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The effect of an ergonomic computer device on muscle activity of the upper trapezius muscle during typing
Applied Ergonomics 34 (2003) 125–130
The effect of an ergonomic computer device on muscle activity of the upper trapezius muscle during typing M. Tepper*, M.M.R. Vollenbroek-Hutten, H.J. Hermens, C.T.M. Baten Roessingh Research and Development, Roessinghsbleekweg 33b, AH Enschede 7522, The Netherlands Received 18 May 2001; accepted 15 November 2002
Abstract Objective: Investigate whether an ergonomic computer device, characterised by an inclined working area and keyboard localisation close to the screen (the Up-Line), decreases the muscle activity of the upper trapezius muscle. Methods: In a crossover design 19 healthy subjects and 19 patients with Whiplash Associated Disorder (WAD) typed during 10 min at the Up-Line and at a standard workstation with 15 min of rest in between. During typing surface EMG was measured of the trapezius muscle. The subjects were asked to rate sitting comfort and complaints. Results: Although most subjects subjectively preferred the Up-Line, on average no significant differences were found in muscle activity between the two workstations for both patients and healthy subjects. Individually in 5 healthy subjects (25%) and in 6 patients (31%) muscle activity was lower when working at the Up-Line. Conclusion: Although some subjects subjectively prefer the Up-Line in sitting comfort, on average the Up-Line did not decrease the muscle activity, both in healthy subjects as in patients with WAD. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Electromyography; Ergonomic device; Occupational rehabilitation
1. Introduction In the last decade the number of subjects with work related musculoskeletal disorder (WMSD) increased dramatically. In The Netherlands over 2 million workers are at risk for WMSD and in 500,000 of them this is due to computer work (Huppes and Schreibers, 1999). Several anatomical sites are mentioned to be at risk for musculoskeletal discomfort during computer work especially neck shoulder region (Waersted et al., 1991; Schuldt . et al., 1987; Hagberg and Kvarnstrom, 1984) and arm (Hales et al., 1994). The musculoskeletal discomfort is hypothesised to be a result of a chronic imbalance between the functional capacity of the worker and the functional workload. This imbalance can be reduced by increasing the functional capacity of the worker and/or by decreasing the functional workload (Goeken, 1998, Mathiassen and Winkel 1996, Bergqvist et al., 1995a, b).
*Corresponding author. E-mail address: [email protected] (M. Tepper).
Different kinds of computer workstation and computer devices like forearm support and ergonomic mice have been developed in the last decade to try to reduce workload (Cook and Kothiyal, 1998; Harvey and Peper, 1997; Bergqvist et al., 1995a, b; Vasseljen and West. et al., 1994; Takala and Viikarigaard, 1995; Fernstrom Juntura, 1991). In occupational rehabilitation, a successful return to work of chronic pain patients is hampered as these patients often have a lowered functional capacity. A category of chronic pain patients is patients with Whiplash Associated Disorder (WAD). These patients complain about chronic pain in the neck and as a consequence they are not able to perform computer work during prolonged period of time. An ergonomic computer workplace adaptation might be able to reduce the functional workload and might due to this have a positive influence on a successful return to work. Such an ergonomic computer workplace adaptation has been developed to assist in a more successful return to work of patients with a WAD. Characteristics of this adaptation, called the Up-Line, are an inclined working area and the keyboard located close to the screen
0003-6870/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 6 8 7 0 ( 0 2 ) 0 0 1 4 5 - X
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M. Tepper et al. / Applied Ergonomics 34 (2003) 125–130
2.2. Protocol 2.2.1. Design Using a crossover design, every subject typed 10 min at the standard workstation and at the Up-Line with 15 min rest in between. Half of the subjects started typing at the Up-Line and the other half started at the standard workstation. The text to be typed was designed in such a way that it required an equal amount of keystrokes from the left and right side of the keyboard.
Fig. 1. The Up-Line: characteristics of the Up-Line are the inclined work area and the position of the keyboard close to the screen.
2.2.2. Computer workstations Before starting, the subject had permission to change the height of the chair and the table to achieve 901 angles in hips and knees and to position the screen with the eye level 10 cm below the upper border of the screen. 2.2.2.1. The standard workstation. The position of the document was fixed on the left side of the screen and the keyboard had a fixed position on the table.
(Fig. 1). These features enable total forearm support, wrist in neutral position and require only slight flexion of the neck to focus on the keyboard. The first subjective experiences were positive. Patients continued their work over a longer period and they indicated having fewer complaints when working at the Up-Line compared to a standard workstation (according the guidelines of the Dutch Ministry of Labour, 1999). In this study it is hypothesised that due to the total forearm support and the keyboard position the Up-Line will cause a decreased muscle activity of the neck. To investigate this, the muscle activation level of the trapezius muscle during typing at the Up-Line is compared to typing at a standard workstation. Beside EMG measurements, the subjective interpretation, using Visual Analogue Scale, concerning sitting comfort and work performance and work posture are measured. Two groups of subjects have been included in this study, healthy subjects and patients with WAD.
