SPINE ERGONOMICS Malcolm H. Pope, Kheng Lim ... - … · Occupational low back pain (LBP) is a great burden for industry and medicine. Kelsey & White (1) found 2% of workers in the

19 Jun 2002 13:38 AR AR164-03.tex Ar164-03.sgm LaTeX2e(2002/01/18)P1: GDL10.1146/annurev.bioeng.4.092101.122107

Annu. Rev. Biomed. Eng. 2002. 4:49–68doi: 10.1146/annurev.bioeng.4.092101.122107

Copyright c© 2002 by Annual Reviews. All rights reserved

SPINE ERGONOMICS

Malcolm H. Pope, Kheng Lim Goh, andMarianne L. MagnussonLiberty Safework Research Centre, Departments of Environmental and OccupationalMedicine and Bio-Medical Physics and Bio-Engineering, University of Aberdeen,Aberdeen, AB25 2ZP Scotland, United Kingdom; e-mail: [email protected]

Key Words low back pain, occupational, prevention

■ Abstract Occupational low back pain (LBP) is an immense burden for both indus-try and medicine. Ergonomic and personal risk factors result in LBP, but psychosocialfactors can influence LBP disability. Epidemiologic studies clearly indicate the role ofmechanical loads on the etiology of occupational LBP. Occupational exposures such aslifting, particularly in awkward postures; heavy lifting; or repetitive lifting are relatedto LBP. Fixed postures and prolonged seating are also risk factors. LBP is found inboth sedentary occupations and in drivers as well as those involved in manual materi-als handling. Any prolonged posture will lead to static loading of the soft tissues andcause discomfort. Standing and sitting have specific advantages and disadvantages formobility, exertion of force, energy consumption, circulatory demands, coordination,and motion control. The seated posture leads to inactivity causing an accumulationof metabolites, accelerating disk degeneration and leading to disk herniation. Driver’spostures can also lead to musculoskeletal problems. Workers in a driving environmentare often subjected to postural stress leading to back, neck, and upper extremity pain.This exacerbates the problems due to the vibration. Prevention is by far the treatmentof choice. Improved muscle function can be preventative. Poor coordination and motorcontrol systems are as important as endurance and strength. Fixed postures should beavoided. Seats offering good lumbar support should be used in the office. A suspensionseat should be used in vehicles whenever possible. Heavy and awkward lifting shouldbe avoided and lifting aids should be made available. Workers should report LBP asearly as possible and seek medical advice if they think occupational exposure is harm-ing them. The combined effects of the medical community, labor, and management arerequired to cause some impact on this problem.

CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50RISK FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Muscle Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Lifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Pulling and Pushing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Posture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

1523-9829/02/0815-0049$14.00 49

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50 POPE¥ GOH ¥ MAGNUSSON

Whole Body Vibration (WBV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57PREVENTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Muscle Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Lifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Pushing and Pulling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Posture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Whole Body Vibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

INTRODUCTION

Occupational low back pain (LBP) is a great burden for industry and medicine.Kelsey & White (1) found 2% of workers in the United States have a compensableback injury. LBP is the most expensive United States health problem in ages 20to 50. The greatest expense is from 25% of the cases, and as the LBP durationincreases, the total cost accelerates. LBP is responsible for 21% of all compensablework injuries and 33% of the cost. Medical costs are 33% of the total and disabilitypayments are the remainder (2).

In this article, we examine the epidemiologic relationships between LBP andphysical loading of the spine and the biomechanical etiology.

RISK FACTORS

General

We emphasize ergonomic factors, but other factors result in LBP and especiallyLBP disability. Psychosocial factors can influence LBP disability (3). According toKarasek et al. (4), work stress and ill-health are related to work load and clarity andconflict at work, but the effect of these work-related characteristics is moderated bycontrol (decision latitude), career development opportunities, and social support atwork. The combination of high demands and low control at work is supposed to bestressful and related to adverse health effects. It is now being recognized that thereis a symbiotic relationship between psychological and physiological stress. Psy-chosocial stress during activity such as lifting under stressful conditions increasesmuscle activity and leads to increased spine compression and lateral shear (5).

Other important patient factors are obesity, physical fitness, smoking history,height, and pregnancy (6). Specific personal factors affect tissue tolerance (i.e., ageand gender). These are important factors to take into account with an aging anddiversified work force.

