اثر دستگاه کامپیوتر بر ارگونومی فعالیت عضلانی تراپز بالایی عضله در طی تایپ

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.

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; Schüldt 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) and Bergqvist et al (1995b)). 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) and Bergqvist et al (1995b); Vasseljen and Westgaard, 1995; Fernström et al., 1994; Takala and Viikari-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 (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.

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 Schüldt 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 less then 90° 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 90°. In accordance to Takala and Viikari-Juntura (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 (Hägg and Åström, 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. Hägg and Åström (1997) on the other hand demonstrated 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. 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.