Click for next page ( 130


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 129
6 Anthropomel~y and Biomechanics in VDT Applications Biomechanics and anthropometry play important roles in the design of VDT workstations. They are intertwined with other ergonomic aspects of the workplace and have implications for both postural and visual task requirements. A classic example of the way in which visual and postural aspects are interlinked is that of an operator who has to bend his or her head back In order to read the screen through the lower part of bifocals. Adapting this posture can cause neck and back pain and possibly visual discomfort and reduction in performance. Similarly, the positions of the keyboard, source document, and screen all have implica- tions for visual accommodation. If the ambient illumination level is raised to enhance the legibility of a source document, the legibility of the characters on the screen may decrease. Since only relatively little adaptation can be expected from human operators, the technical elements of a VDT system must be selected to fit the operator. In fact, all system components, that is, the display, the workplace furniture, the environment, and the work tasks and schedules must be fitted to each other, and all of them to the operator: none of these components can be con- sidered independently. Human factors problems can be analyzed in a systems context. Two major systems are involved in VDT applications: the job and the operator. The job may be divided into two subsystems, work- station design and task characteristics, both of which impose certain demands on the operator. The other major system, the operator, may also be thought of in terms of two subsystems: biomechanical/anthropometric factors and personal factors. Unless the proper balance between job demands and operator capabilities is established, there may be adverse health conse- quences, as indicated by physical complaints and symptoms, absenteeism, and so forth. Problems must be studied at the level of subsystems (e.g. investigating biomechanical and anthropo- 129

OCR for page 129
130 metric factors) to obtain information sufficiently detailed to propose practical solutions. This chapter addresses anthropometry and biomechanics in VDT applications and the ways in which they influence appro- priate design of workstations, particularly the choice of chairs, support stands for keyboard and display tables, footrests, wrist- rests, armrests, and document holders. The first part of this chapter reviews several field surveys of postural strain in VDT work. The second part reviews and analyzes the anthropometric, biomechanical, and physiological factors that influence the appropriate design of VDT workstations. The implications of this analysis for the design of VDT tasks and workstations are discussed in Chapter 9. POSTURAL STRAIN Of the substantial body of literature on the postural strain associated with VDT work, nearly all is based on subjective reports of muscular discomfort. Some researchers support this data with medical observations or measurements of body angles, chair and table heights, and viewing distances. Overall, the research represents an attempt to isolate VDT design problems, but because of problems in the methodologies of many studies, the results are difficult to interpret. Practically all of the research has been performed in the field, using existing groups of subjects. The main sample has always consisted of VDT operators; in some cases investigators classified the operators by their tasks, such as data entry, data acquisition, or interactive work. Approximately one-half of the studies have used a no - VDT comparison group. Several problems are inherent in survey research using static group comparisons (see Chapter 2~. For example, by sensitizing subjects to the issues under investigation, surveys tend to be reactive. Surveys may also be influenced by uncontrolled psychosocial factors. For example, one study (Smith et al., 1980; National Institute for Occupational Safety and Health, 1981) found that, depending on job demands and task requirements, different types of health problems were reported. Workers who were allowed more flexibility and autonomy generally voiced complaints of a psychological nature, such as anxiety and irritability, while workers whose jobs were rigid and offered little control reported more visual and musculoskeletal problems. The authors mentioned that union negotiations at the time of the survey may have influ- enced the results. Task characteristics, psychosocial factors, and personal characteristics influence the reporting of symptoms; the results of survey research studies must, therefore, be interpreted very carefully.

