Thermoregulatory responses of junior lifesavers wearing protective clothing

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ournal of Science and Medicine in Sport (2008) 11, 542—548

RIGINAL PAPER

hermoregulatory responses of juniorifesavers wearing protective clothing

ade H. Sinclair ∗, Melissa J. Crowe,arwick L. Spinks, Anthony S. Leicht

nstitute of Sport and Exercise Science, James Cook University, Queensland, Australia

eceived 23 November 2006; received in revised form 16 May 2007; accepted 16 May 2007

KEYWORDSChild;Exercise;Heat;Protective clothing;Body temperatureregulation

Summary This study investigated the influence of protective clothing worn toprevent marine stinger envenomation on the thermoregulatory responses of pre-pubescent surf lifesavers exercising in situ under hot and humid conditions (27 ◦C,78% relative humidity). Participants performed beach and water activities typicallyassociated with junior surf lifesaving competition in a randomised cross-over designon two separate occasions 7 days apart: one wearing a full-length Lycra stingersuit (S) and one wearing normal swimwear (SW). Skin (TSK) and core (TC) bodytemperatures, skin blood flow (SKBF), heart rate (HR), body mass, thermal com-fort and perceived effort were assessed pre-, mid- (following beach activities) andpost-exercise (following water activities). Sweat rates were compared betweenS and SW. TC was greater following beach activities for S (37.78 ◦C ± 0.06) com-pared to SW (37.60 ◦C ± 0.07; p < 0.05) and male participants experienced greaterTC (37.97 ◦C ± 0.09) than their female counterparts (37.71 ◦C ± 0.07 ◦C). TSK follow-ing both the beach and water activities were lower than pre-exercise (p < 0.05).SKBF was significantly increased for calf across time (p < 0.01). Male participantsexperienced a higher HR for S compared to female participants (p < 0.01) while theopposite applied to SW (p < 0.01). There were no gender or between-condition dif-ferences for sweat rate or perceived effort. There was evidence of heat storage

while stinger suits were worn during beach activities in the absence of any differ-ences in exercise intensity or sweat rate. The results of the present study suggestthat the stinger suits should be limited to water-based activities.

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© 2007 Sports Medicine

Introduction

arine stinger species are endemic along the northustralian coastline between the warmer months of

∗ Corresponding author.E-mail address: [email protected] (W.H. Sinclair).

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440-2440/$ — see front matter © 2007 Sports Medicine Australia. Publisheoi:10.1016/j.jsams.2007.05.010

tralia. Published by Elsevier Ltd. All rights reserved.

ctober and May and are known to initiate severeystemic, noxious and immunological pathologies ornflict death within minutes.1 The north Queens-

and marine stinger season parallels surf lifesavingctivities in the region. As a result, the bathingublic are required to swim within stinger-resistantetted enclosures and it is mandatory for junior

d by Elsevier Ltd. All rights reserved.

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hildren in protective clothing

urf lifesavers to wear full-length protective cloth-ng (stinger suits) during training and competitivectivities. In addition to high ambient tempera-ures and humidity during this period, stinger suitsresent an increased possibility of heat-related ill-ess in these children, while competing and duringmpromptu play between competitive events.2,3

During exposure to extreme climatic heat,he thermoregulatory capabilities of childrenre severely disadvantaged compared to adults,hereby increasing their risk of experiencing heat-elated illness.3—6 The ability of children tohermoregulate while exercising in thermoneutralnvironments is similar to that of adults albeit viaiffering routes.7,8 However, this thermoregulatorybility is deficient when exposed to extreme envi-onmental conditions.4 When compared to adults,hildren have a greater surface-area-to-body massatio4 and rely more on conduction, convectionnd radiation than adults which can become aiability once ambient temperatures exceed thatf the skin.9 Under these conditions, heat isbsorbed from the environment imposing addi-ional thermoregulatory stress on children, therebyncreasing their susceptibility to an increased coreody temperature (TC) and potential developmentf heat-related illnesses. Children exercising inot environments are at a further disadvantageompared to adults because of differing bodyomposition,9 smaller absolute blood volume,7,10

ower cardiac output,10 greater metabolic heat pro-uction kg−1 body mass during work4 and a lessfficient sweating mechanism.4 Additionally, exer-ising while wearing close-fitting clothing in hotnvironments can increase TC.2 Consequently, pre-ubescent surf lifesavers in northern Australia aret an increased risk of developing heat-related ill-ess because of physiological limitations as well ashose imposed by the environment and the protec-ive clothing requirements of their sport.

