Manual Work Designendustri.eskisehir.edu.tr/ipoyraz/ENM 301/icerik/Manual_Work_Design.pdfergonomics...

END 202-ManualWorkDesign -

END202– Workanalysisand design ManualWork Design

Manualwork design

• The design of manual work was introduced by theGilbreths through motion study and the principlesof motion economy

• Theprinciplesarebrokendowninto3basicsubdivisions:– Theuseofthehuman body– Thearrangementandconditionsofthe workplace– Thedesignoftoolsand equipments

• Theprinciplesarebasedonanatomical,biomechanicalandphysiologicalprinciplesofthehumanbody.Theyformthescientificbasis forergonomicsandwork design.


Musculoskeletal system

Musculoskeletalsystemofthe arm

• Thehumanbodyisabletoproducemovementsbecauseofacomplexsystemofmusclesand bones

• Themusclesareattachedtothebonesbya joint• Agonistsactastheprimeactivatorsofthe motion• Antagonistscounteracttheagonistsandopposethe

motion



• Thereare3typesofmusclesinthehumanbody– Skeletal:attachedtothebones,approx.500inthebody

– Cardiacmuscles:found inthe heart

– Smoothmuscle:foundintheinternalorgansandthewallsoftheblood vessels



Skeletal muscles• Myofibrilsaresubdividedintomyofilaments

• Myofilaments:thick(myosine)andthinfilaments (actine)

• Filaments slide over oneanother to contract andextendmuscles– Completecontraction=50% oftheresting length

– Completeextension=180%oftheresting length


Principlesofworkdesign: Humancapabilitiesandmotion economy


Principlesofwork designForce-lengthrelationshipofskeletal muscle

Ataskrequiringanimportantmuscleforceshouldbeperformedattheoptimum position.


Principlesofwork design

Maximumforce state• restinglength:optimalbondingbetweenthick(myosin)

andthin(actin) filaments• stretchedstate:minimaloverlap,decreasedmuscle force• contractedstate:interferencebetweenfilaments,

decreasemuscle force



1. Achieve the maximummuscle strength at themidrange of motion

Midrangeof motion(relaxed posture)

Thepostureassumedbyanastronautinweightlessconditionswhenbothagonistandantagonistmusclesarerelaxed.

Typicalrelaxed posture



Force-velocityrelationship

2.Achievethemaximummusclestrengthwithslowmovements

• Thefasterthemolecularbondsareformed,broken,reformed,thelesseffectiveisthebondingandthelessmuscularforceis produced

• Slowmovementsare themost efficient



3. Use momentum to assist workers whereverpossible; minimize it if it is counteracted bymuscular effort

• Fastermovementsproducehighermomentumandhigherimpactforcesinthecaseof blows

• Downward motions are moreeffective than upwardmotionsbecauseoftheassistancefrom gravity



4.Designtaskstooptimizehumanstrength capability

• Humanstrengthcapabilitydepends on– thetypeof strength:• Dynamicstrength (isotonic)• Staticstrength (isometric)• Psychophysicalstrength(extended time)

– themuscleorjointmotionbeing utilized– posture

• Maximumacceptableloadis40to50percentlessthanaone-timestaticexertion

• Extensive tables for psychophysicalstrength of various frequencies andpostures (pg 134, Tables 4.2,4.3,4.4)


Staticstrength positions


5.Uselargemusclesfortasksrequiring strength

• Musclestrengthisdirectlyproportionaltothesizeofthemuscle

• Forexample,legandtrunkmusclesshouldbeusedinheavyloadlifting,rather thanweakarmmuscles

Staticstrength positions



6.Staybelow15%(evenbelow10%or5%)ofmaximumvoluntary force

• Thenon-linear relationship:– atamaximalcontractionaveryshort

endurancetime(6 sec)– indefiniteendurancetimeatapprox.

15%ofamaximal contraction

T = 1.2 /( f - 0.15)0,618-1.21T : Endurance time (min)f : Required force, expressed as a fraction

of maximum static strength

Ex:T=1.09minforf= 50%


Staticmuscleendurance-exertionlevel relationship


7.Useshort,frequent,intermittent,work/restcycles

• Afastinitialrecoveryperiod,whichthentendstoleveloffwithincreasing time

• Mostofthebenefitisgainedinarelativelyshort period

• Ahigher%ofmaxstrengthcan bemaintainedifthestrengthisexertedasaseriesof repetitivecontractionsratherthan onesustainedstatic contraction Percentageofmaxstaticstrengththatcanbemaintained

inasteadystateduringrhythmic contractions



8. Designtaskssothatmostworkerscando them• Individualfactorsaffectstrengthperformance:gender,age,

handedness,and fitness/training• Musclestrengthappearstopeakinthemid-20s,thendecreases

linearlybythe mid-60s.• Handedness:non-dominanthandproducesabout90%ofdominant

hand’s grip



9. Useoflowforceforprecisemovementsorfinemotorcontrol

– Whentheforceincrease,musclecontrol decreases

10. Donotattemptprecisemovementsorfinecontrolimmediatelyafterheavy work

– Ex:Assemblyoperationsafterliftingheavyparts.Usedifferentworkersfor lifting.

