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Test 2Lectures 9-15Lecture 9PhysiologyFueling and cooling system: blood vesselsFood = FuelCombustion of fuel takes place with oxygenCombustion yields energy for mechanical movementBlood vessels move supplies (oxygen, carbs, fat derivatives) to combustion sites (muscles and organs)Blood vessels move byproducts out (lactic acid, CO2, water, heat) for dissipation by skin and lungsRespiratory System: provides oxygen for metabolism and dissipates byproductsMoves air to and from lungs (some absorbed into blood stream)Removes CO2, water, heat from blood into air for exhaleAir inhaled and exhaled by pumping thorax via muscles connecting ribsThe volume of air exchanged in the lungs depends on the requirements associated with the work being performedVital capacity: maximal inspirationIs a function of age, training, body size, position, genderFemales have 10% less than malesUntrained people only have 60-80% the capacity of athletes Residual capacity: amount of air left when forcing an expirationVital + Residual = total capacityPulmonary ventilation: movement of gas in/out of lungsAt rest we breath 10-20 times/minuteLight exercise: tidal volume increased, not frequencyHard exercise: tidal volume and frequency increasesCirculatory System: carries oxygen from lungs to cells that use and removes metabolic byproducts (CO2, heat, water) (works in a closed loop)Body’s transportation system for blood and cardiovascular system Takes away by-products: CO2, water, heatBloodRed blood cells take oxygen to tissue and remove CO2White blood cells fight germs and infectionPlasma is solution in which blood cells are heldPlatelets stop bleeding Atrium and ventricle: in left and right halves of heartBlood flows from atrium to ventricleChambers DO NOT send blood back and forth 2 Circuits of blood circulationSystemic System (left to right): Blood with oxygen and nutrients passes through the left atrium, pumped out of the left ventricle, flows through arteries/capillaries to muscles/other organs, come back to heart through veinsPulmonary System (right to left): blood with CO2 comes in from the right atrium, is pumped out of right ventricle through two arteries (one for each lung) so CO2 is exhaled, and blood re-oxygenated Returns to left atriumCapillaries: transport blood into muscles, if muscles need more blood, blood vessels dilate, if other organs need blood capillaries are constrictedBlood supply: Quickly distributed as necessary for proper muscle and organ function through blood vessel dilation and constrictionMuscles over digestion, skin over musclesHeart response to workIncrease stroke volume – effective for moderate workIncrease heart rate – once stroke volume is maxed, increasing heart rate is main way to increase cardiac outputAt rest about ? of the volume in the ventricle is ejected, other ? stays in the heartAt rest, output of adult is about 5L/minDuring heavy work, can raise to 25L/minMetabolic System: supports chemical processes in the body that yield energyEnergy inputs are nutrients (food and drink)Energy released through metabolism, process of yielding energyEnergy measured in joules (J) or calories (cal4.2J = 1cal1J = 1NmBasal metabolism: energy to functionResting metabolism: without workWork metabolism: due to additional energy needed to workPhysical WorkOnly possible when there is energy to support the muscular effort neededEnergy required for muscular effort comes from nutrients being metabolized in presence of oxygen (aerobic metabolism) or without oxygen (anaerobic metabolism)Workload: Ratio of capacity to demandsPhysiological, physical, mentalWorkload is the energy required by the body to do workEnergy expenditure rate of work is linearly related to oxygen consumption and heart rateThree ways to measure workloadOxygen consumptionLinear relationship between oxygen consumption and energy expenditureFor every liter of oxygen consumed a person releases on average about 4.8kcal of energyHeart rate# of heart beats per minuteHighly correlated with oxygen consumptionEasier to measure than oxygen consumption, but not as reliableOften measured using portable telemetry device while worker wears a set of electrodes on chestBorg Ratings of Perceived Exertion (RPE)Scale from 6-206 being no exertion at all, 20 being maximum exertionNOTE: All measures could be affected by heat, humidity, psychological stress, cognitively complex work, noiseWhole-Body CapacityMaximum physical work capacity (MPWC)Maximum rate of energy productionShort-Term Work Capacity (aerobic capacity or VO2max)Maximum expenditure for a few minutesVaries as a function of durationWhole body fatigue is experienced with 8-hour energy requirements exceed 33% maximum (30-50% of aerobic capacity)Occurs because body cannot consume enough oxygen to keep up with energy requirementsThe potential for fatigue is based onEffort levelEffort timeRecovery timeIndicators of fatigueSlight tirednessWearinessExhausted (difficulty staying awake)Worn outReduced muscular performanceEngineering controls to reduce fatigueRedesign jobProvide