This study evaluated the thermoregulatoryesponses and cardiovascular strain of pre-ubescent surf lifesavers competing in beach

ctivities in situ under hot and humid conditionshile wearing full-length stinger suits. Whilearticipants competed in junior surf lifesavingeach and water activities, the primary focus of

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Table 1 Descriptive characteristics for male, female and a

n Age (years) Body mass (kg)

Male 7 9.1 ± 0.4 32.0 ± 1.1Female 13 9.9 ± 0.4 34.8 ± 2.2All 20 9.7 ± 0.4 33.8 ± 1.9

Values are presented as mean ± S.E.; AD = surface area; ADM = surfa

543

he study was to evaluate the influence of stingeruits on participants competing in beach activities,s previous research has shown significant heatoss from the body during immersion in water.11,12

t was hypothesised that pre-pubescent surf life-avers exercising in stinger suits would experiencen increased thermoregulatory strain compared toearing normal swimwear while competing in surf

ifesaving beach activities.

ethods

articipants

re-pubescent surf lifesavers (male, n = 7; female,= 13) were recruited from north Queensland surf

ifesaving clubs. The descriptive characteristicsf the participants are presented in Table 1.articipants (aged 7—12 years) were classifieds pre-pubertal based on previous research thatmployed the Tanner stages for assessing mat-ration which identified pre-pubertal childreno be <12.2 years.13,14 Participant and parentalritten informed consent was obtained and the

tudy approved by James Cook University Humanesearch Ethics Sub-Committee and Surf Life Savingueensland in accordance with the 1964 Decla-

ation of Helsinki. Participants were immediatelyithdrawn from the study if exercising HR > 95%ge-predicted HRmax, TC > 39◦C or any signs orymptoms of heat-related illness were evident.15

xperimental procedures

articipants attended two randomised cross-overrials 7 days apart; one wearing a full-length stingeruit (S) and one wearing normal swimwear (SW).W consisted of either one-piece or bikini-stylewimwear for the females and one-piece swimmingriefs for males. The stinger suits comprised a one-iece protective stinger suit of swimsuit material82% nylon, 18% Lycra®) with only the head, feet

nd hands exposed. Each trial consisted of partic-pants competing in beach activities followed byater activities in the same order and identical to

hose conducted during normal training sessions.

ll participants

Height (m) AD (m2) ADM (cm2 kg−1)

1.36 ± 0.03 1.10 ± 0.03 346.27 ± 3.071.37 ± 0.03 1.15 ± 0.04 337.81 ± 7.851.37 ± 0.02 1.13 ± 0.04 340.77 ± 6.54

ce-area-to-body mass ratio.

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Beach activities included beach flags (7—12years; 4—6 multiple 15-m sprints competing fora piece of rubber hose with one person elimi-nated each round until a winner is identified);70-m beach sprint (7—12 years) and an age-group70-m beach relay (9—12 years). Water activitiescommenced and finished on the beach with par-ticipants rounding buoys placed at various lengthswithin the stinger resistant enclosure. Water activ-ities included a 20-m wading race (7—10 years), anage-group 40-m wading relay (7—10 years), 150-m surf swim race (9—12 years), 150-m surfboardpaddle race (9—12 years) and Ironman and Iron-woman races (11—12 years, consisting of a 150-mswim and 150-m surfboard paddle with 40 m ofintermittent beach running). Swim distances weretypical of those used in training sessions held withinstinger resistant enclosures. Rest periods betweenactivities ranged between 5 and 15 min with 20 minbetween beach and water activities.

Experimental protocols

Participant body mass was recorded to the near-est 0.1 kg with participants wearing swimwearonly (Tanita TBF-521, Tanita Corporation, Tokyo).Height was recorded to the nearest 0.01 m, using aportable, wall mounted stadiometer (Handy HeightScale, Mentone Educational Center, Australia). Sur-face area (AD) and surface-area-to-body mass ratio(ADM) were calculated via standard equations.16

Blood pressure (BP) and HR were measured usingan electronic sphygmomanometer (Nissei DS-157,Japan) and HR monitor (Polar S610 HR Monitor, PolarElectro Oy, Finland), respectively.