Muscle Function

Chaffin et al. (7) found that LBP was three times more prevalent in workers whohad less strength than needed for their jobs and that one third of the workforce

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SPINE ERGONOMICS 51

is required to exert at their maximum strength [NIOSH (National Institute ofOccupational Safety and Health) 1981]. Chaffin et al. (8) suggest industry useplacement programs based on strength performance. Isometric testing is the mostcommon, but it may be unrealistic and unsafe (3). In isokinetic tests, the liftvelocity is constant. In isodynamic tests, the velocity varies but the resistance ispreset, which is a realistic functional test, but has not been tested prospectively.The purpose of job simulation is to see if the worker can perform the job required.Strength testing should be as specific as possible to the job for practical, ethical,and legal reasons.

More subtle muscle functions such as motor control are also important.Cholewicki & McGill’s (9) model showed that the spine can buckle during aminor task with low muscle forces where a small motor error causes over-rotationof a functional spinal unit and loading of the soft tissues. Magnusson et al. (10)found that muscles do not respond quickly enough to protect the soft tissues.Krajcarski et al. (11) found that preactivation of trunk extensor muscles mayserve to reduce the flexion displacements caused by rapid loading. The abdominaloblique muscles, especially external oblique, will rapidly increase their activationlevels in response to rapid loading. When preactivation levels are low, there is alower initial trunk stiffness and spine compression force. Adams & Dolan (12)found that soft tissues fail with bending moments of 60 Nm, which could be thetorso weight when bending over with no muscle support. Some injuries occurbecause of motor control errors. These are random events, but are more likely inpeople with poor motor control systems (13). Thus, a back injury can occur in asimple task if there is insufficient warning of an overload.

Endurance is important because workers fatigue with repetitive tasks (14).McGill et al. (15) noted that people differ in their ability to hold a load in theirhands and breathe “heavily.” This may be significant because the muscles requiredto be continuously active to prevent buckling of the spine are also used to breathe.Thus, those with compromised lung elasticity (i.e., smokers) may be at risk whenthe muscles are relaxed. McGill et al. (15) have identified inappropriate musclesequences in subjects loaded while breathing 10% carbon dioxide to elevate breath-ing. Potvin & O’Brien (16) made the important observation that fatigue results in achange in the recruitment patterns of trunk muscles: Muscles serving as antagonistand trunk stabilizers during prolonged axial twist exertions increased their forcelevels in response to fatigue.

The stability of the spine can be considered to consist of three subsystems:the passive musculoskeletal, the active musculoskeletal, and the neural feedbacksubsystem. All systems act together. In recent years, more attention has been givento the neural feedback system. Proprioception is an important factor in stabilizationof the joints and throughout the spine. It is probable that proprioception deficits areassociated with LBP and low back injuries. Proprioception decreases with aging,with lack of exercise, and in those with LBP. A system with feedback has beensuccessfully used to rehabilitate those with chronic LBP. With prolonged exposureto lifting or excessive postures such as full flexion, the viscoelastic structures of the

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lumbar spine will become stretched out due to creep. Gedalia et al. (17) reportedthat laxity in the viscoelastic structures desensitizes the mechano-receptors withinand causes loss of reflexive stabilizing forces from the multifidus muscles. The first10 minutes of rest after cyclic loading results in fast partial recovery of muscularactivity; full recovery is not possible even with rest periods twice as long as theloading period. This may increase the risk of instability, injury, and pain in the spine.

Lifting

LBP is associated with lifting, but instructions for the “proper” technique havebeen controversial. The principle is to hold the object as close to the body aspossible, which is more important than keeping a straight back (18). Brinckmannet al. (19) reported that spinal loading during forward bent posture results ina height decrease or deformation of lumbar vertebrae. Other things affect theway we lift. Chen (20) showed that when muscle fatigue occurs in the arms, thelifting strategy becomes more stressful on the back, and whole-body lifting shouldbe avoided in order to reduce the risk of injury to the lower back. The workershould use smooth lifting technique without jerking to minimize the effect ofdynamic loads on the spine. Davis & Marras (21) concluded from an extensiveliterature review that the ability of the individual to perform a task “safely” may becompromised by muscle coactivity during dynamic exertions, e.g., trunk motionoutside the sagittal plane. Fathallah et al. (22) established that lowering of a loadgives consistently lower trunk muscular activities than lifting. Asymmetric liftingis associated with an increased risk of low back disorders according to Kingma et al.(23); small deviations of a lifting movement from the sagittal plane can increaserisk of low back disorders. Antagonistic cocontraction affects spinal stability andspinal compression. For antagonistic cocontraction to be beneficial, stability mustincrease more than spinal load. Antagonistic cocontraction is beneficial at lowtrunk moments typically observed in upright postures (24). Granata et al. (25)found that weight, task asymmetry, and job experience (level of experience) affectthe magnitude and variability of spinal load during repeated lifting exertions.

