males volunteered to participate in

the experiment [mean (SD); age 24.3 (4.3) years; height

1.78 (0.05) m; mass 73.1 (7.9) kg; body surface area 1.90

(0.10) m2; body fat 15.6 (1.7) %]. The study protocol was

approved in advance by the University of Portsmouth

Ethics Committee and the participants gave their written

informed consent to participate.

General design

Each participant completed four conditions where they

exercised (walking on a treadmill) at a low-intensity

(5 km h-1, 2 % gradient) in a hot environment (*34.0 �C,

50 % RH, WBGT = 28.44 �C) for 72 min, followed by a

33-min period of rest. Each condition was separated by

2–4 days to eliminate any acclimation and the order of

the conditions was balanced using a Latin square. To

2724 Eur J Appl Physiol (2013) 113:2723–2735

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investigate the effects of different types of cooling profiles

on thermal perceptions, the four conditions only differed by

the type of cooling profile that was perfused through a

APV. The four cooling profiles were: (a) continuous per-

fusion of ambient air (35 �C, *14 % RH) into the vest at a

flow rate of 339.8 L min-1 (CP), (b) intermittent perfusion

of ambient air at a flow rate of 339.8 L min-1 in 6 min

ON/OFF periods (IPonoff), (c) a steady increase and

decrease of the flow rate between 0 and 339.8 L min-1 in

1 min increments (IPramp) and (d) an initial increase in flow

rate from 0 to 339.8 L min-1 followed by an incremental

decrease (every 2 min) in flow rate to 0 L min-1 (IPtriang).

Cooling profiles IPonoff, IPramp, IPtriang were designed to

have a total flow of air during each condition of

17,841.61 L which was exactly half of that ventilated

during CP (35,683.20 L) (Fig. 1). Thermal perceptual

measures of overall thermal comfort (TC), torso thermal

comfort (TTC), overall temperature sensation (TS) and

torso temperature sensation (TTS) were collected every

3 min. Physiological measurements of rectal temperature

(Tre), mean skin temperature ( �Tsk), torso skin temperature

( �Tsktorso), torso relative humidity (RHtorso) and heart rate

(fc) were recorded every minute.

Procedure

Upon arrival to the laboratory, the participants were

instructed to empty their bladder before nude and clothed

body masses were measured. The participants then self-

inserted a rectal thermistor (DS18B20, MSR, Switzerland)

12 cm beyond the anal sphincter. They were then instru-

mented for the measurement of local skin temperature

(Tskloc) (DS18B20 T-3, MSR, Switzerland), torso percent-

age relative humidity (RHtorso) (SHT15, MSR, Switzer-

land) and heart rate (fc) (Team Polar, Polar�, Finland). To

prevent high levels of thermal stress obscuring any dif-

ferences that may occur between the cooling profiles, it

was decided that the participants would not wear the type

of PPC (i.e. high thermal and evaporative resistance) that is

typically worn when APV’s are utilised. Therefore, each

participant wore polyester jogging trousers, Lycra briefs,

cotton boxer shorts, a cotton long-sleeved shirt

(*clo = 1.2) and an APV (GORE� Active cooling). The

APV was worn directly over the skin of the torso under-

neath the long-sleeved shirt. The same rationale for the

choice of clothing was applied to the selection of the

ambient conditions used in this study. It was reasoned that

Fig. 1 A schematic diagram of the different cooling profiles that were perfused through the APV throughout the exercise and rest periods. Six

consecutive profiles were ventilated through the APV during exercise and three consecutive profiles during the rest period

Eur J Appl Physiol (2013) 113:2723–2735 2725

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if differences did occur between the cooling profiles (e.g.

intermittent cooling profiles enhance perception when

compared to continuous), further investigations could

clarify the optimal levels of thermal stress for intermittent

cooling profiles to remain an effective alternative to con-

tinuous cooling.

