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shoulder disorders
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Applied Ergonomics 39 (2008) 255–260 www.elsevier.com/locate/apergo
Neck postures in air traffic controllers with and without neck/shoulder disorders Inger Arvidssona,, Gert-A˚ke Hanssona, Svend Erik Mathiassenb, Staffan Skerfvinga a
Division of Occupational and Environmental Medicine, University Hospital, SE-221 85, Lund, Sweden b Centre for Muskuloskeletal Research, University of Ga¨vle, Box 7629, SE-907 12, Umea˚, Sweden Received 1 June 2006; accepted 20 March 2007
Abstract Prolonged computer work with an extended neck is commonly believed to be associated with an increased risk of neck–shoulder disorders. The aim of this study was to compare neck postures during computer work between female cases with neck–shoulder disorders, and healthy referents. Based on physical examinations, 13 cases and 11 referents were selected among 70 female air traffic controllers with the same computer-based work tasks and identical workstations. Postures and movements were measured by inclinometers, placed on the forehead and upper back (C7/Th1) during authentic air traffic control. A recently developed method was applied to assess flexion/extension in the neck, calculated as the difference between head and upper back flexion/extension. Results: cases and referents did not differ significantly in neck posture (median neck flexion/extension: 101 vs. 91; p ¼ 0.9). Hence, the belief that neck extension posture is associated with neck–shoulder disorders in computer work is not supported by the present data. r 2007 Elsevier Ltd. All rights reserved. Keywords: Case-referent; Computer work; Inclinometry
1. Introduction Work-related musculoskeletal disorders (WRMSD) in neck and shoulders are frequently reported among computer operators (Gerr et al., 2002; Brandt et al., 2004). Several studies of clinical effects of disorders, or their mechanisms, have focused on the issue whether subjects with WRMSD show other patterns in motor performance, in terms of postures, movements and muscular load, than healthy subjects (Ha¨gg and A˚stro¨m, 1997; Vasseljen and Westgaard, 1997; Madeleine et al., 1999, 2003; Szeto et al., 2002). These studies have shown conflicting results: Vasseljen and Westgaard (1995) found consistent associations between pain and increased activity in the trapezius muscles among manual workers, but not in office workers. In contrast, Ha¨gg and A˚stro¨m (1997) found that secretaries with neck/shoulder complaints had fewer episodes with relaxed trapezius muscles, compared to healthy referents. Corresponding author. Tel.: +46 46 17 31 75; fax: +46 46 17 31 80.
E-mail address: [email protected] (I. Arvidsson). 0003-6870/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apergo.2007.03.006
Madeleine et al. (1999, 2003) found differences in some kinetic and kinematic parameters between workers reporting pain and pain-free subjects while performing a standardised cutting task, while Vasseljen and Westgaard (1997) did not find any significant differences in trunk and arm postures among office workers with and without disorders. A common clinical conception is that prolonged computer work with an extended neck can lead to neck–shoulder complaints. In the literature, a similar ‘‘forward head posture’’ is described, and defined as a combination of ‘‘extension of the upper cervical spine and flexion of the lower cervical spine’’ (Szeto et al., 2002; McLean, 2005). Chiu et al. (2002) found that 60% of computer operators with neck pain reported such posture. Using a two-dimensional video-based motion-analysis system, Szeto et al. (2002) showed non-significant trends of increased forward head posture among female symptomatic office workers, compared to asymptomatic controls. The sparse, available literature shows varying opinions concerning whether neck–shoulder pain influences motor performance during work. More data is needed comparing
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motor performance in general among subjects with and without disorders, and particularly regarding neck postures in computerised work. In air traffic control, a large group of subjects perform exactly the same work tasks in identical workstations with a large element of computerised activities. The aim of the present study was to find out whether subjects with clinically defined neck–shoulder disorders performed this work in neck postures and movements, as measured by exact technical measurements, different from those of healthy referents. 2. Methods 2.1. Study population At baseline, the study group consisted of all female certified air traffic controllers (n ¼ 70) employed in a Swedish Air Traffic Control Centre. All had been working with air traffic control more than 20 h/week for at least 3 months. All had the same education, the same work tasks and identical workstations. The air traffic controllers were sitting in front of circular radar screens (diameter 500 mm, lower edge 80 mm over the desk top, not adjustable) and kept paper strips with airplanes information in holders at the desk in front of them. Changes in height, speed or course of the aircrafts were noted manually on the strips. The controllers used a microphone and headset for communication with the aircrafts, as well as a keyboard and trackball which were fixed and placed with a distance of 425 and 460 mm, respectively, from the edge of the desk. The desks were 735 mm in height and not adjustable, which were the chairs. The air traffic controller worked for more than 5 h per shift at the workstation. The air traffic intensity, and thus the intensity of work operations, varied widely between different working periods (10–30 aircrafts per hour). 2.2. Selection of cases and referents The selection of cases and referents was based on an interview regarding musculoskeletal complaints and a standardised physical examination of neck, shoulders and upper back, performed twice at an interval of 1.5 years. For inclusion as a case or referent, strict criteria had to be fulfilled by the individual at both occasions. 2.2.1. Interview The subjects were asked about musculoskeletal complaints from neck, shoulders and upper back (Standardised Nordic Questionnaire, SNQ; Kourinka et al., 1987) the last 12 months and 7 days. The frequency of the complaints was reported (never ¼ 0, seldom ¼ 1, sometimes ¼ 2, often ¼ 3 and very often ¼ 4; Holmstro¨m and Moritz, 1991). The subjects were asked if the complaints were caused or associated with the work.
