7. Ergonomic Design of VDT Workstations
7.6. Ergonomic design of office chairs
between 10 and 15° below the horizontal plane and this corresponds very well to the preferred viewing angles of VDT operators watching their screen.
with a lumbar pad fixed at a height between 10 and 20 cm above the seat. Hünting and Grandjean (91) studied chairs with high backrests, both in the laboratory and under practical working conditions, recording sitting habits and reports of physical discomfort in different parts of the body. A tiltable chair and a similar model with a fixed seat were compared with a traditional type of chair fitted with an adjustable but short backrest. The latter and the tiltable chair are illustrated in Figure 71.
The subjects were doing their normal work while using each of the three chairs for two weeks. The most interesting results were related to the reported preferences, as shown in Figure 72.
The survey indicated quite clearly that the office workers favoured the two types of chair with the high backrest. This confirms the view expressed earlier that a high backrest is preferable for office work as most employees often desire to lean back. It is obvious that a high backrest will be more effective in supporting the weight of the trunk than a chair with a small backrest. It follows that any office working place offering the possibility of leaning back—all the time or only occasionally—should be provided with a high backrest. It must be pointed out here that the experimental tiltable chair could not be fixed at the desired inclination; thus it did not provide enough support for the whole body. This was criticized by many subjects and leads to the conclusion that tiltable chairs or chairs
Figure 72 Comparative assessment of three experimental chairs which were used by 66 office employees over a period of two weeks for each chair.
Type I=fixed moulded chair with high backrest. Type II=moulded chair with tilts of 2 forward and 14 backward, freely movable with high backrest. Type III=standard office chair with a rather short but adjustable backrest.
with adjustable backrest inclinations should be fitted with a mechanism allowing the subject to fix the desired degree of inclination.
General experience as well as a number of studies have yielded the following ‘golden rules’ for office chairs:
1. Office chairs must be adapted to both traditional office work and the modern equipment of information technology, especially to jobs at VDT workstations.
2. Office chairs must be conceived for a forward and reclined sitting posture. See Figure 73.
3. The backrest should have an adjustable inclination, and it should be possible to lock the backrest at any desired inclination.
4. A backrest height of 48–52 cm vertically above the seat surface is an ergonomic necessity today. The upper part of the backrest should be slightly concave. A width of 32–36 cm for the backrest is advisable. It may alternatively be concave in all horizontal planes with a radius of 40–50 cm.
5. The backrest must have a well formed lumbar pad, which should offer good support to the lumbar spine between the third vertebra and the sacrum, e.g., at a height of 10–20 cm above the lowest point of the seat surface. These recommendations are illustrated in Figure 74.
6. The seat should measure 40–45 cm across and 38–42 cm from back to front. A slight hollow in the seat, with the front edge turned upwards about 4–6 ° will prevent the buttocks from sliding forward. A light padding of foam
rubber-Figure 73 An office chair must be conceived for a forward as well as backward inclined sitting posture.
The lumbar spine must get proper support from the backrest in both sitting postures.
Golden rules for office chair equipment
2 cm thick, covered with non-slip, permeable material is a great aid to comfort.
7. Foot rests are important, so that small people can avoid sitting with hanging feet.
8. An office chair must fulfil all requirements of a modern seat:
adjustable height (38–54 cm), swivel, rounded front edge of the seat surface, castors or glides, 5-arm base and user-friendly controls. The most important dimensions for a seat and working desk are shown in Figure 75.
Orthopaedics as well as ergonomics recommend frequent or at least occasional changes of position from leaning forward to leaning back and vice versa. This calls for a ‘dynamic’ chair which allows easy changes of the sitting posture. It is obvious that an adjustable back-rest is crucial for such a ‘dynamic’ chair. Such chairs are available with a tiltable shell or with a back-rest tilting independently of the
Figure 74 Recommendations for the backrest of a well designed office chair.
The backrest is crucial for an office chair
Figure 75 Recommended dimensions for the design of the seat and working desk.
seat surface which, for its part, can remain in a horizontal position or be tilted backwards with increasing backrest declination. With the tiltable shell the angle between backrest and seat surface remains the same in all positions. A drawback of this chair is the elevation of the knees with full backrest declination. With the independently tilting backrest a simultaneous inclining of the seat surface is advisable to prevent forward sliding.
The most sophisticated office chair, especially adapted to VDT workstations, is the “Syntop” model which was presented by Hort at the ‘Ergo-design 84’ conference (90) and is produced by Giroflex Entwicklungs A.G. of Koblenz, Switzerland. The development of this chair is based on the fact that the lumbar support of a backrest moves about 4.5 cm upwards when the inclination is increased from 90° to 105°. This corresponds to almost a whole lumbar vertebral segment. A consequence of a declined backrest is therefore that the lumbar support is no longer at the correct level but too high. The main characteristic of the new chair is that the backrest descends as its declination increases. This is shown in Figure 76.
The “Syntop”
chair
Figure 76 A recent office chair design in which the backrest moves down with increasing declination.
This mechanism allows the back to get adequate support at the correct level for any backrest declination, as indicated by the small arrows. The large arrows show the way the backrest descends with increasing declination.
The “Syntop” chair is an interesting example of applying ergonomics to the design of a chair.
The main objection to a good office chair with an adjustable backrest inclination is of course its cost. But one should bear in mind that the life span of a well constructed chair is about 10 years, or about 2000 working days. The price for a good chair which reduces physical discomfort and promotes well-being is, at a few cents per day, certainly a good investment!
7.7. VDT workstation design: preferred settings and