An excess risk attributable to interaction was indicated only for the combination of VDU work and job strain among women, but not men: VDU work only, RR = 1.0; job strain only, RR = 1.4; and VDU work and job strain, RR = 4.2 (95% CI = 1.5–11.2), with AP of excess risk owing to the interaction of 0.67.
Logistic Regression Analyses
The overall risk pattern in the logistic regression analyses was not substantially different from the stratified analyses. The strongest physical risk indicators were work with vibrating tools for men (RR = 1.9; 95% CI = 1.1–3.1; AP = 19%), and repetitive hand/finger movements and work with hands above shoulders for women (RR = 1.6; 95% CI = 1.1–2.2; AP = 18%; and RR = 1.5; 95% CI = 1.0–2.3; AP = 6%), respectively.
The most pronounced risk indicator related to work organization or psychosocial conditions was not having a fixed salary both among men (RR = 2.1; 95% CI = 1.2–3.6; AP = 11%) and women (RR = 1.6; 95% CI = 0.8–3.4; AP = 2%). Solitary work, high time pressure, and night work/shift work indicated harmful effects among women but preventive effects among men (RR = 1.7, 1.3, and 1.4, and RR = 0.4, 0.4, and 0.7, respectively). The effect of job strain observed for women in the stratified analyses disappeared in the logistic regression analyses.
The results indicate that the multifactorial causes of seeking care for neck or shoulder disorders and the corresponding physical and psychosocial risk indicators may differ between men and women in a general working population. The effect of single exposures was moderate, however, and no exposure, except not having a fixed monthly salary, increased the RR twofold or more. The contrasts of exposure were low, and few subjects were highly exposed in this population-based study. Nevertheless, multiple exposures to working conditions associated with an increased risk were common, and the effect increased with increasing number of risk indicators. The limits for being classified as exposed were low, but these moderately high exposures increased the RR. Apart from potential interactions, which we only observed for VDU work and job strain among women, the high prevalence of exposure to several risk indicators may explain the high incidence of neck and shoulder disorders in the general working population.
Heavy work has been identified as a risk indicator for neck or shoulder disorders in several studies. 22–24 These associations might partly be attributable to manual materials handling, awkward postures, and/or exposure to vibrating tools, which commonly exist in physically demanding work. Additionally, a high energetic load per se may cause fatigue and thereby lead to impaired neuromuscular control, which may increase the risk of injury. 11,25 On the other hand, high energy expenditure may also be attributable to health-promoting physical activities with positive effects on the musculoskeletal system, which may prevent musculoskeletal disorders. 12,26,27 Our results do not support heavy work as an independent risk indicator for neck and shoulder disorders. High energetic load was a risk indicator for low back disorders among women in the MUSIC-Norrtälje study. 3 The differences between neck or shoulder and low back disorders among women may be that a high energy expenditure is more often a consequence of activities more harmful for the back than for the neck and shoulders. A recent study indicated adverse effects from long-lasting physically demanding work on trunk muscle strength, but not on hand-arm-shoulder muscle strength, for which a maintaining or training effect was observed among women. 28
Work with elevated arms has frequently been observed as a risk indicator for neck and shoulder disorders. 1,2,23,29,30 In this study, work with hands above shoulder level indicated harmful effects among women, but not among men. Men were, however, more frequently exposed, and the duration of exposure was also longer among men compared with women, 4 which indicates that women may be more vulnerable.
Work involving repetitive hand/finger movements was a marked risk indicator among women, which is in accordance with results from several other studies. 1,2,22,30 Prolonged repetitive hand/wrist movements are assumed to cause maintained static muscle contractions, which may cause neck or shoulder disorders. The lack of association among men may partly be explained by less intensive and/or shorter duration of exposure within the exposed group. Many occupations with long duration of intensive repetitive hand movements are female dominated, for example, data entry operators, secretaries, and garment workers.
Long duration of constrained sitting and VDU work was believed to increase the RR owing to prolonged static contraction of the trapezius muscle, which may result in an overload of type 1 muscle fibers. 31 Several studies indicate that VDU work, especially monotonous data entry with limited rest breaks, may increase the risk of neck and upper extremity disorders. 32–34 This study indicated moderate effects of constrained sitting and VDU work among women but not men. One reason for the difference between men and women may be differences in work content, task variability, and work-rest patterns that were not accounted for in this study.