2. Methods 2.1. Subjects Nineteen healthy persons and 19 patients with WAD have been included in this study. General inclusion criteria were: age between 20 and 50 yr, no history of inflammatory disease, no use of muscle relaxants and a body mass index less then 30. Specific inclusion criteria for healthy subjects were absence of history of neck pain and for patients with WAD no history of musculoskeletal disorders before neck pain started due to WAD. Only patients with WAD type 2 or type 3, according to the Quebec Task Force for WAD (Spitzer et al., 1995) were included.
2.2.2.2. The Up-Line. The position of the document was fixed on the left side of the screen. The keyboard had a fixed position close to the screen. The inclined working area of the Up-Line had a fixed position of 181 with the tabletop (Fig. 1). 2.2.3. Measurements 2.2.3.1. Muscle activity. During typing surface EMG (sEMG) was recorded for the left and right trapezius muscle. Bipolar surface electrodes were placed bilaterally above the descending part of the trapezius muscle 2 cm lateral of the middle of the line between the acromion and the processus of the 7th cervical spine in the direction of the muscle fibres according to international guidelines with a 2.5 cm interelectrode distance with an electrode size of 1 cm in diameter (Mathiassen et al., 1995; Hermens et al., 2000). A reference electrode was placed around the right ankle. The sEMG signal was low-pass filtered at 3–400 Hz filter, sampled at 1000 Hz and AD converted (12 bit). To facilitate comparison between individuals, EMG was normalised using a submaximal isometric reference test. In sitting position the subject was asked to hold both arms in a position of 901 abduction for 15 s. This reference test was repeated 4 times with 1 min rest in between two tests and was performed before the experiment (Mathiassen et al., 1995). 2.2.3.2. Sitting comfort. Immediately after the typing task the subject was asked to mark a Visual Analogue Scale (VAS), representing how comfortable he/she sat during 10 min typing. The VAS is a 10 cm vertical line, with sitting uncomfortable at the bottom of the scale (zero) and sitting as comfortable as possible on the top of the scale (10).
M. Tepper et al. / Applied Ergonomics 34 (2003) 125–130
2.2.3.3. Work performance and work posture. Work performance was characterised by typing speed (words/min) and by number of errors during the typing task. The viewing distance was measured immediately after the 10 min typing. The viewing distance is the distance of a virtual horizontal line from the forehead to the screen. The viewing angle of the keyboard is the angle between a virtual horizontal line at eye level and a virtual line from the eye to the middle of the keyboard. 2.3. Data-analysis 2.3.1. EMG analysis The standard deviation of the sEMG signal was used to estimate the amount of muscle activity and was expressed as percentage of the mean EMG value during the normalisation tests multiplied with 100. The mean EMG value of the normalisation tests was calculated as the mean of the standard deviations of the sEMG signal of the middle 10 s of the 4 submaximal isometric tests. 2.3.2. Statistics Statistical analyses were carried out using Microsoft Excel 97 and SPSS 9.0. 2.3.2.1. Differences in muscle activity. The mean difference in muscle activity between both workstations was investigated using a sampled t-test (po0:05 was considered as statistical significant) and was investigated for healthy persons and patients with WAD separately. A difference of more than 5% between the EMG activity during working at the Up-Line and the EMG activity during working at the standard workstation was considered relevant. Subgroup analyses for age, time of accident, formal work experience, successful return to work, existence of daily complaints and typing performance were done in the group of patients with WAD to search for a common factor in those patients showing a relevant decreased EMG activity when working at the Up-Line (by box-plot).
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3.1.1. Muscle activity On average, the difference in EMG activity of the trapezius muscle between the Up-Line and the standard workstation was not significant (sampled t-test). The interindividual variability of the differences in muscle activation levels between both workstations was however high, which is reflected in the high standard deviation. When applying a 5% difference as relevant, the muscle activity when working at the Up-Line is lower in 5 subjects (25%) and higher in 11 subjects. In 3 subjects no difference in muscle activity between both workstations was found (Fig. 2). 3.1.2. Sitting comfort Considering a difference of more than 2 on the VASscale, as relevant only 4 healthy persons preferred working at the Up-Line and 5 preferred the standard workstation. Ten healthy persons did not experience a difference in sitting comfort between both workstations (Fig. 2). Remarkable is that for those subjects with a subjective preference for a certain workstation, the muscle activation levels are in line with the preference, i.e. the 4 subjects who preferred the Up-Line had lower muscle activation levels during working at the Up-Line. In contrast, of the 10 subjects without a subjective preference there was a discrepancy with muscle activation levels in 7 subjects (Fig. 2).
2.3.2.2. Sitting comfort. The difference in sitting comfort between the two workstations was considered as relevant when a difference in VAS scores>2 was found (Beurskens, 1996).
