Applied Ergonomics 47 (2015) 43e51

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Work with prolonged arm elevation as a risk factor for shoulder pain: A longitudinal study among young adults Therese Nordberg Hanvold a, *, Morten Wærsted a, Anne Marit Mengshoel b, Espen Bjertness b, Kaj Bo Veiersted a a b

Department of Work Psychology and Physiology, National Institute of Occupational Health, Oslo, Norway Institute of Health and Society, Faculty of Medicine, University of Oslo, Oslo, Norway

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 February 2014 Accepted 21 August 2014 Available online

This prospective study aimed at examining if work with prolonged arm elevation predicts shoulder pain among 41 young adults in their first years of working life. Fifteen hairdressers, 15 electricians, 5 students and 6 with various work were followed over a 2.5-year period (2006/7e2009). Arm elevation was measured with inclinometers during a full working day at baseline. Shoulder pain was reported at baseline and twice in the follow-up period. Data were analyzed by generalized estimating equations (GEE-analysis), stratified by gender and adjusted for time, mechanical workload, work demand, physical activity, tobacco use and prior shoulder pain. Work with prolonged arm elevation with angles >60
and >90
were associated with shoulder pain among women. Even though the shoulder pain levels are low the study suggests work with arms elevated as an early work-related risk factor among women, and indicates the importance of early prevention strategies. © 2014 Elsevier Ltd and The Ergonomics Society. All rights reserved.

Keywords: Mechanical workload Musculoskeletal disorders Inclinometer

1. Introduction Shoulder pain is frequently reported in both the general and the working population (Luime et al., 2004; Sterud et al., 2014). Pain in the shoulder region also has substantial influence on sickness absence and health-care costs (Andersen et al., 2012; Nyman et al., 2007). In a recent review, exposure to work postures involving elevated arms was found as an important risk factor for shoulder pain (Mayer et al., 2012). The association between work with arms above shoulder level and shoulder pain has however often been €m et al., 1992; examined by cross-sectional designs (Holmstro Ohlsson et al., 1995), and the exposure of arm elevation during work has frequently been based on self-reported measures (Leclerc et al., 2004; Roquelaure et al., 2011). A study among automobile assembly workers objectively measured the work postures by video recordings (Punnett et al., 2000). They found that working 10% of the work day with elbows over shoulder height (>90
arm elevation) constitutes a risk for shoulder disorders (Punnett et al., 2000). A previous study evaluating the relation between shoulder

* Corresponding author. Department of Work Psychology and Physiology, National Institute of Occupational Health, P.O Box 8149 Dep, 0033 Oslo, Norway. Tel.: þ47 23195100; fax: þ47 23195205. E-mail address: [email protected] (T.N. Hanvold). http://dx.doi.org/10.1016/j.apergo.2014.08.019 0003-6870/© 2014 Elsevier Ltd and The Ergonomics Society. All rights reserved.

disorders and objectively measured episodes with arms above shoulder level, found an exposure-response relation between current arm elevation and shoulder disorders (Svendsen et al., 2004). The literature is however still scarce on the effects of objectively measured prolonged arm elevation and it is unclear how high the arms must be elevated and how long the periods of arm elevation must be before the harmful effects occur. In the current study we used inclinometer to give an accurate measure of arm elevation. In contrast to earlier studies of middle-aged and older workers we have in our study analyzed the prospective associations between arm elevation and shoulder pain among a population in the start of their working carrier, being unexposed to earlier work with arms elevated. Young Norwegian workers report more work with arms elevated compared to middle-aged and older workers (NOA, 2011) and it may therefore be of importance to examine if exposure to work with arm elevation is a risk factor for shoulder pain in the first years of working life. The study population consisted of hairdressers and electricians together with a group of students and subjects in various jobs. The population was chosen based on the knowledge that manual occupations like hairdressers (Veiersted et al., 2008) and electricians (Moriguchi et al., 2013) are commonly exposed to work with elevated arms. The study postulates that work exposures with elevated arms predict shoulder pain. Thus, the aim of the present study was to examine the relative time with prolonged arm elevation >60
and

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T.N. Hanvold et al. / Applied Ergonomics 47 (2015) 43e51