Work with vibration tools was a risk indicator for men, and the effect of vibrations among women could not be investigated because of too few exposed subjects. The increased RR observed among men in the stratified analysis remained high in the logistic regression analysis. Several studies have indicated associations between the use of vibrating tools and neck and shoulder disorders, but because work with vibrating tools implies exposure to static load to stabilize the upper extremity while using the tool and is often associated with awkward postures, the role of the vibrations per se is unclear. 1,2,35,36 Nevertheless, vibrations in the hands and arms have been shown to increase the muscular tension in shoulder muscles, which indicates that vibrations per se may increase the risk for disorders. 37
It seems plausible that the manual material handling, which causes high loads on the neck and shoulder area, may cause disorders. 1,2,30,36 We could not, however, identify manual material handling as a risk indicator, either among men or women. Although the limit for being classified as “exposed” included low loads (≤50 N), the frequency of exposed subjects was low, and it seems that forces prevalent in the general population are too low to have any substantial impact on the occurrence of neck and shoulder disorders.
Work Organization and Psychosocial Exposures
There is increasing evidence that work organization and psychosocial work-related factors play a major role in the development of work-related musculoskeletal disorders. 1,2,38
The etiologic mechanisms are poorly understood, but different plausible explanations have been suggested. 1,2,38,39 In the interpretation of the relation between psychosocial factors and neck or shoulder disorders, the possible influence from such factors on awareness and perception of pain as well as on sickness behavior must also be taken into consideration. 39
Not having a fixed salary was a marked risk factor for both men and women, which is in agreement with several other studies, although often with a more limited definition (piecework salary). The effect may be attributable to an increased work pace, high demands, and low decision latitude in the work situation. 30 The economically often more insecure situation may also entail stress and tensions.
Hindrances at work indicated a harmful effect among women. The most frequent hindrance among women was poor personnel resources in the organization, probably because of reduction of personnel in, for example, the health care and education sectors. 40
Night work/shift work was associated with an increased RR for women, but not for men. The most common occupations among women with night work/shift work were found in the health care system. Night work/shift work may affect the amount and quality of sleep, which reduces the activity of hormones that stimulate anabolism and may therefore increase the tissue vulnerability and thus the risk of disorders. 39 Additionally, some women may have chosen night-shift work to take care of children and manage other domestic work in daytime, which may increase leisure time exposure.
Solitary work was found to be harmful for women but not for men. Lack of work-task support and social support could be an obstacle that may cause stress and tension, maybe to a greater extent among women.
Time pressure showed a moderately increased RR for women and a diminished RR for men. Among the time-pressured men, more than one-third were self-employed; the employed men often seemed to be in an executive position. It is plausible that the time pressure among men was associated with desirable conditions such as successful career and high influence at work. Additionally, these men may be more reluctant to seek care owing to lack of time. Almost no time-pressured women were self-employed. They often worked in health and social work or in restaurants and were to a limited extent in executive positions. These differences may contribute to the difference in effect among men and women.
Job strain, high demands and low decision latitude, has been shown to be moderately associated with neck and shoulder disorders in several studies. 1,2,39 In this study, job strain was associated with an increased RR for women, but not for men, in the stratified analyses. Among women, the most frequent occupations with job strain were connected to work with people and for men with production of things. Perhaps job strain is more stress-provoking in work with people than with things. The effect of job strain disappeared in the logistic regression analyses; however, this finding could be a result of many covarying factors in the model such as, for example, time pressure and the balance between demands and competence.
Low demands in relation to competence was associated with a moderately increased risk among men but not among women. There were mainly two different groups of exposed men. One group included men with actual higher education than their job required, for example, bus drivers and postmen with theoretical university diploma. The other group consisted of men, with relevant formal education for their job, but perhaps with disagreement with the employee of the focus of their work content, for example, teachers.