OCR for page 129
131 Table 6.1 summarizes six studies that reported subjective data on pains in various parts of the body and in which the VDT group was compared with a non-VDT group (but see the discussion in Chapter 2 regarding comparisons between VDT and non-VDT groups). Since the studies used different methods or questions, or both, the percentages in the table can be compared only within each study. As shown in the table, three studies indicate that VDT operators reported more symptoms than non-VDT operators (Arndt, 1981; Eisen, 1981; Smith et al., 1981~. In the study by Hunting and coworkers (1981), traditional office workers had fewer complaints than data entry and interactive VDT users and typists (except for leg pains, for which data entry workers had many more reports than the other three groups). Finally, Coe and coworkers (19g0) found only differences by sex: females reported more problems than males, which was also found by Onishi and coworkers (1973~. OVERVIEW OF BIOMECHANICAL FACTORS Work Posture When a person stands upright, the body's center of gravity is located in the upper part of the pelvis, vertically above the arch of the foot. Any movement of the trunk, head, or arms requires a compensating countermove by the pelvis. For example, if the arms are extended forward, the hips will move backward. In this way the body redistributes its weight and achieves balance. In some postures, such as sitting and lying down, balance is main- tained with only modest contributions of certain groups of muscles. In others, there may be a great deal of effort, with static muscle contractions required. Most of the physiological changes associated with postural stress are caused by static muscle contractions (Troup, 1978~. Posture, implying a more or less static condition, is found in few natural biological systems. Most systems in the body are in a dynamic equilibrium, with continuous changes around an average value. Troup (1978) emphasized that it is generally easy to relieve symptoms of postural stress sinaply by moving around. Kramer (1973) observed that such postural changes result in a massaging action of the discs in the spine. Branton and Grayson (1967) studied train passengers and found that there were cyclic postural changes, which they interpreted as necessary to prevent postural strain. Ostrom (1981) pointed out that "dynamic sitting," implying a change in posture about every five minutes, is important for comfort and promote:; good circulation. The design of a VDT workstation should make it easy to change work postures. For example, a flat seat pan is better than an

OCR for page 129
132 TABLE 6.1 Percentage of Various Populations Reporting Body Pains Pain Location Author and Groups Investigated Back Lum- Head Neck Arms Wrists (General) bar Legs Coe et al. (1980) VDT users 32 Input 45 Creative 20 Editing 41 Dialogue 18 Non-VDT 41 VDT users Male 19 Female 40 Non-VDT Male 22 Female 49 Arndt (1981) VDT users 6Sa Non-VDT 39a Eisen (1981) VDT users Mainly editing 76b 38 19 76 34 Non-VDT 41b 27 7 66 24 Hunting et al. (1981) VDT users Data entry 11 9 9 13 Dialogue 5 6 11 6 Non-VDT Typists 6 9 10 5 Traditional office work 1 2 2 6 Smith et al. (1981) VDT users Clerical 56b 37 78 Non-VDT 19b 20 56 a Head and neck pain. b Neck and shoulder pain.

OCR for page 129
133 "anatomical seat pan," molded after the contours of the posterior, because the latter inhibits movements (Vernon, 1924~. Adjustable or flexible provisions at the VDT workstation, such as a detachable keyboard, a movable document holder, and an adjustable chair allow movements and are therefore important in preventing postural stress. Muscular Load There is an important distinction between dynamic work and static contraction of muscles. Dynamic work is characterized by a rhythmic change of contraction and relaxation of the muscles, which is favorable for blood circulation. Static effort, on the other hand, is characterized by sustained contraction, such as brought about by static postures. During static efforts, blood flow through the involved muscles is impaired (due to contracted muscles and blocked blood vessels) and waste products (e.g., lactic acid) accumulate (Astrand and Rodahl, 1977~. This condition may cause acute pain in the statically loaded muscle. Excessive static loads may also lead to rheumatic diseases that affect the joints, ligaments, and tendons and to the development of arthrosis of an inflammatory and degenerative nature (van Wely, 1970~. The peritendinitus that frequently occurs in the lower arm of typists is caused by excessive static load (Tichauer, 1976~. Static workloads can often be alleviated by providing suitable body supports, such as armrests on chairs and wristrests in front of keyboards. Pi recent innovation is the split keyboard, the halves of which are located on inclined surfaces that provide forearm support. Joint Angles Every joint has a limited range of movement (Grieco et al., 1978~. The closer the joint's position approaches either of its extremes, the more uncomfortable the position becomes. Therefore, work activities should be performed with the joints in about the middle of their range of movement. This is particularly important for the joints of the neck, trunk, and upper limbs (van Wely, 1970~. Two common examples of poor posture caused by working near the limits of joint ranges of movement are the VDT operator with presbyopia who bends his or her head back in order to observe the screen through the lower part of bifocals and the outward angling of the hands (ulnar deviation) in VDT operators and typists. The standard keyboard with horizontal rows of keys requires an

