A Critical Review of Epidemiologic Evidence for Work-Related Musculoskeletal Disorders of the Neck, Upper Extremity, and
Low Back

Chapter 2. Neck Musculoskeletal Disorders: Evidence for Work-Relatedness

Summary

Over 40 epidemiologic studies have examined physical workplace factors and their relationship to neck and neck/shoulder musculoskeletal disorders (MSDs). Among these studies are those which fulfill rigorous epidemiologic criteria and appropriately address important issues so that causal inferences can be made. The majority of studies involved working groups with a combination of interacting work factors, but certain studies assessed specific work factors. Each of the studies we examined (those with negative, positive, or equivocal findings) contributed to the overall pool of data for us to use in assessing the strength of the work-relatedness using causal inference.

There is evidence for a causal relationship between highly repetitive work and neck and neck/shoulder MSDs. Most of the epidemiologic studies reviewed defined “repetitive work” for the neck as work activities which involve continuous arm or hand movements which affect the neck/shoulder musculature and generate loads on the neck/shoulder area; fewer studies examined relationships based on actual repetitive neck movements. The two studies which measured repetitive neck movements by measuring head position (using frequency and duration of movements) fulfilled the most stringent epidemiologic criteria, showing strong associations with neck/shoulder MSDs. In those studies defining repetitive work involving continuous arm or hand movements affecting the neck/shoulder, nine studies were statistically significant and had odds ratios (ORs) greater than 3.0.; eight studies fulfilled all the epidemiologic criteria except the exposure criteria, and measured repetition for the hand/wrist and not for the neck. Of these, three were statistically significant and had ORs greater than 3, five had nonsignificant ORs, all under 2.0.

There is also evidence for forceful exertion and the occurrence of neck MSDs in the epidemiologic literature. Most of the epidemiologic studies reviewed defined “forceful work” for the neck/shoulder as work activities which involve forceful arm or hand movements, which generate loads to the neck/shoulder area; no study examined a relationship based on actual forceful neck movements. Of the 17 studies addressing force as one of the exposure factors, five studies found statistically significant associations, but did not derive ORs; two studies found ORs greater than 3.0, seven studies from 1 to 3.0, and two studies with ORs less than 1.0. Many of the studies relating measured force (as workload, etc.) to MSDs are in the biomechanical and ergonomic literature.

There is strong evidence that working groups with high levels of static contraction, prolonged static loads, or extreme working postures involving the neck/shoulder muscles are at increased risk for neck/shoulder MSDs. Consistently high ORs were found (twelve statistically significant studies with ORs over 3.0) providing evidence linking tension-neck syndrome with static postures or static loads.

The epidemiologic data were insufficient to provide support for the relationship of vibration to neck disorders. At this time, further studies must be done before a decision regarding causal inference is made. The few prospective studies which have included interventions to decrease workplace exposures that include decreasing repetitive work and less extreme working postures showed a decrease in the incidence of neck MSDs and an improvement in symptoms among affected workers. The data on intervention provide additional evidence that these disorders are related to workplace risk factors.

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Introduction

Studies from the United States have generally classified neck disorders separately from shoulder disorders when evaluating work-related risk factors. Scandinavian studies examining work-related factors, on the other hand, have often combined neck and shoulder MSDs into one health outcome variable. This was based on the concept that several muscles act on both the shoulder girdle and the upper spine together. We have divided our reviews of the neck and shoulder MSDs into two chapters: Chapter 2 addresses neck and neck/shoulder MSDs and Chapter 3 addresses shoulder MSDs.

Our discussion of the evidence for work-relatedness of the neck will include criteria Tables 2-1 through 2-6 and Figures 2-1 through 2-6. Shoulder MSDs will be discussed in the next chapter.

Epidemiologic studies have defined neck MSDs in one of two ways: (a) by symptoms occurring in the neck (usually with regard to a specific duration, frequency, or intensity), or (b) by using both symptoms and physical examination findings.

The prevalence of reported MSDs is generally lower when they are defined using both symptoms and physical examination results than when defined using symptoms alone. For example, the prevalence rate of tension neck syndrome (TNS) among male industrial workers in the United States was reported to be 4.9% from interview data and 1.4% when case definitions included physical exam findings [Hagberg and Wegman 1987]. The percent of work-related MSD cases defined by physical examination findings to those defined solely by symptoms has ranged from approximately 50% (Silverstein et al. [1987]; Blåder et al. [1991]; Bernard et al. [1993]; Hales et al. 1994]) to about 85% (Andersen and Gaardboe [1993b]). Forty-seven of the listed studies referenced included physical examination findings in their health outcome assessment criteria.

Many of the neck and neck/shoulder MSD studies referenced in the tables were part of larger studies that inquired about musculoskeletal symptoms and physical findings in multiple body sites. In most of these studies, there were no separate ergonomic exposure observations or measurements made that pertained to the neck region (e.g., there were no neck posture observations, neck angle measurements, neck work-load assessment, trapezius electromyographic testing, etc.). In these studies, the primary interest and measurement strategies focused on the hand and wrist region (e.g., Kuorinka and Koskinen [1979]; Ohlsson et al. [1989]; Hales et al. [1989]; Kiken et al. [1990]; Baron et al. [1991]). In the studies, workers were categorized only by hand/wrist exposures. Hand/wrist categorization will not reflect exposures of the neck region (or other musculoskeletal sites). For example, workers who may have frequent and rapid awkward postures of the neck but less frequent or extreme postures of the hand and wrist region may be misclassified as low risk if classification depends only on hand/wrist exposure. In general, we have given these studies less weight because of a significant potential for misclassification.

The text of this section on neck and neck/shoulder MSDs is organized by work-related exposure factor. The discussion within each factor is organized according to the criteria for evaluating evidence for work-relatedness in epidemiologic studies using the strength of association, the consistency of association, temporal relationships, exposure-response relationship, and coherence of evidence. Conclusions are presented with respect to neck and neck/shoulder MSDs as a single disorder for each exposure factor. Summary information relevant to the criteria used to evaluate study quality is presented in Tables 2-1 through 2-6. A more extensive summary, which includes information on health outcome, covariates, and exposure measures, is presented at the end of this chapter.

Studies Included in Neck MSDs Tables

Forty-six epidemiologic studies dealing with neck MSDs and 23 dealing with neck/shoulder MSDs appear in the summary tables. Of the studies, 38 were cross-sectional, 2 were case-control studies, and 6 were prospective studies. Among all the studies pertaining to the neck or neck/shoulder area, 35 had participation rates of over 70%, 3 had less than 70%, and 8 did not report their participation rates.

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Repetition

Definition of Repetition for Neck and Neck/Shoulder MSDs

For our review of the neck or neck/shoulder region, we chose those epidemiologic studies that examined repetition or repetitive work activities and MSDs. Studies generally address repetition as cyclical work activities that involved either: (1) repetitive neck movements (e.g., the frequency of different head positions during a cycle), or (2) repeated arm or shoulder motions that generate loads to the neck/shoulder area (e.g., trapezius muscle). Most of the studies that examined repetition or repetitive work as a potential risk factor for neck or neck/shoulder MSDs had several concurrent or interacting physical workplace factors that were being evaluated. Therefore, repetitive work was not necessarily considered the primary exposure factor but was considered along with the other work factors.