Furthermore, incidence of eye strain or eyesight difficulties, use of spectacles/contact lenses and if the subjects were satisfied with their spectacles/lenses, was asked. 2.2.2. Physical examination A standardised physical examination (Ohlsson et al., 1994) of the neck, shoulders and upper back, was performed by the same physical therapist. Diagnoses were made according to predefined criteria. In the original method, the diagnoses Tension neck syndrome, Cervical syndrome, Cervicalgia, Thoracic outlet syndrome, Frozen shoulder, Supraspinatus tendinitis, Infraspinatus tendinitis, Bicipital tendinitis, and Acromioclavicular syndrome are included. In the present study, the diagnosis Trapezius myalgia (current pain from the neck and tender and restricted trapezius muscle/s) was added to the original set of diagnoses. Each subject could receive multiple diagnoses. The findings were noticed in three degrees of seriousness (none ¼ 0 point, moderate ¼ 1 point and severe ¼ 2 points). The sum of the points was, for each individual, calculated as the ‘‘findings score’’ for each body segment, and totally for the neck/shoulders/upper back (0–244 points). 2.2.3. Inclusion criteria Inclusion criteria for the cases were reported complaints ‘‘often’’ or ‘‘very often’’ during the last 12 months, and neck/shoulder diagnosis or at least 20 points in the findings score in the physical examinations, at both occasions. Inclusion criteria for the referents were reported complaints ‘‘never’’ or ‘‘seldom’’ during the last 12 months (if ‘‘seldom’’ was reported, no present work related complaints were allowed), no diagnosis and a low finding score (o6) at both occasions. This selection identified 13 cases with neck–shoulder disorders and 11 healthy referents out of the original 70 subjects in the study base. All cases had disorders in the neck, and 12 out of 13 were also affected in shoulders and/or upper back. Characteristics of the cases and referents are shown in Table 1. All certified air traffic controllers regularly undergo occupational eye examinations. Optical corrections were used by 11 out of the 24 included air traffic controllers, seven of the 13 cases (54%) and four of the 11 referents (36%), as specified in Table 1. Most (8/11) were satisfied with their spectacles/lenses, while two of the cases and one of the referents were dissatisfied or quite dissatisfied. Three cases and two referents reported eyesight difficulties. The cases reported eye strain to a higher extent than the referents (nine cases (69%) vs. one referent (9%); p ¼ 0.01). 2.4. Measurements of postures and movements The 24 women performed an ordinary work period in authentic air traffic control, during, in mean 56 min (range 36–66). The subjects were instructed to assume their habitual postures and way of working and to adjust the
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Table 1 Characteristics of cases and referents (mean and range), number of operators who reached at least one diagnosis, and findings score in the first and second physical examination (PE 1 and 2, respectively) Subjects
Cases (n ¼ 13)
Referents (n ¼ 11)
Age (years) Stature (cm) Time of employment (years)
38 (27–55) 172 (167–182) 11 (3–30)
35 (25–51) 168 (160–180) 10 (0.5–29)
Optical corrections (n) Progressive lenses Single vision lenses Contact lenses and single vision lenses
7 2 2 3a
4 1 1 2a
Diagnosis PE 1 (n) PE 2 (n)
10b 10c
0 0
Findings PE 1 total Neck only PE 2 total Neck only
34 22 35 21
3 2 2 2
(15–85) (13–46) (11–59) (11–37)
Fig. 1. Assessment of neck flexion/extension by combining data from inclinometers on the forehead and the upper back (C7/Th1). The reference position and an example of a work posture are illustrated. (0–6) (0–6) (0–6) (0–5)
a All the operators altered between single vision lenses and contact lenses. b Diagnoses in the first physical examination were tension neck syndrome (6), cervical syndrome (2), cervicalgia (2), trapezius myalgia (4), infraspinatus tendinitis (2), supraspinatus tendinitis (3), acromioclavicular syndrome (2), and bicipital tendinitis (1). Sex subjects obtained more than one diagnosis. c Diagnoses in the second physical examination were tension neck syndrome (3), trapezius myalgia (5), cervicalgia (1), infraspinatus tendinitis (2), supraspinatus tendinitis (2), acromioclavicular syndrome (1), and bicipital tendinitis (4). Sex subjects obtained more than one diagnosis.
Posture range was defined as the 95th minus the 5th percentile. 2.5. Statistical analysis Mann–Whitney U test was used to evaluate differences in physical exposures (postures, movement velocities and posture ranges) between cases and referents, and between subjects with and without spectacles/contact lenses or eye strain. Spearman’s correlation coefficients (rS) were used to evaluate associations between physical exposures and age, stature and air traffic intensity. 3. Results
chair the way they use to do. After the work period, the air traffic controllers were asked about their perception of the air traffic intensity (nine-point scale; ‘‘very low’’ to ‘‘very high’’). This did not differ significantly between cases and referents (means 3.9 vs. 4.3; p ¼ 0.65). Inclinometry was used to measure the postures (flexion/ extension and right/left lateral flexion angles, relative to the line of gravity) and movements for the head and upper back (Hansson et al., 2001a, 2006). Data was sampled continuously at 20 Hz. The inclinometers were placed on the head (forehead) and upper back (C7/Th1). The reference position (01 of flexion/extension) was recorded when the subject was standing upright, looking at a mark at eye level. In addition, the difference between the head flexion/extension and upper back flexion/extension was calculated for each sample. This angle is denoted the neck flexion/extension (Fig. 1). Thus, the neck is regarded as the joint between the head and the upper back, distributed across the seven cervical vertebrae. Positive angles denote flexion and negative angles denote extension. Postures and movement velocities were expressed in terms of the 10th, 50th (median value) and 90th percentiles of the cumulated distributions during the recording period.
There was no significant difference in neck posture between cases and referents (Table 2). In both groups, the median neck posture was extended, however, with large inter-individual differences: 101 (SD 81) among the cases and 91 (SD 101) among the referents (p ¼ 0.9; Table 2). Movement velocities and posture ranges for the neck were also similar among cases and referents. The neck flexion/ extension was not related to stature or age. Only minor lateral flexion of the neck was observed in either group (not in table). The head was held in a close to upright posture. There was a small, but non-significant tendency that the cases flexed their head more than the referents (average head flexion 81 vs. 61; p ¼ 0.08; Table 2). Furthermore, there was a tendency that taller women flexed their head more than shorter ones (correlation between 50th percentile of head flexion/extension and stature: rS ¼ 0.36; p ¼ 0.09, not in table). The upper back was held in a flexed posture, with no significant differences between cases and referents. Lateral flexion for the head and upper back was similar in the two groups (not in table). There was no significant correlation between head and upper back postures and age.