3. Results 3.1. Healthy persons Seven men and 12 women with a mean age of 30 yr (range 21–44 yr) were included.
Fig. 2. The distribution of subjective preference for a workstation concerning sitting comfort combined with the preference for a workstation based on muscle activation level in the group of healthy persons (N ¼ 19). No preference means for sitting comfort a difference in VAS score less then 2 and for muscle activity a difference in EMG activity less then 5%. The marked bars ( ) correspondence the same preference for a workstation in sitting comfort and muscle activity.
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3.1.3. Work performance and work posture The mean rate of typing at the Up-Line was 146/min (SD 33) and at the standard workstation 146/min (SD 35). The mean amount of errors was 3 at both workstations. This means that there was no difference in work performance between both workstations. The work posture during typing at the Up-Line and the work posture during typing at the standard workstation differed only in viewing distance and the viewing angle of the keyboard. Half of the healthy subjects were sitting too close to the screen at the standard workstation according to the recommended distance by the Dutch Ministry of labour (50–70 cm). The main viewing angle of the keyboard during work at the Up-Line was 421 (341–571) and during work at the standard workstation 531 (461–661). 3.2. Patients with WAD In total 19 patients, 7 male and 12 female with a mean age of 32 yr (range 23–53 yr) participated. Fifteen patients were diagnosed as WAD type 2 and 4 patients as WAD type 3. The car accident causing the WAD took place 1.5 upto 6 years ago. Seven patients had a successful return to work, 6 patients failed to return to work and 6 patients did not finish their occupational rehabilitation yet.
Fig. 4. The distribution of subjective preference for a workstation concerning sitting comfort combined with the preference for a workstation based on lower muscle activation level in the group of patients with WAD (N ¼ 19). No preference means for sitting comfort a difference in VAS score less then 2. There is a preference for one of both workstations concerning muscle activity when the difference in muscle activity is more then 5%. There is no preference for workstations when the difference in muscle activity is less then 5%. The marked bars ( ) indicate the same preference for a workstation in sitting comfortable and muscle activity.
3.2.1. Muscle activity On average, no significant difference was found in EMG activity of the trapezius muscle between working at the Up-Line and working at the standard workstation (sampled t-test, po0:05). A high standard deviation was found for differences in muscle activity between both workstations, representing a high interindividual variability (Fig. 3, 95% CI). On average the muscle activity during the 10 min typing did not change. Assuming 5% difference as relevant in 6 patients (31%) the muscle activity was lower at the Up-Line and in 7 patients the muscle activity was lower at the standard workstation. In 6 patients no difference was found (Fig. 4).
Subgroup analysis was done to try to characterise the 6 patients with lower muscle activity at the Up-Line. They did not differ from the other 13 subjects by age, time of accident, formal work experience, successful return to work and existence of daily complaints. They also did not differ in typing performance, formal experience of working at the Up-Line, and type of WAD.
Up - Stan ( % )
EMG up - EMG stan 15 10 5 0 -5 -10 min min min min min min min min min min 1 2 3 4 5 6 7 8 9 10
Fig. 3. The mean difference in EMG acidity level during typing at the Up-Line minus EMG level during typing at the standard workstation per minute The results presented are from the group of patients with WAD (N ¼ 19; with 95% CI).
3.2.2. Sitting comfort In 11 patients with WAD there was a relevant difference in sitting comfort in favour of the Up-Line. One patient preferred the standard workstation. In 7 patients there was no relevant difference between both workstations concerning sitting comfort (Fig. 4). Of 11 patients with WAD who preferred the Up-Line concerning sitting comfort only in 6 patients this was accompanied by a preference for the Up-Line concerning muscle activity (Fig. 4). The other 5 patients had higher muscle activation levels during working at the Up-Line. In the 8 patients with WAD with no preference for the Up-Line according to sitting comfort also muscle activation levels did not reveal a preference for the Up-Line.
M. Tepper et al. / Applied Ergonomics 34 (2003) 125–130
3.2.3. Work performance and work posture The mean rate of typing at the Up-Line was 116/min (SD 66) and at the standard workstation 120/min (SD 68). The mean amount of errors was 4 for both workstations. This means that there was on average no difference in work performance between both workstations. Two patients had more then 15 errors at the standard workstation compared to 5 and 7 errors at the Up-Line. Both patients complained about loss of concentration during working at the standard workstation. Another 5 patients reported loss of concentration during working at both workstations. The work postures at the Up-Line and at the standard workstation differed only in viewing distance and the viewing angle of the keyboard. The main viewing distance at the Up-Line was 59 cm (range 50–69 cm) and at the standard workstation 54 cm (range 42–62 cm). The recommended distance according to the Dutch Ministry of Labour is 50–70 cm. The main viewing angle of the keyboard at the Up-Line was 421 (range 331–501) and at the standard workstation 531 (range 421–661).
