ACCLIMATIZATION, DECAY AND RE-ACCLIMATIZATION · • Evidence of acclimatization = plateau in core temperature over last 30 minutes for 3 days • à 6 days for acclimatization 36

ACCLIMATIZATION, DECAY AND RE-ACCLIMATIZATION

Candi D. Ashley [email protected]

ACKNOWLEDGEMENTS

•  NIOSH•  SunshineERC•  Students,facultyandstaffatUSF•  Researchcollaborators:

–  Dr.ThomasE.Bernard–  Dr.EricCoris–  Dr.RebeccaLopez

•  Introductiontoacclimatization•  Physiologicaladaptationsofacclimatization•  Howlongtoacclimatize?•  Whataboutdecayofacclimatization?•  Howlongtore-acclimatizeafterabsence?•  Acclimatization/Re-acclimatizationGuidelines

EXERTIONAL HEAT ILLNESS

•  Exertionalheatillnessisariskforworkersinhotenvironments.

Fatalillnesses(n=14)

Nonfatalillnesses(n=11)

Totalsample(n=25)

Unacclimatizedn(%)

11(78.6) 1(9.1) 12(48.0)

TustinAW,LamsonGE,JacklitschBL,etal.EvaluationofOccupationalExposureLimitsforHeatStressinOutdoorWorkers—UnitedStates,2011–2016.MMWRMorbMortalWklyRep2018;67:733–737.

PHYSIOLOGY OF ACCLIMATIZATION: HEART RATE

•  Decreasedheartrateforagivenintensity•  Increasedplasmavolume•  Increasedstrokevolume•  Increasedskinbloodflow

130

135

140

145

150

155

160

1 2 3 4 5 6 7 8

HeartR

ate

DaysofAcclimatization

HeartRate

PHYSIOLOGY OF ACCLIMATIZATION: CORE TEMPERATURE

•  Decreasedcoretemperatureforagivenworkload•  Decreasedthresholdfortheonsetofsweating

38

38.1

38.2

38.3

38.4

38.5

1 2 3 4 5 6 7 8

CoreTem

perature


CoreTemperature

PHYSIOLOGY OF ACCLIMATIZATION: SWEAT RATE

•  Greatersweatrate•  Moredilutesweat•  Sweatatalowercoretemperature

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1 2 3 4 5 6 7 8 9 10

SweatLoss(L/hr)


SweatRatewithAcclimtization

AND…

•  Decreasedperceivedexertion•  Reducedoxygendemandatagivenintensity•  Fitnessconfersanadvantagetoacclimatization

CURRENT ACCLIMATIZATION GUIDELINES

•  OccupationalSafetyandHealthAdministration:–  5daysbeginningwith50%;increaseto100%byday5

•  NationalInstituteforOccupationalSafetyandHealth:–  5daysbeginningwith20%;increase20%eachday

•  MiningSafetyandHealthAdministration–  6daysbeginningwith50%;increase20%eachday

•  Militaryguidelines–  2weeks

HOW LONG TO ACCLIMATIZE?

•  n=18•  Evidenceofacclimatization=plateauincoretemperatureoverlast30minutesfor3days

•  à6daysforacclimatization

36

36.5

37

37.5

38

38.5

39

39.5

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Tre

time

Tre- Acclimation-S7

Tre 1 Tre 2 Tre 3 Tre 4 Tre 5 Tre 6 Tre7 Tre 8

•  Adaptationsinheartrateandcoretemperatureoccurin5dayswithdailyheatexposure.–  Increaseddurationofdailyheatexposureimprovescoretemperatureadaptations.

–  IncreasedWBGTimprovessweatrateadaptations.

SYSTEMATIC REVIEW

Heat Acclimation Decay and Re-Induction: A Systematic Reviewand Meta-Analysis

Hein A. M. Daanen1 • Sebastien Racinais2 • Julien D. Periard2,3

Published online: 11 November 2017! The Author(s) 2017. This article is an open access publication

AbstractBackground Although the acquisition of heat acclimation

(HA) is well-documented, less is known about HA decay

(HAD) and heat re-acclimation (HRA). The available lit-erature suggests 1 day of HA is lost following 2 days of

HAD. Understanding this relationship has the potential to

impact upon the manner in which athletes prepare formajor competitions, as a HA regimen may be disruptive

during final preparations (i.e., taper).

Objective The aim of this systematic review and meta-analysis was to determine the rate of HAD and HRA in

three of the main physiological adaptations occurring

during HA: heart rate (HR), core temperature (Tc), andsweat rate (SR).

Data Sources Data for this systematic review were

retrieved from Scopus and critical review of the citedreferences.

Study Selection Studies were included when they met the

following criteria: HA, HAD, and HRA (when available)were quantified in terms of exposure and duration. HA had

to be for at least 5 days and HAD for at least 7 days forlongitudinal studies. HR, Tc, or SR had to be monitored in

human participants.