>90
and its associations with shoulder pain during the following 2.5 years among young adults in their first years of working life. 2. Methods and material 2.1. Study design Forty one young adults (median age, 21) were followed over a 2.5-year period (2006/7e2009) in their first years of working life. Arm elevation was measured objectively for each participant during a full working day at baseline (T0 ¼ 2006/7) and considered as a time independent variable. Questionnaire giving information on shoulder pain and covariate variables were completed at baseline and two times in the follow-up period (T1 ¼ 2008 T2 ¼ 2009). In addition, prior shoulder pain was assessed by a questionnaire approximately one year prior to baseline (T-1 ¼ 2005/6). Table 1 illustrates the study design. 2.2. Study population The study population constituted of 41 participants (23 women and 18 men) with different occupations at baseline. Fifteen females were hairdressers, 15 males were electricians, 5 females were students while 3 females and 3 males had other occupations, such as working in the retail business and kindergarten. Most of the electricians and hairdressers had done their apprenticeship and had been working 2e3 years prior to baseline. Two of the 41 participants (5%) had parents from non-western countries (one male electrician and one male with various work). Thirty four participants (83%) reported to have medium or high socioeconomic background. Table 2 presents a further overview of the study group characteristics. They were all sampled from a cohort followed from 2002 consisting of 420 technical school students from the greater Oslo area, representing student hairdressers, student electricians and art/media/design students (Østerås et al., 2006). Forty five subjects agreed to participate in a one-day field registration at baseline where arm elevation was evaluated by bilateral inclinometers. Two of these cancelled their appointment, and another 2 subjects were excluded because of technical problems, leaving data on 41 participants. The sampling procedure is described in more detail in an earlier publication (Hanvold et al., 2013). The study group (n ¼ 41) had a lower frequency of tobacco users when compared to the original cohort (n ¼ 379), (p ¼ 0.06). No other statistically significant difference was found between the two groups when analyzing other background variables (general health, gender, parental origin, and physical activity in leisure time), shoulder pain and self-reported mechanical exposure from the data collected in 2002.

Table 1 Study design (2005/6e2009) and information on complete data from 41 young adults.

Time points

Technical measurement of arm elevation Shoulder pain Mechanical workload Control over work intensity Quantitative work demands Physical activity Tobacco use a

2005/6

2006/7

2008

2009

T-1a Complete (n)

T0 Complete (n) 41

T1 Complete (n)

T2 Complete (n)

38

41 39 41 41 40 41

32 28 32 32 32 32

35 31 34 34 35 35

Questionnaire on shoulder pain to register “prior pain”.

2.3. Data collection 2.3.1. Work with elevated arms Shoulder postures and movements were assessed by an inclinometer on each upper arm. The inclinometers were fixed with double-sided adhesive tape below the deltoid muscle insertion. Data were sampled at 20 Hz and stored on a portable data logger fastened in a waist belt of the worker (Logger Teknologi HB, Åkarp, Sweden (Hansson et al., 2003)). The inclinometers measured the orientation relative to the vertical plane. The reference position ¼ 0
elevation (normalization procedure) of the upper arm was recorded with the subject seated, with the side of the body leaning towards the armrest of the chair, and the arm hanging perpendicular while holding a 1 kg weight in the hand. The weight was used to stabilize the vertical position. Recording equipment was mounted in the morning before the start of the working day and a questionnaire was completed before the normalization procedures were initiated. The participants were instructed by the investigator to perform their ordinary work during the measurement day. The percentage of time spent with the upper arms elevated >30
, >60
and >90
, were used as measures to describe working postures (Hansson et al., 2001). To give the time proportions of prolonged arm elevation, episodes lasting for >5, >10 and >20 s were processed. The lengths of episodes were chosen on the basis of earlier findings (Svendsen et al., 2004). The percentage of time with arm elevation was calculated bilaterally before a mean from the right and left upper arm was used in the analyses. The mean duration of the measurements was 6 h and 5 min (range 3 h. 39 mine8 h. 37 min). 2.3.2. Shoulder pain Shoulder pain during the preceding 4 weeks was assessed using a pain drawing (Margolis et al., 1988). The participants were asked to shade in areas within an outline of a human figure that correspond to areas of their bodies in pain. The test-retest reliability of the pain drawing has been found reliable over time in both location and extent of pain (Margolis et al., 1988). Fig. 1 show the pain drawing used in the current study. It also show the area defined as the shoulder region and how the shoulder pain was calculated. In our study the number of squares that was shaded within a grid covering the shoulder region indicated shoulder pain. This grid was applied during the data processing and was not visible for the participants. The sum of shaded squares in the left (0e9) and right (0e9) shoulder gave an index ranging from 0 to 18. Shoulder pain was considered as a time-dependent variable and measured at 4 time points (T1, T0, T1, T2). 2.3.3. Covariates The participants self-reported mechanical workload, quantitative job demands, control over work intensity, physical activity in leisure time, tobacco use and working status were all considered as time-dependent variables and measured at 3 time points (T0, T1, T2). The mechanical workload was assessed by 12 questions based on body postures (7 questions) and work involving repetitive, precision movements, manual material handling and vibration (5 questions). On the basis of 3 response alternatives 0 (nothing/ hardly nothing), 1 (somewhat), 2 (a great deal), an index was calculated ranging from 0 to 24 (Balogh et al., 2001). Psychosocial work factors were assessed by items selected from the General Nordic Questionnaire for Psychologocal and Social factors at Work (QPSNordic) (Dallner et al., 2000). Quantitative work demands and control over work intensity was each assessed by 2 questions. The questions on quantitative work demand were “is your work load irregular so that the work pile up?” and “do you have too much to do?”. The questions assessing control over work intensity were

