Physical and mental strain at work: Relationships with onset and persistent of multi-site pain in a four-year follow up

1 Physical and mental strain at work: Relationships with onset and persistence of multi-site pain in a four-year follow up Subas Neupane1, 2 *, Clas-Håkan Nygård1, 2 1School of Health Sciences FI – 33014, University of Tampere, Tampere Finland 2Gerontology Research Center, FI – 33014, University of Tampere *Corresponding author: Dr. Subas Neupane School of Health Science FI – 33014 University of Tampere, Finland Email [email protected] Phone +358 40 1909709 Fax +358 3 35516057 Running title: Mental and physical strain and multi-site pain 2 Abstract This study evaluates whether physical and mental strain are associated with an onset and persistence of multi-site pain among younger and older employees in a four year of followup. A questionnaire survey was conducted in a food processing company twice, in 2005 and 2009 with the response from 734 employees in the age between 20 and 66 years (445 younger and 289 older; 65% female). Information on musculoskeletal pain during the preceding week and perceived mental and physical strain was obtained through a structured questionnaire. The association of onset and persistence of multi-site pain with mental and physical strain was estimated with log binomial regression analysis and stratified by agegroup. Risk ratio (RR) with their 95% confidence intervals (CIs) are reported for the estimates. More than 56% of the employees reported multi-site pain at baseline. Among those who reported multi-site pain at baseline 70% reported persistence of multi-site pain and one third had new onset of multi-site pain at the follow-up. Mental strain at baseline strongly predicted persistence of multi-site pain among both younger and older employees (RR from for younger employees = 1.68, 95% CI = 1.01-2.83 and RR for older employees = 2.25, 95% CI 0 1.27-3.98). The high mental strain predicted the risk of persistence multisite pain among both younger and older employees in a four year follow-up but not onset of multi-site pain. Key words: musculoskeletal pain; multi-site pain; mental strain; physical strain 3 Highlights •There is an increased attention to the relationship between work exposures and musculoskeletal disorders in recent research. •Mental strain at baseline was strong predictor of persistence of multi-site pain musculoskeletal pain both among younger and older employees. •Interventions in the workplaces focusing on the individual level would reduce physical and mental strain as a result lower the incidence of multi-site pain for both younger and older employees. 4 INTRODUCTION Musculoskeletal pain in multiple body sites is frequent in the working population (Neupane et al, 2013a; Haukka et al, 2013; Herin et al, 2014). A large body of research however focused on a single body site. Association between certain physical (physical work load, repetitive task, awkward posture) and psychosocial exposures (high job demands, low job control, co-workers and supervisors support) at work and painful conditions at particular anatomical sites has been studied extensively. Evidence shows that pain in multiple body site is associated with decreased functioning and worse prognosis when compared to having pain in only one body site (Neupane et al, 2014; Haukka et al, 2013; Kamaleri et al, 2008; Miranda et al, 2010). However less is known about the relationship of multi-site musculoskeletal pain with mental and physical strain related work exposures. The concept of strain in this study is based on the stress-strain model where the stress on a worker depends on environmental factors (both physical and psychosocial) acting upon the person, whereas strain denotes the effects of stress which differ by individual for a certain stress factor (Rutenfrantz, 1981; Cox et al, 2000). The stress and strain concept has been widely used in studies on the effects of physical work load on both the cardiovascular as well as the