Study Appraisal The level of bias in each study wasassessed using the McMaster critical review form. Multiple

linear regression techniques were used to determine the

dependency of HAD in HR, Tc, and SR from the number ofHA and HAD days, daily HA exposure duration, and

intensity.

Results Twelve studies met the criteria and were system-atically reviewed. HAD was quantified as a percentage

change relative to HA (0% = HA, 100% = unacclimated

state). Adaptations in end-exercise HR decreased by 2.3%(P\0.001) for every day of HAD. For end-exercise Tc, the

daily decrease was 2.6% (P\0.01). The adaptations in Tcduring the HA period were more sustainable when the dailyheat exposure duration was increased and heat exposure

intensity decreased. The decay in SR was not related to the

number of decay days. However, protracted HA-regimensseem to induce longer-lasting adaptations in SR. High heat

exposure intensities during HA seem to evoke more sus-

tained adaptations in SR than lower heat stress. Only eightstudies investigated HRA. HRA was 8–12 times faster than

HAD at inducing adaptations in HR and Tc, but no dif-ferences could be established for SR.

Limitations The available studies lacked standardization in

the protocols for HA and HAD.Conclusions HAD and HRA differ considerably between

physiological systems. Five or more HA days are sufficient

to cause adaptations in HR and Tc; however, extending thedaily heat exposure duration enhances Tc adaptations. For

every decay day, * 2.5% of the adaptations in HR and Tcare lost. For SR, longer HA periods are related to betteradaptations. High heat exposure intensities seem beneficial

for adaptations in SR, but not in Tc. HRA induces adap-

tations in HR and Tc at a faster rate than HA. HRA maythus provide a practical and less disruptive means of

maintaining and optimizing HA prior to competition.

& Hein A. M. [email protected]

1 Department of Human Movement Sciences, Faculty ofBehavioural and Movement Sciences, Amsterdam MovementSciences, Vrije Universiteit Amsterdam, Van derBoechorststraat 7, 1081BT Amsterdam, The Netherlands

2 Athlete Health and Performance Research Centre, AspetarOrthopaedic and Sports Medicine Hospital, Doha, Qatar

3 Research Institute for Sport and Exercise, University ofCanberra, Canberra, ACT, Australia

123

Sports Med (2018) 48:409–430

https://doi.org/10.1007/s40279-017-0808-x

ACCLIMATIZATION, DECAY AND REACCLIMATIZATION: HEART RATE

ACCLIMATIZATION, DECAY AND REACCLIMATIZATION: CORE TEMPERATURE

DECAY OF ACCLIMATIZATION

•  Foreverydecayday,≈2.5%ofadaptationsinheartrateandcoretemperaturearelost.

•  Rateofdecayincoretemperaturecanbereducedwithincreaseddurationanddecreasedexposure.

SYSTEMATIC REVIEW

Heat Acclimation Decay and Re-Induction: A Systematic Reviewand Meta-Analysis

Hein A. M. Daanen1 • Sebastien Racinais2 • Julien D. Periard2,3

Published online: 11 November 2017! The Author(s) 2017. This article is an open access publication

AbstractBackground Although the acquisition of heat acclimation

(HA) is well-documented, less is known about HA decay

(HAD) and heat re-acclimation (HRA). The available lit-erature suggests 1 day of HA is lost following 2 days of

HAD. Understanding this relationship has the potential to

impact upon the manner in which athletes prepare formajor competitions, as a HA regimen may be disruptive

during final preparations (i.e., taper).

Objective The aim of this systematic review and meta-analysis was to determine the rate of HAD and HRA in

three of the main physiological adaptations occurring

during HA: heart rate (HR), core temperature (Tc), andsweat rate (SR).

Data Sources Data for this systematic review were

retrieved from Scopus and critical review of the citedreferences.

Study Selection Studies were included when they met the

following criteria: HA, HAD, and HRA (when available)were quantified in terms of exposure and duration. HA had

to be for at least 5 days and HAD for at least 7 days forlongitudinal studies. HR, Tc, or SR had to be monitored in

human participants.

Study Appraisal The level of bias in each study wasassessed using the McMaster critical review form. Multiple

linear regression techniques were used to determine the

dependency of HAD in HR, Tc, and SR from the number ofHA and HAD days, daily HA exposure duration, and

intensity.

Results Twelve studies met the criteria and were system-atically reviewed. HAD was quantified as a percentage

change relative to HA (0% = HA, 100% = unacclimated

state). Adaptations in end-exercise HR decreased by 2.3%(P\0.001) for every day of HAD. For end-exercise Tc, the

daily decrease was 2.6% (P\0.01). The adaptations in Tcduring the HA period were more sustainable when the dailyheat exposure duration was increased and heat exposure

intensity decreased. The decay in SR was not related to the

number of decay days. However, protracted HA-regimensseem to induce longer-lasting adaptations in SR. High heat

exposure intensities during HA seem to evoke more sus-

tained adaptations in SR than lower heat stress. Only eightstudies investigated HRA. HRA was 8–12 times faster than

HAD at inducing adaptations in HR and Tc, but no dif-ferences could be established for SR.

