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Journal of Occupational & Environmental Medicine:
doi: 10.1097/JOM.0b013e3182572e6a
Original Articles

The Effects of Shift Work and Interaction Between Shift Work and Overweight/Obesity on Low Back Pain in Nurses: Results From a Longitudinal Study

Zhao, Isabella RN, BN, BN (Hons I), PhD; Bogossian, Fiona RN, DipAppSci, BAppSci, MPH, PhD, MACMI; Turner, Catherine RN, BA, Grad Dip Ed, MN, PhD

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Author Information

From the School of Nursing and Midwifery, The University of Queensland, Brisbane, Australia.

Address correspondence to: Isabella Zhao, RN, BN, BN (Hons I), PhD, School of Nursing and Midwifery, The University of Queensland, Brisbane, Australia (i.zhao@uq.edu.au).

This project is supported by grants from the Australian Research Council (LP0562102, SR0566924), Australian National Health and Medical Research Council (2005002108), and New Zealand Health Research Council (456163). Industry Partners providing additional funding include Queensland Health, the South Australian Department of Health, Injury Prevention and Control Australia (Pty Ltd), Nursing Council of New Zealand, and the Macquarie Bank Foundation.

Industry partners providing in-kind support for the project include Queensland Nursing Council, Nurses and Midwives Board of New South Wales, Nurses Board of Tasmania, Nurses Board of Western Australia, Nurses Board of the Australian Capital Territory, and the Nursing Council of New Zealand. Corporate sponsors include Virgin Blue, Virgin Atlantic, and MessageNet.

The authors declare no conflict of interest.

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Abstract

Objective: To examine whether shift work is a risk factor for low back pain (LBP) and the interaction effects of shift work and overweight/obesity on LBP over time among nurses.

Methods: A longitudinal study over 2 years. Measurements included reported LBP, shift work status, and selected potential confounders.

Results: Among 928 LBP-free nurses at baseline, 319 (34.4%) developed LBP over 2 years. After adjusting for confounders, shift workers were 1.15 times more likely to develop LBP (adjusted odds ratio [aOR], 1.15; 95% confidence interval, 1.05 to 1.40; P = 0.03). The interaction analysis showed that overweight/obese shift workers were more likely to develop LBP than day workers (overweight: aOR, 1.23 vs aOR, 1.03, respectively; obesity: aOR, 1.34 vs aOR, 1.10, respectively).

Conclusions: Our findings suggest that shift workers are at a higher risk of developing LBP over time, especially those who are overweight/obese.

Recent research indicates that nurses are at high risk for work-related musculoskeletal disorders (MSDs).13 Low back pain (LBP) is the most frequent musculoskeletal injury/MSD in nurses, followed by neck and shoulder problems.4 Studies conducted in Australia report that the 12-month prevalence of LBP in nursing populations ranges between 66% and 76%,2,3 with point prevalence of 40% to 59%.57 The functional consequences of LBP for nurses include time off work, increased risk of chronicity, as well as associated personal and economic costs. The total annual costs of LBP in 2008 (including both direct and indirect costs) were $9 billion in Australia, £12 billion in the United Kingdom, and €6 billion in the Netherlands.8 Therefore, it is essential to identify the risk factors for LBP through research to target effective intervention strategies to promote workforce productivity and reduce the financial burden of LBP.

Nursing work is often physically demanding, requiring heavy lifting, bending, twisting, and other postures that contribute to MSDs.9 Physical demands increase the risks of injury10,11 and psychological job demands can amplify the effects of physical exertion.12 Furthermore, lack of control over work has also been reported to contribute to LBP and MSDs.13 These contributing factors are inherent in the occupation of nursing. Moreover, shift work is another defining occupational characteristic in the profession and has been described alongside other occupational risk factors for LBP in nurses.1416 However, the mechanism of how shift work contributes to the development of LBP is not clear; it is postulated that shift work as a stressor both physically and psychologically could increase the odds of LBP. Furthermore, our earlier study demonstrated that overweight and obesity is especially prevalent among shift workers17; and high body mass index (BMI) or overweight/obesity has been established as one of the significant risk factors for developing LBP.1820 Therefore, the interaction between shift work and overweight/obesity should be explored when studying LBP among nurses.