Once instrumented, resting physiological measurements

were taken. The participant then entered an environmental

chamber for the commencement of the experimental ses-

sion. To reduce the time taken to reach thermal stability,

each session began with the participant jogging at a self-

selected speed for 10 min (Newton et al. 2009). A reduc-

tion in this stabilisation period was deemed necessary to

prevent fatigue or boredom; both of which could influence

thermal perceptions. A participant’s jogging speed was the

same for each condition. Immediately after the jog, the

participant entered their ‘after jog’ perceptual votes. Air

(35 �C, *14 % RH) was then ventilated into the APV and

the participant started their 72 min walk on the treadmill at

5 km h-1 (1.39 m s-1) at a 2 % gradient. Based on the

mean oxygen consumption of each of the participants that

was measured 10 min into the walking phase [i.e. 1.16

(0.25) L min-1], this exercise intensity equated to *30 %

of the average males _VO2max (ACSM 2010). Each experi-

ment started with an initial 3 min ventilation of air at a

flow rate of 339.8 L min-1. This was incorporated to

ensure that the initial ventilation of air experienced after

the run elicited similar thermal perceptual responses in all

of the cooling profiles. With the exception of the contin-

uous profile, this 3-min period was followed by the stage in

the cooling profiles where the rate of air flow started to be

reduced (Fig. 1). The 72 min walk was followed by a

resting period of 33 min during which the participants sat

on a wooden stool located beside the treadmill. The dura-

tion of the exercise and rest periods was selected to ensure

that each condition had the same number of cooling cycles.

To prevent feelings of thermal discomfort promoted by the

warm-hot sensations due to heating of the facial region

(Zhang 2003), which could confound any perceptual ben-

efits derived from the different cooling profiles, air was

circulated (*0.90 m s-1) towards the participant from a

fan located 2.2 m in front of the treadmill.

Controlling air temperature and flow rate

The air perfused through the APV was sourced from a

compressor (Kaeser Compressions SX 3, Virginia, USA)

and had a constant relative humidity of *14 %. The flow

rate of the air was measured before and after each exper-

iment by a calibrated anemometer (Flowcheck Y630,

Micronel�, Switzerland).

Measurements

Perceptual measurements

Thermal perceptions were assessed using graphic Visual

Analogue Scales (VAS) that have been previously descri-

bed (Davey et al. 2007). Rating of perceived exertion

(RPE) was also recorded (Borg 1971). Each VAS com-

prised 160 units and included the following descriptors;

thermal comfort: 10 = very comfortable, 34 = comfort-

able, 58 = just comfortable, 104 = just uncomfortable,

uncomfortable = 128, very uncomfortable = 153, tem-

perature sensation: very cold = 6, cold = 24, cool = 40,

slightly cool = 64, neutral = 80, slightly warm = 100,

warm = 118, hot = 137, very hot = 156. To make the

entry of perceptions easier for participants when walking,

the VAS were presented on a touch screen monitor. Per-

ceptual scores were recorded every 3 min to ensure that

perceptions were evaluated at every stage of a cooling

cycle (i.e. when air flow was either reduced, increased or

remained stable).

To familiarise the participants to the perceptual scales

and the experimental set up, each participant undertook a

familiarisation session prior to the main experiment.

Each familiarisation session comprised of two phases.

The first phase involved providing each participant with a

written description of the concept behind each of the

perceptual measures that was originally adapted from

Gagge et al. (1967) and testing their understanding of the

concept. The second phase involved familiarising the

participant to the mode of entering their perceptual votes

(i.e. using the touch screen monitor) whilst walking on

the treadmill at 5 km h-1, 2 % gradient. The familiari-

sation session took place in the same laboratory as the

main experiment.

Having undertaken all conditions, participants were

asked to describe the order that they thought the cooling

profiles were ventilated through the APV. They were then

asked to identify which cooling profile was their preferred

choice and explain why.

Calculations

Sweat rate

Sweat rate (sw) was calculated using Eq. (1).

sw L h�1� �

¼ ½Nude body mass ðbefore trialÞ� Nude body mass ðafter trialÞ�þ ðVolume of fluid consumed

� Volume of fluid excretedÞ=Time (h): ð1Þ


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Mean skin temperature

Mean skin temperature ( �Tsk) was calculated according to

the equation of Hardy and Dubois (1938) (ISO 2002):

�Tsk ¼ 0:07THead þ 0:175TChest þ 0:175TBack þ 0:14TForearm

þ 0:05THand þ 0:07Foot þ 0:13TCalf þ 0:19TFront thigh:

