The impact of occupational and personal factors on musculoskeletal pain - a cohort study of female teachers, nurses and sonographers

Musculoskeletal pain is common in the general population and constitutes a major public health problem. A large proportion of these conditions may be work related. This study aimed to explore the relative importance of physical, psychosocial and personal factors, for musculoskeletal single- and multisite pain, among women in ordinary professions with a broad variety of occupational exposures. A cohort of 1115 women responded to a questionnaire of ergonomic, psychosocial, personal and life-style factors, and musculoskeletal pain (based on frequency and intensity of complaints in nine anatomical sites), at baseline and at follow-up (mean 28 months later). Sum-scores of ergonomic- and psychosocial factors were created. The importance of the exposures at baseline for multisite pain (≥four pain-sites) at follow-up were estimated using ordinal regression. Multi-exposure Poisson regression models of exposures at baseline were used for outcomes of single-site pain at follow-up, in the neck, the shoulders, the hands, the lower back and the feet.


Background
Symptoms and diseases in muscles and joints are common in the general population and thus constitute a major public health problem [1]. A large proportion of these symptoms/disorders, regardless of localization, may be work related [2][3]. Increased risk has been demonstrated for both physical and psychosocial factors. A variety of physical exposures, such as constrained postures, high muscular load, monotonously repeated movements, lack of time for recovery and poor workstation layout, have been identified as possible physical risk factors [3][4][5][6][7]. Further, in the previous scientific literature some evidence for an association between musculoskeletal disorders, and various psychosocial factors such as lack of job control, low decision latitude, and high job demands have been established [8][9]. With respect to medical personnel, at least two systematic reviews focusing on medical staff have reported an association between high psychosocial demand and low job control and musculoskeletal complaints/disorders in lower back, neck, shoulders and knees [10][11]. Moreover, the impact of perceived stress on the development of chronic musculoskeletal pain has been scrutinized in a recent systematic review. The results revealed that available evidence support the etiologic role of perceived stress in the development of chronic spinal pain, with an effect size ranging 4 from low to medium [12]. Besides the occupational factors, personal and life-style factors such as age, obesity, smoking, and lack of physical activity in leisure time [13][14], may be of importance for the development of pain.
Musculoskeletal pain tends to come and go [15], with varying intensities, but can also develop into a persistent and severe condition. Pain may be localized to one single anatomical body site, but may also occur in several sites simultaneously, i.e. multisite pain [16]. The consequences, in terms of medical consumption, and absenteeism and restrictions at work, have been found to depend on how many body regions that are affected [17].
However, there are still inconsistencies in the literature about the strengths of evidence for the longitudinal relationship between many of these identified risk factors, and musculoskeletal pain [8,18]. Most of the studies performed are still cross-sectional and have consequently limited value for studying causal relationships.
Hence, the aim of this study was to further explore the relative importance of physical, psychosocial and personal factors, in longitudinal associations with multisite musculoskeletal pain, as well as for pain in specific anatomical sites. The study population consists of females in ordinary professions in the public sector, with a broad variety of occupational exposures. Cross-sectional results from the same study group have previously been presented and reported by Arvidsson and co-workers in 2016 [19].

Study design
Questionnaires were directed to women in five occupational groups [teachers (Te), anaesthetic nurses (AnN), theatre nurses (TN), assistant nurses (AsN), and sonographers (Sg)] at baseline and at follow-up.
Ten items of self-assessed occupational exposures and personal factors at baseline were 5 analysed in relation to reported musculoskeletal pain at follow up. The experience of pain was collected for nine anatomical sites. The importance of the potential risk factors was analysed in relation to multisite pain, i.e. the number of affected pain-sites, as well as for single-site pain in the neck, the shoulders, the hands, the lower back and the feet, respectively.

Study population
At baseline, 1591 female employees from 49 public schools, 22 surgical departments and 45 sonography departments in Sweden participated in the study [19]. The inclusion criteria were work at least 50% of fulltime during a period of at least three months before fulfilling the baseline questionnaire. The participants responded to a questionnaire that included questions on ergonomic-, psychosocial-, personal-and life-style factors and musculoskeletal pain from nine anatomical sites. At baseline, we also performed clinical examinations as well as technical measurements of the physical workload among subgroups of the participants (n = 485 and n = 60, respectively) [19]. The questionnaires  Table 1). Their mean seniority at baseline were 17 (range 0.25-45) years. The participation rate among the employee categories ranged from 66% to 76%. Table 1 Musculoskeletal pain and sum scores of ergonomic-and psychosocial factors in the total study population at baseline; among the participants at follow-up (n = 1115) and among those who dropped out of the study between baseline and follow-up (i.e., "drop-outs"; N = 476).  [19].