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Table 2 Individual and occupational characteristics at baseline (T0, n ¼ 41), by gender and occupational status. Variables

Shoulder pain, last 4 weeksa (0e18) Arm elevation % of work time >30
b Arm elevation % of work time >60
b % of time >60
,duration at least 5 sc % of time >60
duration at least 10 sc % of time >60
duration at least 20 sc Arm elevation % of work time >90
b % of time >90
,duration at least 5 sc % of time >90
duration at least 10 sc % of time >90
duration at least 20 sc Prior shoulder paind (0e18) Quantitiative work demandse (0e4) Control over work intensityf (0e4) Physical activity in leisure time*,g (0e6) Mechanical workload*,h (0e24) Tobacco use Yes No

Female

Male

Hairdresser n ¼ 15

Student/various n ¼ 8

Electrician n ¼ 15

Various work n ¼ 3

Median (range)

Median (range)

Median (range)

Median (range)

1 (0e14) 45 (35e69) 11 (7e19) 5 (3e9) 3(1e4) 1 (0.2e3) 2 (0.4e5) 0.6 (0.05e2) 0.2 (0e1) 0 (0e0.7) 1 (0e14) 1 (0e2) 1 (0e3) 4 (3e5) 11 (5e14) n (%)

0 (0e0) 35 (17e81) 1 (0.2e9) 0.7 (0.03e2) 0.48 (0e2) 0.15 (0e2) 0.4 (0.01e2) 0.1 (0e2) 0.04 (0.2) 0 (0e2) 0 (0e0) 1.5 (1e2) 1.5 (0.5e3) 4 (0e6) 5 (1e19) n (%)

0 (0e2) 47 (24e57) 16 (5e27) 11.6 (2e21) 9 (1.0e0.2) 5.5 (0.8e10.2) 8 (0.6e19) 5 (0.03e14) 4 (0e9) 2 (0e7) 0 (0e2) 2 (0.5e3) 1.5 (0e3) 3 (1e5) 16 (10e18) n (%)

0 (0e1) 37 (22e39) 5 (4e7) 2.3 (0.8e2.3) 1 (0.4e1.7) 0.7 (0e0.8) 0.7 (0.5e0.8) 0.2 (0.1e0.3) 0.08 (0.07e0.09) 0 (0e0.04) 0 (0e1) 2 (1.5e3.5) 1 (0e1) 4 (2e5) 9 (7e10) n (%)

6 (40) 9 (60)

2 (25) 6 (75)

6 (40) 9 (60)

2 (67) 1 (33)

*Variables with missing values (Physical activity ¼ 1 missing, Mechanical workload ¼ 2 missing). a Shoulder pain drawing distribution of pain in the left and right shoulder (0e18), the last 4 weeks. b Relative time with arm elevation >30
,>60
and >90
(0e100%). c Relative time with arm elevation >60
and >90
with duration >5 s (0e100%). d Shoulder pain drawing distribution of pain in the left and right shoulder (0e18), the last 4 weeks. Collected at T1 (one year prior to baseline). e Quantitative work demands (never ¼ 0 to very often ¼ 4). f Control over work intensity (never ¼ 0 to very often ¼ 4). g Physical activity in leisure time (never ¼ 0 to everyday ¼ 6). h Mechanical workload (nothing ¼ 0 to great deal ¼ 24).