OCR for page 129
134 extreme inward rotation of the hands (proration). Lateral down- tilt of each half of a split keyboard may alleviate the static muscle tension required to maintain this position (Kroemer, 1972; Grandjean et al., 1981~. When a person moves from a standing to a sitting position, the hip angle decreases from 180 to 90. Anatomically, this move- ment is fairly complicated; about 60 of the bending takes place in the hip joint, and the remaining 30 is due to the flattening of Me lumbar curve. Thus, while sitting, the lordosis (forward bend) of the lumbar spine is flattened out. Most of the angular change takes place in the fourth and fifth lumbar discs (Schobert, 1962~. Since these discs are involved in many lower back problems, it is important to supply chairs with lumbar supports. Keegan (1953) pointed out that the angular change in the lumbar region when repositioning from a standing to a sitting posture starts when the hip angle reaches about 135. This angle corresponds to a resting position (commonly assumed while sleeping) for which the muscles of the front of the thighs (quadriceps and iliacus) and the muscles underneath the leg (hamstring) are in relaxed balance. It is also important to provide seats that allow operators to bend their knees since the decrease in lumbar lordosis while sitting is further emphasized if a seated person stretches his or her leg. Stretched legs cause a rotation of the pelvis due to the pull action of the hamstring muscle. Sitting brings the ischial tuberosities (the rounded arches of bone at the base of the pelvis) close to the skin. This can be felt by placing a hand between the body and the seat. The bony pro- trusions absorb most of the sitting pressure (Applied Ergonomics, 1970~. Pressure between the body and the seat can be changed by assuming various postures, such as crossing one's legs, propping an arm on a table or armrest, or leaning forward or back in the chair. For distribution of pressure, the seat should be firmly upholstered, neither too soft nor too hard. Nerves can be stimulated and blood flow restricted if there is pressure on body tissue, which may happen if there is not enough space for the leg between the seat and the table. It can also occur if the edge of the seat cuts into the underpart of the knee. The seat edge must, therefore, be well rounded, and there must be clearance for an operator's legs. The more often major or minor adjustments of posture are made, the less likely is discomfort. Many of the complaints of VDT operators refer to sustained postures of the neck, shoulders, upper and lower extremities, and trunk. If operators had free choices they would probably move around, stand in different locations, and sit in various relaxed or upright positions. Choice of different postures, static and dynamic, would certainly alleviate many of the physical and psychological problems found with the confined seated position required at many VDT workstations.

OCR for page 129
135 With current VDT technology, the eyes must remain in a rather f ixed location relative to the display, the hands must be kept over the keyboard, and the feet are confined to the open space under the table or support. Hence, only a limited choice of postures is available to an operator, and the chair should therefore allow various and easy adjustments in its dimensions and angulations. In some instances it might be feasible and desirable for an operator to change between seated and standing positions. To accom- modate a person either sitting or standing at the same work- station would obviously require a rather elaborate workplace design. One can, however, often provide an extra workstation designed for standing operation, which may be used occasionally for a change of posture. Ergonomic data for sit/stand work- stations are available (see U.S. Department of Defense, 1971, 1974, 1981; Ayoub and FIalcomb, 1976; Woodson, 1981~. Anthropometry Body dimensions of the user population are of primary importance for the design of the VDT workstation. The composition of the working population of the United States is changing dramatically. More women are entering occupations traditionally dominated by males and vice versa. These changes must be considered when designing furniture and other equipment for VDT workstations. Most existing anthropometric data for the U.S. population apply to various military populations and cannot be readily used for estimating body dimensions of the working population. Since the military population is a subsample of the entire population, civilian body dimensions have recently been extrapolated from data on military populations. McConville and coworkers (1981) matched individuals from civilian and military samples in stature and weight creating a subsample of the military population that represents civilians on these two dimensions. From this sub- sample, dimensions other than height and weight were selected and compared to civilian data. For males, there was excellent fit: 99 percent of all civilians could be matched with a member of the military. For females the attempt was less successful, but 94 percent of the civilians could be matched. Though not complete, the data represent the most current compilation of body dimensions for the U.S civilian population (see Table 6.2~. In anthropometric investigations a person is measured in a standardized erect/sitting position with joint angles at 0, 90, or 180. Each of these is referred to as an "anatomical position." Unfortunately, this method does not provide functional dimensions that describe the continuously changing positions of a human body. Until better translation techniques are developed to

