7. ergonomics and human factors

7.Ergonomics and Human Factors

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Figure7-1.7-2.

7-3.7-4*7-5.7-6.7-7.7-8,7-9.

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139 ‘

P a g e .

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LIST OF FIGURESI No.Schematic Representation of a Man-Machine System . . . . . . . . . . . . . . .Control Burner Arrangements of Simulated Stove Used in ExperimentAbout Logical Arrangements . . . . . . . . . . . . . . . . . . . . ., . . . . . . . . . . . . .Major Nerves in the Arm and Hand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .The Carpal Tunnel . . . . . . . . . . . .

● ● ● + ● ● . . ● ● ● ● . . . . , . ● . ● . . . ● ● . ● . . . ,, .Flexion and Extension of the Wrist . . . . . . . . . . . . . . . . . . . . . ● .Job Analysis: Assembly Tasks . . . . . . . . . . . . . . . . . . . . . . .

● . . ● ● ● ,1 ●

. * . . . . . . . . . .Good and Bad Designs for Containers and Workbench- . . . . . . . . . . .Good and Bad Designs for Powered Drivers. . . . . . . . . . . . . . . . . . . . . . .A Knife Designed to Reduce Cumulative Trauma Disorders inPoultry Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Potential Ergonomic Risk Factors Associated with VDT Design ... , . .

Page1 . . . 124

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.** 129● . . 130. . . 130● . . 132. . . 133

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7Ergonomics and Human Factors

Neither workers, nor machines, nor workplacesexist apart from each other. To accomplish work,all come together, and injuries and illnesses some-times result from their interactions. The study ofthese interactions is called “ergonomics,” or “humanfactors engineering. ”

The term ergonomics was coined in the UnitedKingdom after World War II to describe a dis-cipline created during the war. It had been notedthat “bombs and bullets often missed their mark,planes crashed, friendly ships fired upon andsunk, and whales were depth charged” (471). Inresponse to this situation, research on designingmilitary equipment to match more closely the ca-pacities of the users began on both sides of theAtlantic. Attention was thus devoted to the inter-action between the human operators and their ma-chines.

One name for this new discipline, ergonomics,is derived from two Greek words, ergo, meaningwork, and nomos, meaning laws. Ergonomics isthe science devoted to understanding the laws orprinciples that govern the design of work systems.A British professional group—the Ergonomics Re-search Society —was formed by the practitionersof this new discipline. An organization with simi-lar aims, the Human Factors Society, was foundedin the United States in 1957. The work of theoriginal members was termed “human factorsengineering, ” or engineering psychology. Whetherthey are called human factors engineers or ergo-nomists, the scientists who practice this disciplinedraw on a number of other disciplines, includingmedicine, physiology, psychology, sociology,engineering, and physics.

Ergonomists are concerned with safety, effec-tiveness, and efficiency wherever people are partof a system (380). The discipline is “an appliedscience concerned with the design of facilities,equipment, tools, and tasks that are compatiblewith the anatomical, physiological, biomechani-cal, perceptual, and. behavioral characteristics of

humans” (250). The principle behind ergonomicdesign is that the machine should fit the worker,rather than forcing the worker to fit the machine.To quote from one ergonomics guide (655):

Man’s physical limits for bending, stretching,and/or compressing are such that the machinemust be made to adapt to the man rather than theconverse. Behavior characteristics are somewhatmore flexible. Man can adjust his sensory-motorbehavior to some degree, and . . . can utilizealternate procedures and make up for certainequipment inadequacies. However . . . as oper-ator load increases due to task complexity, fatiguemay reduce operator reliability; system perform-ance could degrade at a critical time . . . Work-place layout should favor the man’s physical andbehavioral capability in all cases in which a likelyerror in human performance could affect . . .safety . . , The designer cannot assume that per-sonnel selection and training will be a panacea forimproper workplace considerations.

Figure 7-1 is a schematic representation of ahuman-machine system. The person processes in-formation about the environment and acts on itby using the machine’s controls. The importantfeatures of the machine include the controls usedby the worker, the operations of the machine, andthe displays used to feed information back to theworker. The task of the ergonomist is to analyzehow the worker obtains the information neededto operate the machine, how that information isprocessed to reach a decision concerning theappropriate way to control the machine, and whatworker actions are appropriate to control the ma-chine in a fashion that is safe and meets produc-tion criteria (380).