Studies Reporting on the Association of Repetition as a Work Factor for Neck and Neck/Shoulder MSDs

Either the risk factor “repetition” or “repetitive work” was included in 26 studies as a factor for selection of the study population in their examination of neck and neck/shoulder MSDs in the workplace. However, only a handful of these studies examined repetitive movements of the neck. Few of these studies observed or measured: (a) the frequency or duration of tasks pertaining to the neck, (b) the ratio of work-time-to-recovery time for neck or neck/shoulder involvement, or (c) the percentage of the workday spent on repetitive activities involving the neck. Instead, studies tended to compare and contrast the prevalences of neck symptoms and/or physical findings in workers in occupations requiring a combination of forceful, repetitive movements and extreme postures of the upper extremities (mainly of the hand/wrist) to workers in occupations without those requirements.

Twenty studies that mentioned repetitive work or repetitive movements found a statistically significant positive association between repetition and neck or neck/shoulder MSDs; 6 others had non-significant findings (Tables 2-1 and 2-2, Figures 2-1 and 2-2). In terms of magnitude of the association, 11 studies had ORs greater than 3.0, 11 had ORs between 1.0 and 3.0, and none hadan OR less than 1.0. Four studies did not report their results in terms of ORs or Prevalence Rate Ratio (PRRs), although all of these found significant associations (p<0.05).

Studies Meeting the Four Evaluation Criteria
Of the 27 investigations (see Tables 2-1 and 2-2), 2 fulfilled all four evaluation criteria outlined earlier in the introduction section [Ohlsson et al. 1995; Jonsson et al. 1988]. Only the Ohlsson study reported ORs. The investigations assessed repetitive work as an independent variable in terms of frequency and duration of neck movements.

In the cross-sectional study by Ohlsson et al. [1995], female industrial assembly-line workers exposed to repetitive tasks with short (<30 seconds) cycles were compared to 2 referent groups: 68 former assembly workers and 64 other workers with no repetitive exposure at their current jobs. Industrial workers had to perform tasks with a posture requiring an intermittently flexed neck and elevated arms, which were abducted intermittently. Workers and referents reported neck/shoulder symptom(s) and had physical exams performed by a single examiner. The examiner was blinded to exposure status but not completely to group status. Ergonomic exposure assessment was extensive. It included videotaping, observation, and analysis of postures, including measurements of critical angles (15° and 30°) of flexion of the neck. Two independent readers determined frequency, duration, and critical angles of movement for each variable by taking the average of the two readings. Weekly working time, work rotation, patterns of breaks, and individual performance rate (piece rate) were recorded and used in the analysis. The study controlled for age, gender (only females were included), and psychosocial variables (“tendency for stress” and “worry”).

The other study that fulfilled the four criteria concerned a 3-year prospective study written up in a series of articles by Kilbom et al. [1986], Kilbom and Persson [1987], and Jonsson et al. 1988]. Female electronic workers in highly repetitive tasks with static postural loads to the neck and shoulder areas were followed over a 3-year period. In the second year, some of the employees had workplace interventions that decreased the number of repetitive tasks involving extreme neck and shoulder postures, while others continued to work at unaltered tasks. Three separate physical exams were carried out at yearly intervals, the first one initially assessing tenderness on palpation and pain or restriction with active and passive movements. Ergonomic assessments occurred at the outset of the study and included video analysis of postures and movements of the head, shoulder, and upper arm. The evaluation recorded work-cycle time and number of cycles per hour; time at rest for the arm, shoulder, and head; total number of rest periods; and average and total duration per work cycle and hour. (The method was designed to study short-cycle repetitive work under visual control.) The mean number of neck forward flexions >20° per hour was 728 (standard deviation [s.d.] 365) in the initial 96 workers. The participation rate of the study was 72% after 3 years; the investigators analyzed several variables separately for dropouts and found no significant differences with regards to medical status, physiologic capacity, working technique, or work history. The investigators performed step-wise logistic regression with deterioration of disorders or remaining healthy in the different locations (neck and neck/shoulder) as the two dependent variables. Age, muscle strength, job satisfaction, and high productivity were included in the logistic regression analyses of these studies. Video analysis and observation were used to assess repetitive exposure on all subjects, using work cycle time, number of cycles per hour, as well as number of neck flexions per hour as criteria. Work cycle time varied between 4.6 and 9.1 min, with a mean value of 6.6 min.

Strength of Association for Repetition

In the Ohlsson et al. [1995] study, the OR for the association between repetitive work related to the neck and any neck/shoulder diagnoses was 4.6; for a diagnosis of tension neck syndrome, it was 3.6.

For the cohort study carried out by Kilbom et al. [1986], at the 2-year follow up, the number of neck flexions per hour appeared as a strong predictor for deterioration to severe disorders of the neck. Improvement to a “healthy status” classification from Year I to Year II was seen with reallocating workers to more varied work tasks (which required a reorganization of monotonous and repetitive work tasks). The new tasks were characterized as more dynamic and varied and included only occasional sitting tasks, caretaking work, surveillance of machinery, or assembling of bigger and heavier equipment. The article documenting the last phase of the cohort study by Jonsson et al. [1988] did not specifically address the neck but broadened the health outcome definition to include the neck/shoulder area and the rest of the upper extremity using “cervicobrachial region” as the health outcome of interest. A significant association between deterioration of health status of the cervicobrachial region between Year II and Year III of the study and “work cycle, total time” at the p<0.05 level was found (ORs were not given).

Studies Meeting at Least One of the Four Criteria—Strength of Association
Of the studies that found significant ORs over 3.0 but did not mention or fulfill all of the criteria, almost all focused on working groups with a combination of repetitive and forceful work and compared them to either population referents or groups in occupations with lower exposure. Almost all were cross-sectional surveys. These studies used health outcomes from symptom surveys and self-reported workplace exposure (no direct observations) and either compared symptomatic workers (neck MSD cases) to asymptomatic workers in the same workforce (e.g., Yu and Wong [1996]; Bergqvist et al. [1995a]; Schibye et al. [1995]; Hunting et al. [1981]) or in other occupations (e.g., Liss et al. [1995]; Andersen and Gaardboe [1993b]; Milerad and Ekenvall [1990]; Onishi et al. [1976]). Onishi et al. [1976] found significant differences in neck/shoulder MSDs (OR 3.8) between groups involved in repetitive upper limb operations and office workers. They found workers involved in repetitive activity had 10% to 30% maximum voluntary contraction (MVC) of the trapezius muscle. They concluded that habitual neck or shoulder muscle fatigue is caused by repetitive tasks that result in localized tenderness and may be a precursor to chronic MSDs.

Andersen and Gaardboe [1993a] used a cross-sectional design to compare sewing machine operators with a random sample of women from the general population of the same region. A neck case required a strict predetermined symptom and physical examination definition. Exposure was assessed through observation and categorization of jobs, based on the authors’ experience and judgments. However, the main interest for exposure assessment was duration of exposure as a sewing machine operator. Statistical modeling controlled for age, having children, not doing leisure exercise, smoking, and socioeconomic status found a significant trend for “neck/shoulder syndrome” in relation to years of exposure as a sewing machine operator, with ORs from 3.2 to 36.74. The OR for the lowest exposure category, 0-7 years, was not statistically significant, although the higher exposure levels were. For this study, the exposure classification scheme does not allow separation of the effects of repetition from those of force, and there was no precise measure of repetitiveness.

Baron et al. [1991] studied neck MSDs in 124 grocery store checkers and 157 other grocery store workers who were not checkers. The neck MSD case definition met predetermined symptom and physical exam criteria. Physical examinations had higher participation rates among the checkers (85%) than among the referents (55%). Telephone interviews to non-checkers resulted in questionnaire completion by 85% of the non-checkers. The OR for neck disorders among checkers was 2.0 (95% confidence interval [CI] 0.6–6.7), in a model that included age, hobbies, second jobs, systemic disease, and obesity.