Questionnaire
The questionnaire included questions about the physical workload, psychosocial working conditions, personal and lifestyle factors, and musculoskeletal pain. Physical workload: The mechanical exposure index (MEI [20]) comprised 11 items of work postures and movements, and the physical exposure index (PHYI [20]) included seven items concerned with physical activity and lifting. In both scales, the items were answered on a three-point scale; 1:"hardly nothing/not at all", 2:"somewhat" or 3:"a great deal". For each scale, the points [20].  They were also asked about personal relaxation [24] "How much of your leisure time once a week; 3: occasionally or never) and smoking habits (0: never smoked; 1: ex-smoker of at least six months standing; 2: smoker, but not daily; 3: daily smoker). In the analysis, the categories 0-2 were merged into one category of "not daily smokers".
Musculoskeletal pain [19]: The participants were asked about subjective musculoskeletal complaints in nine anatomical sites: the neck, shoulders, elbows, hands, upper back, lower back, hips, knees and feet, during the preceding 12 months, according to the Nordic Questionnaire [26]. For the shoulders, elbows, hands, hips, knees and feet, pain in one or both sides of the body was regarded as one pain-site. In addition, for each anatomical site, information was collected about the frequency of complaints during the past year using a 5-point scale (never, seldom, sometimes, often, or very often [28]) as well as the intensity of complaints on an eleven-point-scale, from 0 (none at all) to 10 (very, very severe [27]). A subject was considered to have considerable musculoskeletal pain (subsequently referred to simply as "pain") if reporting complaints at least "seldom" with an intensity of at least 7 (very severe), or "sometimes" with an intensity of at least 3 (moderate), or "often" or "very often" with an intensity of at least 2 (slight/mild) [19]. The condition was defined separately for each anatomical site.
The number of anatomical sites with pain was calculated for each individual (0-9 possible).
Following the suggested classification by Pereira de Fernandes and Burdorf [17], the participants were divided into five categories; 0: no pain; 1: one pain-site; 2: two painsites; 3: three pain-sites and 4: ≥ four pain-sites. Multisite pain was defined as having pain from four or more sites of pain.
Further, five of the anatomical sites were selected for analysis of single-site pain: the neck, the shoulders, the hands, the lower back and the feet.

Statistical analyses
All statistical analyses were performed with the IBM SPSS software, version 24 (IBM Corp.).
Prevalence ratios (PRs) and 95% confidence intervals (CIs) for pain in the neck, shoulders, hands, lower back and feet at follow-up were first estimated in single-exposure Poisson regression models for all variables collected at baseline (in total eleven factors including pain in the actual anatomical site, sum-scores of ergonomic-and psychosocial factors, computer work, age, BMI, personal recovery, domestic work, physical exercise, smoking and occupational category). In the next step, PRs for pain at follow-up were estimated using Poisson regression with multiple exposures (multi-exposure model), without pain at baseline. In the third step, by adjusting the multi-exposure model for pain at baseline we tried to quantify how much of the explanation from different factors concerning pain at follow-up that was not due to associations with pain that were present already at baseline.
Odds ratios (ORs) and 95% confidence intervals (CIs) for multisite pain at follow-up (number of pain-sites stratified into five categories, i.e. 0, 1, 2, 3 and ≥ 4 sites) were first estimated in single-exposure ordinal regression models for the eleven occupational and personal factors collected at baseline. In addition, the importance of multisite pain at baseline, for multisite pain at follow-up was estimated. Next, ORs for the multisite pain at follow-up were estimated using multi-exposure ordinal regression, without multisite pain at baseline. Finally, the multi-exposure models were adjusted for multisite pain at baseline.
Differences between prevalence of pain at baseline and at follow-up were evaluated with McNemar test.
Due to a high collinearity (strong correlation) between seniority and age, seniority was omitted from the multi-exposure statistical analysis. Seniority is reported as a singleexposure in additional table 1.
The correlation between the difference of multisite pain categories between baseline and follow-up, and the follow-up period (months) was low (Spearmans rho = 0.03).