ð2Þ

Torso skin temperature

�Tsktorso ¼ 0:5TChest þ 0:5TBack: ð3Þ

Torso skin relative humidity

RHtorso ¼ 0:5RHChest þ 0:5RHBack: ð4Þ

Sweat efficiency

Sweat efficiency was determined as the ratio of evaporative

versus non-evaporative sweat loss by the following

equation:

Eactual ð%Þ ¼ D nude body mass� D clothing massð Þ½D nude body mass= � � 100:

ð5Þ

Determination of body fat

Percentage of body fat of each participant was calculated

using the Siri equation (Siri 1961) based on the body

density (BD) that was estimated from the sum of seven

skinfolds (triceps, biceps, subscapular, iliac crest, calf,

thigh, supraspinale) by the equation developed by Withers

et al. (1987). Skin folds were measured using calipers

(Harpenden Instruments, West Sussex, UK).

Amplitude and rate of change of the fluctuations

in skin temperatures

The mean amplitude and rate of change in both �Tsk and�Tsktorso within a cooling profile were calculated for each

participant and then pooled to provide an overall mean for

each cooling profile. The response to the initial 3 min

ventilation of 339.8 L min-1 air after the 10 min jog was

excluded from the calculation.

Changes in physiological and perceptual measures

over time

To evaluate whether the skin temperatures and thermal

perceptions changed over time, the average �Tsk, �Tsktorso,

TS, TTS, TC and TTC was calculated for both the first and

last cooling cycles in each cooling profile, during both the

exercise and rest period.

Data analyses

To compare the effect of the different cooling profiles on

thermal balance, the physiological measures of Tre, �Tsk,�Tsktorso, sw, fc and Eactual were analysed using either a one-

way (condition) or a two-way (time 9 condition) ANOVA

with repeated measures at time points -10, 0, 25, 50, 75

and 105 min. The same analyses were performed to assess

any differences between the cooling profiles within and

between the exercise and rest periods. To evaluate any

difference in thermal perceptions between the cooling

profiles, the same analyses were conducted on the per-

ceptual measures (i.e. TS, TTS, TC, and TTC). For sphe-

ricity violations, the F values were corrected using the

Greenhouse-Geisser procedure. In cases where this occurs,

the modified degrees of freedom are provided after the

adjusted F value. To detect differences amongst means, a

significant F test, was further analysed with a Bonferroni

post hoc comparison test. Paired-sample t tests were used

to detect any differences between the cooling profiles in

their influence on both the physiological and perceptual

measures over time. This was completed for the exercise

and rest periods. A single binominal proportion test was

utilised to determine the participants’ overall preference of

cooling profile, as assessed by the post- experiment

interviews.

All statistical analyses were performed using SPSS 18.0

software (SPSS Inc. Chicago, IL). Statistical significance

was set at a level of a\ 0.05, with a trend in the data being

considered to occur if P C 0.05 \ 0.1. All data are pre-

sented as mean (±SD).

Results

Physiological

Rectal temperatures, �Tsk, �Tsktorso, fc, sw, Eactual and RPE

were similar between all the conditions during exercise and

rest (Table 1). In all of the conditions, thermal balance was

achieved within 25 min of the start of the walk and, upon

cessation of exercise, rectal temperature gradually

decreased to a value similar to that experienced at the

beginning of the experiment (i.e. prior to the initial 10 min

jog) (Fig. 2). During the post-exercise rest period, �Tsk, and�Tsktorso, significantly decreased in all conditions with no

significant difference between the conditions (Fig. 3;

Table 1). Due to continuous air movement removing

moisture from the skin and surrounding air, RHtorso was

significantly lower in CP than all of the other conditions

(P = 0.001) and significantly higher in IPonoff than all the

other conditions (P = 0.001) (F = 121.75, r = 0.931,


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b = 0.989, P \ 0.001). These differences occurred during

both exercise and rest. There was a significant difference in

RHtorso between exercise and rest for IPonoff and IPramp

(P = 0.001) and for IPtriang (P = 0.005), but not for CP. In

addition, during exercise and rest, none of the amplitudes

in RHtorso in the intermittent cooling profiles were lower

than the percentage maintained in CP (Fig. 4; Table 1).