11. Beginandendmotionswithbothhands simultaneously– Workstationscanbedesignedtodo“twoata time”

12. Movethehandssymmetricallyandsimultaneouslytoandfromthecenterofthe body

– Deviationsfromsymmetryresultinslow,awkward movements



13.Usethenaturalrhythmsofthe body

• Optimumworktempoforsome tasks:– Filingmetal:60-78strokesper minute– Chiseling:60strokesper minute– Armcranking:35rpm(numberoffullrotationsover1 min)– Shoveling:14-17tossesper min



14.Usecontinuouscurvedmotions

• Straight-linemotionsinvolvingsuddenandsharpchangesindirectionrequiremoretimeandareless accurate

• Continuouscurvedmotionsdo notrequiredecelerationtomakeadirectionalchangeandperformedfasterperunitof distance

• Pivotaroundajoint (elbow)Forearmmotion is bestwhilepivotingon elbow



15.Usethelowestpracticalclassificationof movement

• Alwaysutilizethelowestpossible: finger→wrist→forearm→shoulder→ body

• Additionaltimeisrequiredforthecentralnervoussystemtoprocessadditionaljointsand muscles

16.Workwithbothhandsandfeet simultaneously

• Relievethehandsofworkthatcanbedonebythefeetifthisworkisperformedwhilethehandsare occupied Classificationsof movements



17.Minimizeeye fixations• Thelocationoftheprimaryvisualtargetsshouldbeoptimizedwith

respecttothe operator• Withintheareainthefigurenoheadmovementsareneededand

eyefatigueis minimized


Principlesofworkdesign– Summary

Humancapabilitiesandmotion economy1.2.3.4.5.6.7.

AchievethemaximummusclestrengthatthemidrangeofmotionAchievethemaximummusclestrengthwithslow movementsUsemomentumtoassistworkerswhereverpossible;minimizeitifitiscounteractedbymusculareffortDesigntaskstooptimizehumanstrength capabilityUselargemusclesfortasksrequiring strengthStaybelow15%(evenbelow10%or5%)ofmaximumvoluntaryforceUseshort,frequent,intermittent,work/rest cycles

8. Designtaskssothatmostworkerscando them9. Useoflowforceforprecisemovementsorfinemotor control10. Donotattemptprecisemovementsorfinecontrolimmediatelyafterheavy work11. Beginandendmotionswithbothhands simultaneously12. Movethehandssymmetricallyandsimultaneouslytoandfromthecenterofthe body13. Usethenaturalrhythmsofthe body14. Usecontinuouscurved motions15. Usethelowestpracticalclassificationof movement16. Workwithbothhandsandfeet simultaneously17. Minimizeeyefixations


Principlesofworkdesign– Summary

• Theprinciplesofhumancapabilitiesandmotioneconomyarebasedonanelementaryunderstandingofhuman physiology

• Theanalystneednotbeanexpertinhumananatomy

• Themotioneconomychecklistsummarizesmostoftheprinciplesinaquestionnaireformat(Figure4.16,pg146:Coursewebpage )


Motion Study


Motion study

• Motionstudyisthecarefulanalysisofbodymotionsemployedindoinga job

• Thepurposeistoeliminateorreduceineffectivemovementsandfacilitateandspeedeffectivemovements

• Gilbrethspioneeredthestudyofmanualmotion anddevelopedbasiclawsofmotion economy

• Gilbrethsconcludedthatallwork,productiveornon-productive,isdonebyusingcombinationsof17 basicmotionscalled therbligs:– Effectivetherbligs:directlyadvancetheworkprogress, theycanbeshortenedbutnotcompletely eliminated

– Ineffectivetherbligs:donotadvancetheworkprogress,shouldbe eliminated


Effective therbligs


Ineffective therbligs


Thetwo-handprocess chart

• Thetwo-handprocesschart(operatorprocesschart)showsallmovementsanddelaysmadebytherightandlefthands,andtherelationshipsbetweenthem