Fig. 1. Pain drawing used in the current study is illustrated on the left without any markings of areas or squares. The mannequin on the right illustrates how the shoulder pain was calculated by counting the squares shaded on each shoulder (9 squares each shoulder, giving a shoulder index of 0e18).

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“Can you set your own work pace?” and ” can you determine the lengths of your own breaks?” (Dallner et al., 2000). All 4 questions had 5 response alternatives from 0 (never/seldom) to 4 (often/very often). The mean of the 2 questions made the score for the 2 psychosocial working conditions. Physical activity in leisure time was assessed by 1 question (how often do you perfome activities that lead to incresed heart rate and shortness of breath?) with 7 response categories from 0 (never) to 6 (everyday) (Wold et al., 2000). Smoking and snuff habits were assessed and the participants were characterized as tobacco users if they smoked or used snuff daily or occasionally. Working status was monitored in the follow-up period to detect changes that might have changed the participant's exposure to arm elevation. 2.4. Data analyses The missing data was either due to unit non-responders (missing because participants did not answer the questionnaire as a whole) or item non-responders (missing because participants did not fill in single items in a questionnaire). The total amount of unit nonresponders ranged from n ¼ 0 at T0 to n ¼ 9 at T1, while item nonresponders was mostly found in the assessment of mechanical workload and ranged from n ¼ 2 at T0 to n ¼ 13 at T1. See Table 1 for more information on the missing data. Multiple imputations were done to impute the missing values and the imputations were based on a linear mixed model. The imputation model included all the variables which were used in the multivariate analysis in this study. Five imputed datasets were made and on the basis of them one average estimate on the association was calculated (Rubin, 1996). Statistical analyses were done using STATA (version 12.0). Chisquare statistics were used to evaluate differences between the study group and the original cohort. Gender differences in occupational factors at baseline were analyzed using ManneWhitney U test. We used Spearman's rank correlation (r ¼ rho) when analyzing shoulder pain at baseline and prior pain in addition to evaluating the correlation between self-reported mechanical workload and objectively measured arm elevation. To evaluate the longitudinal association between arm elevation and shoulder pain, Generalized Estimating Equations (GEE-analysis) was used. This model takes into account the correlation between the repeated measurements within the individual. Due to over dispersion in the discrete outcome variable a negative binomial GEE-analysis was used. For the effect estimates, rate ratio (RR), with corresponding 95% confidence intervals were reported. In all negative binomial GEE-analysis an exchangeable correlations structure was used. The multivariate analyses were adjusted for covariates selected for inclusion a priori. The analyses were adjusted for time, work demands, tobacco use, self reported mechanical exposure, physical activity, prior pain and gender. Because of the gender difference in effect of pain over time the univariate and multivariate analyses were done for the whole group and stratified by gender. Although the study population was small (n ¼ 41), one of the benefits of repeated measures is statistical power relative to sample size, allowing the adjustment of multiple covariates. Incorporating the intraclass correlation coefficient (r ¼ 0.64) of the 3 outcome measures. The effective sample size in our study was calculated to n ¼ 64. 3. Results 3.1. Shoulder pain over time and occupational characteristic at baseline Twelve of the 41 participants (29%) shaded areas in the pain drawing indicating shoulder pain at baseline. The number of painful areas in the shoulder region showed a median of 0 (range