Gagnon & Smith (26) also point out that slower lifts with reduced accelerationwhen lifting moderately heavy work may present less of a risk to low back disorder.With regard to our aging workforce, bending and lifting activities generate loadson the spine that exceed the failure load of vertebrae with low bone mineral density.This yields a high factor of risk (ratio of load on spine to failure load of bone) (27).

In critical tasks with twisting, it is important to have the worker turning with thefeet to reduce the torsional loads on the intervertebral discs. The relative merits ofback-stooped posture versus knee lifts are debated. However, lifting a bulky loadwith an erect back increases the intradiscal pressure (IDP) compared to the back-stooped posture because of the increased moment arm and the vertical componentof body weight and hand forces. However, shear forces are greater when liftingwith the back flexed and the articular capsule and the posterior ligaments maybe overstrained. Workers may lift loads with their backs rather than their legs to

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SPINE ERGONOMICS 53

maximize the available energy. IDP is similar with back lift and leg lift if the loadmoment arm is constant. Intra-abdominal pressure (IAP) has been proposed as amajor source of column support. Asmussen & Poulsen (28) report a maximumlimit of 150 mm Hg for highly trained individuals and 90 mm Hg for the normalpopulation. Krag et al. (29) established that increasing IAP actually increasesmuscle activity and thus spinal load.

Pulling and Pushing

Pushing and pulling activities can cause occupational LBP. Magora (30) foundincreased LBP in those whose jobs involved reaching and pulling, and NIOSHreported that 20% of injury claims for LBP involved pushing or pulling loads.Damkot et al. (31) measured pushing exposure by multiplying the weight of pushedobjects by the number of pushing efforts required each day. Those with severe LBPhave five times as much weight-day units as those without LBP. White & Panjabi(32) have shown that high spine loads result from pulling activities (see Figure 1a).Ayoub & McDaniel (33) found that body posture plays an important role in theforce capability in both pushing and pulling and is probably very important inthe etiology of pulling or pushing LBP injuries. Friedrich et al. (34) found thatcombined walking and pushing under vertical space constraints was associatedwith increased lumbar flexion and thoracic extension. The latter increase was due toan attempt to enhance abdominal muscle strength. The health care professions areat a high risk of LBP, and pushing-pulling activities are no exception. Lavender et al.(35) found the most hazardous tasks performed by emergency medical techniciansincluded pulling a victim from a bed to a stretcher, the initial descent of a set ofstairs when using the stretcher, and lifting a victim on a backboard from the floor.

Posture

The posture at work can cause LBP. We have already mentioned the antagonisticmuscle activity and the effect on spinal loading. McGill et al. (36) pointed outthat anterior shear load on the lumbar spine increases the risk of LBP. Bendingforward allows the spine to fully flex; this changes the line of action of the largestextensor muscles and reduces their effectiveness to support anterior shear forces.In fact, in this position, the muscles become inactive and the person is hanging bytheir soft tissues. McGill et al. (36) found that fully flexing the spine renders thelumbar extensor muscles ineffective for supporting anterior shear forces. Anteriorshear load on the lumbar spine is highly related to the risk of back injury. Jobssuch as seated warehouse shipper, gardener, and construction worker require suchpostures. Potvin et al. (37) also pointed out that during lifting, the risk of injury tothe spine may be increased more by the degree of lumbar flexion than the choiceof stoop or squat technique. McGill & Brown (38) measured creep response of thelumbar spine from nine min of full flexion posture. Full recovery took∼30 min.Many workers have to work in extreme postures. Gallagher et al. (39) found thatthe effect of the kneeling posture, used in construction and mining, is increased

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Figure 1 Forces in (a) pulling and (b) pushing activities.

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SPINE ERGONOMICS 55

Figure 1 (Continued)

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lumbar compression. Adams & Dolan (40) point out that the risk of bending injuryto soft tissues due to posture will depend not only on the loads applied to the spine,but also on loading rate and loading history.

The quantification of potentially harmful postures has been an enigma. Oneapproach to record potentially stressful postures is referred to as posture target-ing. A worker is observed at random times during the workday and the angularconfiguration of various body segments is recorded with the aid of a body dia-gram. This procedure is useful in evaluating workplace layouts when combinedwith worker reports of localized musculoskeletal pain obtained at several intervalsduring a workday. Another technique has been to use a trunk goniometer as shownin Figure 2.