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Table 2 Postures and movements for the neck, head and upper back, in 13 female cases of neck–shoulder disorders and 11 healthy referents, during air traffic control work Region measure Neck Angles (1)
Velocities (1/s)
Head Angles (1)
Velocities (1/s)
Upper back (C7/Th1) Angles (1)
Velocities (1/s)
Percentile distribution
Cases (n ¼ 13)
Referents (n ¼ 11)
p-value
10th 50th 90th 95th–5th 10th 50th 90th
24 (9) 10 (8) 11 (7) 44 (9) 0.8 (0.2) 6.4 (1.7) 36 (8)
22 (8) 9 (10) 11 (12) 42 (10) 0.7 (0.3) 5.5 (2.0) 33 (8)
0.49 0.86 1.00 0.36 0.09 0.19 0.39
10th 50th 90th 95th–5th 10th 50th 90th
3 (7) 8 (6) 29 (9) 39 (8) 0.7 (0.2) 5.4 (1.2) 30 (7)
3 (2) 6 (3) 23 (6) 34 (7) 0.6 (0.2) 4.4 (1.4) 26 (6)
0.17 0.08 0.09 0.17 0.05 0.07 0.23
10th 50th 90th 95th–5th 10th 50th 90th
10 (7) 19 (8) 27 (7) 23 (8) 0.5 (0.1) 3.5 (0.9) 19 (4)
5 (10) 16 (10) 24 (7) 26 (11) 0.4 (0.1) 3.0 (1.2) 18 (5)
0.11 0.42 0.12 0.69 0.08 0.36 0.65
Mean values and standard deviations (within brackets) are shown for the 10th, 50th and 90th percentiles of the distributions, as well as for posture range (95th minus 5th percentile). For postures, positive angles denote flexion and negative angles denote extension. po0.05, statistically significant difference between cases and referents.
The cases tended to have a higher 10th percentile movement velocity for the neck and upper back, and showed a statistically significant higher movement velocity for the head (Table 2). However, the numerical difference between cases and referents was very small, only 0.11/s. When cases and referents were combined, negative correlations between age and movement velocities (50th percentiles) were found for neck and head, though not statistically significant (rS ¼ 0.37 and 0.36, respectively; not in table), while there was a statistically significant negative correlation between the movement velocity and age for the upper back (50th percentile); older subjects moved more slowly (rS ¼ 0.47; p ¼ 0.02; not in table). Subjects using spectacles or contact lenses worked with less neck extension compared to those without spectacles [median flexion/extension 41 (SD 71) vs. 141 (SD 81); po0.01; not in table], and showed tendencies to less median upper back flexion [141 (SD 91) vs. 211 (SD 71); p ¼ 0.07]. There was no significant difference in neck postures between subjects reporting eye strain compared to subjects with no eye strain [median neck flexion/extension 91 (SD 81) vs. 101 (SD 101); p ¼ 1.0; not in table], but a small non-significant trend of more head flexion among subjects with eye strain compared to those without [median head flexion/extension 91 (SD 51) vs. 61 (SD 51); p ¼ 0.08; not in
table] and a significantly larger back flexion in 10th percentile [111 (SD 61) vs. 51 (SD 91); p ¼ 0.05]. Perceived air traffic intensity did not influence postures, movement velocities or posture ranges for the neck, head, or upper back. 4. Discussion Air traffic controllers with neck–shoulder disorders did not perform their work with neck, head and back postures significantly different from those used by healthy referents. 4.1. Methodological considerations Although the study is small, it had a statistical power of 81% to show a difference of 111 in median neck flexion/ extension between cases and referents, at a p-level of 0.05 (two-tailed). We judge that a difference of 111 is consistent with a reasonable minimal level of biological interest, and thus we do not believe that we have failed to detect an important difference between cases and referents due to low power. As an example of a contrast appearing in an earlier study of the air traffic controllers, the corresponding difference in neck flexion/extension between males and females was 161 (Arvidsson et al., 2006a).
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Musculoskeletal disorders are known to have an intermittent pattern of recurrence (A˚kesson et al., 1999; Luime et al., 2005; Silverstein et al., 2006). Hence, to distinguish cases and referents from each other, a longitudinal approach is preferable rather than a cross-sectional one. Our selection of cases with neck–shoulder disorders and healthy referents was based on strict criteria. The interviews and clinical examinations were performed twice with an interval of 1.5 years, resulting in an exclusion of 46 subjects with less severe or fluctuating symptoms. Hence, we measured the postures in cases with chronic neck– shoulder disorders, and compared to referents who were pain free since at least 2.5 years. The definitions of neck and head postures in computer work are not well established in the literature. For example, in the study of Ariens et al. (2001), the definition of ‘‘neck flexion’’ was based on observations of head inclinations with no consideration of the upper back posture. Furthermore, Psihogios et al. (2001) defined neck flexion based on the line through vertebrae C7 and the Tragus. Contrary, we defined the neck as the joint between the head and upper back. In our study, most of the subjects leaned their back forward during the computer work. Since the head per se was, in general, close to upright relative to the line of gravity, this meant that the head was held extended relative to the back, thus resulting in an extension of the neck. A number of methods have been used to measure neck and head postures among computer operators. Chiu et al. (2002) used self-reports based on illustrations of different postures, for characterising ‘‘forward head posture’’. However, compared to technical measurements, questionnaire-assessed exposure data has a low validity (Hansson et al., 2001b). Szeto et al. (2002) used a video-based twodimensional motion-analysis system, assessing postures among subjects during short periods (10 periods of 10 s each during a work day). In contrast, our method gives access to more information, due to continuous (20 Hz) and detailed registrations of the head and neck flexion/ extension during the whole work period. In addition, the method has a low inherent variability (Hansson et al., 2006). A limitation of the present method is that rotation of the neck and head cannot be measured. However, rotation was not an obvious issue for the present work. The cross-sectional study-design limits the possibilities for making conclusions about causal relationships between neck extension during computer work, and neck–shoulder disorders. The cases may have adjusted an earlier awkward habitual posture in response to their pain. However, since the height of the desks, radar screens and equipment were fixed, the opportunity for correcting postures was limited. It could be that cases with a pronounced neck extension, compared to subjects with less neck extension, developed more severe disorders, and thus had been forced to leave the profession. Hence, the lack of association between neck extension and less severe disorders may be due to a healthy workers selection. However, the air traffic controllers