0e14) on a group level. Female hairdressers had a median of 1, which was the highest of the occupational groups (Table 2). Shoulder pain reported at baseline was highly correlated to prior pain (r:0.68 p < 0.01). Among the participants (3 men and 9 women) who reported shoulder pain at baseline, 8 reported pain in both follow-up questionnaires (2 men and 6 women). Twenty nine participants (15 men and 14 women) were pain free at baseline. Seven of these developed shoulder pain during the follow up period. New onset shoulder pain was reported by 1 male with various work at T1, while at T2, 4 male electricians and 1 female hairdresser and 1 female student had developed shoulder pain. Of the twelve participants reporting pain at baseline, eight also reported pain one year prior. Women had a 2.7 times higher rate of shoulder pain compared to male participants (RR ¼ 2.7 95%CI 0.93, 7.93 p ¼ 0.07). A moderate increase of shoulder pain during the study period was seen for the whole study group (RR ¼ 1.10 95%CI 0.75, 1.61 p ¼ 0.62). This indicates a higher number of shaded areas in the shoulder region at the follow-ups. It implies that at T1, the participants had 10% higher rate of shoulder pain compared to T0, and at T2 they had 20% higher rate of shoulder pain compared to T0. Male electricians and female hairdressers were the two occupational groups with highest exposure to work-related arm elevation. Stratifying analyses by exposure (exposed ¼ hairdressers and electricians (n ¼ 30) and unexposed ¼ students and various work (n ¼ 11)) showed that participants exposed to work with arms elevated had almost five times higher rate of shoulder pain compared to the participant less exposed to arm elevation at work (RR ¼ 4.79 95%CI 1.29,17.7 p ¼ 0.02). There was however found a gender difference in occupational work factors at baseline. Arm elevation angles >60
and >90
(p > 0.01), self-reported mechanical workload (p < 0.01) and quantitative work demands (p ¼ 0.02) showed highest levels among the male participants. No significant gender differences were found analyzing tobacco use, physical activity and control over work intensity at baseline. 3.2. Longitudinal associations Examining the longitudinal unadjusted association accounting for the correlation between the repeated pain measurements showed a positive association between arm elevation >60
and shoulder pain. Also prolonged arm elevation >60
lasting >5 s showed a positive association with shoulder pain among the young cohort. Analyses of arm elevation >60
and >90
with episodes lasting >10 and >20 s showed wide confidence intervals, especially among women, as there were few participants with exposures of these lengths. A gender difference in the association was also found suggesting significant associations between elevated arms >60
and >90
and shoulder pain among women (Table 3). This is also illustrated in Figs. 2 and 3. In the multivariate analyses, no significant associations were found between arm elevation and shoulder pain in the whole group adjusting for time, quantitative job demands, physical activity, tobacco use, prior pain and gender (Table 3). Stratifying the analysis by gender showed no association among the male participants. It also showed low levels of shoulder pain among men. Even though the male participants had more new cases with shoulder pain than the female participants during follow-up, there were only 6 males with shoulder pain at T3. The separate analysis for the female participants showed that the rate of shoulder pain increased with increasing arm elevation angles and increasing duration of episodes with arms elevated during work. For every additional unit increase in arm elevation >60
(0e100%) an estimated 28% increase in shoulder pain was found. Analysis done on complete case data (before multiple imputations) did not change the conclusions. However the confidence intervals were wider and the p-values

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Table 3 GEE analyses** of the association between work-related arm elevation (% of working time) and shoulder pain (0e18) [RR ¼ rate ratio, 95% CI ¼ 95% confidence interval].

Unadjusted analyses Arm elevation >60
a Arm elevation >60
, >5 Arm elevation >90
a Arm elevation >90
, >5 Adjusted analyses Arm elevation >60
a Arm elevation >60
, >5 Arm elevation >90
a Arm elevation >90
, >5

sa sa

sa sa

All (N ¼ 41)

Men (N ¼ 18)

Women (N ¼ 23)

No. observations ¼ 123 (T0-2, 2½ years)

No. observations ¼ 54 (T0-2, 2½ years)

No. observations ¼ 69 (T0-2, 2½ years)

RR

95% CI

RR

95% CI

p-value

RR

95% CI

p-value

1.05 1.01 0.96a 0.92a

0.99 0.94 0.88 0.79

1.10 1.07 1.04 1.06

0.06 0.87 0.36 0.26

0.98 0.98 0.97 0.98

0.91 0.90 0.87 0.83

1.06 1.07 1.09 1.16

0.70 0.72 0.72 0.85

1.23 1.71 1.72 3.50

1.13 1.41 1.20 1.67

1.34 2.07 2.45 7.35

5 s, among men (n ¼ 18).