Multiple Exposure and Interactions
Multiple exposure to several risk indicators was common in the general population, and the RR increased with increasing number of risk indicators. The only indication of an excess risk attributable to interaction was noted for coexposure to VDU work and job strain among women. The character of the VDU task influences the risk of neck and upper extremity disorders, and several studies indicate higher risks among data entry workers, compared with more interactive VDU tasks. 32 Data entry is more common among women compared with men 40 and probably constitutes a job strain situation.
In this study group, as in the Swedish labor market, men and women were found in different occupations. 41 The men mainly worked with “things,” and the women mainly with “people,” and their work content differed substantially. The observed difference in risk pattern between men and women depends probably mainly on the gender-segregated labor market and thus on differences in exposure. The higher prevalence of multiple exposure to several risk indicators among women compared with men might explain the higher incidence of neck and shoulder disorders among women compared with men.
The participation rate in this population-based case-control study was approximately 70% among the controls. Additionally, 11% completed the questionnaires but were not interviewed and clinically examined. The self-reported exposure in this group did not differ substantially from the participants, and only minor differences in effects were observed when this group was added to the analyses based on questionnaire data (Fredriksson K, Alfredsson L, Ahlberg G, Josephson M, Wiktorin C, Wigaeus Tornqvist E, Ving˚ard E, manuscript submitted). Because all possible caregivers within a defined geographic area were involved in the study and the study population lived and worked within the same geographic area, it is most likely that the vast majority of cases sought care from any of the involved caregivers. The caregiver may, however, have forgotten to report some of the cases, but such forgetfulness would hardly be selective toward subjects with specific exposures.
The interview-based information on work-related physical exposure has been compared with observations and technical measurements, and the intermethod reliability was high. 6,9,10 The duration of awkward postures was, however, systematically overestimated, whereas the duration of sitting was somewhat underestimated. This misclassification did not differ between subjects with and without musculoskeletal complaints, which indicates that there was no differential misclassification of exposure.
For some of the physical exposures, the data obtained by questionnaire and interview could be compared. The correlation between questionnaire and interview responses was high to moderate for the work-related exposures and the cases estimated as correctly or incorrectly as the controls. Thus, no differential misclassification of exposure was indicated. 4
The outcome definition in the present study was “seeking care because of neck or shoulder disorder.” Subjects who had sought care or been treated for low back, neck, or shoulder disorders during the previous 6 months were excluded, because we wanted to study risk indicators for “new” episodes of the disorder and thus tried to reduce the effect of factors influencing the duration of disorders as well as changes of exposure caused by the disorders. The outcome, seeking care because of neck or shoulder disorders, is not synonymous with an acute episode of neck or shoulder disorders but has the advantage of being a feasible method to identify “incident” cases in a general population, and it may restrict the outcome to more severe cases. Care-seeking behavior may, however, differ between, for example, socioeconomic classes, sex, age, and occupational groups. Thus, the risk indicators observed in this study may be explained by care-seeking behavior. Notably, however, almost all effect estimates were unaffected or increased when the analyses were restricted to cases with a confirmed diagnosis (and controls without a diagnosis), which indicates that the observed results are not attributable to malingering of psychosocially dissatisfied subjects.
Potential confounding from age, socioeconomic class, previous symptoms, different life-style factors, and social support has been taken into consideration in the analyses. According to present knowledge, there is no reason to believe that any other factor would have any marked influence on the observed associations between work-related exposure and seeking care because of neck or shoulder disorders.
In conclusion, the RR for seeking care because of neck or shoulder disorders in the general working population was only moderately increased by single work-related exposures, and the risk indicators differed between genders. A large proportion of the population was, however, exposed to several of these conditions, and their concomitant effect may explain the high incidence of neck and shoulder disorders in the population.
We thank all of the caregivers in Norrtälje; the administrators in the Norrtälje part of the County Council; the advisory board to the MUSIC network; and all of the collaborators such as the project coordinator and administrators, physiotherapists, psychologists, statisticians, and nurses for excellent work before, during, and after the study.
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Keywords:© 2001 Lippincott Williams & Wilkins, Inc.
ergonomics; general population; neck and shoulder disorders; physical exposures; psychosocial exposures; work-related exposures