• Analystcandeterminewhatimprovementscanbe introduced

• Therbliganalysischecklist(Figure4.18,pg151:Coursewebpage)


Manualworkanddesign guidelines


Manualworkanddesign guidelines

• Energyexpenditureandworkload guidelines• Heartrate guidelines• NIOSHlifting guidelines• Multitasklifting guidelines• Generalguidelines:Manual lifting


Energyexpenditureandworkload guidelines

• Energyisrequiredformuscle contraction• ATP(adenosinetriphosphate)molecule:immediate

energysourcebutverylimited,lastingonlyfew seconds• ATPmustbereplenishedfromCP(creatinephosphate)

molecule,lastingforlessthan1 min• CPisregeneratedfromthebasicfoods:carbohydrates,

fats, proteins• TwodifferentmodesforCP generation:

– Aerobic(requiresoxygen):muchmoreefficient,generates38ATPsforeachglucosemolecule,butitis slow

– Anaerobic(withoutoxygen):veryinefficient,generatesonly2ATPsforeachglucosemolecule,butmuch quicker.(Glucosemoleculeisonlypartiallybrokendowninto2lactatemolecules,inwateryenvironmentofthebodytheyformlacticacidwhichcauses fatigue)



Sourcesofenergyduringthefirstfewminutesofmoderatelyheavy work



• Bywarmingupandstartingheavyworkslowly,theworkercanminimizetheamountofanaerobicmetabolismandthebuildupoflacticacidassociatedwithfeelingsof fatigue.

• Thedelayoffullaerobicmetabolismistermedoxygendeficit

• Theenergyexpendedonataskcanbeestimatedbyassumingthattheenergyisproducedthroughaerobicmetabolismandmeasuringtheamountofoxygenconsumedbythe worker



• Theamountofinspiredairisassumedtocontain21%oxygen

• Typically4.9kcalofenergyisproducedforeachliterofoxygenusedin metabolism

• Energy expenditure:E(kcal/min)=4.9xoxygen consumption

=4.9*V*(0.21– EO2)– V=volumeofairinspired,L/min(canbemeasuredbya flowmeter)– EO2=fractionofoxygeninexpiredair(canbemeasuredbyan

oxygenmeter)



• Theenergyexpendedonataskvariesbythetypeoftask,thepostureandtheload carriage.

• Dataonseveralhundreddifferenttypesoftaskshavebeen collected

• Formanualmaterialhandling,themannerinwhichtheloadiscarriedismost critical

• Balancedloadscarriedclosesttothecenterofgravityofthebodydemandlowest energy– Ex:backpacksupportedbythetruckmusclesis easier than

twosuitcasesofequalweightineach arm• Posturealsoplaysanimportantrole,withlessenergy

forsupported postures



• 5.33kcal/ministhelimitofacceptableenergyexpenditureforan8hworkdayfora man,– Thisnumbercorrespondsto1/3themaximumenergyexpenditureoftheaverageU.S. Male

– 16 kcal/min x1/3=5.33 kcal/min– Forfemales: 12 kcal/min x1/3=4 kcal/min



Examplesof energycostsofvarioustypesofhumanactivity (kcal/min)



• Iftheoverallworkloadisexceeded(morethan5.33kcal/minformalesand4kcal/minforfemales)restisrequiredtoallowthebodytorecoverfromfatigueandrecyclethelactic acid

• Guidelineforrestallocation:

R=(W– 5.33)/(W– 1.33)– R:timerequiredforrest,aspercentoftotal time– W:averageenergyexpenditureduringwork, kcal/min– 1.33kcal/min:energyexpenditureduring rest

• Example:ShovelingcoalW=9.33 kcal/min R= 0.5Foran8hofworkspend4hfor resting



• Thedurationoftheworkcycleisimportantforanoptimal recovery• Forheavyworks,withoutresting,therecoveryprocesstendstobe

exponential• Shortburstsofheavyworks(1/2to1min)followedbyshortrest

periodsprovidemaximum benefit• Micropausesof1to3sarealso useful• Activebreaks,duringwhichtheworkeralternateshandsoruses

othermuscles,relievesfatiguedmuscles• Itisbestforworkerstodecidewhentotakebreaks,wheneverthey

feeltheneedforrestasopposedtoprescribed breaks• Theuseoffrequentshortwork/restcyclesishighly recommended


Heartrate guidelines

• Themeasurementofoxygenconsumptioncanbecumbersomeandexpensive(equipmentcostsseveralthousanddollarsandinterfereswiththe worker)

• Analternativewaytomeasuretheenergyexpenditureistheheartrate level

• Thehighertheenergyexpenditurethehighertheheart rate• Lessexpensive(100$foravisualreadout,several hundred

dollarsforaPC interface)