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T.N. Hanvold et al. / Applied Ergonomics 47 (2015) 43e51

T1(2008)

T2 (2009)

0

5

10

15

T0 (2006/7)

0

5

10

0

5

10

0

5

10

Relative time with arm elevation >60degrees, with duration >5seconds (0-100%) Female various workers/students

Female hairdressers

Graphs by time

Fig. 3. Shoulder pain at T0, T1, T2 and the association with relative time with arms elevated >60
with a duration of >5 s, among women (n ¼ 23) (At T0 three female hairdressers had a shoulder pain index of 2. Two of them had an overlapping relative time with arm elevation >60
>5 s of 4.63 and 4.67 respectively).

among young adults in their first years of working life. Studies among young workers are rare. In contrast to earlier studies the current study population consists of young adults with minimal previous work exposure. They were recruited during school and followed into working life which reduces a possible healthy worker effect. Most studies are based on questionnaires assessing both exposure and health effects (Luime et al., 2004; Sterud et al., 2014). Selfreported mechanical exposures are considered only to give limited insight and increase the risk of information bias (van der Beek and Frings-Dresen, 1998). To counteracted information bias arising from measurement errors in exposure the current study used objectively measurements of arm postures at work. However, objective measurements based on short duration samples (e.g. duration from minutes to a two-three hours) have been done in many studies and have a risk of bias and low precision (Mathiassen et al., 2012). To overcome this shortcoming our data was collected from a full working day which increases the precision of the estimates. Nevertheless this sampling strategy reflects the current exposure level at one particular working day, which limits our possibility to detect the variability of arm elevation between days within individuals. It could also be that the exposure measurement at baseline is not representative for the exposure over the 2.5 year follow-up period. An earlier study on upper arm postures was done using inclinometers during 4 working days the same week from 28 €m et al., 2010). They found that female hairdressers (Wahlstro posture variability between days within hairdressers was in the same order of magnitude as that between hairdressers, suggesting €m et al., 2010). This that 'typical' workdays do not exist (Wahlstro could also be relevant for electricians. In our study self-reported mechanical workload and occupational status were however monitored during the follow-up period and changes in these variables were believed also to reflect variations in arm elevation over time. The correlation between the self-reported workload and objectively measured arm elevation decreased over time and could indicate that the objective measurement done at baseline did not reflect the exposure during follow-up. This decline in correlation was less pronounced when removing the participants who changed occupational status in the follow-up period. The assumption that arm elevation at work measured at baseline influences the shoulder

pain reported at follow-up nevertheless implies that the exposure was stable during the follow-up period. Shoulder pain in the current study was assessed by the number of squares shaded in the shoulder region, not reflecting the shoulder pain intensity or duration. It has however been found that the number of separate symptom areas in pain drawings was associated with the symptoms chronicity and severity in the neck and shoulder region (Toomingas, 1999). Self-reported pain drawings have also been found to have high sensitivity and specificity comparing it with clinical assessment of neck and shoulder di€ rkste n et al., 1999). The number of studies with lonagnoses (Bjo gitudinal design and repeated measurements of health effects are increasing (Mayer et al., 2012). Shoulder pain has a fluctuating nature and repeated assessments are recommended  ttir et al., 2004). One of the strengths in our study (Steingrímsdo was the 4 pain measurements on each participant: 1 assessing prior pain, 1 assessment at baseline and 2 follow-up assessments. Adjusting for prior shoulder pain in the multivariate analyses were done to eliminate biases that are stable over time e.g. habitual pain tendency. The pain assessments were done with one-year intervals. Knowing that shoulder pain does not always persist for a long time there could have been fluctuations in pain between the measurements which were not detected. One of the limitations in the current study was the low number of participants. Even if measuring shoulder pain at three different occasions for each participant increased the total number of observations, the number of participants was still very low in analyses stratified by gender. Low participation rate from the larger cohort study increased the possibility of selection bias. However, analyses showed that the participants did not differ from the rest of the cohort in background and outcome variables, increasing the population validity. Nevertheless generalizations to work as hairdressers and electricians are uncertain and the stratified analyses by gender should be interpreted carefully especially among men. 4.2. Discussion of the results Mechanical workload, more specifically work with hands above shoulder level as a risk factor for shoulder pain and disorders has