Other considerations, especially improving theproductivity of workers, fit easily into the goalsof ergonomics. Opportunities exist for workplacedesigns that simultaneously improve productionoutput and reduce the risks of injury. For instance,C. G. Drury reported that new handtools in thecomponent-assembly department of a large com-

123

124 . Preventing Illness and Injury in the Workplace

puter company and changes in seating, lighting, and savings, and the number of rejected compo-and workbenches led to improved production and nents fell to half the previous level. The workersthe elimination of repetitive motion injuries. The involved expressed increased satisfaction withcosts of the redesign” were earned back 4.3 times their jobs (340).within one year through increased productivity

Figure 7-1.—Schematic Representation of a Human-Machine System

SOURCE (292)

CLASSIFICATIONS OF ERGONOMICSErgonomists usually subdivide the field into in-

formation ergonomics and physical ergonomics.Information ergonomics is concerned with the col-lection, display, sensing, and processing of infor-mation. Physical ergonomics is concerned withworker size, strength, capabilities for motion, andworking posture.

Ergonomists use a number of techniques thatinclude the evaluation of the transmission of in-formation between the machine and the worker(link analysis), discovery and evaluation of sys-

tem failures (critical incidence analysis), detailedexamination of the sequence of actions taken byworkers (task analysis), and analysis of situationsthat may arise from unprogrammed events or hu-man errors (contingency analysis) (380). Ergon-omists also make extensive use of anthropometricdata concerning the physical dimensions and ca-pabilities of the human population. In addition,the techniques of biomechanical analysis are usedto measure expected physical stresses encounteredby parts of the body while performing; work tasks.

Ch. 7—Ergonomics and Human Factors 125

Information Ergonomics

Information ergonomics is generally concernedwith what the worker senses in the workplace andhow that information is processed by the worker,The two major sources of sensory information arevisual and auditory.

Visual displays in the workplace include me-ter scales, control labels, warning signs, cathode-ray tube (CRT) displays, and printed text. To thislist can be added video display terminals (VDTs),which can be CRT displays, liquid crystal dis-plays, or other technologies. Three factors impor-tant to the operator must be considered in design-ing a visual display: the size of the critical detailin the display, its brightness, and the contrast ofthe display against the background. There are anumber of techniques available to achieve thesegoals in design and to evaluate existing equipment.There are also many guidelines for special applica-tions, including the special needs of groups suchas older workers (23).

Workers also receive information throughsound. These can include tones (bells, whistles,beepers, etc. ) as well as speech. Designers andergonomists often face the question of whetherto provide information through visual or auditorymeans. In general, if the message is simple, short,or calls for immediate action, if the worker isalready overburdened with visual messages, orif the workplace is too dark or too bright forvisual displays, an auditory presentation is rec-ommended. If the message is complex, long, anddoes not necessitate immediate action, or theauditory system of the worker is already over-burdened, or if the workplace is too noisy, avisual display may be preferred (23).

Finally, information ergonomics is concernedwith the processing of information by the workerand the design of the workplace, including the de-sign of controls. Research that has measured theabilities of humans to accurately process infor-mation has shown that people often will not beable to make quick, accurate responses in com-plex or unexpected situations, Second, the studieshave also shown that “compatibility” in display-control design is very important. Compatibilityrefers to the relationships between stimuli and re-

sponses, and generally falls into three categories:spatial (the compatibility of physical features orspatial arrangement for displays and controls),movement (the direction of movement of displaysand controls), and conceptual (the associationspeople hold concerning the meaning of signals,such as in the United States the association of thecolor green with “go”) (292).

One example of an everyday problem in com-patibility will perhaps clarify this notion. Figure7-2 presents four possible arrangements of theburners and burner-controls for a stove. For sev-eral of these patterns, the relationships betweenburners and controls can be difficult to learn andhard to remember because the arrangements lackspatial compatibility. To examine this, research-ers in the 1950s setup an experiment in which theytold a group of subjects to turn on specific burners.The subjects’ reaction times were measured andthe number of errors was noted. Design I, asso-

Figure 7-2.–Control Burner Arrangements ofSimulated Stove Used in Experiment About

Logical Arrangements

I I II

I

Ill

I

Iv I

Number of errors in 1,200 trials:arrangement

1 0II 76Ill 116IV 129

SOURCE: (292)

I


ciated with both the fewest errors (zero) and the The cherry-picker accident discussed in box C il-best reaction time, was considered to be the most lustrates the importance of controls layout.compatible (292).