Sweat rate was determined via body masschanges and water consumed and expressed rel-ative to time (mL h−1). During the first trial,participants consumed water ad libitum while inthe assessment area with the time and volumeof water consumed recorded so that participantscould be issued with the same volume at identicaltimes during the second trial in order to minimisefluid consumption effects.

looseness1 TC was assessed via tympanic mem-brane infrared thermometry (FirstTemp 2000A,Intelligent Medical Systems, USA) from the partic-ipant’s right ear and in an area sheltered fromairflow by the same researcher.17 Prior to com-mencement of this study, the accuracy of thetympanic thermometer was confirmed as ≤0.1 ◦C by

comparing skin temperature assessments betweenthe unit (using the ‘surface’ setting) and skinsurface temperature probes (SST-1, PhysitempInstruments Inc., NJ, USA). Previous research hasshown the FirstTemp 2000A to record tympanic tem-

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eratures within an average of 0.13 ◦C of rectalemperature.18

Following the removal of the stinger suit, andurface water and sand from the skin, TSK was mea-ured via skin surface temperature probes. SKBFas measured by laser Doppler flowmetry (ML191lood FlowMeter, PowerLab AD Instruments, Aus-ralia) and four-channel PowerLab (PowerLab/4SP,D Instruments, Australia). Blood flowmeter out-ut was 4 mV for each recorded blood perfusionnit at a sampling rate of 10 Hz. SKBF data wereollected using Chart for Windows software (Chart4.0.1, AD Instruments, Australia) and data pointsere obtained from 5 s of continuous segmentsith minimal artefact noise for each site. Fournthropometric landmarks (forehead, back, fore-rm and calf) were marked on the participant’sight side prior to commencing competition activi-ies to enhance the reproducibility of TSK and SKBFeasurements.Participant perceptions of exercise effort was

ssessed via the 10-point Children’s Effort RatingCERT) Scale.19 Environmental parameters (dry andet bulb, black globe, %RH and WBGT) were logged

or the duration of the trials (CR10X Measurementnd Control System, Campbell Scientific Pty. Ltd.,ustralia).

tatistical analyses

ata analysis was conducted using the Statis-ical Package for Social Sciences (v.11. SPSS,hicago, USA). Distribution of data was initiallyssessed using the Kolmogorov—Smirnov test oformality with all skewed data transformed.20

nalysis was conducted via three-way repeatedeasures ANOVA (condition × time × gender) of

ondition (S or SW) × time (pre-, mid- andost-exercise) × gender (M/F). Two-way repeatedeasures ANOVA (time × gender) was conducted

or sweat rate data. Post hoc analysis was con-ucted using the Tukey test. Where all assumptionsf ANOVA were not met, nonparametric compar-sons were performed via the Friedman Test withubsequent post hoc analysis by Nemenyi’s test.

one-way ANOVA or the nonparametric Wilcoxonigned Ranks test was conducted to assess between-ender differences for age, height, body mass, ADnd ADM. Alpha was set at 0.05 and all values areresented as mean ± S.E.

esults

nvironmental conditions for the trials are pre-ented in Table 2 with wet bulb (TWB) and wet bulb

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Table 2 Environmental conditions experienced byparticipants during surf lifesaving activities on days1 and 2

Day 1 Day 2

Dry bulb (◦C) 27.3 ± 0.3 27.8 ± 0.3Wet bulb (◦C) 24.6 ± 0.1 24.9 ± 0.3**Black globe (◦C) 28.9 ± 0.6 30.7 ± 0.7Relative humidity (%) 81.6 ± 2.4 76.8 ± 1.5*WBGT (◦C) 25.7 ± 0.1 26.3 ± 0.3*

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Table 3 Forehead, back, forearm and calf skintemperatures (◦C) for all participants prior to(pre-exercise) and following beach activities (mid-exercise) and following water activities (post-exercise)

Pre-exercise Mid-exercise Post-exercise

Forehead 34.6 ± 0.1 33.7 ± 0.1b 33.5 ± 0.2b

Back 33.3 ± 0.1 32.8 ± 0.2a 32.1 ± 0.2b,c

Forearm 34.6 ± 0.1 33.7 ± 0.1b 33.5 ± 0.2b

Calf 32.2 ± 0.1 31.7 ± 0.2a 30.2 ± 0.1b,c

Values are presented as mean ± S.E.a p < 0.05.b p < 0.01 significantly lower than pre-exercise.c p < 0.01 significantly lower than mid-exercise.