Theanalystmustbe careful:• Thismeasurementismostappropriatefordynamic work

• Canvaryconsiderablybetweenindividuals,dependingontheirfitnesslevelsand age

• Canbeconfoundedbyotherstressorsincludingheat,humidity,emotionallevels,andmentalstress



• Averageworkingheartrate=40beats/min+restingaverageheartrate(≈72 beats/min)

• averageincreaseinheartrate/increaseinenergyexpenditure(slope)=10beats/minper1 kcal/min

• A5.33kcal/minworkload(4kcal/minabovetherestinglevel of1.33kcal/min)producesa40beats/minincreaseinheart rate



• Heartratebetween1/2to 1minaftercessation: HR1

• Heartratebetween2.5to3minaftercessation:HR2

• Acceptableheartraterecovery:– HR1≤110 beats/min– HR2– HR1≥20 beats/min

• Theincreaseintheheart rateduringsteady-statework,calledheartratecreep,indicatesanincreasingbuildupoffatigueandinsufficientrecoveryduringrestpausesandmustbe avoided

Averageheartratemeasurementfortwodifferent workloads



Subjectiveratingsofperceived exertion

• ThescaledevelopedbyBorg (1967)

• Ratingsthrough6to 20correspondtotheheartratesdividedby 10

• Theratingsshouldbeusedwithcaution andnormalizedtoeachindividual’smaximumrating

BorgRatingofPerceivedExertion (RPE)


Lowbackcompressive forces

• 40%ofworkaccidentsarecausedduringmaterialhandling

• 70%ofmanualmaterialhandlingaccidentsconcernlow back

• Lowbackaccidentsandillnessesconsists25%ofthetotal compensations

• Theaveragecostoflowbackaccidentsis 60,000$



• Vertebraeisdividedinto:cervical,thoracic,lumbarandsacrum regions

• Spinalcordisprotectedbyvertebrae,spinalnerverootsareseparatedfromthespinalcordtoachieveinternalorgansand extremities

• Vertebralbonesareseparatedbyasoftertissue:intervertebral disks.

• Theyserveasjoints,allowingalargerangeofmotion,andcushionsbetween vertebral

• Mosttrunkflexionoccursinthetwolowestjoints:L5/S1ve L4/L5

Anatomyofthehuman spine



Lowback problems• Aging,heavymanualworkexposure:thediskscan

weaken• Cartilageendplatecansuffermicrofractures,releasing

somegelatinousmaterial,thecenterstartsdrying up• Thediskspacenarrows,vertebralbonescomecloser

andtouch:irritationandpain,motor impairments• Diskherniation(slipped disk)• Softtissueinjuries(ligaments,muscles, tendons)



(a) Normal state(b) Narrowingofthediskspace,allowing

thenerveroottobe pinched(c) Herniateddisk,allowingthegel

materialtoextrudeandimpingeuponthenerve root

Anatomyofavertebraandtheprocessofdisk regeneration



Somecausesforlow-back problems• Heavy work

– Frequentliftingoflarge loads– Forward-bendingtrunkposturesforlong periods

• Longperiodsofimmobility,eveninsitting postures• Wholebody vibration• Geneticpredisposition(weakerconnectivetissues,disks,

ligaments, etc.)• Personallifestyleconditions(smoking,obesity, etc.)



AnalogybetweenL5/S1diskandfirst-class lever

• Centerofthediskactsasthe fulcrum• FindthemuscleforceFMandthetotalcompressiveforce Fcomp

2xFM=30x50=>FM=750lb(341kg)Fcomp=750+50=800lb (364kg)

• Acompressiveforceof770lb(350kg)isconsideredthedangerthreshold



Moreaccuratevaluesforvariousloadsandhorizontal distances


NIOSHlifting guidelines

• NIOSH:NationalInstituteforOccupationalSafetyand Health• Keyoutputistherecommendedweightlimit (RWL)• RWLisbasedontheoptimumweight,withadjustmentsfor

variousfactorsrelatedtotask variables• RWListheloadthatcanbehandledbymost workers

