7. Ergonomic Design of VDT Workstations
7.2. Body size and the design of workstations for traditional office jobs
7.2. Body size and the design of workstations for
wearing shoes 2.5 cm should be added to men’s heights and 4 cm to women’s.
The ergonomic recommendations for the dimensions of workstations are only to some extent based on anthropometric data;
behavioural patterns of employees and specific requirements of the work itself must be considered too. Thus the recommended dimensions given in textbooks or in various standard works are compromise solutions which may often be quite arbitrary. Another critical remark is necessary: most standard specifications for ergonomic workstations were worked out by committees, in which ergonomics, economics, industry as well as unions or employers were represented. The resulting recommendations seem reasonable and suitable in most cases, but they were seldom seriously tested under practical conditions. It is therefore not surprising when field studies of practical experience do not always confirm recommended standard dimensions.
Working height is of critical importance in the design of working places. If the working height is too high the shoulders or the upper arms have to be lifted to compensate, which may lead to painful symptoms and cramps at the level of neck and shoulders. If, on the other hand, the working height is too low, the back must be excessively bowed, which may cause backache. Hence the work table must be of such a height as to suit the body length and the activity of the operator. This also applies to sedentary work in offices. A few studies in this field demonstrate the adverse effects of inappropriate desk heights:
Figure 47 Sitting postures of 378 office employees, as shown by a multimoment observation technique.
4920 observations. The percentage quoted indicates how much of the working period was spent in that posture. The two lower observations were seen simultaneously with the three upper postures, which is why the sum of all five characteristics exceeds 100% (67).
Recommendations are compromises
Working height
How office employees sit
as early as 1962, a survey carried out by Grandjean and Burandt (67) on 261 men and 117 women engaged in traditional office work revealed interesting links between desk heights and musculoskeletal troubles. The work-sampling analysis gave particulars about the different sitting postures, shown in Figure 47.
An upright trunk posture was observed only about 50% of the time, with the trunk leaning against the back-rest about 40% of the time, although most of the chairs were provided with rather poor back supports.
Figure 48 presents the results of the survey on musculoskeletal complaints.
The principal anthropometric data and desk heights were assessed and compared with the reports on muscoloskeletal troubles. From a large number of results and calculated correlations, the following conclusions emerged:
1. 24% reported pains in neck and shoulders which most of the subjects, especially the typists, blamed on a too high desk top.
2. 29% reported pains in the knees and feet, most of them small people who had to sit on the front edge of their chair, probably because they had no footrests.
3. A desk top height of 74–78 cm gave the employees most scope for adaptation to suit themselves, provided that a fully adjustable seat and footrests were available.
4. Regardless of their body length, the great majority of the workers preferred the seat to be 27–30 cm below the desk top. This seems to permit a natural position of the trunk, obviously a point of first priority with these employees.
5. The incidence of backache (57%) and the frequent use of the
Figure 48 Incidence of bodily aches among 246 employees engaged in traditional sedentary office jobs.
Multiple answers were possible.
Links between desk height, sitting behaviour and pains
backrest (42% of the time) indicate the need to relax the back muscles periodically and may be quoted as evidence of the importance of a well-constructed backrest.
Several authors have recorded the electrical activity of the shoulder muscles while the subjects worked with different desk heights. It should be remembered here that the electrical activity of a muscle is an indicator of the exerted muscular force; the procedure is called electromyography. Already in 1951, Lundervold (135) investigated the electrical activity of shoulder and arm muscles of subjects operating typewriters at high and lower levels. (At that time electrical impulses were obtained with ink-writer records). The Electromyography
of shoulder muscles
Figure 49 Electromyographic recording of shoulder muscle activity.
The figures refer to the percentage of time in the maximum voluntary contraction position. A=optimal height of the typewriter (i.e., home row at elbow height). B=too high, resulting in elevation of shoulders by the trapezius muscle. C=too high, compensated by a sideward elevation of the upper arms by the deltoid muscle.
According to Hagberg (79).
author concluded: “the smallest number of action potentials (electrical impulses) were recorded when the person undergoing the experiment was sitting in a relaxed and well balanced state of equilibrium, or was using a backrest”. High-level typewriting was associated with raised shoulders and a strongly increased electrical activity of the trapezius and deltoid muscles. (The trapezius muscle lifts the shoulders, the deltoid the upper arms). Recently Hagberg (79) made a quantitative analysis of the electromyograms of shoulder and arm muscles when typing at different heights. These results are shown in Figure 49.