Figure 2 Heart rate (b min−1) for male and female par-ticipants with (S) and without (SW) stinger suits whenpa

Values are presented as mean ± S.E.; *p < 0.05, **p < 0.01significantly different to Day 1.

lobe temperature (WBGT) greater and %RH loweror day 2 compared to day 1 (p < 0.05).

TC for S (37.78 ◦C ± 0.06) was significantlyreater compared to SW (37.60 ◦C±; p < 0.05) whenombined for gender and time. Following the beachctivities, TC was higher for S (37.81 ◦C ± 0.06)ompared to SW (37.52 ◦C ± 0.08; p < 0.01) and sig-ificantly increased for S (p < 0.01) and decreasedor SW (p < 0.01) from pre-exercise. TC after theater activities for both S and SW was lower thanoth pre-exercise and following the beach activitiesp < 0.01). TC was significantly greater following theeach activities for male participants compared tore-exercise values and female TC after the beachctivities (Fig. 1).

TSK at all sites was significantly lower than pre-xercise after both the beach (p < 0.05) and waterctivities (p < 0.05; Table 3). There was no effect ofondition or gender on TSK (p > 0.05).

SKBFCALF following the beach events39.59 ± 2.76 mV) was increased compared toaseline (29.22 ± 3.15 mV; p < 0.01) and wasignificantly lower following the water events

igure 1 Core body temperature (◦C) for male andemale participants with (S) and without (SW) stingeruits prior to (pre-exercise) and following beach activ-ties (mid-exercise). Values are presented as mean ± S.E.p < 0.01 greater than female S; bp < 0.01 greater thanpposite condition for same gender.

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ooled for time. Values are presented as mean ± S.E.p < 0.01 greater than opposite gender; bp < 0.01 greaterhan opposite condition for same gender.

27.61 ± 2.18 mV; p < .01). No differences wereound for SKBF at any of the other sites.

Male HR for S was significantly greater comparedo SW and females for S (Fig. 2). Female HR for SWas significantly greater than S and males for SW

Fig. 2).No differences were identified for the effects

f condition, time or gender for sweat rate whichveraged 195 mL h−1 (0.58% initial body mass h−1)nd 126 mL h−1 (0.37% initial body mass h−1) forW; p > 0.05). No differences were identified forERT between S (4.14 ± 0.38) and SW (3.63 ± 0.41;> 0.05).

iscussion

he results suggest an increased thermoregula-ory strain on pre-pubescent surf lifesavers wearing

stinger suit and exercising in warm, humidnvironments. Participants experienced higher TC

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while wearing the full-length protective stingersuits during the beach activities compared to pre-exercise. However, the observed increase (0.29 ◦C)is not considered biologically important. Con-versely, significantly lower TC and TSK for both Sand SW following the water activities was mostlikely because of an enhanced thermal gradientin the water resulting in an increased conduc-tive and convective heat transfer between thecore and the skin and the skin-to-water boundary,respectively.11,21,22 Although the incidence of TC≥38 ◦C following beach activities (32% for S and11% for SW) parallels previous research,7,8,14,23—25

children in those studies were exposed to greaterclimatic heat stress (34—49 ◦C, 18—45%RH) thanthose in the present study.

Tympanic infra-red thermometry has beendescribed as problematic.26,27 However, the coef-ficient of variation (cv) for TC in this study(cv ≤ 0.93%) was found to be comparable tothose previously reported for rectal temperature(cv ≤ 0.97%) and tympanic TC (cv ≤ 1.07%) under hotenvironmental conditions.28 The higher TC and HRidentified in the male participants parallels previ-ous research28,29 but contrasts with other studiesin which females had significantly higher HR whenexercising,10 particularly under hot conditions.7

Previously, lower TC for females exercising inhot and humid environments has been attributedto their greater ADM or lower lean muscle mass.29

In individuals of similar body mass, greater thermalstress is required to elevate TC for low adipositylevels due to the respective specific heat of adi-pose tissue (1.67 kJ kg−1 ◦C−1) and fat free mass(3.35 kJ kg−1 ◦C−1).9 A plausible explanation forhigher TC in male participants in the present studymay be higher adiposity levels than in their femalecounterparts. Pre-pubertal girls have lower % bodyfat than adult females and pre-pubertal boys haveslightly higher levels of adiposity than adult males.9