musculoskeletal system (Rutenfranz, 1981, Nygård et al, 1988). The model emphasizes the role of individual characteristics such as age, gender, health status and work ability as modifiers of the relationship between strain and musculoskeletal symptoms or pain. 5 Industrial workers are mostly exposed to physical strain (Nicot, 2007). The high strain is mostly due to the physical work environment in the industry. In a cross-sectional study among men and women employed in different occupations, physical strain was associated with musculoskeletal symptoms (Johansson, 1995). One earlier study among health care employees, mental strain was found to be a risk factor for musculoskeletal symptoms in a three year follow-up (Josephson et al, 1997). It also found that the risk was higher when it was combined with perceived high physical exertion. Psychological or mental strain has also been found to be linked to poor outcomes such as early retirement (Laine et al, 2008) and mortality (Kivimäki et al, 2002). In a 28 years of follow-up study, work strain (both mental and physical) during midlife among municipal employees was found to be strongly related to declined work ability in old age (von Bonsdorff et al, 2012) and stress symptoms in midlife predicted disability after 28 years (Kulmala et al, 2013). Earlier studies have reported the relationship of strain and age with inconclusive findings. One study has shown that older employees experience higher strain than younger employees (Nygård et al., 1997). However, Pailhe (2005) reported that younger cohorts are exposed to physical strain more frequently than older cohorts. In this study of 4 years of prospective follow-up of industrial employees we evaluated whether physical and mental strain were associated with an onset and persistence of multi-site pain. We were also interested whether the association were different for younger and older employees. METHODS 6 This study is based on questionnaire surveys conducted among all employees of one of the leading food industries in Finland (1985 employees) in spring 2005 and spring 2009. The questionnaires were distributed at the workplaces to every employee (response rate 60%). It was possible to reply anonymously or to sign the consent for individual follow-up of the surveys and for linking to the personnel registers of the company including information on age, gender occupational status, workplace and duration and interruptions of the job contract. The replies were placed in sealed envelopes which were collected and forwarded to the researchers. As the question forms were not addressed to individual employees, no reminders could be sent. The study was approved by the ethical committee of Pirkanmaa Hospital District. Measurement of variables A modification of the validated Nordic Musculoskeletal questionnaire (Kuorinka et al. 1987) was used to assess musculoskeletal pain. It included a question regarding whether the employee had felt pain, aching or numbness in four anatomical areas (hands or upper extremities; neck or shoulders; lower back; and feet or lower extremities) during the preceding week, with the reply scale being from 0 (not at all) to 10 (very much). The variables were dichotomised at the median (less than median: 0= mild; more than median: 1= severe). The cut-off values for upper extremities, neck and shoulder, lower back and lower extremities were 4, 5, 2 and 2, respectively. The dichotomised variables were summed up into a variable expressing the number of areas with severe pain (from zero to four) (Neupane et al. 2013a; Neupane et al. 2013b). The summed variable was further 7 categorized into two, leaving zero and one as ‘no multi-site pain’ and ‘multi-site pain’ by combining two, three and four sites pain. Mental strain Perceived mental strain was assessed by a modified version of the occupational stress questionnaire (Elo et al., 1992), using a single question (“Stress means a situation in which a person feels excited, apprehensive/concerned, nervous