Bergqvist et al. [1995a] carried out a study comparing office workers using video display terminals (VDTs) to those who did not. A physiotherapist’s diagnosis of tension-neck syndrome was used to define a case. Exposure assessment was based on both self-reports and the investigators’ observation of work postures, movements, and measurements of heights of work-station equipment in conjunction with the user. Statistical modeling included several individual factors, organizational factors, and ergonomic factors. For “tension neck” syndrome, no factor related to repetitive work was found to be significantly related.

Blåder et al. [1991] surveyed 199 sewing machine operators from 4 plants. Of the 155 who reported shoulder or neck pain, 131 were examined. Exposure assessment was by questionnaire and addressed employment duration and hours per week. Authors stated that the study involved a control group and took into account psychosocial factors, but the results were not included in the article. Both employment duration and working more than 30 hours per week were found to be statistically significant at the p<0.05 levels. For this study, the exposure as duration of work (per week and per years) does not allow separation of the effects of repetition from those of force. There was no direct measure of repetitiveness.

Ekberg et al. [1994] carried out a case-control study involving cases from a semi-rural community in southern Sweden who had consulted a community physician for MSDs of the neck, shoulder, arm, or upper thorax. Cases had to have been ill immediately prior to physician visit and have been on sick leave less than 4 weeks. Cases were excluded for trauma, infectious causes, accident, malignancy, rheumatic disease, abuse, or pregnancy. Controls were randomly selected from the Swedish insurance registry. Exposure was obtained by questionnaire. The analysis showed that for neck disorders with precise repetitive movements the OR was 3.8 for medium exposure and 15.6 for high exposure comparing jobs with low force and low repetition. Gender, immigrant status, work pace, and current smoking were also analyzed in the logistic model.

Ekberg et al. [1995] surveyed 637 Swedish residents for the presence of neck symptoms in the past six months. Exposure was based on questionnaire responses. Twenty questionnaire items on physical work conditions were factor analyzed. Age, smoking, exercise habits, and family situation with preschool children were not significantly associated with symptoms. Repetitive movements demanding precision was found to have an OR of 1.2 for neck pain.

Hales and Fine [1989] compared 89 female workers in 7 high exposure jobs to 25 female poultry workers in low exposure jobs employed in poultry processing. Neck case definition required symptoms and physical examination findings that met predetermined criteria. Exposure assessment was based on hand/wrist assessment of forceful and repetitive jobs. No assessment of neck repetition was performed. Twelve percent of workers in high risk jobs versus none in low risk jobs were found to have neck MSDS.

In a study of VDT users in a range of jobs (data entry to “conversational” VDT use), Hünting et al. [1981] used a case definition requiring symptoms and physical exams and an extensive exposure assessment using questionnaire, observation, and measurements of workstations, and body posture measurements using a prescribed method. Data entry terminal users, whose tasks required more extensive repetitive work than traditional office workers, found an OR of 9.9 with the comparison. There were no adjustments for confounders in this analysis.

Kamwendo et al. [1991] compared 420 medical secretaries with frequent, significant neck pain to those with few episodes based on questionnaire responses. Exposure was also questionnaire based. The analysis was controlled for age and length of employment. A surrogate for repetitive work consisted of hours sitting or working with office machines with high exposure equal to 5 hrs or more/day.

Kiken et al. [1990] also studied poultry workers at two plants with exposure to highly forceful, highly repetitive jobs and compared them to other poultry workers with less exposure. Neck case definition required symptoms and physical examination findings that met predetermined criteria. Exposure assessment was based on hand/wrist assessment of forceful and repetitive jobs. No assessment of neck repetition was performed. Job turnover was around 50% at plant 1 and 70% at plant 2 making survivor bias a strong possibility.

Kuorinka and Koskinen [1979] studied occupational rheumatic diseases and upper limb strain among 93 scissor makers and compared them to the same group of department store assistants (n=143) that Luopajärvi et al. [1979] used as a comparison group. Temporary workers and those with recent trauma were excluded from the scissor makers group. Exposure assessment included videotape analysis of scissor maker tasks, however exposure assessed for the hand and wrist region and not the neck. No formal exposure assessment was conducted on the shop assistants. Health assessment involved an interview and physical examination by a physiotherapist following a standard protocol. Diagnoses of tension neck syndrome were determined using predetermined criteria [Waris et al. 1979]. In problem cases, orthopedic and physiatric teams determined case status. It is unclear whether cashiers were excluded from the comparison group in this study as they were in the Luopajärvi et al. [1979] study. The study group was 99% female.

Luopajärvi et al. [1979] compared the prevalence of neck/shoulder disorders among 152 female assembly line packers in a food production factory to 133 female shop assistants in a department store. Exposure to repetitive work, awkward hand/arm postures, and static work was assessed by observation and videotape analysis of factory workers. No formal exposure assessment was conducted on the department store workers; their job tasks were described as variable. Cashiers were excluded, presumably because their work was repetitive. No formal assessment occurred for neck/shoulder repetition. The health assessment consisted of interviews and physical examinations conducted by a physiotherapist, and diagnoses of tension neck syndrome were later determined by medical specialists using these findings and predetermined criteria (95% CI 2.63–6.49). Age, hobbies, and housework were considered in the analysis.

Milerad and Ekenvall [1990] compared the self-reported neck and neck/shoulder symptoms between dentists and pharmacists. Dentists had been considered the high risk group because of awkward postures and repetitive use of small hand tools. Exposure was based on self-reports. The authors examined several covariates and stratified by gender for their analysis. No difference between groups in leisure time, smoking, systemic disease, and exposure to vibration.

Ohlsson et al. [1989] studied 148 electrical equipment and automobile assemblers, 76 former female assembly workers who quit within 4 years and compared these two groups to 60 randomly sampled females from the general population. A case was determined by questionnaire; exposure was based on job categorization and questionnaire responses. Repetitive exposure was based upon the number of items completed per hour. The work pace was divided into four classes: (1) Slow: <100 items/hr; (2) Medium: 100 to 199 items/hr; (3) Fast: 200 to 700 items/hr; (4) Very Fast: >700 items/hour. The OR increased with increasing work pace, except at very high paces, where there was a decrease. This was attributed to “selective quitting of subjects with complaints, only the healthiest being left in the assembly work.”

Onishi et al. [1976] compared several groups of workers with varying exposure to repetitive tasks. Health outcome was based on symptoms of shoulder stiffness, dullness, pain, numbness; pressure measured by strain transducer at which a subject felt pain; and a physical exam. Observation and measurements of some job tasks, including some measures of repetition, were performed then job categorization was done. Based on job categorization and job analysis, and taking into account shift length, activities, number of breaks, repetitive movements of the hands, arm manipulations, and length of employment, there was not a difference between workers with tenderness threshold above 1.5 kg/cm² and those below with respect to age, height, weight, skinfold thickness, grip strength, upper arm abduction strength, and back muscle strength.

Punnett et al. [1985] compared neck/shoulder MSDs based on symptom reporting alone in 162 women garment workers and 76 women hospital workers such as nurses, laboratory technicians, and laundry workers. There was a low participation rate among the hospital workers. Eighty-six percent of the garment workers were sewing machine operators and finishers (sewing and trimming by hand). The sewing machine operators were described as using highly repetitive, low force wrist and finger motions, while the finishers had shoulder and elbow motions as well. The exposed garment workers likely had more repetitive jobs than most of the hospital workers. The neck/shoulder cases were found to lift both the “typical” and “heaviest” loads with greater frequency than non-cases.