Although body composition was not assessed inthe current study, ADM was not significantly dif-ferent between genders. When matched for ADMand fitness levels, females maintain similar or lowerTC and lower HR than males,28 suggesting maleadiposity levels as causative of the higher TC inthe present study. Furthermore, higher male HRsuggests males may have been more competitiveduring the beach activities than the females,30

resulting in an increased exercise intensity andthus increasing metabolic heat production. Conse-

quently, male participants may have experiencedincreased sympathetic activity initiating vasodi-lation to meet the increased thermoregulatorydemand.31 Another contributing factor could alsobe the resultant higher TC having a direct influence

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n the sinoatrial node.31 Previous research on thehermoregulatory responses of pre-pubertal chil-ren exercising in hot environments is inconclusivend warrants further investigation.

Children experience a reduced sensitivity in ini-iation of the sweating response with increasesn TC.9 However, the absence of a significantecrease in SKBF in participants in the presenttudy suggests cooler TSK resulted from sweat evap-rating from the skin or sweat absorbed in thetinger suit cooling the skin surface. Sweating isn effective heat loss mechanism for children dur-ng mild heat exposure.6 However, childrens sweatate can be ineffective during periods of com-ined heat and exercise stress when compared todults.7,32 In addition, the major contributor tonvironmental stress is %RH, with high %RH com-romising the evaporative heat loss potential ofweat.33 The humid conditions experienced in theresent study (76.8—81.6%RH) limited the evapo-ative capacity of the participants exemplified byweat rates being indicative of non-acclimatisedhildren (260—300 mL h−1) rather than children whoave resided in the region for 7.0 ± 0.8 years andould therefore be acclimatised.8,14,23 Sweat rates

n the present study ranged between 0.4 and 0.6%nitial body mass h−1 potentially increasing TC by.2 ◦C and/or HR by 3—5 beats min−1,34 therebyxacerbating thermoregulatory and cardiovasculartrain. A loss of 0.3% initial body mass h−1 results inreater increases in the TC of children compared todults for similar levels of hydration.35 Therefore,he combined thermal stress, exercise intensity andweat loss in the current study was sufficient tolicit an increased thermoregulatory and cardiovas-ular strain.

Previous studies conducted indoors under con-rolled climatic conditions have occurred in theear absence of any airflow.7,8,14,23—25 Airflowould substantially influence convective and evap-rative heat loss.36 The TSK results for all sitesn the present study were significantly lowerollowing both the beach and water activities pos-ibly as a consequence of airflow experienced≥6.5 km h−1 ± 2.4) prior to the assessment of TSK

36

iding evaporative heat loss or conductive heat lossetween the skin and water.11,21 Caution shoulde extended to comparisons between indoor andutdoor studies because TC, TSK, HR, perceivedxertion and sweat rate are all significantly influ-nced by the velocity of circulating air.36,37

Taken together, the present results suggest thatull-length protective stinger suits worn by juniorurf lifesavers in north Queensland promote heattorage when worn while competing in beachctivities typically associated with surf lifesaving

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ompetition. Despite all participants being asymp-omatic of heat-related illness during both trials,he higher TC for participants while wearing stingeruits and in the absence of any differences in sweatate and perceived exercise intensity is potentiallyelated to heat stored during stinger suit use.38

herefore, these junior athletes are at an increasedisk of developing heat-related illness becausef higher TC resulting from ambient environmen-al conditions, poor hydration status, clothingorn and intensity of the exercise undertaken.2,3,5

igher TC and HR in males wearing stinger suitsuggest they experienced additional cardiovascularnd thermoregulatory strain during beach activi-ies compared to females. Collectively, the presentesults suggest that protective stinger suits shoulde limited to water-based activities rather thansun protection role on the beach and caution

xtended to environments hotter or more humidhan those assessed in this study.

Practical implications

• During beach activities, wearing stinger suitsmay place pre-pubertal surf lifesavers at anincreased risk of developing heat-related ill-nesses.

• Pre-pubertal boys exhibit greater physiologi-cal stress than pre-pubertal girls while wearingstinger suits under hot and humid conditions.

• As a safeguard for junior surf lifesavers,stinger suits use should be confined to water-based activities.

cknowledgement

he authors gratefully acknowledge the assistancend support given throughout this research by Surfife Saving Queensland and the members of the Pic-ic Bay and Arcadian Surf Life Saving Clubs.

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