Perceptual data

Rating of perceived exertion

The rating of perceived exertion (RPE) was not different

between the four cooling profiles, with a mean RPE of 10

(±2) for each cooling profile (P [ 0.05).

Thermal perceptions

The participants’ mean whole body thermal comfort (TC)

and temperature sensation (TS) were not different between

the four cooling profiles during both exercise and rest

(P [ 0.05). However, during exercise, with CP, there was a

trend for participants to feel more uncomfortable over time

(P = 0.063) and during rest there was a trend for TC to

improve over time with IPonoff (P = 0.059) (Fig. 5;

Table 1). Thermal comfort was also similar between

exercise and rest with CP and IPonoff (P [ 0.05), but TC

was significantly improved in the transition from exercise

to rest with IPtriang (P = 0.023) and IPramp (P = 0.024)

(Table 1).

Torso thermal comfort was similar between each of the

cooling profiles during exercise and rest. With IPtriang and

IPramp, TTC was significantly improved in the transition

from exercise to rest (P = 0.009 and P = 0.038, respec-

tively), with a trend for participants to feel more com-

fortable with IPonoff during rest than exercise (P = 0.094).

No significant difference in TTC was found with CP

between exercise and rest (Table 1).

There was a main effect for condition in TTS during

exercise (F = 4.520, r = 0.334, b = 0.831, P = 0.011).

Table 1 Physiological and psychological responses to each of the different cooling profiles during both exercise and rest (±SD) (n = 10)

Variables Exercise Rest

Type of cooling profile Type of cooling profile

CP IPonoff IPtriang IPramp CP IPonoff IPtriang IPramp

HR (beats min-1) 107 (13) 107 (15) 105 (11) 106 (14)

BM reduction due

to water loss (%)a-0.54 (0.48) -0.46 (0.48) 1.22 (0.29) 1.30 (0.29)

Eactual (%)a 93.98 (4.22) 81.24 (24.80) 78.89 (28.17) 92.03 (5.44)

Sweat loss (L)a 1.14 (0.29) 1.16 (0.34) 1.22 (0.29) 1.30 (0.29)

Final Tre (�C) 38.12 (0.38) 37.97 (0.35) 37.94 (0.36) 37.93 (0.39) 37.83 (0.38) 37.77 (0.30) 37.70 (0.30) 37.78 (0.38)

�Tsk (�C)b 35.85 (0.57) 35.84 (0.49) 35.85 (0.45) 35.89 (0.41) 35.52 (0.79) 35.50 (0.72) 35.40 (0.71) 35.46 (0.75)

�Tsktorso (�C)b 35.80 (0.54) 35.96 (0.47) 35.93 (0.52) 35.93 (0.66) 35.25 (0.86) 35.45 (0.32) 35.11 (0.87) 35.07 (0.95)

RHtorso (%) 46.31 (8.62)c 68.37 (7.81)d,e 61.20 (8.92)e 62.03 (8.20)e 42.77 (7.36) 61.29 (12.05) 55.28 (12.44) 54.29 (9.10)

TC 69.25 (31.32) 70.52 (25.13)f 72.50 (26.77)g 69.60 (27.60)g 62.31 (27.98) 62.88 (22.78) 59.73 (23.12) 59.76 (23.31)

TS 91.33 (26.51) 94.66 (18.51) 91.56 (27.32)h 91.83 (20.34)h 86.84 (22.60) 89.60 (16.28) 87.53 (26.02) 86.53 (20.67)

TTC 59.27 (27.77) 64.50 (22.54) 65.97 (26.67)i 59.71 (27.60)i 53.93 (20.55) 58.37 (19.07) 53.70 (20.38) 53.33 (19.47)

TTS 76.73 (29.00) 82.85 (18.60)j 87.20 (19.73) 75.88 (19.04)k 77.41 (24.00) 77.10 (16.75) 82.55 (16.75) 73.28 (19.66)

a Measurements calculated over the whole experimental session (i.e. exercise and rest)b Significant difference between exercise and rest for all cooling profilesc Significant difference between CP and all (P \ 0.001)d Significant difference between IPonoff and all (P \ 0.001)e Significant difference between exercise and rest, IPonoff and IPramp (P = 0.001), and for IPtriang (P = 0.005)f Trend for a difference between exercise and rest (P = 0.094)g Significant difference between rest and exercise, IPtriang (P = 0.023) and IPramp (P = 0.024)h Trend for a difference between rest and exercise, IPtriang (P = 0.074) and IPramp (P = 0.059)i Significant difference between exercise and rest, IPtriang (P = 0.009), IPramp (P = 0.038)j Significant difference between exercise and rest (P = 0.021)k Significant difference between Ipramp and IPonoff (P = 0.029) and IPtriang (P = 0.000)