Sakakibara et al. [1995] found among orchard workers that neck shoulder MSDs based on symptom and physical findings were significantly higher when performing pear bagging than when apple bagging. Exposure was based on measurements of specific angles of the neck and shoulder and job tasks in a representative worker. ORs were not derived in this study. Confounders were not checked for in this study.

Sakakibara et al. [1987] did not include physical exam findings in the case definition of neck and neck/shoulder MSDs when comparing workers bagging pears versus apples. Exposure was again based on measurements of job tasks by a representative worker.

Schibye et al. [1995] followed up 303 sewing machine operators at nine factories representing different technology levels who completed a questionnaire in 1985. In April 1991, 241 of 279 traced workers responded to the same 1985 questionnaire. Operators still working were compared to those who moved to other employment in 1991. Exposure was assessed through a questionnaire asking type of machine operated, work organization factors, workplace design factors, units produced per day, the payment system, and the duration of employment as a sewing machine operator. Although the authors state that the analysis did not show that neck symptoms among workers who had worked as a sewing machine operator to be significantly related to exposure, exposure time, or age, there was a significant drop-out rate of those above 35 years.

Rossignol et al. [1987] chose 38 random sites from Massachusetts workers with more than 50 employees, and selected 191 workers from computer and data processing services, and public utilities and the Commonwealth Government. Subjects were selected after the observation of the work site. A self-administered questionnaire case definition was used for neck MSD. Exposure was also based upon self-reports of number of hours worked each day with a keyboard machine with a VDT. Analysis controlled for the following confounding factors: age, cigarette smoking, industry, and educational VDT training.

Yu and Wong [1996] chose to compare 90 data entry, data processing, and computer programmers from an International Bank in Hong Kong and 61 infrequent users of VDTs. Both neck MSD case definition and exposure assessment were based on symptom data. Analysis controlled for “age and gender, and other covariates” (as stated in the paper). For frequent VDT use an OR of 28.9 was found.

Kuorinka and Koskinen [1979] found a significant difference in neck MSDs between scissor makers (an occupation chosen for study because of its assembly-line repetitive hand tasks) and shop assistants (non-stereotypic, non-repetitive jobs) with an OR of 4.1. In the same study, comparing the different stereotypic, repetitive jobs in scissor-making, those in short-cycled tasks (2–9.5 sec) had no significantly different prevalence of neck disorders than workers in longer-cycled tasks (7.3–26 sec) (OR 1.6, 95% CI 0.7– 3.8). It is important to note that both the longer-cycled tasks and short-cycled tasks in Kuorinka’s study would have been classified as “highly repetitive” in most other ergonomic studies [Silverstein et al. 1987; Chiang et al. 1993; Viikari-Juntura et al. 1991a; Kurppa et al. 1991]. When comparing two groups in which the level of repetitive exposure may not differ by much (in this case, where both groups have highly repetitive tasks), it is unlikely that one will find a significant difference because there is not enough variance between the exposures.

Three studies [Ekberg et al. 1994, 1995; Milerad and Ekenvall 1990] used health outcomes and exposure assessments based on self-reports and found significant associations between symptoms and repetitive work. The Ekberg studies specifically asked about “precise repetitive movements” in their questionnaire and controlled for confounders and effect modifiers (age, gender, having pre-school children) in their analyses. Milerad and Ekenvall [1990] compared dentists and pharmacists, stratified by gender, and found no association between neck or neck/shoulder MSDs with metabolic disease, smoking, leisure time, exposure, or vibration. Significant ORs of 2.0 to 2.6. for neck MSDs were reported for dentists compared to pharmacists.

Of those studies reporting no significant association between repetition and neck or neck/shoulder MSDs, none included exposure assessment or observations of the neck or neck/shoulder area that were both objective and independent of the hand/wrist. Several of these studies [Baron et al. 1991; Kiken et al. 1990; Hales et al. 1989; Ohlsson et al. 1989; Luopajärvi et al. 1979] categorized workers into high and low exposure groups based strictly on hand/wrist exposure and not arm, shoulder, or neck exposure. All of these studies reported ORs below 2.0.

In the study of VDT users by Bergqvist et al. [1995a], exposure was based on self-reports of “the presence of repeated work movements” for all work tasks and not specifically focused on the neck or neck/shoulder area. They found no significant association with neck/shoulder MSDs when the variable “repeated work movements” was analyzed in the logistic model alone, but found a significant relationship with a combination of variables: (1) workers wearing glasses, (2) who reported VDT use, and (3) VDT use for more than 20 hours/week. In this case, it was the combination of variables at higher levels of exposure (VDT use more than 20 hours per week) that was found to be statistically significant.

Temporal Relationship—Repetition and Neck/Shoulder MSDs

Of the prospective studies of neck MSDs that can be used to establish a temporal relationship between exposure to repetitive work and neck or neck/shoulder disorders, the study by Jonsson et al. [1988] fulfills all the four study criteria. Jonsson’s study was a follow up of the cohort studied by Kilbom et al. [1986], electronic workers who entered the study without MSDs. Exposure assessment pertaining specifically to the neck/shoulder area was completed three times over 3 years.

In the longitudinal study by Ohara et al. [1976], the authors attributed the increase in neck symptoms in cash register operators to the introduction of new electronic cash registers placed at unsuitable heights. They noted an increase in repetitiveness and an increase in awkward and static postures by cash register operators using the new registers. The authors reported a relationship between static loading and MSDs and found that a subsequent reduction in exposure to static loading resulted in less worker disability (sick leave).

Although temporality cannot be obtained from cross-sectional studies, several studies attempted to insure that disorders developed following the exposure being studied. In certain studies [Baronet al. 1991; Kiken et al. 1990; Hales et al. 1994; Hoekstra et al. 1994], the health outcome definition excluded persons reporting symptoms prior to the job or reporting acute injury thought to be unrelated to work, insuring that exposure preceded MSD occurrence. Other studies excluded participants with less than 6 months (or even longer) of job experience, thereby omitting from their study workers who may have developed their MSDs prior to working at the job of interest, or who had experienced discomfort or fatigue due to new activities or a “break-in period” at work. It is reasonable to assume that in those studies, given the exclusions required by the case definitions, the onset of exposure was prior to the onset of neck/shoulder MSDs in the majority of participants.

Consistency in Association for Repetition and Neck/Shoulder MSDs

In the studies fulfilling the four criteria [Ohlsson et al. 1995; Jonsson et al. 1988; Kilbom et al. 1986], significantly positive associations between neck MSDs and repetitive work were found. Many more studies involved workers in repetitive work from a range of industries (VDT workers, dentists, electronic assembly, sewing machine operators, etc.), comparing symptom prevalences to those in less repetitive jobs. There was also significant association between neck and neck/shoulder MSDs and jobs with repetitive tasks, with ORs between 1.6 and 5.9 [Onishi et al. 1976; Kuorinka and Koskinen 1979; Rossignol et al. 1987; Vihma et al. 1982; Kamwendo et al. 1991; Andersen and Gaardboe et al. 1993b; Ekberg et al. 1994, 1995; Schibye et al. 1995] indicating that workers exposed to higher levels of work risk factors have greater rates of neck and neck/shoulder symptoms. None of the studies that failed to find significant associations carried out exposure assessment of the neck or neck/shoulder.