or distressed or she/he cannot sleep because of the things on her/his mind. Do you feel this kind of stress nowadays?”) (Elo et al., 1992), with the reply scale from 0 (not at all) to10 (very much). For this analysis, mental strain was categorized into low (0-2), medium (3-6) and high (7-10) by their tertile values. Physical strain Perceived physical strain was elicited with the rating of perceived exertion (RPE) with the question “How physically hard/ exhausting do you feel your job is in a normal work day?” on a scale from 6 (not at all) to 20 (very much) (Borg, 1970). Physical strain in this study was categorized into three low (6-11), medium (12-15) and high (16-20) based on the tertile values. Covariates: Work ability was assessed with one question from the work ability index as a subjective assessment of current work ability compared with a person’s self-identified lifetime best (i.e. with the question “Assume that your work ability at its best has a value of 10 points. 8 What score would you give your current work ability?”). Work ability was categorized into excellent (score 10), good (score 9), moderate (score 8) and poor (scores 0-7) work ability in this study (Gould et al, 2008). Body mass index (BMI) was calculated using workers’ self-reported weight (kg) and height (m). The level of physical exercise during the last month was elicited on a scale from 0 (not at all) to 7 (strenuous physical activity for more than 3 hours a week). Statistical Analysis Descriptive statistics of the study population was presented first in the form of frequencies and percentages in total stratified by age group (<45 years ‘younger’ and ≥45 ‘older’). Log binomial regression analysis was performed to examine whether baseline mental and physical strain were associated to multi-site pain after four years of follow-up. Risk ratios (RR) with their 95% confidence intervals (CI) are presented. The analyses were conducted separately for those who had multi-site pain at baseline ('persistence of multi-site pain') and those with no multi-site pain at baseline (‘onset of multi-site pain’). And again both analyses were stratified by age group in order to see the difference between age group. The models were built up in 3 steps: Model I: crude risk ratios, Model II: adjusted for gender and occupational status and lastly Model III: includes gender, occupational status, physical exercise, BMI and baseline work ability. All the analyses were carried out with the statistical package SPSS version 21.0. RESULTS 9 Table 1 shows the descriptive characteristics of the study population at baseline. Within the total of 734 study population, 60% were less than 45 years of old. About two third of the study population were female and more than 70% were blue-collar employees. There were more male and blue-collar employees in the younger age group and more female and white-collar employees in the older age. One third of the study population had high physical exercise which not much differed between age groups. A little more than one fifth of the people had BMI ≥ 29.0 Kg/m2 and almost 30% of the younger employees had low (<23.0 kg/m2) BMI and 25% of the older employees had high (≥ 29.0 Kg/m2) BMI. About 15% of the study population had poor work ability at baseline and about the same number (16%) had reported excellent work ability. Within the age group, almost one fifth of younger employees had excellent work ability compared to 11% in their older counterparts. Physical strain was comparatively high (30% vs. 27%) among younger employees while mental strain was high among older employees (30% vs. 21%). More than 56% of the employees reported multi-site pain at baseline. Comparatively younger employees reported multi-site pain more often than their older counterparts. Risk ratios and their 95% CI for multi-site pain at follow-up due to physical and mental strain among the cohort with no multi-site pain at baseline are presented in Table 2. One third of the study population had new onset of multi-site pain. High physical strain was found to be associated with onset of multi-site pain among all in the crude model (RR = 1.48, 95% CI = 1.01-2.18) but significance association was lost when adjusted in model II and model III. 