job aidsDesign for the least fitPhysical loads that are within strength and endurance capacities for 5% femaleProvide accommodationsConveyor belts to reduce carryingAutomated material-handling devices to reduce liftingRest break as a fraction of total work time = (PWC-Ejob)/(Erest – Ejob)PWC – physical work capacity of the workersEjob – energy requirement for jobErest – 1.5 kcal/minResulting fraction is multiplied by 8hrs to determine total break time in a shiftPreferably divide total break time over course of 8hr shiftLecture 10Simplified AnatomyBones: internal frameworkMuscles: generate force and movementLigaments: connect bones Tendons: connect muscles to boneJoints: degrees of freedom to move, synovial fluid, types: ball and socket and pivotCardiac: your heart, only self-exciting muscle in the body, don’t have to tell it to beatSmooth: digestive muscles, not under voluntary control, anything close to gut Skeletal: under voluntary control, tell them when/how to moveMusclesEach muscle is made up of thousands of muscle fibers surrounded by connective tissue, blood vessels and nerve fibersGenerates force and produces movementContraction of muscles allows bones to act like leversMuscles can also perform eccentric contractions when they are lengthened involuntarily (when you hold something too heavy and your muscles start to give away)Muscle Fibers:Each fiber is made up of parallel cylindrical myofibrils (each fiber is like needles and threads)Myofibrils are made up of sacromeres (cross sections) which form a repeating pattern along the length of the myofibrilSarcomeres are the contractile units of skeletal muscle. You can tell if your muscles are contracting or expanding by seeing if the sacromeres are getting smaller or largerMuscular Effort - Dynamic (moving)Rhythmic alternation between contraction and relaxation of musclesMuscle acts as a pump to the blood circulationContinuously supplied with sugar and oxygen, and waste products are removedMuscular Effort - Static (needs a lot of blood but doesn’t get a lot)Prolonged state of contraction (postural stance)Muscles are tense, not relaxedBlood vessels compressed and blood no longer flowsMuscle does not receive sugar or oxygen, waste products not removedMuscle Fatigue: Influenced by strength and contraction time – not enough recovery timeLeads to oxygen deficit, lactic acid build up, waste build up/no nourishmentControlling muscle fatigueAllow for strongest postureMinimize static muscle loadingProvide handles for carried objects (reduce moment arm)Rest breaks Job rotationHow the back worksSpine is the major support structure, reaching from neck to hip areaSpine: series of separate bones (vertebrae) 24 total, 7 cervical, 12 thoracic, 5 lumbar, and sacrum and tailbone Discs: separate vertebraeHelp to cushion spinal loadingCompression, flexion, lateral bending, torsion Spine houses and protects spinal cord The muscles of the back and of the stomach support the back in different postures and create motionAll movements of the back require muscular effortNatural position of spine is S-shapedTypes of curves in spineCervical Lordosis: inward curve at the neck Thoracic Kyphosis: outward curve at the mid backLumbar Lordosis: inward curve at lower backStanding: lordotic (inward)Sitting: kyphotic (outward) Advantages: rest your legs, don’t need as much blood flow (muscles recover)Disadvantages: curve of spine, disrupts organsThe neck contains top 7 vertebrae (cervical) Each vertebrae move a little over 10 degrees to move side to sideYour neck can move 90 degrees in each wayThe position and movements of the neck are controlled by a complex of about three dozen muscles which attach from the skull and cervical vertebrae to bones all around the base of the neckWorking of the arms and handsUpper arm controlled by shoulders and forearmHolding arms without support requires static effort gives way to fatigueForearm controls finger and hand movement (these muscles connect via tendons from elbow to wrist)Abduction: move body part away from midlineAdduction: move body part towards midlineFlexion: bend towards body, make smaller angleExtension: away from body, larger angleMedial: inward toward midline of bodyLateral: outwards away from midline of bodySupination: face upPronation: face downTorque = moment arm (meters) X load (newtons)Reduce either moment arm or load to decrease torque B is worseLecture 11 & 12Anthropometry – the science of measurement of body sizeAnthrop(s) – humanMetricos – of or pertaining to measurmentVariability in dimensions comes fromAgeGenderRace and ethnicityGenerations Where to find the dataCivilian and military databases, other booksUS military: problem: gender and race/ethnicity Types of data:Structural: measures while body is static, not movingFunctional: measures while body is doing some work (reach)Some body dimensions are highly correlated, stature and leg length Determine the user, body dimensions, percentile range and population (let the small person reach and large person fit)Design Principles:Custom fit individual (near impossible)Several fixed