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receive a specific and costly education and the work is regarded to be of high socioeconomic status. The turnover among the controllers is very low, as is the sick-leave rate (Arvidsson et al., 2006b). Hence, a selection bias because of a healthy workers effect seems unlikely. 4.2. Postures and movements Eye problems were more prevalent among cases than among referents, and cases more often used glasses. The air traffic controllers received frequent ophthalmologic examinations (at 1 or 2 years intervals). Hence, for the cases, the possibility must be considered that eye problems could originally have led to awkward neck postures and therefore neck complaints, and that a subsequent introduction of spectacles improved neck postures without significantly affecting complaints. This would have diminished the possible difference in neck postures between cases and referents. Indeed, subjects with glasses had less extended neck than those without. However, it is less plausible that this hypothetical scenario contributes to any major extent in explaining the lack of difference between the groups. We found a tendency that stature was associated with head flexion. In another study of the air traffic controllers, we found that stature was related to neck–shoulder complaints, but not to diagnoses (Arvidsson et al., 2006b). Since the cases were somewhat taller than the referents (in mean 4 cm), a causal pathway from stature via increased head flexion to disorders must be considered. According to a prospective study of Ariens et al. (2001), an increased risk for neck pain was found among subjects in various occupations, working with the neck flexed ( ¼ head inclination) more than 201 for at least 70% of the working time. In the present study, the head was only slightly flexed for both cases and referents (81 and 61 for the 50th percentile, respectively), which can be compared to other occupational groups measured with the same methods, e.g. dentists (head flexion 391; A˚kesson et al., 1997) and mink fur sorters (head flexion 401; Hansson and Mikkelsen, 1997). Hence, we do not believe that the present slight head flexion represents a significant risk. Compared to the referents, the cases showed slightly larger 10th percentile movement velocities (by up to 0.21/s), which was statistically significant for the head and tendencies for neck and upper back. These small differences are probably not associated with an increased risk of disorders, but they might indicate differences in motor performance: maybe the pain induced more frequent changes of postures. Indeed, fewer periods of relaxation among cases were found in a trapezius EMG study of secretaries by Ha¨gg and A˚stro¨m (1997). There was a difference in age (3 years) between cases and referents, but age was not associated with the neck, head and upper back postures, and could thus not confound the results. As a conclusion, subjects with pain seemed to perform computerised work with a similar motor strategy as healthy
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referents. This is in contrast to studies of work with higher physical demands, e.g. in simulated meat cutting, where an adaptation of work technique due to pain has been observed (Madeleine et al., 1999, 2003). The authors suggested that the pain-related motor changes could play a role in the progression of disorders. In a previous study of the air traffic controllers, we found a high prevalence of neck–shoulder disorders, but a low level of sick leave (Arvidsson et al., 2006b). Consistent with this finding, the present study indicates that air traffic control can be carried out as usual in spite of prevalent pain. This result may apply even to other jobs, e.g. in an office setting, which are characterised by low load levels, slow movements, and limited variation. Furthermore, our results indicate that more aspects than neck flexion/extension must be considered when ergonomics risk factors for neck–shoulder disorders are to be assessed in computer work. Acknowledgements This study was supported by the county councils of southern Sweden, the Swedish Council for Working Life and Social Research, AFA Insurance, and the Medical Faculty, Lund University. The kind co-operation of the Swedish Board of Civil Aviation and the air traffic controllers is acknowledged. References A˚kesson, I., Hansson, G.-A˚., Balogh, I., Moritz, U., Skerfving, S., 1997. Quantifying work load in neck, shoulders and wrists in female dentists. Int. Arch. Occup. Environ. Health 69, 461–474. A˚kesson, I., Johnsson, B., Rylander, L., Moritz, U., Skerfving, S., 1999. Musculoskeletal disorders among female dental personnel—clinical examination and a 5-year follow-up study of symptoms. Int. Arch. Occup. Environ. Health 72, 395–403. Ariens, G.A.M., Bongers, P.M., Douwes, M., Miedema, M.C., Hoogendoorn, W.E., van der Wal, G., et al., 2001. Are neck flexion, neck rotation, and sitting at work risk factors for neck pain? Results of a prospective cohort study. Occup. Environ. Med. 58, 200–207. Arvidsson, I., Hansson, G.-A˚., Mathiassen, S.E., Skerfving, S., 2006a. Changes in physical workload with implementation of mouse-based information technology in air traffic control. Int. J. Ind. Ergon. 36, 613–622. Arvidsson, I., Arvidsson, M., Axmon, A., Hansson, G.-A˚., Johansson, C.R., Skerfving, S., 2006b. Musculoskeletal disorders among female and male air traffic controllers performing identical and demanding computer work. Ergonomics 49 (11), 1052–1067. Brandt, L.P.A., Andersen, J.H., Funch Lassen, C., Kryger, A., Overgaard, E., Vilstrup, I., Mikkelsen, S., 2004. Neck and shoulder symptoms and disorders among Danish computer workers. Scand. J. Work Environ. Health 30, 399–409. Chiu, T.T.W., Ku, W.Y., Lee, M.H., Sum, W.K., Wan, M.P., Wong, C.Y., Yuen, C.K., 2002. A study on the prevalence of and risk factors for neck pain among university academic staff in Hong Kong. J. Occup. Rehabil. 12, 77–91. Gerr, F., Marcus, M., Ensor, C., Kleinbaum, D., Cohen, S., Edwards, A., Gentry, E., Ortiz, D., Monteilh, C., 2002. A prospective study of
computer users: 1. Study design and incidence of musculoskeletal symptoms and disorders. Am. J. Ind. Med. 41, 221–235. Ha¨gg, G., A˚stro¨m, A., 1997. Load pattern and pressure pain threshold in the upper trapezius muscle and psychosocial factors in medical secretaries with and without shoulder/neck disorders. Int. Arch. Occup. Environ. Health 69, 423–432. Hansson, G.-A˚., Mikkelsen, S., 1997. Kinematic evaluation of occupational work. Adv. Occup. Med. Rehabil. 1, 57–69. Hansson, G.-A˚., Asterland, P., Holmer, N.-G., Skerfving, S., 2001a. Validity and reliability of triaxial accelerometers for inclinometry in posture analysis. Med. Biol. Eng. Comput. 39, 405–413. Hansson, G.-A˚., Balogh, I., Unge Bystro¨m, J., Ohlsson, K., Nordander, C., Asterland, P., Sjo¨lander, S., Rylander, L., Winkel, J., Skerfving, S., Malmo¨ Shoulder-Neck Study Group, 2001b. Questionnaire versus direct technical measurements in assessing postures and movements of the head, upper back arms and hands. Scand. J. Work Environ. Health 27, 30–40. Hansson, G.-A˚., Arvidsson, I., Ohlsson, K., Nordander, C., Mathiassen, S.E., Skerfving, S., Balogh, I., 2006. Precision of measurements of physical workload during standardized manual handling. Part II: inclinometry of head, upper back, neck and upper arms. J. Electromyogr. Kinesiol. 16, 125–136. Holmstro¨m, E., Moritz, U., 1991. Low back pain—correspondence between questionnaire, interview and clinical examination. Scand. J. Rehabil. Med. 23, 119–125. Kourinka, I., Jonsson, B., Kilbom, A˚., Vinterberg, H., Andersson, G., Jørgensen, K., 1987. Standardized Nordic questionnaires for the analysis of musculoskeletal symptoms. Appl. Ergon. 18, 233–237. Luime, J.J., Koes, B.W., Miedem, H.S., Verhaar, J.A.N., Burdorf, A., 2005. High incidence and recurrence of shoulder and neck pain in nursing home employees was demonstrated during a 2 year follow-up. J. Clin. Epidemiol. 58, 407–413. Madeleine, P., Lundager, B., Voigt, M., Arendt-Nielsen, L., 1999. Shoulder muscle co-ordination during chronic and acute experimental neck–shoulder pain. An occupational pain study. Eur. J. Appl. Physiol. 79, 127–140. Madeleine, P., Lundager, B., Voigt, M., Arendt-Nielsen, L., 2003. The effects of neck–shoulder pain development on sensory–motor interactions among female workers in the poultry and fish industries. A prospective study. Int. Arch. Occup. Environ. Health 76, 39–49. McLean, L., 2005. The effect on postural correction on muscle activation amplitudes recorded from the cervicobrachial region. J. Electromyogr. Kinesiol. 