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Health 51, 180-192. 20 Table 1: Baseline characteristics of the study population Characteristics All Younger (<45 years) Older (≥45 years) N=734 % N=445 % N=289 % Gender Male 255 34.7 173 38.9 82 28.4 Female 479 65.3 272 61.1 207 71.6 Occupational status Blue-collar 518 70.6 327 73.5 191 66.1 White-collar 216 29.4 118 26.5 98 33.9 Physical exercise Less 322 44.1 193 43.6 129 44.9 Moderate 162 22.2 94 21.2 68 23.7 High 246 33.7 156 35.2 90 31.4 BMI <23.0 180 24.5 129 29.0 51 17.6 23.0-25.9 230 31.3 136 30.6 94 32.5 26.0-28.9 153 20.8 82 18.4 71 24.6 ≥29.0 171 23.3 98 22.0 73 25.3 Work ability Poor 106 14.5 63 14.3 43 14.9 Moderate 235 32.1 130 29.4 105 36.3 Good 274 37.5 165 37.3 109 37.7 Excellent 116 15.9 84 19.0 32 11.1 Physical strain Low 247 33.7 140 31.5 107 37.2 Medium 273 37.3 169 38.1 104 36.1 High 212 29.0 135 30.4 77 26.7 Mental Strain Low 267 36.4 166 37.4 101 34.9 Medium 288 39.3 186 41.9 102 35.3 High 178 24.3 92 20.7 86 29.8 Musculoskeletal pain No multi-site 321 43.7 191 42.9 130 45.0 Multi-site pain 413 56.3 254 57.1 159 55.0 21 Table 2: Risk ratios (RR) and 95% confidence interval (CI) for multi-site pain in relation to physical and mental strain at work among the cohort with no multi-site pain at baseline (N=108), younger and older employees RR and 95% CI for multi-site pain Model I Model II Model III All Physical strain Low 1.0 1.0 1.0 Medium 1.12 (0.81-1.55) 1.02 (0.70-1.48) 1.09 (0.74-1.60) High 1.48 (1.01-2.18) 1.29 (0.83-2.00) 1.38 (0.87-2.18) Mental Strain Low 1.0 1.0 1.0 Medium 1.22 (0.89-1.67) 1.27 (0.92-1.75) 1.14 (0.82-1.60) High 1.19 (0.80-1.78) 1.28 (0.85-1.94) 1.23 (0.80-1.88) Younger Physical strain Low 1.0 1.0 1.0 Medium 1.20 (0.79-1.82) 1.16 (0.71-1.91) 1.40 (0.82-2.41) High 1.39 (0.80-2.41) 1.34 (0.72-2.47) 1.69 (0.88-3.25) Mental Strain Low 1.0 1.0 1.0 Medium 1.14 (0.76-1.71) 1.17 (0.78-1.77) 1.05 (0.68-1.62) High 0.90 (0.49-1.66) 0.99 (0.53-1.87) 1.08 (0.56-2.10) Older Physical strain Low 1.0 1.0 1.0 Medium 1.11 (0.65-1.87) 0.93 (0.52-1.65) 0.91 (0.50-1.66) High 1.53 (0.89-2.64) 1.18 (0.62-2.24) 1.12 (0.57-2.20) Mental Strain Low 1.0 1.0 1.0 Medium 1.31 (0.79-2.17) 1.38 (0.82-2.30) 1.28 80.74-2.21) High 1.36 (0.77-2.40) 1.46 (0.82-2.61) 1.39 (0.75-2.55) Model I: Crude risk ratio Model II: Adjusted for gender and occupational status Model III: Adjusted for model II + physical exercise, BMI and work ability at baseline 22 Table 3: Risk ratios (RR) and 95% confidence interval (CI) for multi-site pain in relation to physical and mental strain at work among the cohort with multi-site pain at baseline (N=291), younger and older employees RR and 95% CI for multi-site pain Model I Model II Model III All Physical strain Low 1.0 1.0 1.0 Medium 1.02 (0.74-1.41) 1.05 (0.73-1.50) 0.97 (0.66-1.42) High 1.46 (1.04-2.04) 1.52 (1.02-2.24) 1.28 (0.85-1.94) Mental Strain Low 1.0 1.0 1.0 Medium 1.44 (1.07-1.95) 1.45 (1.07-1.97) 1.40 (1.01-1.93) High 2.07 (1.47-2.92) 2.24 (1.57-3.20) 1.90 (1.30-2.78) Younger Physical strain Low 1.0 1.0 1.0 Medium 1.00 (0.65-1.53) 1.03 (0.64-1.67) 0.97 (0.58-1.62) High 1.57 (1.03-2.41) 1.63 (1.00-2.73) 1.40 (0.81-2.42) Mental Strain Low 1.0 1.0 1.0 Medium 1.37 (0.93-2.01) 1.37 (0.93-2.01) 1.21 (0.80-1.84) High 1.96 (1.24-3.10) 2.15 (1.33-3.45) 1.68 (1.01-2.83) Older Physical strain Low 1.0 1.0 1.0 Medium 1.04 (0.64-1.71) 1.07 (0.62-1.83) 0.92 (0.51-1.65) High 1.25 (0.72-2.17) 1.30 (0.70-2.41) 1.00 (0.52-1.93) Mental Strain Low 1.0 1.0 1.0 Medium 1.54 (0.94-2.53) 1.56 (0.94-2.56) 1.63 (0.96-2.78) High 2.24 (1.33-3.77) 2.42 (1.41-4.15) 2.25 (1.27-3.98) Model I: Crude risk ratio Model II: Adjusted for gender and occupational status Model III: Adjusted for model II + physical exercise, BMI and work ability at baseline