4. Discussion The purpose of this study was to investigate whether a workplace adaptation characterised by inclined working area and keyboard position close to the screen reduces the muscle activity of the trapezius muscle. The first clinical subjective experiences with the use of this workplace in patients with WAD encourage us to verify whether these subjective findings could be quantified in more objective terms in order to develop an indication for the workplace. 4.1. Muscle activity The results show that the Up-Line does not demonstrate such positive effect on muscle activity. Individually in 5 healthy subjects (25%) and in 6 patients (31%) muscle activity was lower when working at the Up-Line. The Up-Line decreases the viewing angle of the keyboard as intended but on average, for both healthy subjects and patients with WAD, no difference in muscle activity between the two workstations is found. This is not in accordance with Schuldt . et al. (1987), who found in 10 healthy subjects the lowest muscle activity of the neck with the cervical spine in vertical position and the thoraco-lumbar spine in slightly backward-inclined position. This working posture is highly comparable to the posture when working at the Up-Line. One possible reason for not finding a decreased muscle activity at the Up-Line could be that due to the inclined working area the arms are forced in a less ergonomic position; especially the elbow is forced in a
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less then 901 position. Some subjects changed their position by more anteflexion in the shoulder joints to return to a more comfortable sitting position with the elbows in 901. In accordance to Takala and ViikariJuntura (1991) small changes of the upper arm angle strongly influence the muscle activity. More anteflexion in the shoulder joint is associated with more muscle activity of the trapezius. The 6 patients with less muscle activity during working at the Up-Line could not be characterised in more detail. They did not differ from the other 14 patients by age, time of accident, successful return to work, work posture and complaints during typing. To characterise these patients other factors like psychosocial influences as job satisfaction might be important ( om, . (H.agg and Astr 1997). 4.2. Sitting comfort Eleven patients with WAD (58%) and only 4 healthy persons (21%) subjectively preferred the Up-Line according to sitting comfort. An explanation might be that patients are positively influenced by the idea that the Up-Line was developed especially for them. Therefore patients expected that the Up-Line should be an improvement compared with the standard workstation. In this study we expected to find an association between sitting comfort and muscle activation levels. We assumed that lower EMG levels would reflect lower effort and consequently higher experienced sitting comfort. In the group of healthy persons we found such an association. In all 9 healthy subjects, who had a preference in workstation according to sitting comfort, the muscle activity showed the same preference. In contrast to the association found in healthy subjects, in only 6 of the 11 patients with WAD who preferred the Up-Line according to sitting comfort, the muscle activity showed the same preference. In contrast in the 8 patients not preferring the Up-Line the muscle EMG activity was in line with their subjective preference in 7 out of 8 patients (88%). Next to the already mentioned explanation of higher expectations of patients with WAD this discrepancy between healthy persons and patients might be explained by a decreased awareness of muscle activation in patients as described in the chronic pain model of Vlaeyen et al., 1997. Vasseljen and Westgaard (1995) also did not show consistent association between the subjective experienced amount of pain and an increased activity of the upper trapezius muscle in the group of office workers. ( om . (1997) on the other hand demonH.agg and Astr strated a correlation between work related complaints in medical secretaries, EMG activity and work task satisfaction. A correlation between EMG activity and other psychological factors could not be demonstrated in their study.
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Based on the found relationship between subjective reported sitting comfort and muscle activation levels it can be concluded that the subjective preference to some extend could be useful as an indicator for prescribing the Up-Line. When healthy subjects have a subjective preference for the Up-Line it is likely that the Up-Line will reduce their muscle activation level. However when healthy subjects and patients with WAD have a preference for the standard workplace it is unlikely that the Up-Line will reduce their muscle activation level. 4.3. Conclusion This study showed on average no difference in muscle activity of the trapezius muscle between working at the Up-Line and working at the standard workstation in both healthy subjects and in patients with WAD.