Earlier studies suggest that previous back problems were one of the most common predictors for back pain in nurses.3,21,22 To remove such an effect, we decided to study a group of LBP-free nurses and follow them to determine both whether shift work is a risk factor for LBP and the interaction between shift work and overweight/obesity on LBP over time among nurses.

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METHODS

Participants

The participants were selected from the Nurses and Midwives' e-cohort Study (NMeS), which is a longitudinal population-based study funded by the Australian Research Council and a range of industry partners. The purpose of NMeS is to examine factors associated with both workforce and health outcomes in a cohort of nurses and midwives in Australia, New Zealand, and the United Kingdom. The NMeS is conducted completely electronically from recruitment to follow-up. Recruitment procedures have been published elsewhere23 and the NMeS was granted ethical clearance by The University of Queensland. Each participant was given a unique participant ID and was not personally identified in the analysis data sets. The baseline survey (survey 1 [S1]) was launched in April 2006 when the study recruitment began. Registrations to the online study via the study Web site (www.e-cohort.net) continued over a 2-year period until April 2008. There have been 10,120 total registrants, of whom 7604 registrants completed S1 (75%) and 5280 registrants completed both S1 and survey 2 (S2), yielding a follow-up rate of 69%.

In our study, participants were selected from the 5280 registrants who completed both surveys. To be included in the study sample, a participant must be currently working in the nursing profession at S1, have reported on LBP status at S1 and S2, not identify as being pregnant at S1 or S2, and have completed questions relating to shift work status at S1. We excluded those participants working as midwives because analysis of MSDs/musculoskeletal injuries in this profession is being conducted separately in the NMeS in recognition of the different working contexts and occupational demands faced by midwives.

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Variables and Measurements
Outcome Variable

Low back pain was defined as “ache, pain, or discomfort in the area of the back between the 12th ribs (bottom of rib cage) and gluteal folds (bottom of buttocks) whether or not it extends from there to one or both legs (sciatica).”24 A question from the Nordic Musculoskeletal Questionnaire (NMQ) was used to determine whether the participants ever had LBP at S1. Only the participants who reported to have never had LBP in S1 were selected for follow-up. At S2, these participants were asked whether they had LBP at any time during the last 12 months (yes/no). The validity and reliability of NMQ have been established and it has been widely used in studies of MSDs.24,25

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Exposure Variable

Shift work is defined as work performed primarily outside typical daytime hours and includes evening shifts, rotating shifts, irregular shifts, and flex time.26 In S1, shift work was measured by asking the respondent to identify which of the following categories described her or his current shift schedule: (1) day shifts only, without weekends; (2) day shifts only, with weekends; (3) continuous shift work; (4) evening shifts only; (5) night shifts only; (6) morning and evening shifts only; and (7) evening and night shifts only. For the purposes of this study, the first two response categories were considered to be day work and the remaining five categories were considered to be shift work.

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Potential Confounding Variables Selected

All the potential confounding variables identified from the literature and included in the analysis were selected from S1. These variables were categorized as demographic, occupational, and general health factors.

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Demographic Factors

Common demographic variables, including age, sex, and marital status, were selected. Age was measured in years. Marital status was categorized into never married, married, or de facto, divorced or separated or widowed.

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Occupational Factors

Job classification and work setting were selected to understand the nature and context of the participants' employment. Because our study excluded those who were working as midwives, job classification comprised registered nurses; enrolled nurses and nurse assistants; specialist registered nurses; administrators and managers; and academics and educators. It is important to consider work setting in this analysis because nursing homes and residential care facilities have a higher incidence rate of injuries than that of hospitals and other settings.27 In this study, work setting, a categorical variable, consisted of hospital, residential age care, community health, integrated hospital/clinic/community, medical practice, and government department/university.