Thus a too high working level can be compensated either by lifting the shoulders through contraction of the trapezius muscles or by lifting the upper arms with the deltoid muscles. Moreover, the contraction force of the shoulder-lifting muscle reaches 20% of the maximum force, which would certainly suffice to eventually generate great pains in the shoulder muscles.
There was a classical study carried out in 1951 by Ellis (52) which is often advanced when working heights are discussed. Ellis was able to confirm an old empirical rule: the maximum speed of operation for manual jobs carried out in front of the body is achieved by keeping the elbows down at the sides and the arms bent at right angles. This is a generally accepted basis for the assessment of working heights.
For sedentary office jobs, working heights must also take note of the optimum visual distance. In some cases the working surface must be raised until the operator can see clearly without having to bend the neck too much. A slight forward stoop, with the arms on the desk, is only minimally tiring when reading or writing, but in order to relax the back the distance from seat surface to desk top must be between 27 and 30 cm. As mentioned above, employees sitting at an office desk first of all look for a comfortable and relaxed position of the trunk and often accept a seat height that is bad for legs or buttocks, rather than sacrifice a comfortable trunk posture.
The height of tables which are not adjustable is primarily based on average body measurements and makes no allowance for individual variation. Therefore all table heights recommended are too high for short people, who will need some kind of footrest. On the other hand, tall people will have to bend the neck over the work table which will cause musculoskeletal troubles in the neck and back. Table heights for traditional office work (with the exception of typing!) thus follow the rule that it is more practical to choose a height to suit the tall rather than the short person; the latter can always be given a footrest so that he/she can raise the seat to a suitable level. On the other hand, a tall person given a table that is Lifting the
shoulders is strenuous static work
Most important:
seat to desk distance
too low can do nothing about it except fix the seat so low that it might be uncomfortable for the legs.
Office desks to be used without a typewriter should have a height of 74–78 cm for men and 70–74 for women assuming that the chairs are fully adjustable and foot rests are available. These figures are slightly higher than most standard specifications recommending desk top heights of between 72 and 75 cm, which are certainly not ideal for tall male employees.
It is important that office desks allow plenty of space for leg movement and it is an advantage if the legs can be crossed without difficulty. For this reason there should be no drawers above the knees, and no thick edge to the desk top. The table top should not be thicker than 3 cm and the space for legs and feet under the table should be at least 68 cm wide and 69 cm high.
Many employees, especially those who lean back, periodically like to stretch their legs under the table. It is therefore necessary to leave enough depth as well. At knee level the distance from table edge to back wall should not be less than 60 cm increasing to 80 cm at ground level. These recommendations are also valid for workplaces with typewriters or VDTs.
The above mentioned classical guideline, requiring a straight upright trunk position with the elbows at the sides and bent at right angles has for a long time been the basis for the design of typing desks. Since the height of the keyboard defines the working level, the middle row (or so-called home row) should be at about elbow level. However, the need for such a low desk conflicts with the necessity for enough knee space under the table. This can be a Conclusion for
office desk without typewriter
Vertical and horizontal leg room
Typing desks calls for height adjustability
Figure 50 Recommended desk-top heights for traditional office jobs.
Left: Range of adjustability for typing desks.
Right: Desk-top heights for reading and writing without typewriter.
limiting factor and calls for height-adjustable typing desks. Indeed, today most experts recommend the height of such desks to be adjustable between 60 and 70 cm. These recommendations are illustrated in Figure 50.
The height of a non-adjustable typing desk is a very problematical dimension. Until now most experts have recommended a fixed top height of 65 cm. This recommendation is based on two assumptions which are questionable for the following reasons:
1. The assumed straight upright trunk posture is not normally adopted by typists if the work lasts for a few hours or more. They often lean back or forwards in order to relax the back muscles or to get a suitable viewing distance.
2. The old mechanical typewriters required stroke forces of several hundred grams. Modern electrical typewriters need much lower stroke forces of 40–80 g; the keys are easily operated by the fingers alone and no dynamic muscular effort is required from the forearms and hands, which therefore need not be kept in a horizontal plane!
Thus it is concluded that the recommended height of 65 cm for a fixed typing desk has yet to be demonstrated in a convincing manner by taking into account the preferred and actually assumed postures of typists and by providing operators with comfortable office chairs as well as with electric typewriters.
7.3. Field studies on musculoskeletal troubles of office