Results
Changes in the presence of pain between baseline and follow up Multisite pain, i.e. pain from four or more anatomical sites, was reported by 26% of the participants at baseline vs. 28% at follow-up (p = 0.22). Among those, 182 participants (18% of the study population) reported multisite pain at both occasions ( Fig. 1). Eleven percent (117 individuals) did not report any pain-site, neither at baseline nor at follow-up.
In a majority of the participants, the number of pain-sites changed between baseline and follow-up, in both directions: thirty one percent reported more pain-sites at follow-up, while 27% improved.
In the total study population, the prevalence of neck pain increased between baseline and   Single-exposure models of risk factors for single-site pain Results from the single-exposure models, for all of the five specific anatomical sites (the neck, the shoulders, the hands, the lower back and the feet), are given in additional Table   2. The main results from these models were that high sum-scores of both ergonomic-and psychosocial factors at baseline were statistically significant risk factors of pain in the neck, the shoulders, the hands and the lower back at follow-up. A high sum-score of ergonomic factor was also a risk factor for pain in the feet. Additional factors that were of importance for subsequent single-site pain were complaints on the computer work station arrangement (pain in the neck), high age (pain in the hands and the feet), a high bodymass index (pain in the feet), lack of time for personal recovery (pain in the neck and the shoulders), a low frequency of physical exercise (pain in the neck and in the lower back) and occupational category (all five anatomical sites). However, having pain in a specific anatomical site at baseline was the strongest predictor of pain in the actual site at followup, with PRs ranging from 2.7 to 4.7 between the sites. Much domestic work was a protective factor for hand pain and daily smoking was a protective factor for neck pain.

Multi-exposure models of risk factors for single-site pain
A high sum-score of ergonomic factors at baseline was of importance for pain in the neck, the shoulders, the hands and the feet (Table 3). When adjusting the multi-exposure models for pain in the actual anatomical site at baseline, adverse ergonomic factors remained as a significant risk factor of pain in the neck, the hands and the feet ( Table 4). Table 3 Multi-exposure models in the total study population (n = 1115) of associations between self-reported ergonomic-, psychosocial-and personal factors at baseline, and musculoskeletal pain in the neck,  Table 4 Multi-exposure models adjusted for pain at baseline, in the total study population (n = 1115) of associations between self-reported ergonomic-, psychosocial-and personal factors at baseline, and musculoskeletal pain in the neck, shoulders, hands, low back and feet at follow-up, calculated with Poisson regression, with overall p-values, prevalence ratio (PR) and 95% confidence intervals (CI Complaints on the computer work station arrangement was a risk factor for pain in the neck and the shoulders ( Table 3), but these associations declined in the adjusted multiexposure model (Table 4).
A high sum-score of psychosocial factors at baseline was a risk factor of pain in the neck, the shoulders, the hands and the lower back (Table 3). When adjusting for pain at baseline, the psychosocial factors remained as a statistically significant risk factor or for pain in the neck and the shoulders (Table 4).
High age was of importance for subsequent pain in the hands and the feet (Table 3).
However, when adjusting for pain at baseline the statistically significant association remained for feet pain only (Table 4).
A high body-mass index was of importance for pain in the feet in the multi exposure model (Table 3), but the association was no longer statistically significant after adjustments for foot pain at baseline ( Table 4).
Lack of time for personal recovery was a risk factor for pain in the neck and the shoulders (Table 3), but in the adjusted multi-exposure model, it remained significant for neck pain only (Table 4).
Among the other life-style factors, a low frequency of physical exercise and much domestic work, respectively, did not predict pain in any anatomical site (Table 3 and 4).
On the contrary, much domestic work was a protective factor for pain in the hands and the feet (Table 4). Further, daily smoking at baseline was a protective factor for neck pain in 22 both multi-exposure models (Table 3 and 4).
For all of the specific anatomical sites, pain at baseline was the strongest risk factor for pain in that anatomical site at follow-up, with PRs ranging from 2.4 to 4.2 between the sites; Table 4).
After adjustments of all studied occupational and personal factors, and pain at baseline, there was a residual of unknown factors associated with the occupational category, which were of importance for all of the anatomical pain-sites at follow-up (Table 3 and 4). Among the specific occupational categories, it was observed that among sonographers there was an increased risk of shoulder pain, but a decreased risk of pain in the feet, when pain at baseline was taken into account (Table 4).