Physical exposures were measured specifically as manual handling frequency assessed using an eight-item, specially developed instrument (total score, 0 to 24 points).14 The instrument enquires about the average daily frequency of manual handling activities such as pushing/pulling, lifting equipment, and assisting patients to transfer from sitting to standing. Test–retest reliability has been established and reported elsewhere.14 To assess perceived general physical demands, we used the Job Content Questionnaire (JCQv1.1) Physical Demands subscale measuring physical effort and rapid and continuous physical activity. The reliability of the JCQv1.1 has been established internationally and this instrument has been widely used in various occupations.28

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Health Factors

Several general health and psychological factors that might have potential confounding effects on the association between shift work and LBP were selected. Overweight/obesity has been reported in relation to back injuries among nurses.18,19 Consequently, BMI was calculated from weight and height reporting and classified as per the World Health Organization and categorized into underweight, normal, overweight, and obesity.29

A meta-analysis of 27 cross-sectional studies and 13 cohort studies reported that both current and former smokers had a higher prevalence and incidence of LBP than never smokers.30 Therefore, self-reported smoking status was categorized as never smoked, ex-smoker, and current smoker.

Perceived health status has been found to be correlated to LBP in cross-sectional studies.31 To further explore the relationship, we decided to include this variable in the analysis. We chose the 36-item Short Form Health Survey (version 2) (SF-36-v2) as a generic outcome measure designed to examine a person's perceived health status.32 The SF-36-v2 can be divided into two aggregate summary measures of the physical component and the mental component,33 which reflect one's perceived general physical health and general mental health, respectively. Higher scores indicate better physical/mental health. The SF-36-v2 has been widely used in research.

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Work-Related Psychosocial Factors

Work-related social factors were assessed with the JCQv1.1,28 which measured skill discretion, decision authority, psychological demands, coworker support, and supervisor support. Guided by an earlier publication from NMeS on a similar topic,14 we decided to include decision latitude (the sum of skill discretion + decision authority subscales), job support (the sum of coworker support + supervisor support subscales), and psychological demands in the analysis. These factors were suggested to have an impact on LBP in two systematic reviews.34,35

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Statistical Method

All statistical analyses were undertaken using SPSS for Windows version 17 (SPSS, Chicago, IL). A significance level of 0.05 was determined for all statistical tests. Descriptive statistics were carried out to calculate medians for continuous variables, and percentages were presented for categorical variables. At the bivariate level of inferential statistics, a t test was performed on the continuous variables between the two groups as the assumptions were all met, and Pearson chi-squared tests were carried out for the categorical variables. The selected potential confounding variables were all adjusted in the multivariable logistic regression to calculate adjusted odds ratio (aOR). Two-way interaction terms between shift schedule and BMI classification were also entered into the model. Interactions are denoted by an asterisk: Shift Schedule * BMI Classifications.

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RESULTS

Among the 5280 participants who completed S1 and S2, 928 nurses (17.5%) met the inclusion criteria. Among the participants (n = 4352) who did not meet the inclusion criteria, the majority were excluded because they reported having LBP at S1 (n = 3239) and they were registered as midwives or were not working in the nursing profession at S1 (n = 812). In addition, 274 participants did not provide information about their shift work status at S1 and 27 participants reported being pregnant at either S1 or S2. The excluded participants were comparable with the included participants on the selected demographic variables at baseline (age, sex, and marital status; data not shown).

Our study included 928 nurses aged 21 to 67 years (mean ± SD, 43.4 ± 9.9 years). Those nurses who reported having LBP at S2 were 3.4 years older on average than the nurses without LBP at S2 (45.3 vs 41.9 years; P < 0.0001) (Table 1). Of all the participants, 6% were male nurses; those with LBP and those without LBP were similar on the distribution of sex (P = 0.13; Table 1). Almost half of the participants were married or in de facto relationships and 12.1% of them were either never married or divorced/separated/widowed (Table 1). The nurses with LBP had a higher percentage of being divorced/separated/widowed than those who did not have LBP (14.9% vs 8.8%; P = 0.01; Table 1). Among the 928 LBP-free nurses at baseline, 319 (34.4%) developed LBP over the 2-year follow-up. Among the 319 nurses who had LBP at S2, 145 were day workers and 174 were shift workers at baseline (45.5% vs 54.5%; P = 0.02; Table 1).