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medical secretaries with and without shoulder/neck disorders. Int. Arch. Occup. Environ. Health 69, 423–432. Hales, T.R., Sauter, L.S., Peterson, M.R., Fine, L.J., Putz-Anderson, V., Schleifer, L.R., Ochs, T.T., Bernard, B.P., 1994. Musculoskeletal disorders among visual display terminal users in a telecommunications company. Ergonomics 37 (10), 1603–1621. Hermens, H.J., Freriks, B., Disselhorst-Klug, C., Rau, G., 2000. Development of recommendations for SEMG sensors and sensor placement procedures. J. Electromyogr. Kinesiol. 10, 361–374. Harvey, R., Peper, E., 1997. Surface electromyography and mouse use position. Ergonomics 40 (8), 781–789. Huppes, G., Schreibers, K.B.J., 1999. Handboek RSI: risico’s, oplossingen, behandeling. Sdu Uitgevers, Den Haag The Netherlands. Part C Risico’s, pp. 91–184. Mathiassen, S.E., Winkel, J., 1996. Physiological comparison of three interventions in light assembly work: reduced work pace, increased break allowance and shortened working days. Int. Arch. Occup. Environ. Health 68, 94–108. Mathiassen, S.E., Winkel, J., H.agg, G.M., 1995. Normalisation of surface EMG amplitude from the upper trapezius muscle in ergonomic study - a review. J Electromyogr. Kinesiol. 5 (4), 197–226. The Dutch Ministry of Labour (Ministerie van sociale Zaken en Werkgelegenheid), 1999. Arboinformatie A1–2: Werken met beeldschermen. Derde oplage, SDU Uitgevers, Den Haag. Schuldt, . K., Ekholm, J., Harms-Ringdahl, K., Arborelius, U.P., Nemeth, G., 1987. Influence of sitting postures on neck and schoulder e.m.g during arm-handwork. Clin. Biomech. 2 (3), 126–139. Spitzer, W.O., Skovron, M.L., Salmi, L.R., Cassidy, J.D., Duranceau, J., Suissa, S., Zeiss, E., 1995. Scientific monograph of the Quebec Task Force on Whiplash-Associated Disorders: redefining ‘‘whiplash’’ and its management. Spine 20S, S1–73. Takala, E.-P., Viikari-Juntura, E.V., 1991. Loading of shoulder muscles in a simulated work cycle: Comparison between sedentary workers with and without neck-shoulder symptoms. Clin. Biomech. 6, 145–152. Vasseljen, O.V., Westgaard, R.H., 1995. A case-control study of trapezius muscle activity in office and manual workers with schoulder and neck pain and symptom-free controls. Int. Arch. Occup. Environ. Health 67, 11–18. Vlaeyen, J.W.S., Kole-Snijders, A.M.J., Heuts, P.H.T.G., Eek, vH., 1997. Behavioral analysis, fear of movement/(re)injury and behavioral rehabilitation in chronic low back pain. In: Vleeming, M., Dorman, et. al., (Eds.), Movement, Stability & Low Back Pain. Churchill Livingstone. Waersted, C.T.M., Bjorklund, R.A., Westgaard, R.H., 1991. Shoulder muscle tension induced by two VDU-based tasks of different complexity. Ergonomics 34 (2), 137–150.
The effect of an ergonomic computer device on muscle activity of the upper trapezius muscle during typing M. Tepper*, M.M.R. Vollenbroek-Hutten, H.J. Hermens, C.T.M. Baten Roessingh Research and Development, Roessinghsbleekweg 33b, AH Enschede 7522, The Netherlands Received 18 May 2001; accepted 15 November 2002
Abstract Objective: Investigate whether an ergonomic computer device, characterised by an inclined working area and keyboard localisation close to the screen (the Up-Line), decreases the muscle activity of the upper trapezius muscle. Methods: In a crossover design 19 healthy subjects and 19 patients with Whiplash Associated Disorder (WAD) typed during 10 min at the Up-Line and at a standard workstation with 15 min of rest in between. During typing surface EMG was measured of the trapezius muscle. The subjects were asked to rate sitting comfort and complaints. Results: Although most subjects subjectively preferred the Up-Line, on average no significant differences were found in muscle activity between the two workstations for both patients and healthy subjects. Individually in 5 healthy subjects (25%) and in 6 patients (31%) muscle activity was lower when working at the Up-Line. Conclusion: Although some subjects subjectively prefer the Up-Line in sitting comfort, on average the Up-Line did not decrease the muscle activity, both in healthy subjects as in patients with WAD. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Electromyography; Ergonomic device; Occupational rehabilitation
1. Introduction In the last decade the number of subjects with work related musculoskeletal disorder (WMSD) increased dramatically. In The Netherlands over 2 million workers are at risk for WMSD and in 500,000 of them this is due to computer work (Huppes and Schreibers, 1999). Several anatomical sites are mentioned to be at risk for musculoskeletal discomfort during computer work especially neck shoulder region (Waersted et al., 1991; Schuldt . et al., 1987; Hagberg and Kvarnstrom, 1984) and arm (Hales et al., 1994). The musculoskeletal discomfort is hypothesised to be a result of a chronic imbalance between the functional capacity of the worker and the functional workload. This imbalance can be reduced by increasing the functional capacity of the worker and/or by decreasing the functional workload (Goeken, 1998, Mathiassen and Winkel 1996, Bergqvist et al., 1995a, b).
*Corresponding author. E-mail address: [email protected] (M. Tepper).
Different kinds of computer workstation and computer devices like forearm support and ergonomic mice have been developed in the last decade to try to reduce workload (Cook and Kothiyal, 1998; Harvey and Peper, 1997; Bergqvist et al., 1995a, b; Vasseljen and West. et al., 1994; Takala and Viikarigaard, 1995; Fernstrom Juntura, 1991). In occupational rehabilitation, a successful return to work of chronic pain patients is hampered as these patients often have a lowered functional capacity. A category of chronic pain patients is patients with Whiplash Associated Disorder (WAD). These patients complain about chronic pain in the neck and as a consequence they are not able to perform computer work during prolonged period of time. An ergonomic computer workplace adaptation might be able to reduce the functional workload and might due to this have a positive influence on a successful return to work. Such an ergonomic computer workplace adaptation has been developed to assist in a more successful return to work of patients with a WAD. Characteristics of this adaptation, called the Up-Line, are an inclined working area and the keyboard located close to the screen
0003-6870/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 6 8 7 0 ( 0 2 ) 0 0 1 4 5 - X
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M. Tepper et al. / Applied Ergonomics 34 (2003) 125–130
2.2. Protocol 2.2.1. Design Using a crossover design, every subject typed 10 min at the standard workstation and at the Up-Line with 15 min rest in between. Half of the subjects started typing at the Up-Line and the other half started at the standard workstation. The text to be typed was designed in such a way that it required an equal amount of keystrokes from the left and right side of the keyboard.