Physical ErgonomicsMany workplaces have dozens of incompati-

ble control configurations, which often lead to Physical ergonomics, concerned with the workeroperator errors and subsequent serious injury (23). as a physical component of the work process,

—.

Ch. 7—Ergonomics and Human Factors ● 127

often uses the techniques and equipment of an-thropometry and biomechanics. Anthropometryis the measurement of the physical dimensions ofthe human body, including both the structure ofthe body in fixed positions and the extent of move-ment possible, such as reach or lifting capacity.

Biomechanics is the study of the mechanicaloperations of the human body. The muscles,bones, and connective tissue can be analyzed as

a mechanical system using the fundamental lawsof Newtonian mechanics. The forces acting on themuscles, bones, joints, and spine can be deter-mined for the lifting of an object, for instance.Through such analysis, it is possible to calculatethe size and direction of forces acting on the body.These can be compared with expected human tol-erances to judge whether the activity in questionwill cause harm.

ERGONOMICS AND PREVENTION OF MUSCULOSKELETAL INJURIES

Ergonomic principles can be applied to preventboth overt and cumulative traumas. An examplein the overt category is the risk of falling froma ladder, which can be reduced by considering thesizes and mobility of people when deciding howfar apart to place a ladder’s rungs (250). Cumu-lative traumas are not the result of single eventsor stresses; they stem from the repeated perform-ance of certain tasks. Back problems are by farthe most common cumulative trauma injuries.Evaluation and redesign of tasks to prevent backinjuries is discussed later in this chapter.

Repetitive motion disorders are a type of cumu-lative trauma associated with repeated, oftenforceful movements, usually involving the wristor elbow. Some 20 million workers on assemblylines and in other jobs that require repetitive,strain-producing motions are at increased risk ofdeveloping such disorders. Redesigning work sta-tions, equipment, and handtools can significantlyreduce the awkward, forceful movements com-mon to many jobs on assembly lines, in food proc-essing, in the garment industry, and in offices.Carpal tunnel syndrome, one of this class ofdisorders, illustrates the potential for preventionoffered by the integration of ergonomics, medi-cal surveillance, and treatment.

Carpal Tunnel Syndrome

A wide variety of workers (see table 7-I), fromaircraft assemblers to upholsterers, are amongthose at risk for carpal tunnel syndrome (CTS),a progressively disabling and painful condition

Table 7.1.—Occupations and Activities AssociatedWith Carpal Tunnel Syndrome

Aircraft assemblyAutomobile assemblyBuffingCoke makingElectronic assemblyFabric cutting/sewingFruit packingGardeningHay makingWaitressingHousekeepingSOURCE (60)

InspectingMeat processingMetal fabricatingMusiciansPackagingPostal workersTextile workersTire and rubber workersTypingUpholstering

of the hand. Because the musculoskeletal strainfrom repeatedly flexing the wrist or applying arm-wrist-finger force does not cause observable in-juries, it often takes months or years for workersto detect damage.

The incidence and prevalence of CTS in thework force is not known. The National Institutefor Occupational Safety and Health (567) reportsthat 15 to 20 percent of workers employed in con-struction, food preparation, clerical work, pro-duction fabrication, and mining are at risk forcumulative trauma disorders. The Bureau of La-bor Statistics (603) reports 23,000 occupationallyrelated repetitive motion disorders in 1980, al-though the number of CTS cases is not specified.

CTS is undoubtedly underreported in aggregatestatistics. Research in particular high-risk plantsprovides some insight into the extent of the prob-lem. In a study at an athletic products plant, 35,8percent of workers had a compensable repetitivetrauma disorder. In some jobs within the plant,

128 ● Preventing Illness and injury in the Workplace

Photo credit: OSHA, Office of Information and Consumer Affairs

Work on an automobile assembly line can involve cramped working positions. A Volvo assembly plant in Kalmar,Sweden, uses “tipper trolley s.” These trolleys hold the automobile bodies and can be tipped 90 degrees to allow

work on the underside of the car

the rate was as high as 44.1 percent, and carpaltunnel syndrome occurred in 3.4 percent of theworkers (21,23). Many industries claim that theincidence of CTS is increasing and is one of theirmost disabling and costly medical problems (60).