OCR for page 129
136 lo o Go N 6 o - :~: ILL a) ._ a o m - ._ > ._ on j Cal J m ~ O ~ ._ CO ~ .= > Cal Cat ~ A) O a, - C) a: o ._ ._ O C,) my ~ tS ~ AS cr~ ~ _ oo ~ ~ ~ 0 0 . . . . . . . . . . . ~D ~ ~ U~ ~ ~ ~ ~ ~ ~ ~ ___________ cr~ O V~ ~ 00 ~ ~ ~ 00 . . . . . . . . . . . . r~ ~ ~ ~ ~ O O ~ ~ ~ 00 . . . . . . . . . . . . . ~ ~ ~ ~ 0 ~ ~ ~ ax ~ oo 00 ~ ~ _ 00 cr~ 00 ~ ~ ~ ~ _ _ _ _ _ _ ______ ______ 00 ~ ~ ~ ~ _ ~ ~ ~ ~ . . . . . . . . . . . . . _ ~ _ 00 ~ O 00 _ ~ ~ ~ O ~ ~ ~ ~ ~ ~ ~ _ _ _ _ _ _ 00 0- ~ ~ \0 d ) ~ 1 ~t d" O oo C~ ~ ~ d" O ~ ~ ~ ~ ~ ~ ~ _ _ _ _ _ ~ _____ ____ __ _ ~ ~ . . . . . O oo O ~ ~ ~ O ~ _ _ _ _ O ~ ~ ~ oo oo ~ ~ . . . . . . . . ~ ~ ~ ~ ~ ax ~ ~ oo t ~ ~ ~ ~ _ _ oo _ ~ O oo ~ ~ ~ O ~ C~ . . . . . . . . . . . _ _ ~ 0 crx O ~ oo ~ tn _ _ _ _ _____________ _ ~ ~ ~ V~ ~ _ ~ ~ ~ ~0 . . .. . . . . . . . . . . oo _ ~ ~ oo ~ C~ oo ~ ~ O ~ d. ~ ~ C~ ~ ~ ~ ~ d~ ~ _ _ _ _ ,S: _ - .oo o.~) _ - -= oo _ _ co e~o ~ o o 04) ~ ~ a~ a S ',o-= ~o ~,SC08 S _ S S o ~1] >, ~J ~) S C ~ t4 0 E - a, C~ a

OCR for page 129
137 oo ~ ~ ~s ~ ~ cr~ o . . . . . . . _ o ~ ~ o o _ _ _ _ _ _ _ _ _ o o ~ oo . . . . . . . . . . . _ o ~ ~ ~ o o ~ U~ ~ ~ ~ ~ oo o . . . . _ _ oo _ o _ _ ~ o oo . . . ~ o ~ ~ ~ ~ ~ ~ ~ ~ ~ o ~ _ . . . . . . . . . . . . . . =\ ~ oo _ o ~ ~ ~ o o ~ ~ o oo ~ ~ ~ ~ V~ ~ ~ ~ _ ___ __ ___`____ o o ~ ~ ~ ~ ~ ~ ~ C~ . . . . . . . . . . . . . . ~D ~ ~ ~ oo ~ ~ ~ ~ ~ ~ ~ ~ d" _ _ ~ ~ ~ ~ C~ 00 o ~ ~ ~ ~ V~ ~ o C" ~ ~ ~ ~ ~ ~ ~ o ~D ~ ~ ~ 0N oo _ ~ _ oo ~ ~ ~ ~ ~ ~ ~ ~ ~ ________ _ __ oo 0 ~ ~ ~ ~ ~ ~ ~ ax _ oo _ . . . . . . . . . . . . . . - . ~ _ ~ ~ ~ oo ~ oo ~ ~ ~ oo ~ ~ ~ _ _ oo V~ ~ ~ ~ ~ ~ ~ o ~ o oo o C~ . . . . . . . . . . . . . . ) v) ~ ~ o ~ ~ o ~ ~ ~ ~ oo ~ - ~ . . . . . . . . . . . . . . ) ~ oo ~ ~ ~ - ~4 ~ ~ ~ ~ - ~ ~ ~ ~ ~ ~ v ~: c) ~ c c#O ~ ;- ~ 'e ~ - . c~ - a, ~: ~ - e c~ c~ - - ~ e tts C: . - o s ct, - ~ r~ ~ ~ ~ ~ v ~ a~ c~ _ S: a ~ ~ ~ s~ s - ^ ~ ~ ~ ~ 0} :5 C 3 o ~ ~ ~ ~q ~o D Ct C~ ~ ,= ~4 ~1 ~ := :~: V Ct ~0 o - o S C=> E~ - CO a~ ._ Fll - e~ ._ U, o a' ._ cn Ct C) ._ S o o ._ U, U, .E S - ._ 3 os ._ s ~ - _ ._ '~ .. = ~ t,.o ~ ~D.= ~ 3 .~ a) o ~ 3 ~ C~ ~ D S o ~ O <_ _, co D ._ ~ O O ~ ~ ~ m ~ ~ 3 a' ._ . ~_ - - 0 ~ ~ _ ~ ~ O ~ ._ c~ u~ _d 0 v, ~