Coherence of Evidence for Repetition

Studies outside the epidemiologic literature give supportive evidence that repetitive work is related to neck/shoulder disorders. Stevens et al. [1966] found that the neck injuries among fork-lift truck drivers were from repetitive, extreme head rotations needed for the operation of fork lift trucks and introduced the sideways sitting driver forklift. Eklund et al. [1994] reported following up on a “sideways-sitting” forklift (in an unpublished study); these drivers experienced neck pain three times as often as other drivers on traditional forklifts—indicating that moderate head rotations during long periods of time can be more risky than short term and extensive head rotations. Nicholas [1990] reported in his discussion on pathophysiologic mechanisms of sports injuries that a low-load force with high repetition results in a gradual deterioration of tissue strength from strain to fatigue to deformation, with prefailure symptoms, such as pain on use, a common clinical sign of early inflammation from overuse.

Exposure Response Relationship for Repetition

There were no studies reviewed that showed a clear dose-response relationship between repetition and neck and neck/shoulder MSDs.

Conclusions Regarding Repetition

The association between neck or neck/shoulder MSDs and repetitive work was found to be statistically significant in 19 studies using different epidemiologic approaches and under different circumstances of exposure. Twenty-seven studies found ORs above one; of these, 13 were above 3.0. Almost all the studies (6 of 8) with non-significant associations used hand/wrist exposure assessments for their analyses and did not conduct specific neck, shoulder, or upper extremity (apart from hand/wrist) exposure assessment. (Only one of the studies finding significant associations did so using hand/wrist exposure assessment.) The possibility of misclassification affecting the results must be a consideration.

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Force

Definition of Force for Neck and Neck/Shoulder MSDs

For our review, we included studies that examined force or forceful work or heavy loads to the neck and neck/shoulder, or described exposure as strenuous work involving the upper extremity that generates loads to the trapezius muscles. Most of the studies that examined force or forceful work as a risk factor for neck/shoulder had several concurrent or interacting physical work load factors.

Force has generally been defined as: (1) either externally as a load or internally as a force on a body structure, or (2) a force magnitude expressed in newtons or pounds or as a proportion of an individual’s strength capacity, that is, of a person’s MVC, usually measured by EMG. Most studies that have dealt with force loading of the neck or stress generated on the neck structures are from biomechanical studies performed in the laboratory. These studies are not included in this document. In the epidemiologic studies reviewed, force is usually estimated by either questionnaire, biomechanical models, in terms of weight lifted, electromyographic activity, or the variable, “ heavy physical workload.”

Seventeen studies reported results on the association between force or forceful work (in combination with repetition) and neck and neck/shoulder MSDs. Of the 17 studies of force and neck MSDs, 11 found a statistically significant positive association between force and neck or neck/shoulder MSDs; six others had non-significant findings. In terms of magnitude of the association, two studies had ORs greater than 3.0, seven were between 1.0 and 3.0, and two were less than 1.0. Six studies did not report their results in terms of ORs or prevalence rate ratios (PRRs) but reported that the findings were statistically significant at the p<0.05 level.

Studies Meeting the Four Criteria for Force and Neck/Shoulder MSDs

There were no studies that met the four epidemiologic evaluation criteria for forceful exertion of the neck.

Studies Not Meeting the Four Criteria for Force and Neck/Shoulder MSDs

Aaras [1994] carried out a cohort study of four groups, 15 female assembly workers making telephone exchanges, 27 female VDT users, 25 female data entry operators, and 29 male VDT users. Case definition for neck MSD was based on self-reports. However, musculoskeletal sick leave per man-labor years was also used as an endpoint. For force estimate the load on the trapezius was measured by electromyography (EMG).

Quantification of the muscle load was done by ranking the interval estimate (0.1 s) to produce an amplitude probability distribution function. Both the total duration and number of periods per minute when muscle activity was below 1% maximum voluntary contraction (MVC) were calculated. Post-intervention (which involved changes to the workstation, tools, and organization of work)—see Table 2-4 at the end of the chapter for further explanation, the mean static trapezius load in assemblers was reduced from 4.3% MVC to 1.4%, the mean static trapezius load in VDT users reduced from 2.7% MVC to 1.6% MVC (post-intervention). Sick leave also decreased considerably. Because so many interventions were involved in this study, it is not clear to what intervention changes the decrease in sick-leave per man-labor years might be attributed.

Bjelle et al. [1981] compared 13 workers of an industrial plant consecutively seen at a health clinic with acute, nontraumatic shoulder-neck pain not due to causative disease or malformation compared to 26 controls, matched on age, gender and place of work.

In another cohort study, Veiersted and Westgaard [1994] followed 30 female chocolate manufacturing workers, 17 of whom contracted trapezius myalgia within 6 to 51 weeks compared to those workers who did not. Diagnosis was based on both symptoms and physical exam. There were prospective interviews every 10 weeks to detect symptoms of muscle pain. Daily “pain diaries” were also kept by subjects.

Exposure assessment consisted of measured static muscle tension recorded by EMG. Interviews concerning exposure at work were also conducted prospectively every 10 weeks for 1 year. Only 55% of the subjects were retained during the full study; however, the ‘drop-outs’ were follow-up subjects and had no significant differences in static muscle tension compared to the participants.

Viikari-Juntura et al. [1994] , the third longitudinal study discussed under force and neck and neck/shoulder MSDs, used questionnaire to assess neck symptoms and based exposure on job category, comparing 688 machine operators, 553 carpenters, and 591 office workers. For the initial evaluation, observation of work sites were performed. In multivariate analysis occupation, age, and current smoking were significant predictors in change from no neck trouble to severe neck trouble (ORs were not given for logistic model.)

Wells et al. [1983] evaluated letter carriers with an increased load on the shoulder from a mailbag. Letter carriers were compared to gas meter readers (without heavy loads) and postal clerks. A telephone survey was used to obtain both symptoms and exposure. This analysis was adjusted for age, number of years on the job, quetelet (body mass) ratio and previous work experience.

Of the studies in the tables, five (that did not fulfill all the inclusion criteria) examined the risk factor, force, either as trapezius muscle load (using EMG), or as forceful work in combination with other risk factors [Aaras 1994; Wells et al. 1983; Onishi et al. 1976; Andersen and Gaardboe 1993a; Punnett 1991]. Wells et al. [1983] found a significant difference (p<0.05) in reported neck pain between letter carriers and postal clerks and attributed it to weight from carrying heavy mail bags on shoulder straps. In the Wells study, confounding due to age, number of years on the job, previous work experience, or quetelet ratios was ruled out. As noted above, Onishi et al. [1976] reported that the operations studied required continuous contraction of the trapezius muscle to sustain the arms, estimated to be about 10 to 30% of the maximum contraction of the trapezius. This level, 10 to 30% of the maximum contraction, was found by Tanii et al. [1972] to induce static fatigue significant enough to produce electromyographic changes. Hales et al. [1989] and Kuorinka and Koskinen [1979] reported statistically significant ORs (1.6 and 4.1, respectively) for the association between neck MSDs and high levels of force combined with high levels of repetition estimated for the hand/wrist areas. There were no separate force measurements for the neck area. Both studies controlled for age, gender, and length of employment in the current job. Two of the four studies that used estimated hand and wrist exposure measurement combinations of force and repetition (but carried out no neck, shoulder, or upper extremity exposure measurements) found non-significant associations between neck MSDs and force/repetition exposure [Baron et al. 1991; Kiken et al. 1990].

Temporal Relationship—Force and Neck/Shoulder MSDs

See temporal relationship above in Repetition and Neck/Shoulder MSDs.