T.N. Hanvold et al. / Applied Ergonomics 47 (2015) 43e51

been extensively studied and our results are partly in agreement with earlier findings of work with arms elevated as a risk factor for shoulder pain (van der Windt et al., 2000; Mayer et al., 2012; van Rijn et al., 2010). A review of the cross-sectional and case control studies found work with elevated arms as an important risk factor for shoulder pain (van der Windt et al., 2000). A recent systematic review of the longitudinal studies regarded the evidence as strong (Mayer et al., 2012). Another review assessing studies where some had objective measures of exposure, provided indications that the occurrence of shoulder specific disorders was associated with working with the hand above shoulder level 1 h/day (van Rijn et al., 2010). In our study the association between objectively measured arm elevation at work and shoulder pain was found only among women. This is in contrast to the current literature (Mayer et al., 2012). One reason for this may be that our cohort was substantially younger than the populations in earlier studies, and that this gender difference in association may differ with age. The most important reason might however be that there were too few male participants with shoulder pain, making it difficult to draw any conclusions on the associations. A case control study showed long term cumulative effects of work with highly elevated arms and heavy lifting on symptomatic supraspinatus tendon tears among male patients and controls (Seidler et al., 2011). The self reported duration of work with highly elevated arms was added up over the entire working life, and points to a potential etiologic role of long term cumulative effects of arm elevation. These findings indicate that the lack of an association among men in our cohort may be explained partly by the short exposure time of work with arm elevation. The low number of male participants reporting shoulder pain increases the uncertainty of the estimates and could lead to type II error, failing to detect a relationship. Another reason for this gender difference in association may be that high mechanical work demands may be substantially more strenuous for women compared to men. Knowing that women on average have lower muscle strength than men this vulnerability hypothesis could be a potential explanation (Hooftman et al., 2004). Women could be more vulnerable to lower levels of mechanical work exposures. Differences among men and women in muscle tendons and fibers may also be of importance. Finally the gender difference in our study may also be explained by the difference in their occupation. Disentangling the gender difference from the possible occupational difference is difficult, as most of the males were electricians and most of the females were hairdressers. Gender difference in the association between exposure to arm elevation and shoulder pain is however found earlier. One longitudinal study found that self-reported exposure to work with arms above shoulder level were associated to the incidence of shoulder pain after 3 years, only among women (Leclerc et al., 2004). An association has also been seen among women in cross-sectional and caseecontrol studies (Fredriksson et al., 2002; Nahit et al., 2001). Comparing results were challenging as the study design, population, outcome and exposure often varies. An earlier study (Svendsen et al., 2004) which used the same objectively measure of arm elevation as in our study, found an association between arm elevation >60
and >90
and shoulder disorders among men. The difficulty comparing our results with the study by Svendsen and colleagues is partly that we only found an association among women, while in their study women were not included. There is also a difference in outcome, as they assessed shoulder disorders, while we had shoulder pain as health effect. Another caseecontrol study with objectively measured arm elevation showed that increasing intensity and duration of arm elevation at work among automobile assembly workers were associated with shoulder disorders (Punnett et al., 2000). Both studies found an increase in the

49

risk estimates with increasing time with arm elevation. Despite the methodological differences between these studies and our study, it strengthens the hypothesis that prolonged exposure to work with arms elevated may be harmful and an important factor in the development of shoulder complaints. Even though there is a statistically significant association between shoulder pain and work with arms elevated among the women in our study, the clinical relevance can be questioned due to the low levels of pain. Nevertheless, we argue that these findings are of clinical importance when considering the short exposure time and the participant's young age and most importantly when similar associations are found in middle-aged and older working populations. These findings are also biological plausible as prolonged arm elevation is believed to impair circulation due to impingement of the rotator cuff tendons or increase intramuscular pressure due to static muscle work. This exposure is hypothesized to play an important role in the development of shoulder pain (Armstrong et al., 1993; Hagberg, 1984; Winkel and Westgaard, 1992). A longitudinal study found an increasing risk of surgery for subacromial impingement syndrome with increasing shoulder load at work (Svendsen et al., 2013). Even though our cohort of young workers have low levels of shoulder pain early in working life, the study by Svendsen and colleagues indicates the importance of targeting those with high levels of shoulder load to prevent future pain and surgery. Even though our study population is considerably younger than populations in comparable studies, they had an exposure to arm elevation in the same order as shown among middle-aged and older professionals (Svendsen et al., 2004). The upper arm elevation >60
, >90
for the hairdressers in our study were similar to female hairdressers in an earlier study (Veiersted et al., 2008). Among the male electricians the exposure to upper arm elevation were found similar to a group of Norwegian and Brazilian electricians (Moriguchi et al., 2013). The gender difference in shoulder pain found in the current study is in accordance to other studies where women more frequently report pain in the shoulder region compared to men (Treaster and Burr, 2004). We also found that hairdressers reported more shoulder pain compared to the other occupations. These finding are similar as in a recent caseecontrol study where 147 hairdressers and 67 non-hairdressing controls were analyzed with adjustment for age, smoking and years worked, showing that hairdressers reported significantly higher levels of musculoskeletal problems, including work-related shoulder pain (Bradshaw et al., 2011). When studying a cohort of young healthy adults entering working life it is not surprising that low pain levels are reported. This made it however statistically challenging when stratifying analyses by gender because of the low pain levels among the male participants. Although the pain levels in the current study may be considered low, studying young adults in their first year of working life to identify early risk factors for shoulder complaints can be important as long-term effects of physical work load has been seen (Descatha et al., 2012; Miranda et al., 2008). Even though the effect estimates in this study are relatively small, the associations found may nevertheless be important as early indicators of more severe shoulder pain. A recent prospective study has showed that occupational mechanical loading increases the risk of a subsequent clinical shoulder disorder. In this Finnish study, 909 participants were re-examined 20 years after a national survey, which had gathered data on occupational exposures. The risk of developing a chronic, physician-diagnosed shoulder disorder was increased by 80e150% when workers were consistently exposed to a combination of heavy lifting, working in awkward postures or repetitive movements (Miranda et al., 2008).