OCR for page 129
138 convert static anthropometric measures to functional dimensions, the following guidelines might be used to design a VDT workstation: 1. Stature, sitting height, eye height, and shoulder height should be reduced by 3 to 5 percent from the erect sitting measures (Brown and Schaum, 1980; Kroemer and Price, 1982~. 2. For comfortable reach, forward and lateral reach should be decreased by 30 percent from the erect sitting measures. 3. Popliteal height, knee height, and elbow height should not be changed from the standard positions. The simplest design model is the "average person" (50th percentile). Unfortunately, even one person who is average in many or all body measures is extremely hard to find. It was shown three decades ago by Daniels (1952) that simultaneous use of several average measures is false and results in a design that fits nobody well. Other misleading assumptions are that a large female can be represented by the body dimensions of an average male and that the dimensions of a small male are similar to those of an average female. It is becoming standard practice to design to accommodate 90 percent of the population, disregarding the upper and lower 5 percent. For this reason, anthropometric data are often expressed in terms of 5th, 50th, and 95th percentile measures. These con- cepts are described in Table 6.3. Since body measurements usually have a normal distribution, any percentile measure can be derived from the values of the mean (x) and the standard deviation (S.D.~. Anthropometric tables are available for males and females. For mixed civilian populations (e.g., 20 percent male and 80 percent female), there are special formulas for the calculation of per- centiles (see Roebuck et al., 1975 and Kroemer, 1982~. WORKSTATION DESIGN Effects of Chair Design Features on the Spine Several studies have analyzed how pressure in lumbar discs and electromyographic (EGG) potentials from the muscles in the lower back are affected by various sitting postures (Nachemson and Elfstron, 1970; Andersson and Ortengren, 1974, 1979~. Disc pressure was measured by inserting a needle with a strain-gauge measuring device into the third lumbar disc, and the EMG was recorded using skin electrodes. The investigations showed that disc pressure and EMG potentials were considerably less for standing than for sitting, with or without support. Simultaneous

OCR for page 129
139 TABLE 6.3 50th, 95th, and 5th Percentile Anthropometric Measures Percentile Description 50th 95th 5th Average value of body dimension (x); 50% of the population is smaller 95% of the population is smaller; can be calculated from the formula x + 1.65 S.D.a 5% of the population is smaller; can be calculated from the formula x - 1.65 S.D. a S.D. is standard deviation. X-ray investigations showed that the lordosis of the lumbar region decreased by an average of 38 when subjects sat down. The highest pressure was obtained for a person lifting a weight, and the lowest pressure was obtained during a relaxed, reclining position. It should be noted that typing (without wrist support) induced disc pressures far higher than ordinary handwriting activities in which forearms were supported. Disc pressures decreased monotonically with backrest inclination and the size of the lumbar support. Complementary investigations showed that a seat inclination of 6 towards the back decreased disc pressure in comparison with a horizontal seat. Summarizing these investigations, Andersson and Ortengren (1979) pointed out that there was 35 percent more disc pressure while sitting than while standing. When the backrest inclination increased from 80 to 130, the pressure decreased by about 50 percent. An increase in lumbar support to +4 cm decreased pressure by 30 percent. The maximum combined decrease due to simultaneous change in backrest angle and lumbar support was 65 percent. These findings support the notions that the optimum seat back angle is about 1 10-120, that a suitable backward inclination of the seat pan is about 15, and that there should be a pronounced lumbar support from the seat back (Kroemer, 1971; Yamaguchi et al., 1972; Andersson and Ortengren, 1974; Grandjean, 1980~. However, different design solutions have also been proposed. Staffel (1889), for example, maintained that the seat should have a forward slope. This opinion was recently supported by Mandal (1981), who reported that lordosis of the lumbar spine is main- tained if the seat surface is tilted 20 forward, which increases the hip angle to about 120. The resulting seating postures are quite unusual. Adjustability of the seat height is considered mandatory in most VDT design guidelines (Helander, 1981~. Since people differ