Symptoms

The onset of symptoms of CTS is usually in-sidious. Frequently, the first complaint is of at-tacks of painful tingling in one or both hands atnight, sufficient to wake the sufferer after a fewhours of sleep. Accompanying this is a subjectivefeeling of uselessness in the fingers, which aresometimes described as feeling swollen. Yet littleor no swelling is apparent. As symptoms increase,attacks of tingling may develop during the day,but the associated pain in the arm is much lesscommon than at night. Patients may detect changesin sensation and power to squeeze things but somepeople suffer severe attacks of pain for many yearswithout developing abnormal neurological signs.Ultimately, in advanced cases, the thenar muscleat the base of the thumb atrophies, and strengthis lost.

Compression of the median nerve is the imme-diate cause of CTS. The median nerve comesdown the arm, through the wrist, then branchesin the hand, supplying the thumb, forefinger, mid-dle finger, and half the ring finger with nerves (fig.7-3). The carpal tunnel itself, located in the wrist,

is formed by the concave arch of the carpal bonesand is roofed by the transverse carpal ligament(fig. 7-4). These structures form a rigid compart-ment through which nine finger tendons and themedian nerve must pass. Any compromise of thisunyielding space usually compresses the mediannerve.

Risk Factors

Repetitive motions, such as those required inmany jobs, is one of a number of risk factors forCTS. It is probably the most readily controllablecause, however. Certain diseases, acute trauma,congenital defects, wrist size, pregnancy, oral con-traceptive use, and gynecological surgery all maycontribute to the likelihood of developing CTS.Overall, the incidence of CTS is higher in womenthan in men, perhaps because of some of theserisk factors.

Occupational tasks responsible for the devel-opment of CTS include physical exertions withcertain hand postures or against certain objects,and exposures to vibration or cold temperatures.Repeated and forceful up-and-down motions ofthe wrist (flexion and extension) (fig. 7-5), causethe finger tendons to rub on the structures form-ing the carpal tunnel. This constant rubbing cancause the tendons to swell (tenosynovitis), even-tually putting pressure on the median nerve in-side the carpal tunnel. The nerve itself is stretched

Ch. 7—Ergonomics and Human Factors . 129

Figure 7-3.—Major Nerves in the Arm and Hand

Pal mar(a)

(b)

SOURCE (60)


Figure 7-4.--The Carpal Tunnel

Median nerve

Finger flexor

SOURCE (60)

Figure 7-5.— Flexion and Extension of the Wrist

a.

Bending the wrist causes the finger flexor tendons to rub onadjacent surfaces of the carpal tunnel.

SOURCE’ (60)

Flexor retinaculum(transverse carpal ligament)

by repeated exertions, and compressed betweenthe walls of the carpal tunnel.

Forceful movements and the direction of themovement are only two of the underlying causesof tenosynovitis that can lead to CTS. The speedof movements and incorrect posture while work-ing also are important (275). Median nerve com-pression also can be caused by tasks that requirea sustained or repeated stress over the base of thepalm (247). Examples include the use of screw-drivers, scrapers, paint brushes, and buffers.

Although the mechanism is not yet understood,low frequency vibration is a recognized risk fac-tor for CTS (405). Vibration exposure may resultfrom air- or motor-powered drills, drivers, saws,sanders, or buffers. Cannon (95) examined med-ical records at an aircraft company and found astrong association between CTS and use of vibrat-ing tools.

Control of CTS

Control of CTS requires a two-pronged ap-proach. The primary strategy to prevent cases isthe use of ergonomic principles to modify hand-tools and to improve work-station design and

Ch. 7—Ergonomics and Human Factors 131

work practices. Even a successful ergonomic pro-gram will not prevent all cases of CTS, however.The second important element, therefore, is amedical surveillance program. This is particularlyimportant now when so little is known about theindividual factors that cause some people to de-velop CTS. Thus far, no programs focusing onthe medical evaluation of CTS seem to exist (60).