Table 1
Table 1
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Statistically significant associations between all the selected factors and LBP are presented in Table 1. Regarding occupational factors, LBP was related to working as enrolled nurses and nurse assistants (10.3% vs 6.6%; P = 0.01) and working in residential aged care (14.3% vs 8.8%; P = 0.01). The nurses who developed LBP reported having a higher frequency of manual handling activities (6.1 vs 4.4; P < 0.001) and they also reported having higher physical demands (5.1 vs 4.6; P = 0.02) at work compared with those without LBP. In terms of health and psychological factors, LBP was related to being overweight or obese (32.6 vs 30.8 and 31.5 vs 27.3, respectively; P < 0.001). The nurses who had LBP perceived their general physical (48.5 vs 52.3; P < 0.001) and mental (45.0 vs 48.5; P < 0.001) health to be poorer than those who did not have LBP. Those with LBP reported having less job support from their coworkers and supervisors than those without LBP (11.7 vs 12.2; P = 0.04).

Without adjusting any potential confounders, shift workers were found to be 1.24 times more likely to develop LBP over time compared with day workers (OR, 1.24; 95% confidence interval [CI], 1.08 to 1.48; P = 0.02) (Table 2). When considering all the selected potential confounding variables in the multiple logistic regression, shift workers were found to be 1.15 times more likely to develop LBP over time compared with day workers (aOR, 1.15; 95% CI, 1.05 to 1.40; P = 0.03) (Table 2). The Shift Schedule * BMI Classification interaction showed that shift workers who were overweight and obese were more likely to develop LBP than day workers (overweight: aOR, 1.23, vs aOR, 1.03, P < 0.01; obesity: aOR, 1.34, vs aOR, 1.10, P < 0.01) (Table 3).

Table 2
Table 2
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Table 3
Table 3
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DISCUSSION

Our study followed 928 LBP-free nurses over 2 years; 319 of them (34.4%) developed LBP by the second data collection. Among those who reported having LBP at S2, 145 were day workers and 174 were shift workers at baseline data collection (45.5% vs 54.5%). Crude analysis of the association between shift work and LBP revealed that shift workers were found to be 1.24 times more likely to develop LBP over time compared with day workers. This effect has been attenuated after adjusting the selected confounders, suggesting that shift workers were found to be 1.15 times more likely to develop LBP over time compared with day workers. Nevertheless, the result was still statistically significant and shift work could increase the risk of LBP by as much as 40%. The interaction analysis suggested that shift workers who were overweight and obese had even higher risks of developing LBP compared with day workers, by as much as 61%. The mechanism of how shift work contributed to the development of LBP is unclear; it is postulated that stress caused by shift work both physically and psychologically might increase the odds of LBP. Our study results were in line with several other studies investigating the associations between occupational characteristics of the nursing profession and LBP. Some cross-sectional studies reported that nurses working rotating shifts experienced more musculoskeletal symptoms (P < 0.002);36 or shift work was associated with an increased risk of LBP only when combined with weekend work (OR, 2.08; 95% CI, 1.35 to 2.96)37; or shift work increased the risk of LBP by 30%.20 One cohort study that followed up almost 4000 nurse aids over 15 months concluded that working night shifts increased the risk of LBP (aOR, 1.64; 95% CI, 1.09 to 2.49).19 Another longitudinal study of 2617 registered nurses over 15 months reported that frequently working on weekends and being on call were associated with higher risk of back problems (aOR, 1.18; 95% CI, 1.05 to 1.34; and aOR, 1.88; 95% CI, 1.16 to 3.16, respectively).38

To the best of our knowledge, this is the first longitudinal study examining specifically the impact of shift work as well as the interaction between shift work and overweight/obesity on LBP among nurses recruited from Australia, New Zealand, and the United Kingdom. The interaction analysis explains the risks of developing LBP when combining the effects of shift work and overweight/obesity, after knowing that both factors contribute to LBP individually. Moreover, our study considered potential confounders suggested by the relevant literature in the analysis of the association, including demographic factors, various occupational factors, health and psychological factors, and social factors.