Discussion
Principal findings Adverse ergonomic-and psychosocial factors at baseline were risk factors for multisite pain at follow-up, with statistically significant associations in the single-multi-and adjusted multi-exposure models. However, the strongest risk factor for multisite pain at follow-up was multisite pain at baseline. Further, unknown factors associated with occupational category were of importance for multisite pain, in all of the three statistical models.
In the adjusted multi-exposure models, different risk factors were found for subsequent single-site pain in the five anatomical regions: A high sum-score of ergonomic factors at baseline was a risk factor for pain in the neck, the hands, and the feet. A high sum-score of psychosocial factors was of importance for pain in the neck and the shoulders.
Moreover, lack of personal recovery was a risk factor for neck pain, and high age was a risk factor for pain in the feet. Unexpected findings were that much domestic work was a protective factor for hand-and foot pain, and that daily smoking was a protective factor 23 for neck pain. For all of the specific anatomical sites, the strongest risk factor of pain at follow-up was pain in the actual site at baseline. Further, the occupational category was of importance for pain in the neck, the shoulders, the lower back and the feet at follow-up.
Several of the demonstrated relationships found in the multi-exposure analyses became less strong when the models were adjusted for pain at baseline. This indicate that an important part of the associations were present already at baseline.

Strengths and weaknesses of the study
A strength of the study is the longitudinal study design, and that we used common and tested indicators for physical and psychosocial working conditions. Another advantage is the outcome measure of pain that comprise a combination of the frequency and intensity of complaints of a certain dignity, on the individual level. Thus, mild complaints which do not interfere with daily life activities were not included as an outcome. The fact that the study population included women in common professions from in total 116 different workplaces in Sweden is also a strength, as it increases the generalizability of the study.
The participants at follow-up and the drop-outs reported, with a few exceptions, similar frequencies of pain at baseline. Thus, we do not believe that the overall selection of participation at follow-up has had a significant impact on the longitudinal results.
An obvious limitation of this longitudinal study was that all data were self-reported.
Further, we only had two assessment rounds with quite a long separation in time. Thus we could not identify any potential changes that may have occurred between the two measurements. As pain includes all pain episodes during the 12-months period preceding the reporting time, this should have a limited influence on results.
Another limitation was that the data collection, at both baseline and follow-up, went on for an extended period of time. This was due to administration of the high number of different work places, and that time-consuming assessments (objective measurements of the 24 physical workload and clinical examinations) were performed in subgroups at baseline [19]. However, at both baseline and follow-up, we administered the questionnaires alternately to the teachers, the surgical staff and the sonographers. Thus, any societal changes that may have occurred during this time period would affect the occupational groups similarly. Consequently, the mean follow-up period was about as long in the five occupational groups, although there was a large variation between the workplaces and between the participants. This variation in follow-up periods may have influenced the results. However, the correlation between the length of follow-up and change in multisite pain was very low (rho 0.03) and thus we believe that this was not a major problem.
For the multi-exposure models, sum scores of ergonomic and psychosocial factors were created, to obtain a gradually increasing effect of several ergonomic-and psychosocial factors, respectively. The underlying assumption was that the experience of two or more adverse ergonomic or psychosocial dimensions is worse than the experience of none or only one such dimension.
The results in relation to other studies The present study population comprises women with contrasting occupational exposures.
The work tasks among the surgical staff (i.e. anaesthetic-, theatre-and assistant nurses) include physical exertions when handling of patients and equipment, and prolonged twisted and static postures during surgery [28]. The work tasks among the sonographers is to a high extent characterized by prolonged sitting in constrained postures [29]. In contrast, the physical workload among the teachers is varied and physically relatively light. While time-pressure and adverse psychosocial working conditions may be present in all of the five occupational groups, the mental strain is particularly high among the teachers [19,30,31]. Hence, these occupational groups were exposed to working conditions that previously have been recognized as risk factors for musculoskeletal pain, concerning ergonomic- [4,32,33] and psychosocial factors [8][9]. Thus, in the present study of nurses, teachers and sonographers, the scientific evidence of both ergonomicand psychosocial factors as risk factors for musculoskeletal pain, was confirmed.