– 350kgcompressionforcecreatedbyRWLcanbetoleratedbymostyoung,healthy workers

– Over75%ofwomenand99%ofmenhavethestrengthcapabilitytoliftaloaddescribedby RWL

– Maximumresultingenergyexpendituresof4.7kcal/minwillnotexceedrecommended limits

• IfLoad>RWL,thentheinjuriesandlowbackproblemsincrease



• TheformulationforRWLisbasedonamaximumloadthatcanbehandledinanoptimum posture

RWL=LCxHMxVMxDMxAMxFMx CM– LC=LoadConstant=51 lbs– HM=HorizontalMultiplier= 10/H

H:Horizontallocationoftheloadforwardofthemidpointbetweentheankles,10≤H≤25in

– VM=VerticalMultiplier=1– 0.0075|V-30|V:Verticallocationoftheload,0≤V≤70 in

– DM=DistanceMultiplier=0.82+ 1.8/DD:Verticaltraveldistancebetweenoriginanddestinationoflift,10≤D≤70 in

– AM=AsymmetryMultiplier=1– 0.0032*AA:Angleofsymmetrybetweenthehandsandfeet,0≤A≤135 derece

– FM=FrequecnyMultiplier=Table4.7– CM=CouplingMultiplier=Table 4.8

• Thesemultipliersrangefromaminimumvalueof0forextremeposturestoamaximumvalueof1foranoptimalpostureor conditions



END202– Workanalyisand design ManualWork Design

NIOSHlifting guidelinesCouplingmultiplier

Coupling Type

V < 30 in (75 cm)

V ≥ 30 in (75 cm)

Good 1.00 1.00Fair 0.95 1.00Poor 0.90 0.90

• OptimalContainer:Boxesandcrateswithwell-definedhandlesorhand-holdcutouts

• Optimalhandle:cylindrical,withasmooth,nonslip surface

• Forlooseobjects,agoodcouplingwouldconsistofacomfortablegripinwhichthehandcancomfortablywraparoundtheobjectwithoutanylargewrist deviations



LiftingIndex (LI):LI=loadweight/ RWL

• Toprovideasimpleestimateofthehazardlevelofliftingagiven load– Valuesexceeding1.0deemedtobe hazardous

• Usefulinprioritizingjobsforergonomic design• Controlthehazardbyredesigningjobsand workplaces

– Avoidhighandlow locations– Useliftandtilttables– Usehandlesorspecializedcontainersforhandling loads– Reducethehorizontal distance



Example:Liftingaboxintothetrunkofa car

• 30lbboxfromgroundintothe trunk• Twist90° topickup→A= 90• Groundlevel→Vorigin= 0• Trunk→Vdest=25,D= 25• Horigin=10andHdest= 25• One-timelift,FM= 1• Boxisfairlysmallandcompactbuthas no

handles→CM= 0.95



Example:Liftingaboxintothetrunkofa carRWL=51(10/H)(1-0.0075|V-30|)(0.82+1.8/D)(1-0.0032A)xFMxCM

RWLorigin=51(10/10)(1-0.0075|0-30|)(0.82+1.8/25) (1-0.0032*90)(1)(0.95)

=23.8 lbRWLdest=51(10/25)(1-0.0075|25-30|)(0.82+1.8/25)(1-0.0032*0) (1)(0.95)

=16.6 lbLI=loadweight/RWL→30/16.6= 1.8

• Only16.6lbcouldbeliftedsafelyandthe30lbboxwouldcreateahazardalmosttwicetheacceptable level

• Decreasethehorizontaldistanceto10in→RWLdest=41.5 lb• Movethefeetandeliminatethetwist→RWLorigin=33.4 lb


Multitasklifting guidelines

• Usedforjobswithavarietyoflifting tasks• Overallphysical/metabolicloadisincreasedcomparedtothe

single lifting• ThisisreflectedinadecreasedRWLandanincreased LI• Compositeliftingindex(CLI)representsthecollective

demandsofthe job• CLIequalsthelargestsingle-taskliftingindex(STLI)and

increasesincrementallyforeachsubsequent task• CalculationsforCLIandSTLIarepresentinthebook(pg 166)• Whenthenumberoftasksexceedsthreeorfouravarietyof

softwareprogramscanbeused(Design Tools)


Safelifting procedure

a. Planthe lift

b. Determinethebestlifting technique

c. Getasecure grip

d. Pulltheloadinclosetoyour body

e. Alternateliftingandlightwork tasks


Generalpostureandtaskevaluation checklist


Manualworkdesign– key points

• Designworkaccordingtohumancapabilitiesandlimitations

• Formanipulative tasks– Usedynamicmotionsratherthanstatic ones– Keepthestrengthrequirementbelow15%of maximum– Avoidextremerangesof motion– Usethesmallestmusclesforspeedand precision– Usethelargestmusclesfor strength

• Forliftingandotherheavymanual work– Keepworkloadsbelowone-thirdofthemaximumwork

capacity– Minimizehorizontalload distances– Avoid twisting– Usefrequent,shortwork/rest cycles


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