SITTING LC According to Magora (41, 42), there is increased risk of LBP in seden-tary jobs. There is also an increase in symptoms in those with LBP who sit forprolonged periods. In sitting, body weight is transferred to supporting areas by theischial tuberosities and the soft tissues. Some body weight will be transferred to thefloor, the backrest, and armrests. IDP when sitting with no lumbar support is 35%higher than standing (43). Increased IDP in sitting occurs due to (a) increased trunkmoment when the pelvis rotates backward and (b) disk deformation caused bythe lumbar spine flattening. Inclination of the backrest backward and increasedlumbar support reduce IDP because (a) body weight is transferred to the backrestand an increased backrest inclination is beneficial, and (b) lumbar supports increaselordosis and reduce disc deformation. In addition to the advantage of backrests,armrests offer support that unloads the spine.

Sitting postures are usually classified as anterior, middle, or posterior. In themiddle posture, the center of mass is directly above the ischial tuberosities and isunstable due to them acting as a pivot. The lumbar spine is either straight or inslight kyphosis. The anterior (forward leaning) posture is used when deskwork isperformed. The center of mass is in front of the ischial tuberosities. In the posterior(backward leaning) posture, the center of mass is behind the ischial tuberosities.The posterior posture is obtained by a backward pelvic rotation resulting in lumbarkyphosis. The seated posture depends on factors such as the chair design, sittinghabits, the task, seat height and inclination, backrest position, shape and inclination,and other supports. The chair should permit easy adjustment because continuoussitting in one position is a risk factor for LBP. Writing decreases IDP when thearms are supported by the desk, whereas typing and lifting a phone increase IDPbecause of larger load moments. However, there are physiologic reasons to changeone’s working posture to improve both disc nutrition and soft tissue tone. Griffing(44) noted that postural fatigue, sickness, and absence decrease when posturalchanges are required.

STANDING LC Pope et al. (6) have reported the positive relationship between pro-longed standing and LBP. However, there is no information that prolonged stand-ing increases IDP unless accompanied by twisting, lifting, or other risk factors.

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SPINE ERGONOMICS 57

Muscular activity is required to maintain an upright posture, but as long as thebody segments are well aligned with respect to the center of gravity, the activityis small. Any shift in the center of gravity of the trunk requires active counterbal-ancing by muscle force to maintain equilibrium. Muscle forces are also requiredto counterbalance the moment caused by an outstretched arm, an external weight,or any other force applied to the trunk, head, and upper extremities. The combinedeffect of all these forces upon the lumbar spine produces a moment that must becounterbalanced by the spinal muscles to maintain equilibrium.

It was first reported by DePuky (45) that the height of the human spine decreasesduring the day and increases at night while resting. It has been shown, by use of aprecise height-measuring device called the stadiometer (Figure 3), that the overallheight decreases with loading but increases in the hyperextended posture. Thus,creep occurs throughout the day.

AWKWARD POSITIONS LC Muscle force, and thus IDP, is increased in postures in-volving lateral bending or twisting (46). A much more complex situation occurswhen asymmetry prevails. In postures such as lateral flexion, rotation, and com-binations thereof, other appropriate muscles will contract. In rotation and lateralbending, high levels of activity occur contralateral to the direction of posturalasymmetries, whereas the activities on the ipsilateral side are small. The asym-metry in muscle activity can lead to unequal stress concentrations on the differentcomponent structures of the spine. Electromyographic (EMG) studies demonstratethe antagonistic activity and resulting higher IDP in awkward postures. Antago-nistic cocontraction of trunk muscles was beneficial in improving stability at lowtrunk moments observed in upright postures (24). Antagonistic cocontraction wasless in high trunk moments and more in low trunk moments.

Troup et al. (47) and Frymoyer et al. (48, 49) found twisting to be related toLBP. Basmajian (50) reported antagonistic EMG activity of the deep trunk musclesduring axial rotation. Thus, the high level of muscle activity and the resultantIDP may explain the relationship between twisting and LBP. Sudden loading andawkward postures occur from slipping, tripping, and falling and range from 36%to 70% of LBP injuries (51, 52, 53). Grieve (54) pointed out that slipping is oftenaccompanied by lifting. Measurement of slip resistance is not difficult. Strandberg(55) listed over 60 different methods to measure the frictional characteristics ofthe floor. If the slip resistance is greater, a worker is less likely to slip.