15, 527–535. Ohlsson, K., Attewell, R.G., Johnsson, B., Ahlm, A., Skerfving, S., 1994. An assessement of neck and upper extremity disorders by questionnaire and clinical examination. Ergonomics 37, 891–897. Psihogios, J.P., Sommerich, C.M., Mirka, G.A., Moon, S.D., 2001. A field evaluation of monitor placement effects in VDT users. Appl. Ergon. 32, 313–325. Silverstein, B.A., Viikari-Juntura, E., Fan, Z.J., Bonauto, D.K., Bao, S., Smith, C., 2006. Natural course of nontraumatic rotator cuff tendonitis and shoulder symptoms in a working population. Scand. J. Work Environ. Health 32 (2), 99–108. Szeto, G.P.Y., Straker, L., Raine, S., 2002. A field comparison of neck and shoulder postures in symptomatic and asymptomatic office workers. Appl. Ergon. 33, 75–84. Vasseljen, O., Westgaard, R., 1995. A case-control study of trapezius muscle activity in office and manual workers with shoulder and neck pain and symptom-free controls. Int. Arch. Occup. Environ. Health 67, 11–18. Vasseljen, O., Westgaard, R., 1997. Arm and trunk posture during work in relation to shoulder and neck pain and trapezius activity. Clin. Biomech. 12, 22–31.
Applied Ergonomics 39 (2008) 255–260 www.elsevier.com/locate/apergo
Neck postures in air traffic controllers with and without neck/shoulder disorders Inger Arvidssona,, Gert-A˚ke Hanssona, Svend Erik Mathiassenb, Staffan Skerfvinga a
Division of Occupational and Environmental Medicine, University Hospital, SE-221 85, Lund, Sweden b Centre for Muskuloskeletal Research, University of Ga¨vle, Box 7629, SE-907 12, Umea˚, Sweden Received 1 June 2006; accepted 20 March 2007
Abstract Prolonged computer work with an extended neck is commonly believed to be associated with an increased risk of neck–shoulder disorders. The aim of this study was to compare neck postures during computer work between female cases with neck–shoulder disorders, and healthy referents. Based on physical examinations, 13 cases and 11 referents were selected among 70 female air traffic controllers with the same computer-based work tasks and identical workstations. Postures and movements were measured by inclinometers, placed on the forehead and upper back (C7/Th1) during authentic air traffic control. A recently developed method was applied to assess flexion/extension in the neck, calculated as the difference between head and upper back flexion/extension. Results: cases and referents did not differ significantly in neck posture (median neck flexion/extension: 101 vs. 91; p ¼ 0.9). Hence, the belief that neck extension posture is associated with neck–shoulder disorders in computer work is not supported by the present data. r 2007 Elsevier Ltd. All rights reserved. Keywords: Case-referent; Computer work; Inclinometry
1. Introduction Work-related musculoskeletal disorders (WRMSD) in neck and shoulders are frequently reported among computer operators (Gerr et al., 2002; Brandt et al., 2004). Several studies of clinical effects of disorders, or their mechanisms, have focused on the issue whether subjects with WRMSD show other patterns in motor performance, in terms of postures, movements and muscular load, than healthy subjects (Ha¨gg and A˚stro¨m, 1997; Vasseljen and Westgaard, 1997; Madeleine et al., 1999, 2003; Szeto et al., 2002). These studies have shown conflicting results: Vasseljen and Westgaard (1995) found consistent associations between pain and increased activity in the trapezius muscles among manual workers, but not in office workers. In contrast, Ha¨gg and A˚stro¨m (1997) found that secretaries with neck/shoulder complaints had fewer episodes with relaxed trapezius muscles, compared to healthy referents. Corresponding author. Tel.: +46 46 17 31 75; fax: +46 46 17 31 80.
E-mail address: [email protected] (I. Arvidsson). 0003-6870/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apergo.2007.03.006
Madeleine et al. (1999, 2003) found differences in some kinetic and kinematic parameters between workers reporting pain and pain-free subjects while performing a standardised cutting task, while Vasseljen and Westgaard (1997) did not find any significant differences in trunk and arm postures among office workers with and without disorders. A common clinical conception is that prolonged computer work with an extended neck can lead to neck–shoulder complaints. In the literature, a similar ‘‘forward head posture’’ is described, and defined as a combination of ‘‘extension of the upper cervical spine and flexion of the lower cervical spine’’ (Szeto et al., 2002; McLean, 2005). Chiu et al. (2002) found that 60% of computer operators with neck pain reported such posture. Using a two-dimensional video-based motion-analysis system, Szeto et al. (2002) showed non-significant trends of increased forward head posture among female symptomatic office workers, compared to asymptomatic controls. The sparse, available literature shows varying opinions concerning whether neck–shoulder pain influences motor performance during work. More data is needed comparing
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motor performance in general among subjects with and without disorders, and particularly regarding neck postures in computerised work. In air traffic control, a large group of subjects perform exactly the same work tasks in identical workstations with a large element of computerised activities. The aim of the present study was to find out whether subjects with clinically defined neck–shoulder disorders performed this work in neck postures and movements, as measured by exact technical measurements, different from those of healthy referents. 2. Methods 2.1. Study population At baseline, the study group consisted of all female certified air traffic controllers (n ¼ 70) employed in a Swedish Air Traffic Control Centre. All had been working with air traffic control more than 20 h/week for at least 3 months. All had the same education, the same work tasks and identical workstations. The air traffic controllers were sitting in front of circular radar screens (diameter 500 mm, lower edge 80 mm over the desk top, not adjustable) and kept paper strips with airplanes information in holders at the desk in front of them. Changes in height, speed or course of the aircrafts were noted manually on the strips. The controllers used a microphone and headset for communication with the aircrafts, as well as a keyboard and trackball which were fixed and placed with a distance of 425 and 460 mm, respectively, from the edge of the desk. The desks were 735 mm in height and not adjustable, which were the chairs. The air traffic controller worked for more than 5 h per shift at the workstation. The air traffic intensity, and thus the intensity of work operations, varied widely between different working periods (10–30 aircrafts per hour). 2.2. Selection of cases and referents The selection of cases and referents was based on an interview regarding musculoskeletal complaints and a standardised physical examination of neck, shoulders and upper back, performed twice at an interval of 1.5 years. For inclusion as a case or referent, strict criteria had to be fulfilled by the individual at both occasions. 2.2.1. Interview The subjects were asked about musculoskeletal complaints from neck, shoulders and upper back (Standardised Nordic Questionnaire, SNQ; Kourinka et al., 1987) the last 12 months and 7 days. The frequency of the complaints was reported (never ¼ 0, seldom ¼ 1, sometimes ¼ 2, often ¼ 3 and very often ¼ 4; Holmstro¨m and Moritz, 1991). The subjects were asked if the complaints were caused or associated with the work.