Consistency in Association for Force and Neck/Shoulder MSDs

Both Kilbom et al. [1986] in their cross-sectional study and Jonsson et al. [1988] in their follow-up cohort studies found that

“time spent in physically heavy work before the present employment” appeared as a strong risk factor for deterioration of health of the neck/shoulder area (specifically, the health outcome was for the cervicobrachial region in the Jonsson study). Jonsson et al. [1988] noted that the physical demands of the previous jobs had only been assessed at the initial interview and constituted a subjective estimate. However, the relationship was strengthened by the consistency of findings in the prospective and cross-sectional studies.

Coherence of Evidence for Force

There is coherence with the biological mechanisms proposed by Hagberg [1984] for occupational muscle-related disorders, such as tension neck syndrome. The first mechanism concerns stress on the trapezius and surrounding muscles of the neck from heavy physical exertion that causes rupture of the muscle’s z-discs, and an outflow of metabolites from the muscle fibers, and activation of pain receptors through edema or other mechanisms. This temporary high, local stress involving eccentric contractions in the shoulders improves with time through a re-orientation of collagen in the muscles. This mechanism is offered as an explanation for MSDs in workers unaccustomed to the work. The second mechanism is from local decreased blood flow (ischemia), as seen in assembly workers whose tasks involved dynamic, frequent contractions above 10 to 20% of the MVC and few rest breaks. Reduced blood flow was found to be correlated with myalgia (muscle pain) and ragged red fibers in 17 patients with chronic myalgia thought to be associated with static load during repetitive assembly work [Larsson et al. 1990]. The third pathophysiologic mechanism for muscle pain deals with energy metabolism disturbance, caused by long-term static contractions of the muscles. Supporting this theory was a study finding a correlation between muscle tension and plasma myoglobin among patients with regional muscle tenderness and pain [Dammeskiold-Samsøe et al. 1982].

Other laboratory studies have examined muscle damage that may arise during static muscle contractions used to maintain static postures. Hagg et al. [1990] proposed that while maintaining static postures (that have low force levels), the same low-threshold motor units are contracted repeatedly for prolonged periods, during which time they work close to their maximal capacity. This may lead to injury of these units, despite the fact that the total workload is low. This hypothesis was recently supported by a longitudinal study by Veiersted et al. [1993] who investigated the number of rest-pauses during muscle fiber activity using EMG recording from neck and shoulder muscles. Among subjects performing machine-paced repetitive packing work, those with symptoms had fewer rest-pauses (0.9 versus 8.4 per minute) and a tendency toward shorter total duration of rest-pauses in the muscle fiber activity of their trapezius muscle when compared with those without symptoms. These mechanisms of decreased blood flow, increased metabolite concentration, and prolonged activation of certain small units at near maximum capacity may explain the chronic myofascial shoulder pain seen in workers performing repetitive assembly work with static loading of the trapezius muscles [Hagberg and Kvarnström 1984; Larsson et al. 1988].

Exposure Response Relationship for Force

Aaras [1994] reported that by reducing static muscle loading (an indication of force measurement) through equipment changes among VDT users, as well as improving workplace organization, he was able to decrease the prevalence of neck pain, decrease the number of sick days taken, and cause a significant reduction in trapezius load measured by EMG in VDT operators.

Conclusions Regarding Force

There is evidence for forceful exertion and neck MSDs in the epidemiologic literature. Most of the epidemiologic studies reviewed defined “forceful work” for the neck/shoulder as work activities that involve forceful arm or hand movements that, in turn, generate the loads to the neck/shoulder area; no study examined a relationship based on actual forceful neck movements. Of the 17 studies addressing force as one of the exposure factors, 5 found statistically significant associations but did not derive ORs; 2 found ORs greater than 3.0, 7 found ORs from 1 to 3.0, and 2 studies showed ORs less than 1.0. Many of the studies regarding measured force (as workload, etc.) and MSDs are in the biomechanical and ergonomic literature.

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Posture

Definition of Posture for Neck and Neck/Shoulder MSDs

We included those articles that mentioned neck or head postures, adverse or extreme head or neck postures, or static postures of the head and/or neck.

Studies Reporting on Posture as a Work Factor for Neck and Neck/Shoulder Musculoskeletal Disorders

We included 31 studies of the association between extreme or static posture and neck and neck/shoulder MSDs, including TNS. Studies usually focused on the different prevalences of neck symptoms and/or physical findings in workers in occupations or tasks requiring some combination of forceful, repetitive movements, and extreme or static postures of the upper extremity, and compared them to workers in occupations without those requirements.

Twenty-seven studies that considered extreme or static posture found a statistically significant positive association between posture and neck or neck/shoulder MSDs; three had non-significant findings (Table 2-1. Overall, in terms of magnitude of the association, looking at both significant and non-significant findings, 13 studies had estimations of risk (ORs or PRRs) greater than 3.0, 9 had risk estimates between 1 and 3, and none had an estimate less than 1.0. Eleven studies did not report their results in terms of ORs or PRRs; of these, all but one found a significant relationship.

Studies Meeting the Four Evaluation Criteria
Of the 31 studies evaluating neck postures and neck MSDs, the four investigations mentioned above [Ohlsson et al. 1995; Jonsson et al. 1988; Kilbom and Persson 1987; Kilbom et al. 1986] fulfilled the four evaluation criteria. Three of these studies [Jonsson et al. 1988; Kilbom et al. 1986; Kilbom and Persson 1987], dealt with the same cohort; female electronics workers followed for 3 successive years. These studies found significant association between posture variables and neck MSDs; however, none used methods that reported ORs.

Studies Not Meeting the Four Criteria for Posture and Neck/Shoulder MSDs
Bernard et al. [1993] carried out a cross-sectional study of 894 newspaper employees using a questionnaire survey for case definition based on frequency, duration, and intensity of symptoms in the neck. Exposure was based upon both questionnaire and job analysis. Time spent on the telephone was associated with an increased prevalence of neck MSDs, with a slightly elevated OR of 1.4. Analysis was controlled for age, gender, height, psychosocial factors, and medical conditions.

Kukkonen et al. [1983] compared 104 data entry operators with 57 female workers in varying office tasks. Neck MSD was based on pre-determined symptom and physical exam. Exposure was based on observation of posture, movements and working techniques, assessment of equipment, interview with workers and supervisors. An intervention consisting of adjustment of office furniture and equipment was carried out. The study group was given a short course of basic training on pertinent aspects of ergonomics. Four lessons on relaxation was given by means of exercises. There was no controlling of confounders. There was a significant decrease in tension neck syndrome among the cases involved in the intervention compared to those workers who had no change.

Linton and Kamwendo [1989] surveyed 22,180 employees undergoing screening examinations at their occupational health care service in Sweden. Neck cases defined from questionnaire responses as those persons reporting “yes” to having seen a health care professional for neck pain in the last year. Cases were compared to “non-cases” defined by outcome (neck pain). Exposure was based on questionnaire responses regarding heavy lifting, monotonous or assembly line work, sitting, uncomfortable work postures (bending and twisting), and vibration. The psychosocial work environment was also studied; the analysis was stratified for age and gender.