Ways are needed to identify the earliest sign ofCTS, to evaluate progression of the disease, andto examine the role of predisposing risk factors.The purpose of such a medical surveillance pro-gram is prevention of advanced disease by in-stituting therapy at early stages.

Although medical surveillance for CTS is stillin very early stages, ergonomic interventions havebeen remarkably successful where they have beeninstituted. Armstrong (21) describes the steps in-volved in developing appropriate controls. First,plants and specific departments within plants inwhich there is a documented high rate of CTSshould be identified. Then each job should besystematically analyzed. Traditional time-and-motion studies, in which each movement or actis recorded, can be used. Each element of the jobcan then be checked against factors known to beassociated with CTS development. These includeposture of the hand and wrist, strength, stress con-centrations over the palm, vibration, cold tem-perature, and the presence of gloves.

Armstrong presents a typical work task as anexample. Figure 7-6 shows a worker taking partsout of a container and placing them on a con-veyor. The six elements involved in this task arereach, grasp, move, position, assemble, and re-lease. Reaching into the container involves wristflexion and pinching, during which the worker’swrist is likely to rub on the edge of the box. Theforearm is also likely to rub on the edge of thework bench while the part is positioned. The re-designed work station should reduce stress on thehand and wrist, and eliminate sharp edges. Goodand bad designs for the container and the work-bench with jig in this hypothetical case are illus-trated in figure 7-7.

Powered handtools can also be designed andused to minimize stress. As illustrated in figure7-8, good designs allow the work to be done withlittle or no flexion or extension of the wrist.

Armstrong and his colleagues have investigatedcumulative trauma disorders in a poultry proc-essing plant using the procedures described above.They discovered that workers in the “thigh bon-ing” section had the highest incidence of cumula-tive trauma disorders of all departments. Thighboning involves grabbing the thigh with one handon a moving overhead conveyor, then makingfour cuts with the other to separate the meat fromthe bone. Each worker makes an estimated 15,120cuts per shift. Ergonomic improvements to theprocess recommended by Armstrong and col-leagues include training workers in the “properwork methods and knife maintenance to minimizethe time and, hence, the distance that must bereached and force that must be exerted on thethigh.” The work station could be modified tominimize the distance to be reached. The work-ers wear wire mesh gloves with rubber glovesunderneath, which increase the force necessaryto grasp the thigh and pull the meat away. Glovesshould fit well, and the addition of barbs on thepalm of the wire mesh glove might facilitate thehand actions. A new knife handle design, to re-duce the force required to hold the knife and makethe cuts—e.g., that pictured in figure 7-9—is sug-gested (22). Such a design would also minimizewrist flexion.

A high incidence of repetitive trauma disorders,including carpal tunnel syndrome, in a telephoneassembly plant prompted management to considerhow to prevent future cases. McKenzie and col-leagues (299) noted the highest rates in areas usingvibratory air screwdrivers, and in jobs requiringrepetitive grasping, squeezing, or clipping mo-tions. Ergonomic changes recommended includedmodifying the screwdrivers with sleeve guards andchanging work positions to minimize hand andwrist stress. The changes were instituted withalmost immediate results: from 2.2 percent annualincidence of repetitive trauma disorders in 1979


Figure 7-6.-Job Analysis: Assembly Tasks

Job analysis1. Reach for part - 2. Grasp part

.

4. Position part i5. Assemble part

• Assembling parts on a moving conveyor can be described by a series of six elements,

SOURCE (21)

to 0.79 in 1981. Lost and restricted workdays fellfrom 5,471 in 1979 to 1,111 in 1981, and furtherreductions were expected in subsequent years.

Back Disorders

The Bureau of Labor Statistics completed a sur-vey of workers who had incurred back injurieswhile lifting, placing, carrying, holding, or lower-ing objects (605). Of the 900 workers included inthe tabulated results, more than 75 percent werelifting at the time of the injury, Surprisingly, theback injuries were concentrated in younger work-ers; almost 75 percent occurred in 20- to 44-year-olds. Both the weight and the bulkiness of thelifted objects were often associated with injuries.

About half the injured workers had received someinstruction about proper lifting.