Furthermore, incidence of eye strain or eyesight difficulties, use of spectacles/contact lenses and if the subjects were satisfied with their spectacles/lenses, was asked. 2.2.2. Physical examination A standardised physical examination (Ohlsson et al., 1994) of the neck, shoulders and upper back, was performed by the same physical therapist. Diagnoses were made according to predefined criteria. In the original method, the diagnoses Tension neck syndrome, Cervical syndrome, Cervicalgia, Thoracic outlet syndrome, Frozen shoulder, Supraspinatus tendinitis, Infraspinatus tendinitis, Bicipital tendinitis, and Acromioclavicular syndrome are included. In the present study, the diagnosis Trapezius myalgia (current pain from the neck and tender and restricted trapezius muscle/s) was added to the original set of diagnoses. Each subject could receive multiple diagnoses. The findings were noticed in three degrees of seriousness (none ¼ 0 point, moderate ¼ 1 point and severe ¼ 2 points). The sum of the points was, for each individual, calculated as the ‘‘findings score’’ for each body segment, and totally for the neck/shoulders/upper back (0–244 points). 2.2.3. Inclusion criteria Inclusion criteria for the cases were reported complaints ‘‘often’’ or ‘‘very often’’ during the last 12 months, and neck/shoulder diagnosis or at least 20 points in the findings score in the physical examinations, at both occasions. Inclusion criteria for the referents were reported complaints ‘‘never’’ or ‘‘seldom’’ during the last 12 months (if ‘‘seldom’’ was reported, no present work related complaints were allowed), no diagnosis and a low finding score (o6) at both occasions. This selection identified 13 cases with neck–shoulder disorders and 11 healthy referents out of the original 70 subjects in the study base. All cases had disorders in the neck, and 12 out of 13 were also affected in shoulders and/or upper back. Characteristics of the cases and referents are shown in Table 1. All certified air traffic controllers regularly undergo occupational eye examinations. Optical corrections were used by 11 out of the 24 included air traffic controllers, seven of the 13 cases (54%) and four of the 11 referents (36%), as specified in Table 1. Most (8/11) were satisfied with their spectacles/lenses, while two of the cases and one of the referents were dissatisfied or quite dissatisfied. Three cases and two referents reported eyesight difficulties. The cases reported eye strain to a higher extent than the referents (nine cases (69%) vs. one referent (9%); p ¼ 0.01). 2.4. Measurements of postures and movements The 24 women performed an ordinary work period in authentic air traffic control, during, in mean 56 min (range 36–66). The subjects were instructed to assume their habitual postures and way of working and to adjust the
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Table 1 Characteristics of cases and referents (mean and range), number of operators who reached at least one diagnosis, and findings score in the first and second physical examination (PE 1 and 2, respectively) Subjects
Cases (n ¼ 13)
Referents (n ¼ 11)
Age (years) Stature (cm) Time of employment (years)
38 (27–55) 172 (167–182) 11 (3–30)
35 (25–51) 168 (160–180) 10 (0.5–29)
Optical corrections (n) Progressive lenses Single vision lenses Contact lenses and single vision lenses
7 2 2 3a
4 1 1 2a
Diagnosis PE 1 (n) PE 2 (n)
10b 10c
0 0
Findings PE 1 total Neck only PE 2 total Neck only
34 22 35 21
3 2 2 2
(15–85) (13–46) (11–59) (11–37)
Fig. 1. Assessment of neck flexion/extension by combining data from inclinometers on the forehead and the upper back (C7/Th1). The reference position and an example of a work posture are illustrated. (0–6) (0–6) (0–6) (0–5)
a All the operators altered between single vision lenses and contact lenses. b Diagnoses in the first physical examination were tension neck syndrome (6), cervical syndrome (2), cervicalgia (2), trapezius myalgia (4), infraspinatus tendinitis (2), supraspinatus tendinitis (3), acromioclavicular syndrome (2), and bicipital tendinitis (1). Sex subjects obtained more than one diagnosis. c Diagnoses in the second physical examination were tension neck syndrome (3), trapezius myalgia (5), cervicalgia (1), infraspinatus tendinitis (2), supraspinatus tendinitis (2), acromioclavicular syndrome (1), and bicipital tendinitis (4). Sex subjects obtained more than one diagnosis.
Posture range was defined as the 95th minus the 5th percentile. 2.5. Statistical analysis Mann–Whitney U test was used to evaluate differences in physical exposures (postures, movement velocities and posture ranges) between cases and referents, and between subjects with and without spectacles/contact lenses or eye strain. Spearman’s correlation coefficients (rS) were used to evaluate associations between physical exposures and age, stature and air traffic intensity. 3. Results
chair the way they use to do. After the work period, the air traffic controllers were asked about their perception of the air traffic intensity (nine-point scale; ‘‘very low’’ to ‘‘very high’’). This did not differ significantly between cases and referents (means 3.9 vs. 4.3; p ¼ 0.65). Inclinometry was used to measure the postures (flexion/ extension and right/left lateral flexion angles, relative to the line of gravity) and movements for the head and upper back (Hansson et al., 2001a, 2006). Data was sampled continuously at 20 Hz. The inclinometers were placed on the head (forehead) and upper back (C7/Th1). The reference position (01 of flexion/extension) was recorded when the subject was standing upright, looking at a mark at eye level. In addition, the difference between the head flexion/extension and upper back flexion/extension was calculated for each sample. This angle is denoted the neck flexion/extension (Fig. 1). Thus, the neck is regarded as the joint between the head and the upper back, distributed across the seven cervical vertebrae. Positive angles denote flexion and negative angles denote extension. Postures and movement velocities were expressed in terms of the 10th, 50th (median value) and 90th percentiles of the cumulated distributions during the recording period.