Whole Body Vibration (WBV)

Extensive reviews of the literature have been made by Hulshof & van Zanten (56)and Bovenzi & Hulshof (57), and it has been concluded that there is a positiverelationship between LBP and WBV. There are a few studies with dose–responserelationships. From a survey of occupational drivers (of many types), Schwarzeet al. (58) concluded that, with increasing dose, it is probable that LBP is causedby exposure to WBV. Fritz (59) found the risk from exposure to WBV increases

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Figure 3 Stadiometer system for measuring in vivo vertical creep ofthe seated subject.

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SPINE ERGONOMICS 59

with age. Interestingly, Hinz et al. (60) suggested that a person with a low vertebralstrength may be more at risk to degenerative changes in the lumbar spine duringrepetitive exposures to moderate transient whole-body vibrations. Magnusson et al.(61) measured WBV while the drivers were traveling on their usual routes accord-ing to the recommendations set out by ISO (International Standards Organization)2631 E (62, 63). Male bus drivers and lorry drivers were age matched to a cohortof sedentary workers. There was a significant difference between length of time offwork due to LBP between the three occupations. Bus drivers took more time offwork than truck drivers in both subjects in Sweden and the United States. Of the busdrivers, the Swedes were exposed to higher vibration levels, whereas the UnitedStates truckers were exposed to higher vibration levels than the Swedish truckdrivers. Swedish city bus drivers experienced more vibration exposure than didthe United States long-haul bus drivers. This is probably due to the difference inbus types, road conditions, and driving demands. The United States truck driverswere vibrated more than anyone in the study, possibly due to travel on poorer roadsand less vibration attenuated seats and driver’s cabs.

With mounting epidemiologic evidence associating LBP with WBV environ-ments there has been an increasing focus on the mechanical effect of occupationalvibrations on LBP. WBV has many, and quite varied, effects on the human. Mechan-ical studies have focused on the resonant frequency, transmissibility, impedance,spinal muscle activity, and effects on the spinal tissues. When the spinal systemis excited at the resonant frequency, there are major mechanical consequences be-cause the associated stress can lead to the structure’s mechanical failure. Failurecan occur because the structure oscillates at the maximum excursion, creating thegreatest possible strains on the tissues. The natural frequency of a single degree offreedom structure can be determined by two means: driving point impedance andacceleration transmissibility. In impedance studies, the driving force is divided bythe resultant velocity. Resonance occurs when both the driving force and resultantvelocity are in phase and the impedance versus frequency curve reaches a maxi-mum. They found resonant frequencies to occur between 4 and 6 Hz when theupper torso vibrates vertically, and between 10 and 14 Hz when there is a bendingvibration of the upper torso with respect to the lumbar spine. Studies also reportresonant frequencies of standing and supine subjects, and resonant frequencies ofseated subjects as affected by side to side or fore to aft vibrations. The effect canalso be characterized by a transfer function, describing the relationship of inputacceleration and measured output acceleration at a point in the body. In frequencyranges where attenuation is low, resonance occurs causing increases in the transferfunction magnitude. In sitting subjects, resonance occurs at the shoulders at 5 Hzand also, to some degree, at the head. A significant resonance from shoulder tohead occurs at approximately 30 Hz.

The acceleration transmissibility method determines the output accelerationcaused by a given driving acceleration. Transmissibility is defined as the ratio ofthe output acceleration,Aout, as measured on the body to the input acceleration,Ain,as measured on the system. At resonance,Aout exceedsAin. Using accelerometers

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Figure 4 Impact system to test the frequency response in seated or standingsubjects.

implanted in the lumbar vertebrae and an impact introduced by a pendulum(Figure 4), Pope et al. (64) showed that the resonant frequency was 4.5 Hz. Thesestudies demonstrated that much of the dynamic response is due to the combinedrotation and vertical compression of the pelvis-buttocks system. The frequency re-sponse of a tested subject is influenced by posture. Lateral bending and rotation ofthe trunk, which have a higher risk of disc herniation, also lead to a greater transmis-sion of vibration. Due to fatigue during whole body vibration, there is a significantincrease in EMG response of paravertebral lumbar muscles to a sudden unexpectedload applied to the upper trunk. Thus, truckers who unload their truck right awayafter longer driving have a high risk of soft tissue or muscle injury. Of concern to thespine is the fatigue life of the tissues. This can occur with relatively small stresses

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compared to those required for a static stress failure. A tissue’s fatigue life dependson the range of stress imposed. We have found that an intervertebral disc can fatigueand herniate and can develop a tracking tear due to cyclic vertical vibration. Thisis increased if the vibration is asymmetric or the specimen is preloaded.