Fig. 1. The Up-Line: characteristics of the Up-Line are the inclined work area and the position of the keyboard close to the screen.
2.2.2. Computer workstations Before starting, the subject had permission to change the height of the chair and the table to achieve 901 angles in hips and knees and to position the screen with the eye level 10 cm below the upper border of the screen. 2.2.2.1. The standard workstation. The position of the document was fixed on the left side of the screen and the keyboard had a fixed position on the table.
(Fig. 1). These features enable total forearm support, wrist in neutral position and require only slight flexion of the neck to focus on the keyboard. The first subjective experiences were positive. Patients continued their work over a longer period and they indicated having fewer complaints when working at the Up-Line compared to a standard workstation (according the guidelines of the Dutch Ministry of Labour, 1999). In this study it is hypothesised that due to the total forearm support and the keyboard position the Up-Line will cause a decreased muscle activity of the neck. To investigate this, the muscle activation level of the trapezius muscle during typing at the Up-Line is compared to typing at a standard workstation. Beside EMG measurements, the subjective interpretation, using Visual Analogue Scale, concerning sitting comfort and work performance and work posture are measured. Two groups of subjects have been included in this study, healthy subjects and patients with WAD.
2. Methods 2.1. Subjects Nineteen healthy persons and 19 patients with WAD have been included in this study. General inclusion criteria were: age between 20 and 50 yr, no history of inflammatory disease, no use of muscle relaxants and a body mass index less then 30. Specific inclusion criteria for healthy subjects were absence of history of neck pain and for patients with WAD no history of musculoskeletal disorders before neck pain started due to WAD. Only patients with WAD type 2 or type 3, according to the Quebec Task Force for WAD (Spitzer et al., 1995) were included.
2.2.2.2. The Up-Line. The position of the document was fixed on the left side of the screen. The keyboard had a fixed position close to the screen. The inclined working area of the Up-Line had a fixed position of 181 with the tabletop (Fig. 1). 2.2.3. Measurements 2.2.3.1. Muscle activity. During typing surface EMG (sEMG) was recorded for the left and right trapezius muscle. Bipolar surface electrodes were placed bilaterally above the descending part of the trapezius muscle 2 cm lateral of the middle of the line between the acromion and the processus of the 7th cervical spine in the direction of the muscle fibres according to international guidelines with a 2.5 cm interelectrode distance with an electrode size of 1 cm in diameter (Mathiassen et al., 1995; Hermens et al., 2000). A reference electrode was placed around the right ankle. The sEMG signal was low-pass filtered at 3–400 Hz filter, sampled at 1000 Hz and AD converted (12 bit). To facilitate comparison between individuals, EMG was normalised using a submaximal isometric reference test. In sitting position the subject was asked to hold both arms in a position of 901 abduction for 15 s. This reference test was repeated 4 times with 1 min rest in between two tests and was performed before the experiment (Mathiassen et al., 1995). 2.2.3.2. Sitting comfort. Immediately after the typing task the subject was asked to mark a Visual Analogue Scale (VAS), representing how comfortable he/she sat during 10 min typing. The VAS is a 10 cm vertical line, with sitting uncomfortable at the bottom of the scale (zero) and sitting as comfortable as possible on the top of the scale (10).
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2.2.3.3. Work performance and work posture. Work performance was characterised by typing speed (words/min) and by number of errors during the typing task. The viewing distance was measured immediately after the 10 min typing. The viewing distance is the distance of a virtual horizontal line from the forehead to the screen. The viewing angle of the keyboard is the angle between a virtual horizontal line at eye level and a virtual line from the eye to the middle of the keyboard. 2.3. Data-analysis 2.3.1. EMG analysis The standard deviation of the sEMG signal was used to estimate the amount of muscle activity and was expressed as percentage of the mean EMG value during the normalisation tests multiplied with 100. The mean EMG value of the normalisation tests was calculated as the mean of the standard deviations of the sEMG signal of the middle 10 s of the 4 submaximal isometric tests. 2.3.2. Statistics Statistical analyses were carried out using Microsoft Excel 97 and SPSS 9.0. 2.3.2.1. Differences in muscle activity. The mean difference in muscle activity between both workstations was investigated using a sampled t-test (po0:05 was considered as statistical significant) and was investigated for healthy persons and patients with WAD separately. A difference of more than 5% between the EMG activity during working at the Up-Line and the EMG activity during working at the standard workstation was considered relevant. Subgroup analyses for age, time of accident, formal work experience, successful return to work, existence of daily complaints and typing performance were done in the group of patients with WAD to search for a common factor in those patients showing a relevant decreased EMG activity when working at the Up-Line (by box-plot).