sizesMake it adjustable (fit 5% Female to 95% male)Design for extremes (escape hatch -100% F, brake pedal width 50% length 5%F, control knob 5%F, stretcher 99% M)Select persons whose bodies fit the last design (last resort)Principle of adjustability to design or redesignWhen you need to accommodate a range of users, and fit depends on adjustmentPrinciple of adjustability to accommodateAdjust worker position relative to workplace (height adjustable chair to make reaching easier, platforms)Adjust work piece (life tables, jigs, pallet jacks)Adjust the tool (adjustable length handles)Principles for designing for extremesWhen a minimum or maximum allows everyone else to be accommodated (doorway clearance – 95% male stature)AverageThis is done to save money (the idea is that most people won’t be too uncomfortable)Americans with Disabilities Act of 1990: requires design accommodation’s for less able, accommodate wheel chairsLecture 13Manual Material Handling (MMH)Most significant cause of back problemsBack ProblemsIncreased muscle fatigue to hold postures for long timeFlattening of the lower back when we sit downDiscs may degenerate and lose their strength (flattened)Allows nerves to be strained and pinchedUnnatural postures speed up deterioration of discs MMH TasksBiomechanicalCalculate force on body joints (especially low back L5/S1)Design for 5% strength capacity of compressive forcesGood for occasional liftsPhysiologicalCalculate energy consumptionDesign for least fit person – 33% VO2maxGood for MMH over shiftWe can measure energy consumption in three waysOxygen consumptionBorg scaleHeart ratePsychophysicalSubjective evaluation of strengthMaximum load with out strain, discomfort, fatigue = Maximum acceptable weight of load (MAWL) Design for %5 female MAWLGood for occasional and over shiftMAWL TablesMaximum acceptable weight load increases with horizontal distance, increases with lift distance, increases in walking distance, and varies depending on lift locations and frequenciesIn these tables they are using horizontal distance (box from body), distance of lift, and frequency (one time every eight hours, etc.)Lifting – most dangerous MMH activityCorrect way to lift is straight-back, bent kneeIf load can’t fit between your knees, stoop or freestyle is better. Stoop requires less energy than straight back lift because less musclesObject Characteristics for liftingSize – influences forcesShape – bags have higher MAWL because they are carried closer to body and require less forceStability – center of gravity outside centerline is BADHandlesNIOSH lifting equation: calculate RWL (recommended weight limit)National Institute for Occupational Safety and Health (NIOSH)Used for lifting tasks: two hands without mechanical assistanceDON’T USE: one hand, over 8-hours, wheel barrow/person (assistance), carrying/pushing/pulling, seated/kneelingRWL: weight of load could be performed during 8hour shift with low risk of back painRWL= LC x HM x VM x DM x AM x FM x CMLC – Load ConstantMetric: 23kg. US: 51lb. HM – Horizontal MultiplierDistance of the hands away from the midpoint of anklesVM – Vertical MultiplierDistance of the hands above floorDM – Distance MultiplierDistance between object’s starting point and ending pointAM – Asymmetry MultiplierTwisting of the body, looking straight ahead is zero and twisting up to 90degFM – Frequency MultiplierHow often the object is being liftedCM – Coupling MultiplierThe quality of the hand to object coupoling – handles are bestLifting index (LI) = Load Weight (L)/ RWL (goal <1)LI provides a relative estimate of the level of physical stress associated with a particular manual lifting taskIf LI is greater than 1, redesign because we don’t know who is going to be doing the task. This represents increased risk of low-back pain and injury for some workers. Wickens calls this the Action Limit (AL). If LI is over 3, you must redesign. It represents high risk of low-back pain for most workers. Wickens call this the maximum permissible limit (MPL)Healthy workers refers to workers who are free of adverse health conditions that would increase their risk of musculoskeletal injuryEngineering ControlsReduce weight of objectsAllow pushing or pulling instead of carrying Lift/carry aidsImprove NIOSH multipliers Decrease distance of box from bodyControlling MMH ProblemsWork/rest schedulesJob rotationWorker selectionTraining – weak but criticalBack belts do not workLecture 14Neck StrainEven in the upright position, there is a constant tension on the muscles of the neck to keep the head balanced verticallyPsychological stress contributes to neck muscle tensionLoads to the shoulder are transmitted to the neck musclesCauses of neck strainPlacement of work materialsArm postureVisual problemsRules for avoiding neck strainAvoid high frequencies of extreme twists or tits of the neck and headAvoid long period of fixed postures of the head and neckPeriodically support the arms or wristsPosture of the neck should be so that the gaze angle is in the range of 5-20 degreesShoulder strainCauses of shoulder strainJobs