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5. Conclusion In light of the methodological considerations and earlier studies the results from the current study strengthens the evidence of an association between work with elevated arms and shoulder pain among women. The association among women is however based on relatively few new cases of shoulder pain. It is nevertheless worth noticing that an association was found between repeated measures of shoulder pain and prolonged arm elevation, early in their working life. This indicates that work with prolonged arm elevation >60
and >90
can be harmful. The identification of early work related risk factors may improve intervention strategies among this group of young adults. The low levels of shoulder pain in the male participants made it impossible to draw any conclusion of the association among men. Ethics The Norwegian Data Inspectorate and the scientific ethical committee system approved the project. All participants gave their written consent and parental consent was obtained for those younger than 18 years in 2002. Funding This study was supported by a grant from the NHO (S-2555), Confederation of Norwegian Enterprises. Conflict of interest We herby state that none of the authors has any financial or other relationship that gives any conflict of interest. Acknowledgments We would like to thank Liv Berit Hæg, Ada Ingvaldsen, Jorid Thrane Stuenæs and Birgitta Jarmark for helping with the data collection and Rune Madsen for help with analysis of inclinometer data. We would also like to thank Hein Stigum for assistance in the use of STATA. References Andersen, L.L., Clausen, T., Mortensen, O.S., Burr, H., Holtermann, A., 2012. A prospective cohort study on musculoskeletal risk factors for long-term sickness absence among healthcare workers in eldercare. Int. Arch. Occup. Environ. Health 85 (6), 615e622. Armstrong, T.J., et al., 1993. A conceptual model for work-related neck and upperlimb musculoskeletal disorders. Scand. J. Work Environ. Health 19 (2), 73e84. Balogh, I., et al., 2001. Questionnaire-based mechanical exposure indices for large population studies - reliability, internal consistency and predictive validity. Scand. J. Work Environ. Health 27 (1), 41e48. € rkste n, M.G., Boquist, B., Talba €ck, M., Edling, C., 1999. The validity of reported Bjo musculoskeletal problems. A study of questionnaire answers in relation to diagnosed disorders and perception of pain. Appl. Ergon. 30 (4), 325e330. Bradshaw, L., Harris-Roberts, J., Bowen, J., Rahman, S., Fishwick, D., 2011. Self-reported work-related symptoms in hairdressers. Occup. Med. (Lond) 61 (5), 328e334. Dallner, M., et al., 2000. Validation of the General Nordic Questionnaire (QPSNordic) for Psychological and Social Factors at Work. Nordic Council of Ministers, Copenhagen. Nord 12:(171 pages). Descatha, A., et al., 2012. Long-term effects of biomechanical exposure on severe shoulder pain in the Gazel cohort. Scand. J. Work Environ. Health 38 (6), 568e576. Fredriksson, K., et al., 2002. Work environment and neck and shoulder pain: the influence of exposure time. Results from a population based case-control study. Occup. Environ. Med. 59, 182e188. Hagberg, M., 1984. Occupational musculoskeletal stress and disorders of the neck and shoulder: a review of possible pathophysiology. Int. Arch. Occup. Environ. Health 53, 269e278.

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