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Post hoc analysis identified that TTS was significantly

cooler with IPramp than both IPtriang (P \ 0.001) and

IPonoff (P = 0.029), but similar to CP (P [ 0.05). Dur-

ing rest, TTS was not different between the cooling

profiles, but there was a trend for TTS to increase over

time with CP (P = 0.091). TTS between exercise and

rest was only significantly different with IPonoff

(P = 0.021), with participants feeling cooler (Fig. 6;

Table 1).

Subjective comments

In the post-trial interviews, all participants could accurately

recall the chronological order of the different cooling

profiles that were assigned to them. Therefore, it was

assumed that their subjective comments were specific to

each of the cooling profiles.

Out of the four cooling profiles, participants mostly

preferred CP (4/10), with IPramp being the second most

Fig. 2 Average Tre during

exercise and rest in each cooling

profile (n = 10), *significant

difference between -10 and all

time points except for 105 min

(P \ 0.001), �significant

difference between 25 and all

time points except for 50 and

75 min (P \ 0.001),Usignificant difference between

0 all time points except for 75

and 105 min (P \ 0.05). Error

bars are removed to aid the

clarity of the diagram

Fig. 3 Average mean torso skin

temperature during exercise and

rest in each cooling profile

(n = 10). Error bars are

removed to aid the clarity of the

diagram


123


preferred (3/10). However, the proportions of votes were

not enough to be statistically distinct from the other two

cooling profiles, IPtriang (2/10), IPonoff (1/10) (P [ 0.05).

By examining the subjective comments of the participants,

the factors that influenced the participants’ choice of

cooling profile fell into two common themes: (1) the

cooling profile that felt the coolest, and (2) the cooling

profile that felt the most ‘‘stable’’. The top three reasons

why the participants may not have chosen a cooling profile

as their preferred choice were because it felt either: (1) too

hot, (2) too cold, or (3) had too many fluctuations. In some

cases, participants indicated a preference for a constant,

warmer air temperature provided by the APV (in all cases

the participants believed the temperature of the air was

changed rather than the air flow) over a fluctuating cooler

temperature, as they preferred an element of stability.

Amplitudes of fluctuations and rate of change in the skin

temperatures

The fluctuation amplitudes of both �Tsk and �Tsktorso were

similar between the intermittent cooling profiles during both

exercise and rest (P [ 0.05). However, there was a signifi-

cant difference in the rate of change in �Tsktorso between the

Fig. 4 Average mean torso

relative humidity during

exercise and rest in each cooling

profile (n = 10). Error bars are

removed to aid clarity of the

diagram

Fig. 5 Mean thermal comfort

during exercise and rest in each

cooling profile (n = 10). Error

bars are removed to aid clarity

of the diagram. A trend exists

for TC to differ between the first

and last cooling cycle

(P = 0.063)


123


two intermittent cooling profiles of IPtriang and IPonoff, [0.300

(±0.08) �C min-1 vs. 0.201 (±0.02) �C min-1, respec-

tively, (P = 0.029)] and a trend for a difference between

IPtriang and IPramp [0.300 (±0.08) �C min-1 vs. 0.200

(±0.05) �C min-1, respectively (P = 0.080)].

Discussion

In the current study, three different intermittent cooling

profiles [i.e. step-change (IPonoff), sinusoidal (IPramp), and

sawtooth (IPtriang)] were perfused through an APV and

assessed for their effect on both thermal balance and

thermal perceptions in comparison to continuous cooling.

The results indicate that reducing the airflow through the

APV by 50 % using an intermittent cooling profile does not

negatively affect an individual’s thermal balance during

exercise in a hot environment. Therefore, the first

hypothesis that there would be no difference between the

four cooling profiles in thermal balance is accepted.