Manual lifting presents a risk of overexertioninjuries and cumulative damage to the soft tissuesaround the spine. Overexertion injuries to theback constitute the largest single category ofworkers’ compensation claims, amounting to 25to 30 percent of all disability cases Lower-backinjuries are often extremely painful and signifi-cantly diminish the quality of life of the afflictedworkers. Many of these can be prevented by joband equipment redesign.

The conventional wisdom about how to liftsomething is to squat, pick up the object, and,while keeping the back straight, lift straight up


, a. Container. ’.

,.,.

Iedges too highand too sharp

Side cut out withrounded edge Elbow height

tilted

Elbow heightedges too high

Containers should be designed so that workers can reach all locations without flexing their wrist. All edges that come intocontact with the worker should be well rounded.

b. Workbench and jigs

Bad Bad

!Sharp edge of work surface Flat and too high

\ OK OK

. .

Tilted away

sOURCE: (21)

I&! . Preventing Illness and Injury in the Workplace

Figure 7-8.—Good and Bad Designs for Powered Drivers

Powered drivers

OKPistol handlevertical surface

BadPistol handlehorizontal surfaceelbow height

OKInline handlehorizontal surface

OK

Pistol handlehorizontal surfacebelow waist height

Bad Inline handhorizontalsurfacebelow elbowheight

Wrist posture is determined by the elevation and orientation of the work surface with respect to the workers and the shapeof the tool..-

SOURCE: (21)

with the legs. This procedure is thought to pre-vent injury to the back. In many cases this pro-cedure is justified, but there are many other situ-ations where it is not.

Work-Load Evaluation

There are at least four basic sets of criteria fordetermining acceptable work loads: biomechani-cal, physiological, psychophysical, and epidemi-ological (453). Biomechanical criteria are basedon pressures and stress exerted on the body, par-ticularly on the spinal column. Limits of tolerancehave been developed by observing damage tocadavers when pressure is applied. Physiologicalcriteria are primarily metabolic, e.g. oxygen con-sumption and heart rate. The psychophysical

method incorporates workers’ perceptions andsensations into the assessment of work load forboth static and dynamic strength. Epidemiologiccriteria are derived from aggregate data concern-ing the incidence, severity, and distribution of lowback pain. These four approaches can be inte-grated and guidelines established with input fromeach. The National Institute for OccupationalSafety and Health has developed recommenda-tions in this way.

Maximum acceptable work loads are often ex-pressed in terms of the weight and frequency oflifting. For instance, 75 percent of the industrialmale population can lift a 13-kilogram box offixed proportions every 5 seconds, and a 34-kilogram box every 30 minutes without trigger-ing or aggravating low-back injury symptoms.


Figure 7-9.—A Knife Designed to Reduce Cumulative Trauma Disorders in Poultry Processing

One possible knife handle with three blades for reduced wrist deviations. The handle is designedto reduce the tendency for the knife to fall out of the hand in thigh boning.

SOURCE (22)

There is great practical significance to having compensable back problems. For case histories ofvery specific work-load recommendations. They task evaluation and redesign carried out by thecan be used both for determining that a task is Liberty Mutual Insurance Co., see boxes D andunacceptable and as guidelines for redesigning the E. According to Snook (453), “most industrialtask. Recognition of a problem often occurs only tasks with unacceptable work loads can be mod-after a number of compensation claims have been ified for less than the average cost of a single lowawarded, triggering investigation. Insurance com- back compensation claim. ”panics have a direct incentive to cut down on

A P P L Y I N G E R G O N O M I C S T O V D T D E S I G NThe first reports from Europe concerning po- cluded that there were no known radiation

tential adverse health effects associated with video hazards and that the real hazards were ergonomicdisplay terminals included accounts of many re- problems: musculoskeletal problems, visual prob-ported musculoskeletal and visual problems. Early lems, and fatigue. Poor design of equipment orU.S. studies of potential VDT hazards also con- poor job design may produce such problems.


Musculoskeletal problems among office work-ers range from discomfort to pain and medicaldisability. The back, neck, and shoulders are themost frequent sites of problems. Table 7-2 sum-marizes the results from several studies of VDTworkers. There is general agreement that muscu-loskeletal problems are associated with poorworking positions, repetitive motions, and thelength of work time without a break.