As part of a longitudinal study, Viikari-Juntura et al. [1994] studied 154 subjects from Helsinki, Finland that originally entered the study in 1955, and had repeated cross-sectional exams from 1961 to 1963. During that time, 1084 subjects underwent cross-sectional examination. In 1985, a questionnaire was sent to all subjects; 801 (74%) responded. Of the respondents, 180 lived in the Helsinki area. It was from this group that 162 responded. Eight were excluded due to illnesses. Outcome was based on questionnaire data for this study — because of small number of abnormal physical findings, the physical exam was eliminated from analysis. Exposure was also based on survey, asking the amount of work with hands overhead, work in forward bent position, and work in twisted or bent position. This analysis was controlled for physical and creative hobbies, with no interactions seen.

In a cross-sectional study of machine operators, carpenters were compared to office workers by Tola et al. [1988], who used a postal questionnaire to obtain both health outcome and exposure information. Analysis used “occupation” to examine relationships. Pain Drawing Diagrams were used to distinguish body areas. For the logistic regression model a 12 month prevalence of neck and shoulder symptoms on 8 days or more was used. The logistic regression models were adjusted for years working in an occupation and age.

Welch et al. [1995] examined 39 electricians at a screening convention using surveys to collect information on symptoms and exposures. The questionnaire included questions concerning the frequency of tasks performed, including the percent of time spent hanging duct work. The analysis did not control for confounders except for length of employment.

Strength of Association for Posture

Ohlsson et al.'s [1995] study, discussed previously, compared female industrial workers performing repetitive tasks to referents without such exposure and found significant associations (p<0.05) between (1) neck and neck/shoulder diagnoses with time spent in neck flexion, with critical angles greater than 15°; and (2) neck/shoulder diagnoses and time spent with upper arm abduction greater than 60°.

Kilbom et al. [1986], in the initial paper concerning the electronic workers, reported two findings: (1) that the more dynamic the working technique, the fewer neck symptoms experienced by electronic workers; and (2) that the greater the average time per work cycle spent in neck flexion, the greater the association with symptoms in the neck and neck/shoulder angle. A statistically significant association (p<0.05) was also obtained from the job analysis variables describing neck forward flexion and upper arm elevation and neck and neck/shoulder disorders. Jonsson et al. [1988], in the follow-up study, performed an analysis that grouped the different parts of the neck and upper extremity into a health outcome labeled “cervicobrachial disorder” (unlike the cross-sectional study by Kilbom et al. [1986] that used “neck” and shoulder”). They found that the relationships between MSDs and neck forward flexion, upper arm elevation, and cervicobrachial disorders weakened (compared with the results that Kilbom et al. [1986] had found), but that the results still remained statistically significant in some of the multifactorial analyses (no numerical results were reported). The most important finding, according to the authors, was that reallocation to more varied work tasks was a strong predictor of improvement over the second year. This change would have decreased static loading and increased the dynamic pattern of movements of the workers.

Of those studies not fulfilling the four criteria, results regarding extreme or static posture were similar to those of the studies which did fulfill them. Sakakibara et al. [1995] found a significant difference in the prevalence of neck MSDs when they examined orchard workers who picked and bagged pears and two months later picked and bagged apples. Exposure was assessed by job analysis and posture measurements of two representative workers. Arm and neck elevation was significantly greater for bagging pears (more than 90° for 75% of the time) than for bagging apples (less than 40% of the time). The same authors found similar results in 1987 when only the symptoms of orchard workers were studied. They found significant a positive association between posture and neck MSDs, reporting histograms (not ORs) in their article.

Although they did not mention the participation rates in their methods, Aaras [1994], Veiersted and Westgaard [1994], and Bjelle et al. [1981] found significant relationships between postures and neck MSDs (they fulfilled the other three criteria). Veiersted and Westgaard [1994] found an association between “perceived strenuous postures” and neck MSDs (OR 7.2), but found that these perceived postures were not reflected in any of the conventional EMG parameters (static, median or peak loads) measured in the participants. One explanation for these results may be information bias, if the data concerning perceived strenuous posture are from questionnaires. Another explanation may be that EMG testing results reflect parameters for a single day, whereas symptoms were asked about concerning the entire previous year.

Several studies that carried out no independent assessment of ergonomic factors, but relied on self-reported exposure found significant relationships between posture variables and neck disorders. Ekberg et al. [1994] found an OR of 4.8 for the variable “work with lifted arms,” and an OR of 3.6 for “uncomfortable sitting position” and neck MSDs. Hales et al. [1994] found that “use of bifocals” (OR 3.8) in VDT users was significantly associated with neck MSDs; this variable was interpreted to be a surrogate for neck posture, as bifocals require either neck flexion or extension for eye accommodation when viewing a VDT screen. Bernard et al. [1994] reported that as workers’ time spent on the telephone increased, so did the ORs for neck symptoms, and interpreted this variable as a surrogate for static posture requiring neck deviation to cradle the telephone receiver. Holmström et al. [1992] found that the odds of workers with neck MSDs reporting working with hands above their shoulders for greater than 4 hrs/day compared with those reporting less than 1 hr/day was 2.0, a statistically significant finding. Bergqvist et al. [1995a] reported an OR of 4.4 for workers using highly placed keyboards in their logistic modeling of neck MSDs. Kuorinka and Koskinen [1979] found an increased OR (4.1) of neck MSDs for scissor makers (chosen for their stereotypic, repetitive work using extreme postures) compared to shop assistants, although no quantitative measurements or observations of neck posture were reported. One study by Hunting et al. [1981] showed a fairly strong association (OR 4.9) with constrained postures and neck MSDs in those workers having neck flexion of more than 56° and an OR of 9.9 from the comparison of groups. Several articles with significant posture and neck MSD associations dealt with comparisons of workers in occupations chosen for higher observed combinations of exposure factors and compared them to workers with fewer observed exposure stressors: Viikari-Juntura et al. [1994], OR 3.9 to 4.2; Milerad and Ekenvall [1990], OR 2.6; and Wells et al. [1983], OR 2.57.

For those studies that did not find a significant relationship, 2 out of the 3 did not carry out observation or measurement (ergonomic assessment) of the neck or upper extremity postures. Ferguson [1976] stated that seven body dimensions were measured in the telephonists studied, but that neither discomfort nor aching were linked with any of these body postures. The article does not mention the body postures that were measured. Ferguson’s conclusion, that “physical complaints in telephonists are probably due to static load on joints and muscles occasioned by the fixed forward bent position determined by visual, auditory and manipulative tasks.” Ferguson's data are contrary to the conclusions presented. These conclusions may then only be speculative.

Temporality for Extreme or Static Postures

The prospective study by Veiersted and Westgaard [1994] followed the development of trapezius myalgia among 30 female chocolate manufacturing workers. Seventeen workers developed the MSD within 6 to 51 weeks of starting work. Perceived strenuous postures on the assembly line were found to contribute to the disorders. Although retention of subjects was low (55%), the authors found that the “drop-outs” did not differ in exposure estimates and symptom reporting from those retained in the study. The prospective study of Viikari-Juntura et al. [1994] used self-reported symptoms and exposure defined by occupational status to find a temporal relationship between the development of severe and persistent severe neck pain and jobs involving dynamic work, static posture, and whole body vibration, as compared to office work.

Consistency in Association for Extreme or Static Postures and Neck/Shoulder MSDs

Of the 31 studies we reviewed reporting results on the association between specific or static posture and neck and neck/shoulder MSDs, 27 found statistically significant associations. There were many different studies reporting ORs of greater than 3.0 with CIs above 1, indicating that the effects were not explained by chance. Consistent associations were also found in those studies dealing with specific postures and neck MSDs across many industries, from fish workers [Ohlsson et al. 1995] to fruit pickers [Sakakibara et al. 1995], to assembly line workers [Jonsson et al. 1988], to garment workers [Vihma et al. 1982; Andersen and Gaardboe 1993a,b].