Reducing the airflow by 50 % also does not negatively

impact an individual’s local and overall thermal comfort

and temperature sensation. In addition, this reduction did

not negatively affect the rating of perceived exertion; a

subjective measure that can be influenced by increases in

thermal comfort (Robertson 1982). Therefore, the second

hypothesis is rejected. The sinusoidal intermittent cooling

profile (IPramp) led to a cooler torso skin sensation com-

pared to the other intermittent cooling profiles. Therefore,

the third hypothesis is accepted for torso temperature

sensation but rejected for the other thermal perceptions.

The findings from the post-experiment interviews suggest

that the main characteristics determining participants’

preference for a cooling profile were that it was ‘the

coolest’ and/or ‘most stable’. These factors should, there-

fore, be considered in any recommended optimal cooling

profile. In summary, the results from this study imply that,

if battery life is a concern for the design of air-perfused

garments, then an intermittent cooling profile (such as

sinusoidal) should be considered.

Maintenance of thermal balance

The results from the current study support previous

observations that reducing the cooling capacity of a PCG

via intermittent cooling does not negatively affect thermal

balance (Cadarette et al. 2006; Stephenson et al. 2007). In

this study, Tre, �Tsk, HR and sweat production were similar

between conditions despite the experimental manipulation

of cooling profiles. In studies comparing continuous with

intermittent cooling in liquid-perfused garments, the two

main reasons suggested for the maintenance of thermal

balance were: (1) the perfused liquid still acts as heat sink

when not circulated through the garment (Cadarette et al.

2006) and (2) fluctuating temperatures maintain a �Tsk

above the boundaries of the onset of vasoconstriction

(*33 �C) thereby maintaining a gradient between deep

body temperature and �Tsk necessary for heat dissipation

(Cadarette et al. 2006; Stephenson et al. 2007). Even

though the thermal conductivity of air is 25 times less than

that of water (0.024 vs. 0.58 W mK-1) and the main ave-

nues for heat loss in air-perfused garments are evaporation

Fig. 6 Mean torso temperature

sensation during exercise and

rest in each cooling profile

(n = 10). Error bars are

removed to aid clarity of the

diagram


123


and convection rather than conduction, these explanations

may also hold true for APVs. It appears that as long as the

air adjacent to the skin remains relatively unsaturated and

remains above the skin temperature threshold for vaso-

constriction (*33 �C), intermittent cooling profiles in

APVs can also maintain thermal balance similar to that of a

continuous cooling profile. Other possible explanations for

the maintenance of thermal balance in APVs are: (1) the

suggestion that fluctuations in skin temperature enhance

heat transfer at the skin by increasing the convective heat

transfer coefficients between air and the skin surface

(Mayer 1987) and (2) the thermal load in each condition

was sufficiently matched by the amount of sweat evapo-

rated from the regions of the body not covered by the APV.

Thermal perceptual responses to continuous vs.

fluctuating temperatures

Figures 3 and 4 demonstrate that varying the flow rate in

the intermittent cooling profiles was successful in produc-

ing fluctuations in �Tsktorso and RHtorso when compared with

continuous cooling. However, these fluctuations in skin

temperature did not appear to be of a sufficient magnitude,

or rate, to influence average thermal perceptions between

the cooling profiles. There are several possible explana-

tions for this result. Firstly, both mean body temperature

(as indicated by similar values of skin temperature and

rectal temperature between cooling profiles) and skin

wettedness (as indicated by similar values of % of sweat

evaporated from the skin, sweat rate and RHtorso between

the cooling profiles) were similar between the cooling

profiles. Given that thermal comfort is closely associated

with mean body temperature and/or skin wettedness, and

skin temperature is associated with temperature sensation

(Cabanac et al. 1971; Flouris and Cheung 2009; Frank et al.