Forklift drivers, farmers, and construction workers are exposed to long periodsof twisted posture. The twisted posture in a vibrational environment has been shownto cause increased energy consumption compared to twisted posture or vibrationas single exposure variables (65). Seated whole body vibration in a position thatensured muscular activity of the erector spinae muscles caused faster and morepronounced muscular fatigue in the lumbar erector spinae muscles when comparedto the absence of vibration (66). Both static sitting and seated whole body vibrationcaused increased height loss in subjects, suggesting increased spinal load (67).

PREVENTION

General

Because LBP disability is due to more than ergonomics, it is advisable to considerthem in a prevention strategy. Psychosocial factors can influence LBP disability, soit would seem to be advisable to improve this aspect of the working environment.A conflict-free workplace is probably healthier. Personal fitness programs aimedat obesity, physical fitness, and smoking history are also likely to be beneficial.Such programs should take into account the role of age and gender.

Muscle Function

LBP is more prevalent in workers who have less strength than needed by their jobs,and thus, it makes sense to build up the trunk strength of these workers. Screeningor placement programs based on strength performance are problematic unlessclearly legal in that jurisdiction. Emphasis should also be placed on endurance,motor control, and proprioception.

Lifting

The NIOSH guide, which is a very useful and important contribution, is onlyapplicable to smooth lifting, two-handed symmetrical lifting in the sagittal plane,and the lifting of moderate-width objects. However, Dolan et al. (68) report thatspinal loading during lifting, which is controlled by muscles from the trunk andfascia, depends as much on the speed of movement as the size of and positionof the object lifted. Gagnon & Smyth (26) showed that rapid lifting increasesmoments and forces in the spine and the musculature. NIOSH also prescribes thatthe standing postures should be unrestricted, there should be good handles on theobject, favorable environmental conditions, and no other activities going on at thesame time. If any of these other provisions do apply, then the load should be reducedeven further. When the object is large and heavy, it is desirable to make available

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a hoist or crane to assist in its movement. Obviously, this must be convenientlyplaced, otherwise the worker will not use it. The load acting on the lumbar spineduring lifting is a combined result of the object weight, the upper body weight, theback muscle forces, and their respective lever arm to the disc center. Therefore,the distance of the object from the body is a very important ergonomic factor. Thisdistance should be considered in designing a container or other things that are oftenlifted. If a container is compact, a worker can minimize the spinal load momentby keeping the object’s center of mass close to the body. If the container mustbe lifted from the floor, and it is too large to pass between the knees, it requiresthe person to lift the object in front of the knees. This causes a larger spinal loadmoment than would be the case if the object could be lifted between the knees. Anyobject larger than approximately 30 cm cannot easily be lifted between the knees,thus increasing the horizontal distance. Thus, the admonition to lift by the kneesand not by the back is nonsense—it depends entirely on the load shape and size!Most likely, what is of importance is the pelvic orientation. Delitto & Rose (69)reported that trunk muscle activity occurring with the anterior tilt position of thepelvis may ensure muscular support for the spine while handling loads. Granataet al. (25) found that box weight, task asymmetry, and job experience (level ofexperience) affect the magnitude and variability of spinal load during repeatedlifting exertions and may be responsible for LBP. Low lifting-index values wereobserved by Kee & Chung (70) for relatively high L5/S1 compressive forces fromthe EMG-assisted model. This suggests that the 1991 NIOSH lifting equationsmay not fully evaluate the risk of dynamic symmetric lifting tasks. Marras et al.(71) found that the position from which the worker lifted a box on a pallet affectsspine loads with the lower level of the pallet being the greatest. Thus, the effectson spine loading can be controlled depending on its position on the pallet. Handleson the box also reduce spine loading.

Commissaris & Toussaint (72) suggest the absence of load knowledge, andhence overestimating the load to be lifted, may lead to an increased spinal loadand to an increased risk of losing balance in lifting tasks. Underestimating the loadmass to be lifted may lead to similar problems. Thus it is important to label objectswith the weight and to encourage testing of the load by the worker.

Pushing and Pulling

To avoid high compressive forces in the disc, the pulling or pushing hand forceshould be below 225 N. Several factors such as vertical handles that can be graspedat various heights, large low-friction rubber wheels, and two wheels that easilypivot are important. The workplace floors must be clean and free of hazards. Thefloors and shoes must be clean and dry and be high friction. Pushing or pullingforce requirements should be made low, below 225 N, and the hands should bebetween the hip to waist level so as to minimize spine load moments. The floorsurfaces and areas where pushing and pulling activities occur must be kept cleanand dry and workers should be instructed to use shoes with traction.