There was no significant difference in neck posture between cases and referents (Table 2). In both groups, the median neck posture was extended, however, with large inter-individual differences: 101 (SD 81) among the cases and 91 (SD 101) among the referents (p ¼ 0.9; Table 2). Movement velocities and posture ranges for the neck were also similar among cases and referents. The neck flexion/ extension was not related to stature or age. Only minor lateral flexion of the neck was observed in either group (not in table). The head was held in a close to upright posture. There was a small, but non-significant tendency that the cases flexed their head more than the referents (average head flexion 81 vs. 61; p ¼ 0.08; Table 2). Furthermore, there was a tendency that taller women flexed their head more than shorter ones (correlation between 50th percentile of head flexion/extension and stature: rS ¼ 0.36; p ¼ 0.09, not in table). The upper back was held in a flexed posture, with no significant differences between cases and referents. Lateral flexion for the head and upper back was similar in the two groups (not in table). There was no significant correlation between head and upper back postures and age.
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Table 2 Postures and movements for the neck, head and upper back, in 13 female cases of neck–shoulder disorders and 11 healthy referents, during air traffic control work Region measure Neck Angles (1)
Velocities (1/s)
Head Angles (1)
Velocities (1/s)
Upper back (C7/Th1) Angles (1)
Velocities (1/s)
Percentile distribution
Cases (n ¼ 13)
Referents (n ¼ 11)
p-value
10th 50th 90th 95th–5th 10th 50th 90th
24 (9) 10 (8) 11 (7) 44 (9) 0.8 (0.2) 6.4 (1.7) 36 (8)
22 (8) 9 (10) 11 (12) 42 (10) 0.7 (0.3) 5.5 (2.0) 33 (8)
0.49 0.86 1.00 0.36 0.09 0.19 0.39
10th 50th 90th 95th–5th 10th 50th 90th
3 (7) 8 (6) 29 (9) 39 (8) 0.7 (0.2) 5.4 (1.2) 30 (7)
3 (2) 6 (3) 23 (6) 34 (7) 0.6 (0.2) 4.4 (1.4) 26 (6)
0.17 0.08 0.09 0.17 0.05 0.07 0.23
10th 50th 90th 95th–5th 10th 50th 90th
10 (7) 19 (8) 27 (7) 23 (8) 0.5 (0.1) 3.5 (0.9) 19 (4)
5 (10) 16 (10) 24 (7) 26 (11) 0.4 (0.1) 3.0 (1.2) 18 (5)
0.11 0.42 0.12 0.69 0.08 0.36 0.65
Mean values and standard deviations (within brackets) are shown for the 10th, 50th and 90th percentiles of the distributions, as well as for posture range (95th minus 5th percentile). For postures, positive angles denote flexion and negative angles denote extension. po0.05, statistically significant difference between cases and referents.
The cases tended to have a higher 10th percentile movement velocity for the neck and upper back, and showed a statistically significant higher movement velocity for the head (Table 2). However, the numerical difference between cases and referents was very small, only 0.11/s. When cases and referents were combined, negative correlations between age and movement velocities (50th percentiles) were found for neck and head, though not statistically significant (rS ¼ 0.37 and 0.36, respectively; not in table), while there was a statistically significant negative correlation between the movement velocity and age for the upper back (50th percentile); older subjects moved more slowly (rS ¼ 0.47; p ¼ 0.02; not in table). Subjects using spectacles or contact lenses worked with less neck extension compared to those without spectacles [median flexion/extension 41 (SD 71) vs. 141 (SD 81); po0.01; not in table], and showed tendencies to less median upper back flexion [141 (SD 91) vs. 211 (SD 71); p ¼ 0.07]. There was no significant difference in neck postures between subjects reporting eye strain compared to subjects with no eye strain [median neck flexion/extension 91 (SD 81) vs. 101 (SD 101); p ¼ 1.0; not in table], but a small non-significant trend of more head flexion among subjects with eye strain compared to those without [median head flexion/extension 91 (SD 51) vs. 61 (SD 51); p ¼ 0.08; not in
table] and a significantly larger back flexion in 10th percentile [111 (SD 61) vs. 51 (SD 91); p ¼ 0.05]. Perceived air traffic intensity did not influence postures, movement velocities or posture ranges for the neck, head, or upper back. 4. Discussion Air traffic controllers with neck–shoulder disorders did not perform their work with neck, head and back postures significantly different from those used by healthy referents. 4.1. Methodological considerations Although the study is small, it had a statistical power of 81% to show a difference of 111 in median neck flexion/ extension between cases and referents, at a p-level of 0.05 (two-tailed). We judge that a difference of 111 is consistent with a reasonable minimal level of biological interest, and thus we do not believe that we have failed to detect an important difference between cases and referents due to low power. As an example of a contrast appearing in an earlier study of the air traffic controllers, the corresponding difference in neck flexion/extension between males and females was 161 (Arvidsson et al., 2006a).