involving repeated or prolonged abduction of the armsSolutionsKeep the upper arms close to the bodyIf repeated or prolonged extensions, flexions or abductions of the arms are required, armrests or palm rests may relieve some of the stressElbow strainUnhealthy posture: elbow bent, like in typing postureTension is around the ulnar nerve that wraps around the elbowThere is little protection of the ulnar nerve from external forces (such as resting on a hard surface)Lateral Epicondylitis: Elbow tendon disorder where there is radiating pain from elbow down to forearm caused by tendon strain (tennis elbow). Throwing motions. Wrist strainUnhealthy posturesHands bent backwardsHands bent outwardsCompression of the contents of the carpal tunnel (tendons, nerves and blood vessels) may damage the median nerveWrist tendon injuriesTendon tear = strainSome tendons are surrounded by a sheathSheath has lubricant called synovial fluidWith overuse synovial fluid may be reduced, causing friction between tendon and sheath (inflammation)Carpal Tunnel Syndrome9 tendons and the median nerve are running through the carpal tunnelUnnatural postures combined with high frequency and/or high forces can lead to tendon swelling and compression of median nerveRisk factors associated with hands/wrists problemsPoor postures that constrict the carpal tunnel pathwaysHigh forcePressure resulting from repeated contact on hard surfaces or sharp edgesHigh repetitionVibration Syndrome: (aka white finder/Raynaud’s syndrome)Finger blanching due to close of finger arteriesFrom forceful gripping of vibrating toolsVibrationTwo major types of vibration exposureWhole body vibrationHand-arm vibration (HAVS)Controlling hand-arm vibration exposureSelect tools with lowest vibration Maintain toolsDamping glovesMinimize grip forceAlternate tasks: vibrating -> non vibratingLimit daily use of vibrating toolsLong rest breaksLimit number of days tools are usedAdd springsIncrease stiffnessReduce velocity Controls of arm/hand disordersSupport or rest the arms and handsAvoid highly deviant arm and hand posturesNo hard or cutting edges of chair, table and work surfacesVaried tasks in terms of motion patternsRest breaksWork Systems and Hand ToolsUlnar Nerve (blue) out side of hand, pinky/half ring fingerMedian Nerve (orange) thumb, fingers/finger tips, palmDorsiflection: top of hand moves back towards bodyPalmar Flexion: palm moves downHand toolsMost common hand tool related injuries are associated with knives, wrenches and hammersJust holding a tool creates static loads in forearm, upper arm, neck, shoulder, backPosture is determined byTool designSeated or standing work height of personWork surface orientationWork surface heightPrinciples of hand tool designNeutral posture (straight wrists)Bend tool, NOT wristUse power grips for tasks requiring forceUse pinch grips for tasks requiring precisionProvide adequate grip span (men and women generate the most force with a 7-8 cm opening for open handle tools)Provide finger and glove clearanceEliminate pinch pointsAvoid tissue compressionAvoid repetitive finger action (trigger finger: moving fingers difficult and gets locked in one position)Safe operationRemember lefties and variable hand sizesMinimize/ eliminate vibrationLecture 15Work Station DesignANSI/HFES 100-2007 Human Factors Engineering of Computer WorkstationsISO Series 9241New title: “The Ergonomics of Human-system Interaction”Ergonomic Design: Product/equipment, job aid, user selection (less desirable), trainingStanding: no heel cause forward leaning and increase torque on spineHips parallel to floor, equal weight on both feetShoes should be half to full size bigger for standing workPurchase shoes after work (feet swell during day)Keep weight off heelsFloorsHard floors cause fatigue, metal gratings worstCarpet is best, reduce standing heart rate, compressibility 3-5%Slips and FallsSlips in heal strike (fall back) and push offs (fall forward)Trips when foot swingStep on air when floor level change goes unnoticed Video Display Terminal (VDT) Distance from eye to screen adjustable (15-32 in)Screen tilt-able (5-15) to eliminate reflectionsCenter of screen 15-25 below eye levelKeyboard angle 5-15 (7-11 preferred)Keys should have feedbackStandardization on keyboardChair DesignAdjustableWeight supported by two bones (ischial tuberosities)25% weight supported by feetNot contouredUpholstery should give about 25mm (not too soft)Curved front edgeSlope backwards 1-5 deg (forward slant gives knee problems)Seat DimensionsSeat HeightMeasure from work height not from floorWork slightly below elbow heightUse footrests for shorter workers, if necessaryBackrestsMust be adjustable and should be spring loadedConcave shape to provide lumbar supportArmrestsThey take weight off your backWrist supports for typing workLegs/PedestalSeat should swivel except when operating pedalSwivel permits micro adjustment to prevent fatigueMust have 5 horizontal supports to prevent tippingWheel base about 300 mm in diameter (maximum 350 mm) ................
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