Limitations The available studies lacked standardization in

the protocols for HA and HAD.Conclusions HAD and HRA differ considerably between

physiological systems. Five or more HA days are sufficient

to cause adaptations in HR and Tc; however, extending thedaily heat exposure duration enhances Tc adaptations. For

every decay day, * 2.5% of the adaptations in HR and Tcare lost. For SR, longer HA periods are related to betteradaptations. High heat exposure intensities seem beneficial

for adaptations in SR, but not in Tc. HRA induces adap-

tations in HR and Tc at a faster rate than HA. HRA maythus provide a practical and less disruptive means of

maintaining and optimizing HA prior to competition.

& Hein A. M. [email protected]

1 Department of Human Movement Sciences, Faculty ofBehavioural and Movement Sciences, Amsterdam MovementSciences, Vrije Universiteit Amsterdam, Van derBoechorststraat 7, 1081BT Amsterdam, The Netherlands

2 Athlete Health and Performance Research Centre, AspetarOrthopaedic and Sports Medicine Hospital, Doha, Qatar

3 Research Institute for Sport and Exercise, University ofCanberra, Canberra, ACT, Australia

123

Sports Med (2018) 48:409–430

https://doi.org/10.1007/s40279-017-0808-x

RE-ACCLIMATIZATION TO HEAT

•  OSHA:–  5daysafter2weeksaway

•  MSHA:–  4daysafter8daysaway;Beginningwith50%

•  NIOSH–  4daysbeginwith50%

RE-ACCLIMATIZATION TO HEAT

•  Research:–  4daysafter2weeksaway–  5daysafter4weeksaway

PROPOSED HEAT ACCLIMATIZATION TABLES

•  Heatstressisbasedon:– Environmentalconditions– Metabolicrate– Clothing

ACCLIMATIZATION GUIDELINES ACCORDING TO TEMPERATURE AND WORK RATE

RecommendationsforHeatAcclimatizationforWarmConditions LightWork(125-275W) ModerateWork(275-375W) Hard Work (375 - 475 W)

TimeSpent

workinginhotenvironment

HeatAcclimatization

Days

TimeSpentworkinginhotenvironment

HeatAcclimatization

DaysTimeSpentworkinginhotenvironment

HeatAcclimatization

DaysWBGT℉78-81.9 90-100% 2 - 3 70-100% 3 - 5 50-100% 682-84.9 80-100% 2 - 4 70-100% 3 - 5 50-100% 685-87.9 70-100% 3 - 5 60-100% 4 - 6 50-100% 688-89.9 60-100% 4 - 6 50-100% 6 50-100% 690+ 50-100% 6 50-100% 6 50-100% 6

RE-ACCLIMATIZATION GUIDELINES ACCORDING TO TEMPERATURE AND WORK RATE

RecommendationsforRe-AcclimatizationforWarmConditionsRoutineAbsence AbsenceDue

toIllness

GREEN

Day1 Day2 Day3 Day4 Day5< 4 -- 100 4-5 1-3 90 100 6-12 4-5 80 90 100 12-20 6-8 60 80 90 100

>20 >8 50 60 80 90 100

RoutineAbsence AbsenceDue

toIllness

YELLOW

Day1 Day2 Day3 Day4 Day5< 4 -- 90 100 4-5 1-3 80 90 100 6-12 4-5 70 80 90 100 12-20 6-8 60 70 80 90 100

>20 >8 50 60 70 80 90

RoutineAbsence AbsenceDue

toIllness

RED

Day1 Day2 Day3 Day4 Day5< 4 -- 80 90 100 4-5 1-3 60 80 90 100 6-12 4-5 50 60 80 90 10012-20 6-8 50 60 70 80 90

>20 >8 50 60 70 80 90

CONCLUSION

•  Establishingheatacclimatizationpoliciesiscrucialinensuringworkersafety.

•  Adaptationsofacclimatizationcanbeaccruedin5to6dayswithgreaterbenefitswithlongerexposures.

•  Acclimatizationislostwhenworkersareawayfromtheheat,andare-acclimatizationscheduleiswarranted.

•  Prudentacclimatizationandreacclimatizationguidelinesshouldtakeintoaccountenvironmentalconditionsandworkrate.

THANK YOU FOR YOUR TIME!

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ACCLIMATIZATION, DECAY AND RE-ACCLIMATIZATION · • Evidence of acclimatization = plateau in core temperature over last 30 minutes for 3 days • à 6 days for acclimatization 36