1999; Toftum et al. 1998), it is unsurprising that no sig-

nificant differences were evident in the thermal perceptions

(i.e. TC or TS) between the cooling profiles. Secondly, the

magnitude of change in �T sk (±0.5 �C) caused by the

fluctuations in air flow in the intermittent cooling profiles

may not have been enough to promote the fluctuations in

TS that would distinguish them from CP. Third, the rate of

change in skin temperature caused by the intermittent

cooling profiles (i.e. between 0.2 and 0.3 �C min-1) may

not have been enough to promote significant differences

between the intermittent and continuous cooling profiles in

thermal perceptions. In a warm environment, rates of

change in local skin temperature of *-0.41 to

-0.49 �C min-1 have been suggested to be the thresholds

for eliciting significant changes in thermal comfort (Davey

et al. 2009; Zhang 2003). Lastly, the higher RHtorso in the

intermittent cooling profiles may have confounded the

influence that any fluctuations in skin temperature had on

the thermal perceptions. This possibility is supported by the

observation that during the experiment, in each intermittent

cooling profile, the amplitude of change in RHtorso never

reached a level lower than that of CP. However, during the

transition from exercise to rest, RHtorso was significantly

reduced in all the intermittent cooling profiles, whereas in

CP it remained the same. This reduction in RHtorso in the

intermittent cooling profiles coincided with improved

thermal perceptions (i.e. TC, TTC and TTS) for the inter-

mittent cooling profiles, whereas in the continuous profile

thermal perceptions remained the same.

The improvements observed in TC and TTC in the

intermittent cooling profiles due to the reduction in RHtorso

may have been caused by the well acknowledged positive

relationship between skin wettedness and thermal dis-

comfort (Gagge and Berglund 1986; Toftum et al. 1998).

The improved TTS score in IPonoff may have been caused

by an increase in the rate of sweat evaporation from the

skin of the torso during rest. Theoretically, the cessation of

exercise would have caused a gradual decrease in sweat

output due to a reduction in heat production and conse-

quential gradual fall in deep body temperature. A lower

sweat output would decrease the vapour pressure in the air

surrounding the torso skin and the saturated water vapour

pressure at the skin itself, thus creating a more favourable

gradient for the evaporation of sweat. Ackerley et al.

(2012) suggested that the rate of evaporation of water from

the skin is sensed by the faster-conducting cool Ad affer-

ents and, to an extent, a decrease in the firing of warm

unmyelinated afferents which, as a result, elicit cooler

temperature sensations. They also suggest that the skin is

very sensitive to changes of evaporation rates and therefore

they may not always be associated with differences in skin

temperature. No differences in skin temperature were

observed in the current study between the four cooling

profiles, but it is possible that different rates of evaporation

of sweat may have been experienced between the profiles

as the magnitude in fluctuations in RHtorso differed between

the cooling profiles (Fig. 4). Nevertheless, it is clear that

RHtorso was a significant driver of thermal perception at

key points in the present study.

Although no differences between the intermittent and

continuous cooling profiles were established in thermal

perceptions, there were differences between the intermit-

tent profiles. In the IPramp condition, TTS was significantly

cooler than the other intermittent cooling profiles, but

similar to CP. Our results, and those of others investigating

the effects of different types of air movements on thermal

perceptions (de Dear et al. 1993; Tanabe and Kimura 1994;

Zhou et al. 2006), suggest that the cooler sensations do not

appear to be related to absolute skin temperatures or the


123


rate or amplitude of change in skin temperature; these

values were similar between IPramp and the other inter-

mittent cooling profiles. Therefore, another explanation is

required. One possibility is that the ability of sinusoidal

wave patterns to enhance temperature sensation is due to a

greater rate of recruitment of the thermoreceptors (Ring

et al. 1993; Lv and Liu 2007). Other explanations include:

firstly, a level of adaptation within the thermoreceptors

may have occurred in IPonoff during the ON periods when

the highest airflow rate was perfused through the APV.

This ON period (i.e. 6 min) was twice as long as the period

where the same flow rate of 339.8 L min-1 was perfused in

IPramp. As thermal perceptions are processed centrally, the

adaptation time for temperature sensation could approach

several minutes (Hensel 1981). Therefore, as the last 3 min

of the ON period only improved TC by -0.85 (a very small

amount of the TS scale), suggests a level of adaptation may

have occurred in IPonoff. This adaptation could have

attenuated any changes in TTS and TS. Secondly, the

significantly higher RHtorso present in IConoff may have

confounded the influence that any reductions in skin tem-

perature had on TTS and TS. Finally, within a cooling

cycle in IPtriang, three quarters of the time is spent warming

the skin, whereas in IPramp the cooling and warming peri-

ods were equal.