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3.1.1. Muscle activity On average, the difference in EMG activity of the trapezius muscle between the Up-Line and the standard workstation was not significant (sampled t-test). The interindividual variability of the differences in muscle activation levels between both workstations was however high, which is reflected in the high standard deviation. When applying a 5% difference as relevant, the muscle activity when working at the Up-Line is lower in 5 subjects (25%) and higher in 11 subjects. In 3 subjects no difference in muscle activity between both workstations was found (Fig. 2). 3.1.2. Sitting comfort Considering a difference of more than 2 on the VASscale, as relevant only 4 healthy persons preferred working at the Up-Line and 5 preferred the standard workstation. Ten healthy persons did not experience a difference in sitting comfort between both workstations (Fig. 2). Remarkable is that for those subjects with a subjective preference for a certain workstation, the muscle activation levels are in line with the preference, i.e. the 4 subjects who preferred the Up-Line had lower muscle activation levels during working at the Up-Line. In contrast, of the 10 subjects without a subjective preference there was a discrepancy with muscle activation levels in 7 subjects (Fig. 2).
2.3.2.2. Sitting comfort. The difference in sitting comfort between the two workstations was considered as relevant when a difference in VAS scores>2 was found (Beurskens, 1996).
3. Results 3.1. Healthy persons Seven men and 12 women with a mean age of 30 yr (range 21–44 yr) were included.
Fig. 2. The distribution of subjective preference for a workstation concerning sitting comfort combined with the preference for a workstation based on muscle activation level in the group of healthy persons (N ¼ 19). No preference means for sitting comfort a difference in VAS score less then 2 and for muscle activity a difference in EMG activity less then 5%. The marked bars ( ) correspondence the same preference for a workstation in sitting comfort and muscle activity.
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3.1.3. Work performance and work posture The mean rate of typing at the Up-Line was 146/min (SD 33) and at the standard workstation 146/min (SD 35). The mean amount of errors was 3 at both workstations. This means that there was no difference in work performance between both workstations. The work posture during typing at the Up-Line and the work posture during typing at the standard workstation differed only in viewing distance and the viewing angle of the keyboard. Half of the healthy subjects were sitting too close to the screen at the standard workstation according to the recommended distance by the Dutch Ministry of labour (50–70 cm). The main viewing angle of the keyboard during work at the Up-Line was 421 (341–571) and during work at the standard workstation 531 (461–661). 3.2. Patients with WAD In total 19 patients, 7 male and 12 female with a mean age of 32 yr (range 23–53 yr) participated. Fifteen patients were diagnosed as WAD type 2 and 4 patients as WAD type 3. The car accident causing the WAD took place 1.5 upto 6 years ago. Seven patients had a successful return to work, 6 patients failed to return to work and 6 patients did not finish their occupational rehabilitation yet.
Fig. 4. The distribution of subjective preference for a workstation concerning sitting comfort combined with the preference for a workstation based on lower muscle activation level in the group of patients with WAD (N ¼ 19). No preference means for sitting comfort a difference in VAS score less then 2. There is a preference for one of both workstations concerning muscle activity when the difference in muscle activity is more then 5%. There is no preference for workstations when the difference in muscle activity is less then 5%. The marked bars ( ) indicate the same preference for a workstation in sitting comfortable and muscle activity.
3.2.1. Muscle activity On average, no significant difference was found in EMG activity of the trapezius muscle between working at the Up-Line and working at the standard workstation (sampled t-test, po0:05). A high standard deviation was found for differences in muscle activity between both workstations, representing a high interindividual variability (Fig. 3, 95% CI). On average the muscle activity during the 10 min typing did not change. Assuming 5% difference as relevant in 6 patients (31%) the muscle activity was lower at the Up-Line and in 7 patients the muscle activity was lower at the standard workstation. In 6 patients no difference was found (Fig. 4).
Subgroup analysis was done to try to characterise the 6 patients with lower muscle activity at the Up-Line. They did not differ from the other 13 subjects by age, time of accident, formal work experience, successful return to work and existence of daily complaints. They also did not differ in typing performance, formal experience of working at the Up-Line, and type of WAD.
Up - Stan ( % )
EMG up - EMG stan 15 10 5 0 -5 -10 min min min min min min min min min min 1 2 3 4 5 6 7 8 9 10
Fig. 3. The mean difference in EMG acidity level during typing at the Up-Line minus EMG level during typing at the standard workstation per minute The results presented are from the group of patients with WAD (N ¼ 19; with 95% CI).
3.2.2. Sitting comfort In 11 patients with WAD there was a relevant difference in sitting comfort in favour of the Up-Line. One patient preferred the standard workstation. In 7 patients there was no relevant difference between both workstations concerning sitting comfort (Fig. 4). Of 11 patients with WAD who preferred the Up-Line concerning sitting comfort only in 6 patients this was accompanied by a preference for the Up-Line concerning muscle activity (Fig. 4). The other 5 patients had higher muscle activation levels during working at the Up-Line. In the 8 patients with WAD with no preference for the Up-Line according to sitting comfort also muscle activation levels did not reveal a preference for the Up-Line.