The effect of shift work on LBP was attenuated after adjusting for the selected confounders (aOR, 1.15 vs OR, 1.24); it suggested that those confounders could also be important predictors for LBP. Besides some of the demographic factors (age and marital status), the predictors were working as enrolled nurses and nurse assistants; working in residential age care; more frequently performing manual handling activities at work; higher physical demands; being overweight or obese; poorer general physical and mental health; and having less job support from coworkers and supervisors than those without LBP. These findings were consistent with other studies investigating predictors for LBP,14,18,27,31,34,35 which suggests that interventions targeting improved health and psychosocial factors could help to reduce the risk of LBP since it is not possible to eliminate shift work.

Several potential limitations should be considered when interpreting the results. First, study results relied on self-reported data. Low back pain is no doubt a subjective condition, the definition of which probably varies between different countries, cultures, and occupational groups. To account for differences of this nature, we specifically chose the well-known NMQ, which has been established with good validity and reliability and used extensively in various cultures and occupations. Nevertheless, there should be no reason to expect that response bias to the reported LBP would differ between the exposure groups. Second, measurement of the outcome variable was restricted by the NMeS design. Participants were asked at S2 to report any LBP only in the past 12 months rather than for the entire period since S1 (24 months from S1 to S2), resulting in an estimate of 12-month prevalence of LBP rather than the period prevalence between S1 and S2. This may have resulted in the underestimation of the prevalence of LBP compared with that if the participants were asked to report LBP since baseline data collection. In addition, the exposure variable shift work and potential confounding variables were assessed at one time point and this may not capture the effects of changing exposure status. Future research should follow participants who remain in the same shift schedule throughout the study period to better establish the temporal relationship. Finally, this study investigated the impact of shift work in general (including all kinds of shift schedules) on LBP; future studies should examine the associations between specific shift types (eg, rotating shift, permanent night) and LBP in nurses.

Despite these limitations, our study has a number of strengths. First, this is a cohort study examining the effects of shift work and the combined effects of shift work and overweight/obesity on LBP, which offers a higher level of evidence and insight into the causal relationship between the exposure and outcome variables. Second, our study selected LBP-free nurses at baseline then followed them over 2 years to remove the confounding effect of previous back problems, which was identified in the literature as one of the most common predictors for back pain in nurses.3,21,22 Furthermore, our study considered potential confounding factors in the analysis, including the basic demographic factors (age, sex, marital status); occupational factors (job classification, work setting, manual handling frequency, physical demands); health and psychological factors (BMI, smoking status, general physical and mental health); and work-related psychosocial factors (decision latitude, job support, psychological demands); therefore, the confounding effects of such variables could be minimized. Finally, our study participants were selected from the NMeS, a sample that has been demonstrated as representative of the Australian and New Zealand nursing professions,39 making the results more generalizable to the wider Australian and New Zealand nursing workforce.

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CONCLUSION

To the best of our knowledge, this is the first longitudinal study examining specifically the impact of shift work and the interaction of shift work and overweight/obesity on LBP among nurses recruited from Australia, New Zealand, and the United Kingdom, with the consideration of potential confounders including demographic factors, various occupational factors, health and psychological factors, and social factors. Our findings suggested that shift workers were at higher risk of developing LBP over time compared with day workers. This risk was aggravated for those shift workers who were overweight or obese. Although shift work is inherent in the nursing occupation, employers should be more aware of the shift workers in relation to LBP and provide targeted intervention for modifiable health, psychosocial, and occupational factors to prevent LBP.

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©2012The American College of Occupational and Environmental Medicine

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