Figure 7-IO illustrates risk factors contributingto musculoskeletal problems associated with

Box E.—Ergonomics-Task Evaluationand Redesign

This case history involves back injuries tomateriaI handlers in the packing department ofa metal office equipment and desk accessorymanufacturer. The six female material handlersare required to manually lift boxes of varioussizes and weights from a conveyor and stackthem on wooden pallets. There were six com-pensable back injuries reported between Feb-ruary 1980 and September 1982, with a cost of$131,415.

Boxed products are transported by belt con-veyors to the position of the material handler,where they are taken off the line and loweredand/or lifted onto pallets. Due to the wide varie-ty of Sire, shapes, and weights, the boxeshandled vary in weight from a pound to largebulky boxes weighing 50 pounds. Full pallets aretaken by forklift to a warehouse.

Source: (675}.

VDTs, including equipment design (VDTs, workstations, and chairs) and job design (constrainedworking positions, repetitive work, and inade-quate rest breaks). Any of these can be changed.The keyboards and screens of many early VDTwork stations were fixed relative to each other andthe height of the work station was not adjustable.

Ch. 7—Ergonomics and Human Factors . 137

Table 7-2.—Frequency of Reported Musculoskeletal Problems From Various Studiesof VDT Workers

Percent reporting frequent or daily problemsa

Study/type of worker Neck-shoulder Back Arm-hand

Arndt, 1980:Telephone operatorb . . . . . . . . . . . . . . . . . 8-17 15 12Service assistant . . . . . . . . . . . . . . . . . . . 9-14 14 6Accounting clerkb . . . . . . . . . . . . . . . . . . 7-1o 10 5

Laubli, et al., 1980:Data-entry VDT operator. . . . . . . . . . . . . . 11-15 N.A. 6-15Accounting machine operator . . . . . . . . . 3-4 N.A. 8Conversational VDT operator. . . . . . . . . . 4-5 N.A. 7-11Typist. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 N.A. 4-5Traditional office worker . . . . . . . . . . . . . 1 N.A. 1

Hunting, et al., 1981:VDT operator . . . . . . . . . . . . . . . . . . . . . . . 6-12 6-10 6-12Typist , . . . . . . . . . . . . . . . . . . . . . . . . . . . . , 6-12 6-10 6-10Traditional office worker , . . . . . . . . . . . . 2-5 2-5 2-5

Canadian Labour Congress, 1982:VDT operator . . . . . . . ... . . . . . . . . . . . . 10-12 10-15 2-3Non-VDT operator . . . . . . . . . . . . . . . . . . . 7-8 9-14 4

Sauter, et al., 1983:VDT operator . . . . . . . . . . . . . . . . . . . . . . . 24-34 27-35 3-4Non-VDT operator . . . . . . . . . . . . . . . . . . . 19-28 22-29 4-6

reporting almost frequent, often constant Ranges represent Questions 33 percent use

N A =

SOURCE (25)

Figure 7-10.— Potential Ergonomic Risk Factors Associated with VDT Design

Displayheight

keyboardIocat ion

Chairheight

Foot

SOURCE (25)

138 ● Preventing Illness and Injury in the Workplace

Equipment can be redesigned and made to fit thevarious sizes of people. Many manufacturers nowoffer, usually at a higher price, office furnitureand equipment that has been “ergonomically de-signed. ” Although this equipment has been adver-tised as being vastly superior to ordinary officeequipment, its biggest advantage from an ergono-mic perspective is the simplest—it is adjustable.Job redesign to reduce repetitiveness and toalleviate constrained postures is also possible.Finally, rest breaks can be instituted to reducefatigue from working in one position for long peri-ods of time. Rest breaks often result in increasedproductivity as well as fewer musculoskeletalcomplaints (25).

Visual problems reported among VDT users in-clude eyestrain, burning/itching eyes, blurred vi-sion, double vision, color fringes, and deteriora-tion of acuity (table 7-3). A recent review by aNational Research Council panel (321) concluded,however, that there was no evidence that the useof VDTs led to visual system problems differentfrom symptoms reported by workers engaged inother tasks involving intensive close work.