A clear finding of the study was that pain at baseline vas the strongest risk factor for the occurrence of pain at follow-up. Similar results of previous pain episodes as predictors of present pain have been reported previously [34,35]. However, pain was associated with several occupational factors already at baseline [19]. Thus, since many had worked for a long time (mean seniority 17 years) they may have developed work-related pain already prior to the baseline study, which remained (or recurred) at follow up. Of course, persistent pain may also be due to individual susceptibility or other non-work-related factors. Thus, to explore associations between occupational and personal risk factors, and changes in pain during the follow-up period, we adjusted for pain at baseline and found longitudinal effects also of adverse ergonomic conditions and psychosocial factors.
Among the personal and life-style factors, only a few factors were of importance for subsequent single-and multisite pain. The prevalence of musculoskeletal pain is commonly believed to increase at higher age [1,35] but in the present study this was only the case for pain in the hands and the feet. High age did not explain multisite pain. A high body mass index was of importance for pain in the feet, and for multisite pain. However, most likely, since the statistical significances disappeared after adjustment for pain at baseline, those associations were present already at baseline. Further, lack of personal recovery was a risk factor for pain in the neck and shoulders. Unexpectedly, much domestic work at baseline was found to be a protective factor for pain in the hands and the feet at follow up. We have no explanation for this, but a speculation is that participants with persistent pain from hands or feet need more rest after work, and thus avoid to perform much domestic work. Further, since hand-and foot pain was explained by increasing age (in the second model), another possible explanation is that older participants no longer have small children living at home, and thus have less need for domestic work. Daily smoking is considered to be a risk factor for poor health, including musculoskeletal pain [13]. Contrary, we found that daily smoking was a protective factor for neck pain. Smokers may take more frequent breaks at work, which may be beneficial in relation to work-related pain. However, very few participants (4%) were daily smokers, and it cannot be ruled out that the protective effect was as a random finding. There was a large fluctuation in number of sites of pain between baseline and follow-up.
About one third reported an increased number of pain-sites at follow-up, indicating a worsened condition. Fortunately, almost as many reported less pain-sites at follow-up and thus improved health in this regard. Thus, for many individuals, the occurrence of pain is not a static condition, it rather reflects a pattern of recurrence [15,16]. Nevertheless, in spite of the strict criteria, multiple pain-sites were common in the present study population. Almost one fifth (18%) reported persistent multisite pain, i.e. ≥ four pain-sites at both baseline and follow-up. It is notable that only 11% did not report pain in any anatomical site, neither at baseline nor at follow-up. As far as we know, our outcomemeasure of pain has not previously been used in scientific studies of multisite pain, thus making direct comparisons difficult. However, a longitudinal study of Finnish food company workers reported that 36% of the study population (involving an unclear 27 distribution of women and men) had "no pain" [19]. The high numbers of affected workers in the present study is worrying, since the study population was taken from ordinary occupations, which constitute a large proportion of the female labour force in Sweden.
There is no reason to believe that we have caught any particularly loaded/burdened groups among these occupations, as we have studied personnel in many different worksites. Rather, factors associated with the organizational climate [37], such as time pressure and high physical/mental demands, may contribute to the disorders since such conditions affect both the physical and psychosocial working conditions.

Conclusoins
An overwhelming majority of the women in these common occupations were affected by musculoskeletal pain, from one or several body regions. Further, a substantial proportion of the women were classified as having persistent multisite pain. Since the present criteria of pain was rather strict, the reported impaired health conditions should not be overlooked. Among the occupational factors, both ergonomic-and psychosocial factors were of importance for specific pain-sites, as well as for multisite pain. The findings are important for preventive strategies, and points to the need of improved work environment, with actions on individual, organizational and societal level.

Ethics approval and consent to participate
Written informed consent for participation was obtained from all participants. The regional ethics committee at Lund University approved the study (March 10 2010; reference no.

Consent for publication
Not applicable

Availability of data and material
As the database referred to in this manuscript contains direct or indirect identifiers, it is not available in an open access repository, as there is a possibility that the participants may not be completely anonymous. Readers interested in exploring the data should contact the corresponding author.  The association between sum score of ergonomic factors (1)(2)(3)(4) at baseline and number of anatomical sites with pain at follow.