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SPINE ERGONOMICS 63

Posture

SITTING The posture of a seated person depends not only on the design of thechair, but on sitting habits and the task to be performed. The height and inclinationof the seat of the chair, the backrest, and other types of support all influence theresulting posture. The chair should also permit regular and easy alterations inposture. Inclination of the backrest backward from vertical results in decreasedIDP. Support of the lumbar spine via a backrest towards lordosis reduces thedeformation of the lumbar spine and resultant IDP. Van Dieen et al. (73) suggestthat sitting is a risk factor for LBP due to the prolonged and monotonous low-levelmechanical load imposed by a seated posture. Spinal shrinkage was increased ina fixed rather than dynamic chair.

STANDING To maintain low muscle stresses on the spine when standing, an uprightsymmetric posture should always be advocated, and all material should be handledas close to the body as possible. It is often reported that a compliant mat under thefeet will reduce subjective complaints of back fatigue in standing workers. Thesemats may attenuate impact to the discs. Alternating between sitting and standingis also a good idea.

AWKWARD POSTURES Muscle force and thus disc pressure is increased in pos-tures involving lateral bending or twisting (46). In most cases, these postures canbe avoided by redesign of the workplace to ensure axially symmetric postures.Workplaces should be kept clear of hazards that workers must walk around, andparts must be delivered at a convenient height in front of the worker.

Whole Body Vibration

In a vehicle, the primary source of WBV is the interaction of the vehicle and theground surface, but any component of the vehicle could be a source. In some cars,driving 100 km h−1 over California expressways and the joints in the concreteslabs, which are placed at 5 m intervals, cause resonance of the spine. Thus, theterrain, speed, and vehicle characteristics can all be of importance.

There can be a considerable muscular effort associated with driving due tothe forces necessary to maintain posture and to control the vehicle and resist itsmovements. The components of vibration and shock may not be enough to causeacute injury, but lateral thrusts may add to the spinal stress. Thus, there maybe a symbiotic effect of vibration and posture. The seat itself is only one partof the sitting workplace. Cyclic loading should not exceed ISO 2631 E (62, 63)requirements. Vibration isolation seats should be used where possible, and firmcushions with lumbar support should be employed. If possible, support of thearms and the feet should also be used. The posture should permit the backrest to beused where possible. One method of reducing whole body vibration is to reducethe vibration input. This can only be done by the choice of vehicles, by training

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drivers to choose the vehicle that they drive, and to reduce vibrations by changingtheir driving speed and style. Much of this is within the control of the operator andcan be influenced by training.

Other factors to bear in mind are that a hazardous exposure to WBV can be foundin various forms including driving off-road too fast or over a rough route and driv-ing on badly paved surfaces in vehicles with poor suspension. Exposure to WBV isnot the only cause of back pain. Other factors that can cause or increase back paininclude general posture; poor design of controls, making them difficult to operate;and poor driver visibility, making twisting and stretching necessary when driving.The employer should assess the health risks to workers from WBV and identifywhat is needed to control those risks and should ensure that the equipment the em-ployer provides for his employees has been designed or adapted to minimize WBV.

SUMMARY

Occupational LBP is of major importance both economically and in terms of dis-ability. The problem is getting worse each year and major steps should be takentoward prevention. LBP has been called the albatross of industry and the nemesisof medicine. LBP is one of the most important disabling health problems facingindustrialized societies. The medical, industrial, and socioeconomic costs of LBPare immense. In the United States, LBP is the leading cause of industrial disabilitypayments and the second most common medical cause of work loss in industry. Inthe United States, the total cost of LBP currently comes to nearly 80 billion dollarsper annum (74) and can be anticipated to be far more in the future. Occupationalexposures such as lifting, particularly in awkward postures; heavy lifting; or repeti-tive lifting are related to LBP. Fixed postures and prolonged seating are also riskfactors. Any prolonged posture will lead to static loading of the muscles and jointtissues and, consequently, cause discomfort. The human being’s natural behavior isto change posture often. Even during sleep, there is a need for posture adjustments.No single position should be maintained for a long period of time without con-siderable discomfort. LBP is found in both sedentary occupations and in drivers,as well as those involved in manual materials handling. The seated posture leadsto inactivity that may itself be injurious. Lack of motion leads to an accumula-tion of metabolites, which probably accelerates the degeneration of the disks andincreases the probability of disk herniation. Drivers postures can also lead to mus-culoskeletal problems in the neck, shoulder, and arm. At the workplace, standingand sitting are the two basic forms of postures. These postures have their specificadvantages and disadvantages for mobility, exertion of force, energy consump-tion, circulatory demands, coordination, and motion control. Workers in a WBVenvironment are often also subjected to postural stress, whether they are seated orstanding.