The problems reported are related to the visualdemands of VDT work. The intensity and dura-tion of visual demands, the quality of the VDTimage, and illumination are important. VDTshave often been manufactured and introduced inworkplaces without sufficient attention to thethese factors. The National Research Council (321)report notes that “[i]n many instances . . . VDTshave been designed without attention to existingscientific data on image quality, and many VDTson the market do not provide the legibility ofhigh-quality printed material.” Snyder (454) hasestimated that nearly half of all VDTs now in useare improperly designed for intensive office use.

VDTs have usually been placed into workplaceswithout consideration of the available lighting.There has been a tendency to believe that morelight creates a better environment for office work-ers. The result is that many offices are overlit ortoo bright for certain activities. This is a particu-lar problem in offices with VDTs, Increasing thelight level, up to a point, improves contrast andvisibility for most office tasks. VDTs, however,emit their own light and increasing illumination

Table 7-3.--Frequency of Reported Visual Problems From Various Studiesof VDT Workers

Percent reporting frequent problemsa

Burning,itching, Blurring,

Study/type of worker Eyestrain irritation double vision

Holler, et al., 1975:VDT user . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 N.A. 7

Arndt, 1980:Telephone operatorb . . . . . . . . . . . . . . . . . . . . . . . 8 N.A. 1-5Service assistant . . . . . . . . . . . . . . . . . . . . . . . . . 9 N.A. 1-4Accounting clerkb . . . . . . . . . . . . . . . . . . . . . . . . . 6 N.A. 1-3

Laubli, et al., 1980:Data-entry VDT operator. . . . . . . . . . . . . . . . . . . . 20 N.A. N.A.Conversational VDT operator. . . . . . . . . . . . . . . . 28 N.A. N.A.Typist. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 N.A. N.A.Traditional office worker . . . . . . . . . . . . . . . . . . . 5 N.A. N.A.

Canadian Labour Congress, 1982:VDT operator ... , . . . . . . . . . . . . . . . . . . . . . . . . . N.A. 9-17 13-16Non-VDT operator . . . . . . . . . . . . . . . . . . . . . . . . . N.A. 7-8 8-9

Sauter, et al., 1982:VDT newspaper editor . . . . . . . . . . . . . . . . . . . . . 40 43-46 N.A.Non-VDT newspaper editor . . . . . . . . . . . . . . . . . 17 17-22 N.A.

Sauter, et al., 1983:VDT operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 19-27 5Non-VDT operator . . . . . . . . . . . . . . . . . . . . . . . . . 20 21-23 5

aFreqUenCY of problems reported daily, constantly, often, vew often, freWJentlY.bApproximately 33 percent used ‘fDTs.

N.A. = Not available.

SOURCE: (25).


levels actually reduces the contrast and visibilityof the screen image.

It is frequently suggested that the general light-ing level in a room be somewhat reduced forscreen-based work. An alternative recommenda-tion is to provide even lower levels, with sup-plementary task lighting for reading text frompaper and for other non-VDT work. Other illumi-

SUMMARYErgonomics —techniques for examining worker-

machine interactions and design of machines tofit workers better—can be important in reducinginjuries and discomfort from repetitive motionsand other work activities. Information ergonomicsinvolves workers’ receipt of sensory informationand how it is processed; physical ergonomics dealswith worker body size, strength, and stresseswhile working. Generally, when faced with aproblem in the workplace, ergonomists systemat-ically analyze the job using biomechanical, phy-siological, psychophysical, and epidemiologicalanalyses; each job element is compared with fac-tors known to be associated with overt or repeti-

nation problems that must be addressed concernreflections off the VDT screen, glare, and contrast.There are techniques for reducing and eliminat-ing most of these problems and for enhancing theability of each worker to adjust the environmentof the VDT to fit his or her needs. Unfortunately,these techniques have very often been ignored bymanufacturers, vendors, and employers (25).

tive trauma injuries. Changing work practices,tools, and machine design and redesigning plantlayout are among the principal ergonomic solu-tions. There are many examples of the incidenceof carpal tunnel syndrome and back disorders,two common repetitive trauma injuries among in-dustrial and office workers, being reduced throughthe use of ergonomic designs. In addition to pre-venting overt and cumulative trauma injuries,ergonomic design has been shown to increaseworker productivity and efficiency by making themachine or tool more closely fit the worker andthe worker’s capabilities.

7. ergonomics and human factors

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