Coherence of Evidence for Extreme Or Static Postures

See section above under Coherence of Evidence for Force.

Exposure response Relationship for Specific or Static Postures

The study by Ohara et al. [1976], mentioned earlier, not only portrayed the multifactorial nature of neck and shoulder MSDs, but documented that an increase in specific and static postures by cash register operators using new registers placed on unsuitable counter heights increased symptoms in neck MSDs.

Several studies have suggested an exposure response effect between increased level or duration of exposure and an increase in number of cases of neck MSDs. Burt et al. [1990], in their investigation at a major urban newspaper, found that an increase in the self-reported percentage of time spent typing at VDT keyboards was associated with a moderate increase in neck symptoms. (Job analysis found a significant relationship between independent observation of time spent typing and self-reported time) Keyboard time was considered by the authors to be a surrogate for time spent with the neck held in static postures with arms unsupported. Rossignol et al. [1987] found that the prevalence of neck symptoms among 1,545 clerical workers increased with the number of hours per day using VDTs. Knave et al. [1985] found that, among VDT operators, total daily working hours and time spent at the VDT screen were significant risk factors for neck pain. Andersen and Gaardboe [1993a,b] found an exposure response relationship between persistent neck pain and years of being a sewing machine operator, controlling for age.

Conclusions Regarding Extreme or Static Postures

Overall, the strength of the association (OR ranging from about 1.6 [Vihma et al. 1982] to 7 [Veiersted and Westgaard 1994], dropping the outliers) between specific postures and neck MSDs was similar between studies using the most restrictive criteria and carrying out a prospective design and those that used symptom-based health outcome or self-reported exposures to static or specific postures and cross-sectional methods. We conclude that there is strong evidence for support of an association between static or specific postures and neck and neck/shoulder MSDs based on strength of association criteria. A positive relationship has been observed between exposure to this risk factor and neck or neck/shoulder MSDs in studies where chance, bias, and confounding can be ruled out with reasonable confidence.

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Vibration

No study of neck MSDs met the four criteria to address strength of association between vibration and neck MSDs and only one of the reviewed studies in the tables mentioned neck MSDs and vibration. Viikari-Juntura et al. [1994] selected study groups for their longitudinal study based on different work exposures. Machine operators exposed to static work and whole-body vibration were compared to carpenters exposed to dynamic physical work and presumably no vibration to see whether occupational status was related to neck MSDs. Results found that the OR for progressing from no neck pain to moderate to severe neck trouble was from 3.9 to 4.2; for operators compared to carpenters; a significant difference. No vibration measurements were performed in this study, and vibration was likely to be confounded by neck twisting and static loads.

Conclusions—Vibration and Neck or Neck/Shoulder MSDs

We conclude that there is insufficient evidence to support an association between vibration and neck or neck/shoulder MSDs based on strength-of-association criteria. Too few studies of neck or neck/shoulder MSDs have examined the relationship between exposure to vibration and to draw any conclusions about their relationship.

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Neck or Neck/Shoulder MSDs and the Role of Confounders

As in many MSDs, prevalence of neck and neck/shoulder disorders tends to increase with age. Therefore, it is important that studies take into account when examining the strength of occupational versus non-occupational factors. Age and gender were the primary potential confounders that investigators addressed in many of the studies on neck and neck/shoulder MSDs (The tables at the end of the chapter list summaries of each of the articles and include which particular covariates or confounders were considered.) These were either dealt with by logistic regression modeling, as in the case of age (e.g., Andersen and Gaardboe [1993a]; Rossignol et al. [1987]; Tola et al. [1988]; Ohlsson et al. [1989]; Baron et al. [1991]), through matching of case subjects and referents (e.g., Vihma et al. [1982]), or through study of a single gender (e.g., Luopajärvi et al. [1979]; Hunting et al. [1994]), or stratifying by gender [Sakakibara et al. 1995]. Most studies performed univariate analysis prior to logistic regression to consider factors which needed to be introduced into the logistic models as confounders or covariates.

Almost all the studies we reviewed accounted for the confounders of age and gender. Many of the studies controlled for leisure exercises [Andersen and Gaardboe [1993a,b] smoking (Linton [1990]; Milerad and Ekenwall [1990]; Bergqvist et al. [1995a,b]; Viikari-Juntura et al. [1994]), medical conditions [Bernard et al. [1994]; Hales et al. [1994]). Reviewing the methods and results of these studies, the confounding factors do not account for the consistent relationship that is found with the work-related factors.

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Conclusions

Interpreting association for individual workplace factors is difficult, as most epidemiologic studies of MSDs used populations selected because of multiple factors (such as forceful exertion and repetitive tasks). Unlike laboratory experiments, one cannot isolate exposure factors, nor alter some factors while keeping others constant to insure accuracy in examining, recording, and interpreting results. However, one can examine the body of epidemiologic evidence and infer relationships. There have been over 40 epidemiologic studies which have examined work factors and their relationship to neck and neck/shoulder MSDs. Many studies identified individuals in heavier industrial occupations and compared them to workers in light industry or office environments. Other studies identified a symptomatic group of workers, or those with symptoms and physical exam abnormalities, and compared them to asymptomatic workers at the same work site., or to population referents, and looked for differences in exposure. These approaches, although quite different, by and large have chosen to focus on similar workplace risk factors. These include repetition, forceful exertions, and constrained or static postures, usually found in combination.

There is also reasonable evidence for a causal relationship between highly repetitive work and neck and neck/shoulder MSDs. Most of the epidemiologic studies reviewed defined “repetitive work” for the neck as work activities which involve continuous arm or hand movements which affect the neck/shoulder musculature and generate loads to the neck/shoulder area; fewer studies examined relationships based on actual repetitive neck movements. The two studies which measured repetitive neck movements by head position (using frequency and duration of movements), and fulfilled the four criteria, found strong associations with neck/shoulder MSDs. In those studies defining repetitive work as continuous arm or hand movements affecting the neck/shoulder, nine studies found statistically significant ORs greater than 3.0. Eight studies fulfilled all the criteria except for objective exposure assessment and measured repetition for the hand/wrist, not the neck. Of these, three had statistically significant ORs greater than 3, and five had non-significant ORs, all under 2.0.

There is reasonable evidence for forceful exertion and neck MSD found in the epidemiologic literature. Most of the epidemiologic studies reviewed defined “forceful work” for the neck/shoulder as work activities which involve forceful arm or hand movements which generate the loads to the neck/shoulder area; no study examined a relationships based on actual forceful neck movements. Of the 17 studies addressing force as one of the exposure factors, five studies found statistically significant associations but did not derive ORs; two studies found ORs greater than 3.0, seven studies from 1 to 3.0, and 2 studies with ORs less than 1.0.

There is strong evidence that working groups with high levels of static contraction, prolonged static loads, or extreme working postures involving the neck/shoulder muscles are at increased risk for neck/shoulder MSDs. Consistently high ORs (12 studies found statistically significant ORs over 3.0) for tension neck syndrome associated with static postures or static loads have been found.

The epidemiologic data are insufficient to document relationship of vibration and neck disorders. The few prospective studies which have included interventions to decrease workplace risk factor exposures, including decreasing repetitive work and less extreme working postures, have shown a decrease in incidence of neck MSDs, and an improvement in symptoms among affected workers. These data provide additional evidence that these disorders are related to work factors. However, cumulative exposure response data is lacking, although VDT studies using surrogate exposure variables suggests a relationship.

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