Overall preference in cooling profiles

The post-experiment interviews suggested that an individ-

ual’s choice of cooling profile is determined by the tem-

perature sensation it provides and the amplitude of

fluctuations in temperatures; in both cases, the lower the

better. These two requirements are similar to those found in

studies that have examined the effects of different air

movements on thermal perceptions in warm environments

(Fanger et al. 1988; Tanabe and Kimura 1994; Zhou et al.

2006). In these studies, air movements at high velocities, or

turbulence intensity, were not favoured as they produced

feelings of ‘draft’ or in some cases were described as

‘annoying’ (Fanger et al. 1988; Tanabe and Kimura 1994).

This may explain why the IPramp and continuous cooling

profiles were generally preferred over the other two cooling

profiles in the present study.

Determination of the optimal cooling profile

From the results of this study the following recommenda-

tions are provided for the design of cooling profiles in air-

perfused garments to be used in conditions similar to that

of this study.

1. The cooling profile should be that of a sinusoidal wave

pattern. This will not only provide cooler torso

temperature sensations than step-change or sawtooth

intermittent cooling profiles, but also require less

battery power. Generally, the battery power required to

switch from air flows of no velocity to a high velocity

is greater than steadily increasing the velocity of air

flow.

2. The cooling profile must maintain a RHtorso similar to

that in CP. The ratio of 1:1 ON/OFF periods in the

step-change profile investigated in this study produced

the highest levels of RHtorso. Therefore, ratios favour-

ing the ON periods are recommended to increase the

amount of evaporative and convective heat loss within

a cooling cycle. In addition, the periods of no airflow

within a cooling cycle should be \3 min.

3. The possibility that a level of adaptation occurred in

IPonoff during the ON periods suggests that the ON

period (i.e. 6 min) in this profile may have been too

long. Therefore, the peak airflow of a cooling cycle

should be \6 min.

4. The cooling profile should attempt to elicit a rate of

torso skin temperature change [-0.30 �C min-1

(preferably -0.49 �C min-1).

Limitations

The authors acknowledge that the results of this study are

limited to the three types of intermittent cooling profiles

that were investigated in this study and that many other

combinations of cooling cycles could have been tested

(e.g. length of cooling cycle, magnitude of air flow, time

period of air flow). The specific cooling cycles used in this

study were selected as fair comparisons could be per-

formed between them and continuous cooling. To further

clarify the benefit of using intermittent cooling profiles

that require half the amount of air in a given period of

time than continuous cooling, it would be of interest to

determine the effect on thermal balance and thermal

perceptions of continuous cooling at half the rate of

airflow.

It would also be of interest to compare the performance

of intermittent and continuous cooling profiles over a

longer period of time. In the current study, there were

trends in the continuous cooling profile for the participants

to become more uncomfortable over time during exercise

and their torso to feel warmer over time during rest. These

changes were not statistically, nor perceptually, significant.

Therefore, the second hypothesis was rejected. However, it

is possible that, as indicated in previous studies (de Dear

and Brager 2001; Zhang 2003), longer periods of exercise

(or rest) may have been required for a level of thermal

adaptation to occur within the thermoreceptors, or at a

conscious level, that would significantly alter thermal


123


perceptions negatively and, as a result, may alter an indi-

vidual’s choice of cooling profile.

Conclusion

It is concluded that reducing the cooling capacity of a

cooling garment using an intermittent as opposed to a

continuous cooling profile does not compromise thermal

balance or thermal comfort during exercise in a hot envi-

ronment, but does reduce power consumption and thereby

extend battery life. Further investigations are required to

evaluate whether the recommendations provided for the

design of cooling profiles in air-perfused garments are

appropriate for more thermally stressful environments.

Acknowledgments This research is financially supported by Uni-

versity of Portsmouth and W.L. Gore & Associates. The authors

would like to thank Geoff Long, Mark Newton, and Nicola Ferguson

for all their technical support on this project and the participants for

their efforts.

Conflict of interest The authors declare that they have no conflict

of interest.

Ethical consideration This experiment was approved by the Uni-

versity of Portsmouth Biosciences Research Ethics Committee. It

complies with all current legislation, including the Declaration of

Helsinki, as adopted at the 52nd WMA General Assembly, Edin-

burgh, October 2000.

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