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Musculoskeletal disorders are known to have an intermittent pattern of recurrence (A˚kesson et al., 1999; Luime et al., 2005; Silverstein et al., 2006). Hence, to distinguish cases and referents from each other, a longitudinal approach is preferable rather than a cross-sectional one. Our selection of cases with neck–shoulder disorders and healthy referents was based on strict criteria. The interviews and clinical examinations were performed twice with an interval of 1.5 years, resulting in an exclusion of 46 subjects with less severe or fluctuating symptoms. Hence, we measured the postures in cases with chronic neck– shoulder disorders, and compared to referents who were pain free since at least 2.5 years. The definitions of neck and head postures in computer work are not well established in the literature. For example, in the study of Ariens et al. (2001), the definition of ‘‘neck flexion’’ was based on observations of head inclinations with no consideration of the upper back posture. Furthermore, Psihogios et al. (2001) defined neck flexion based on the line through vertebrae C7 and the Tragus. Contrary, we defined the neck as the joint between the head and upper back. In our study, most of the subjects leaned their back forward during the computer work. Since the head per se was, in general, close to upright relative to the line of gravity, this meant that the head was held extended relative to the back, thus resulting in an extension of the neck. A number of methods have been used to measure neck and head postures among computer operators. Chiu et al. (2002) used self-reports based on illustrations of different postures, for characterising ‘‘forward head posture’’. However, compared to technical measurements, questionnaire-assessed exposure data has a low validity (Hansson et al., 2001b). Szeto et al. (2002) used a video-based twodimensional motion-analysis system, assessing postures among subjects during short periods (10 periods of 10 s each during a work day). In contrast, our method gives access to more information, due to continuous (20 Hz) and detailed registrations of the head and neck flexion/ extension during the whole work period. In addition, the method has a low inherent variability (Hansson et al., 2006). A limitation of the present method is that rotation of the neck and head cannot be measured. However, rotation was not an obvious issue for the present work. The cross-sectional study-design limits the possibilities for making conclusions about causal relationships between neck extension during computer work, and neck–shoulder disorders. The cases may have adjusted an earlier awkward habitual posture in response to their pain. However, since the height of the desks, radar screens and equipment were fixed, the opportunity for correcting postures was limited. It could be that cases with a pronounced neck extension, compared to subjects with less neck extension, developed more severe disorders, and thus had been forced to leave the profession. Hence, the lack of association between neck extension and less severe disorders may be due to a healthy workers selection. However, the air traffic controllers
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receive a specific and costly education and the work is regarded to be of high socioeconomic status. The turnover among the controllers is very low, as is the sick-leave rate (Arvidsson et al., 2006b). Hence, a selection bias because of a healthy workers effect seems unlikely. 4.2. Postures and movements Eye problems were more prevalent among cases than among referents, and cases more often used glasses. The air traffic controllers received frequent ophthalmologic examinations (at 1 or 2 years intervals). Hence, for the cases, the possibility must be considered that eye problems could originally have led to awkward neck postures and therefore neck complaints, and that a subsequent introduction of spectacles improved neck postures without significantly affecting complaints. This would have diminished the possible difference in neck postures between cases and referents. Indeed, subjects with glasses had less extended neck than those without. However, it is less plausible that this hypothetical scenario contributes to any major extent in explaining the lack of difference between the groups. We found a tendency that stature was associated with head flexion. In another study of the air traffic controllers, we found that stature was related to neck–shoulder complaints, but not to diagnoses (Arvidsson et al., 2006b). Since the cases were somewhat taller than the referents (in mean 4 cm), a causal pathway from stature via increased head flexion to disorders must be considered. According to a prospective study of Ariens et al. (2001), an increased risk for neck pain was found among subjects in various occupations, working with the neck flexed ( ¼ head inclination) more than 201 for at least 70% of the working time. In the present study, the head was only slightly flexed for both cases and referents (81 and 61 for the 50th percentile, respectively), which can be compared to other occupational groups measured with the same methods, e.g. dentists (head flexion 391; A˚kesson et al., 1997) and mink fur sorters (head flexion 401; Hansson and Mikkelsen, 1997). Hence, we do not believe that the present slight head flexion represents a significant risk. Compared to the referents, the cases showed slightly larger 10th percentile movement velocities (by up to 0.21/s), which was statistically significant for the head and tendencies for neck and upper back. These small differences are probably not associated with an increased risk of disorders, but they might indicate differences in motor performance: maybe the pain induced more frequent changes of postures. Indeed, fewer periods of relaxation among cases were found in a trapezius EMG study of secretaries by Ha¨gg and A˚stro¨m (1997). There was a difference in age (3 years) between cases and referents, but age was not associated with the neck, head and upper back postures, and could thus not confound the results. As a conclusion, subjects with pain seemed to perform computerised work with a similar motor strategy as healthy
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referents. This is in contrast to studies of work with higher physical demands, e.g. in simulated meat cutting, where an adaptation of work technique due to pain has been observed (Madeleine et al., 1999, 2003). The authors suggested that the pain-related motor changes could play a role in the progression of disorders. In a previous study of the air traffic controllers, we found a high prevalence of neck–shoulder disorders, but a low level of sick leave (Arvidsson et al., 2006b). Consistent with this finding, the present study indicates that air traffic control can be carried out as usual in spite of prevalent pain. This result may apply even to other jobs, e.g. in an office setting, which are characterised by low load levels, slow movements, and limited variation. Furthermore, our results indicate that more aspects than neck flexion/extension must be considered when ergonomics risk factors for neck–shoulder disorders are to be assessed in computer work. Acknowledgements This study was supported by the county councils of southern Sweden, the Swedish Council for Working Life and Social Research, AFA Insurance, and the Medical Faculty, Lund University. The kind co-operation of the Swedish Board of Civil Aviation and the air traffic controllers is acknowledged. References A˚kesson, I., Hansson, G.-A˚., Balogh, I., Moritz, U., Skerfving, S., 1997. Quantifying work load in neck, shoulders and wrists in female dentists. Int. Arch. Occup. Environ. Health 69, 461–474. A˚kesson, I., Johnsson, B., Rylander, L., Moritz, U., Skerfving, S., 1999. Musculoskeletal disorders among female dental personnel—clinical examination and a 5-year follow-up study of symptoms. Int. Arch. Occup. Environ. Health 72, 395–403. Ariens, G.A.M., Bongers, P.M., Douwes, M., Miedema, M.C., Hoogendoorn, W.E., van der Wal, G., et al., 2001. Are neck flexion, neck rotation, and sitting at work risk factors for neck pain? Results of a prospective cohort study. Occup. Environ. Med. 58, 200–207. Arvidsson, I., Hansson, G.-A˚., Mathiassen, S.E., Skerfving, S., 2006a. Changes in physical workload with implementation of mouse-based information technology in air traffic control. Int. J. Ind. Ergon. 36, 613–622. Arvidsson, I., Arvidsson, M., Axmon, A., Hansson, G.-A˚., Johansson, C.R., Skerfving, S., 2006b. Musculoskeletal disorders among female and male air traffic controllers performing identical and demanding computer work. Ergonomics 49 (11), 1052–1067. Brandt, L.P.A., Andersen, J.H., Funch Lassen, C., Kryger, A., Overgaard, E., Vilstrup, I., Mikkelsen, S., 2004. Neck and shoulder symptoms and disorders among Danish computer workers. Scand. J. Work Environ. Health 30, 399–409. Chiu, T.T.W., Ku, W.Y., Lee, M.H., Sum, W.K., Wan, M.P., Wong, C.Y., Yuen, C.K., 2002. A study on the prevalence of and risk factors for neck pain among university academic staff in Hong Kong. J. Occup. Rehabil. 12, 77–91. Gerr, F., Marcus, M., Ensor, C., Kleinbaum, D., Cohen, S., Edwards, A., Gentry, E., Ortiz, D., Monteilh, C., 2002. A prospective study of
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