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3.2.3. Work performance and work posture The mean rate of typing at the Up-Line was 116/min (SD 66) and at the standard workstation 120/min (SD 68). The mean amount of errors was 4 for both workstations. This means that there was on average no difference in work performance between both workstations. Two patients had more then 15 errors at the standard workstation compared to 5 and 7 errors at the Up-Line. Both patients complained about loss of concentration during working at the standard workstation. Another 5 patients reported loss of concentration during working at both workstations. The work postures at the Up-Line and at the standard workstation differed only in viewing distance and the viewing angle of the keyboard. The main viewing distance at the Up-Line was 59 cm (range 50–69 cm) and at the standard workstation 54 cm (range 42–62 cm). The recommended distance according to the Dutch Ministry of Labour is 50–70 cm. The main viewing angle of the keyboard at the Up-Line was 421 (range 331–501) and at the standard workstation 531 (range 421–661).
4. Discussion The purpose of this study was to investigate whether a workplace adaptation characterised by inclined working area and keyboard position close to the screen reduces the muscle activity of the trapezius muscle. The first clinical subjective experiences with the use of this workplace in patients with WAD encourage us to verify whether these subjective findings could be quantified in more objective terms in order to develop an indication for the workplace. 4.1. Muscle activity The results show that the Up-Line does not demonstrate such positive effect on muscle activity. Individually in 5 healthy subjects (25%) and in 6 patients (31%) muscle activity was lower when working at the Up-Line. The Up-Line decreases the viewing angle of the keyboard as intended but on average, for both healthy subjects and patients with WAD, no difference in muscle activity between the two workstations is found. This is not in accordance with Schuldt . et al. (1987), who found in 10 healthy subjects the lowest muscle activity of the neck with the cervical spine in vertical position and the thoraco-lumbar spine in slightly backward-inclined position. This working posture is highly comparable to the posture when working at the Up-Line. One possible reason for not finding a decreased muscle activity at the Up-Line could be that due to the inclined working area the arms are forced in a less ergonomic position; especially the elbow is forced in a
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less then 901 position. Some subjects changed their position by more anteflexion in the shoulder joints to return to a more comfortable sitting position with the elbows in 901. In accordance to Takala and ViikariJuntura (1991) small changes of the upper arm angle strongly influence the muscle activity. More anteflexion in the shoulder joint is associated with more muscle activity of the trapezius. The 6 patients with less muscle activity during working at the Up-Line could not be characterised in more detail. They did not differ from the other 14 patients by age, time of accident, successful return to work, work posture and complaints during typing. To characterise these patients other factors like psychosocial influences as job satisfaction might be important ( om, . (H.agg and Astr 1997). 4.2. Sitting comfort Eleven patients with WAD (58%) and only 4 healthy persons (21%) subjectively preferred the Up-Line according to sitting comfort. An explanation might be that patients are positively influenced by the idea that the Up-Line was developed especially for them. Therefore patients expected that the Up-Line should be an improvement compared with the standard workstation. In this study we expected to find an association between sitting comfort and muscle activation levels. We assumed that lower EMG levels would reflect lower effort and consequently higher experienced sitting comfort. In the group of healthy persons we found such an association. In all 9 healthy subjects, who had a preference in workstation according to sitting comfort, the muscle activity showed the same preference. In contrast to the association found in healthy subjects, in only 6 of the 11 patients with WAD who preferred the Up-Line according to sitting comfort, the muscle activity showed the same preference. In contrast in the 8 patients not preferring the Up-Line the muscle EMG activity was in line with their subjective preference in 7 out of 8 patients (88%). Next to the already mentioned explanation of higher expectations of patients with WAD this discrepancy between healthy persons and patients might be explained by a decreased awareness of muscle activation in patients as described in the chronic pain model of Vlaeyen et al., 1997. Vasseljen and Westgaard (1995) also did not show consistent association between the subjective experienced amount of pain and an increased activity of the upper trapezius muscle in the group of office workers. ( om . (1997) on the other hand demonH.agg and Astr strated a correlation between work related complaints in medical secretaries, EMG activity and work task satisfaction. A correlation between EMG activity and other psychological factors could not be demonstrated in their study.
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Based on the found relationship between subjective reported sitting comfort and muscle activation levels it can be concluded that the subjective preference to some extend could be useful as an indicator for prescribing the Up-Line. When healthy subjects have a subjective preference for the Up-Line it is likely that the Up-Line will reduce their muscle activation level. However when healthy subjects and patients with WAD have a preference for the standard workplace it is unlikely that the Up-Line will reduce their muscle activation level. 4.3. Conclusion This study showed on average no difference in muscle activity of the trapezius muscle between working at the Up-Line and working at the standard workstation in both healthy subjects and in patients with WAD.
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