Although it can be a problem, prevention is by far the treatment of choice.Epidemiologic studies clearly indicate the role of mechanical loads on the etiologyof occupational LBP. Methods are available for characterizing the mechanicalenvironment of the worker. This information can be used to adapt the workplace to

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SPINE ERGONOMICS 65

prevent injuries. It is probable that mechanical risk factors along with psychosocialfactors combine as to total risk for LBP disability. Workers should be made awareof the importance of personal factors such as level of general fitness, obesity,smoking, and leisure pursuits. Workers should report LBP as early as possible andseek medical advice if they think occupational exposure is harming them. Thecombined effects of the medical community, labor, and management are requiredto cause some impact on this problem.

The Annual Review of Biomedical Engineeringis online athttp://bioeng.annualreviews.org

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1 Jul 2002 16:18 AR AR164-03-COLOR.tex AR164-03-COLOR.SGM LaTeX2e(2002/01/18)P1: GDL

Figure 2 Trunk goniometer.

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P1: FRK

June 20, 2002 10:12 Annual Reviews AR164-FM

Annual Review of Biomedical EngineeringVolume 4, 2002

CONTENTS

Frontispiece—Kenneth R. Foster xii

HERMAN P. SCHWAN: A SCIENTIST AND PIONEER IN BIOMEDICALENGINEERING, Kenneth R. Foster 1

ROLES FOR LEARNING SCIENCES AND LEARNING TECHNOLOGIES INBIOMEDICAL ENGINEERING EDUCATION: A REVIEW OF RECENTADVANCES, Thomas R. Harris, John D. Bransford, and Sean P. Brophy 29

SPINE ERGONOMICS, Malcolm H. Pope, Kheng Lim Goh,and Marianne L. Magnusson 49

THREE-DIMENSIONAL CONFOCAL MICROSCOPY OF THE LIVINGHUMAN EYE, Barry R. Masters and Matthias Bohnke 69

BIOENGINEERING OF THERAPEUTIC AEROSOLS, David A. Edwardsand Craig Dunbar 93

DENATURATION OF COLLAGEN VIA HEATING: AN IRREVERSIBLERATE PROCESS, N.T. Wright and J.D. Humphrey 109

DNA MICROARRAY TECHNOLOGY: DEVICES, SYSTEMS, ANDAPPLICATIONS, Michael J. Heller 129

PEPTIDE AGGREGATION IN NEURODEGENERATIVE DISEASE,Regina M. Murphy 155

MECHANO-ELECTROCHEMICAL PROPERTIES OF ARTICULARCARTILAGE: THEIR INHOMOGENEITIES AND ANISOTROPIES,Van C. Mow and X. Edward Guo 175

ELECTROMAGNETIC FIELDS: HUMAN SAFETY ISSUES, Om P. Gandhi 211

ADVANCES IN IN VIVO BIOLUMINESCENCE IMAGING OF GENEEXPRESSION, Christopher H. Contag and Michael H. Bachmann 235

PHYSICS AND APPLICATIONS OF MICROFLUIDICS IN BIOLOGY,David J. Beebe, Glennys A. Mensing, and Glenn M. Walker 261

TELEREHABILITATION RESEARCH: EMERGING OPPORTUNITIES,Jack M. Winters 287

BIOMECHANICAL DYNAMICS OF THE HEART WITH MRI, Leon Axel 321

ADVANCES IN PROTEOMIC TECHNOLOGIES, Martin L. Yarmushand Arul Jayaraman 349

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June 20, 2002 10:12 Annual Reviews AR164-FM

vi CONTENTS

ON THE METRICS AND EULER-LAGRANGE EQUATIONS OFCOMPUTATIONAL ANATOMY, Michael I. Miller, Alain Trouve,and Laurent Younes 375

SELECTIVE ELECTRICAL INTERFACES WITH THE NERVOUS SYSTEM,Wim L.C. Rutten 407

INDEXESSubject Index 453Cumulative Index of Contributing Authors, Volumes 1–4 475Cumulative Index of Chapter Titles, Volumes 1–4 477

ERRATAAn online log of corrections to Annual Review of BiomedicalEngineering chapters (if any, 1997 to the present) may be found athttp://bioeng.annualreviews.org/

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