OCR for page 129
140 in size and postural preferences, adjustability of the equipment appears necessary despite the fact that few experiments have been done to quantify how desirable and how necessary it is. McLeod and coworkers (1980) performed an experiment with chair height and backrest height adjusted in three different combina- tions, all used with the same desks. Overall, a medium setting was perceived as most comfortable. (Interestingly, the subjects thought the desk heights were manipulated and not the chair adjustments; this also supports the notion that system components interact with each other.) Another study observed the adjust- ments in seat and backrest height actually made by VDT users in a new library. Of those responding to a questionnaire, 30 percent adjusted their seat height and 35 percent the back support. Both studies concluded that maladjusted seating produces negative effects. There are obviously trade-offs among the benefits of adjustability and its cost. The actual use of adjustable features depends on the user population, the task, and on the ease of adjustment. Effects of Working Height on Postural Strain Choice of the working height of the hands, and associated arm and trunk postures, is among the most controversial design aspects for keyboard work. Lundervold (1951) quoted three contradictory schools of thought indicating that the working height should be such that a keyboard operator (typist) can work with the elbows at about 90, with the forearms sloping downward, or with the forearms sloping upward. These recommendations have obvious effects on the height of the keyboard and its support stand and also on the seat height to be selected. The first recommendation, having the elbows at right angles and the upper arms hanging vertically, is reflected in the proposed German regulation for VDT operators (Helander, 1981~. Assuming a knee angle of 90 and the thighs about horizontal (as is the usual recommendation found in the literature in order to achieve a "correct" sitting height), an extremely thin table top and a very thin keyboard must be used to allow the right angle at the elbow, with the upper arm hanging vertically. As Table 6.2 indicates, the average differences between elbow rest height and the upper side of the thigh are between 10 cm and 13 cm. In order to allow sufficient height of the open leg room, the table top and keyboard combined cannot be thicker than about 10 cm. With such a small margin, the relative height adjustments of chair and keyboard support must be very fine. Recent studies raise questions about the appropriateness of the 90 elbow recommendations. Zipp and coworkers (1980) measured

OCR for page 129
141 EMG in several muscles during a typing task. They concluded that EMG increases with increasing elbow angle, and they found a flat minimum in EMG activities for angles between 90 and 75. These measurements do not support the idea that the forearms should slope downwards (i.e., that the elbow angle should be larger than 90~; furthermore, such a posture would leave almost no room between thighs and hand position, which would make it difficult to accommodate a suitable keyboard without infringing on needed leg space. The findings of Zipp and coworkers are supported by those of Grandjean and coworkers, who found that typists generally preferred to elevate the forearms, that is, to decrease the elbow angle, when they were provided with divided keyboards and wrist supports. Furthermore, various studies show that the upper arms are not usually hanging down vertically, but are slightly elevated both sagitally and frontally. In the Grandjean and coworkers study, subjects used a chair with a high backrest that allowed them to lean back fully and comfortably. This is indicative, again, of the various interactions between components of workstation and work habits. Effects of Display Position on Postural Strain Human engineering guidelines agree on a preferred downward slope of the line of sight of from 15 to 30 (Cakir et al., 1980; Rupp, 1 98 1; U.S. Department of Defense, 1 981 ). Consequently, the center of the display should be distinctly lower than a VDT operator's eyes. The height difference depends on the declination of the line of sight, a, against the horizontal, and on the preferred viewing distance, D: AH = D sin al Both the preferred viewing distance and the preferred declination are highly subjective. For example, Grandjean and coworkers2 reported (for their special experimental conditions) that their subjects preferred angles of between 4 and 14. This finding indicates a need for further research. It should be noted in this context that the optical correction for near-work routinely provided presbyopic individuals is likely to be at an inappropriate distance for reading a VDT screen. Multi- iEtienne Grandjean, Swiss Federal Institute of Technology, Zurich, Personal communicator, January 1982. See footnote 1.

OCR for page 129
142 focal lenses may not be designed to allow a person to view a VDT screen through the segment for near work without tilting his or her head to an uncomfortable level. It is obvious that many traditional exhortations and recom- mendations are somewhat questionable in the light of current knowledge. For example, people should not and will not "sit straight and upright"--if the location of the hands and the screen, the use of glasses (even with inappropriate lenses), and the avail- ability of a good back support prompt or allow other postures. A modern workseat allows good support and a relaxed seating posture that uses a full backrest and permits an operator to lean back. This is likely to bring about a working height slightly above elbow rest height (measured while sitting). This in turn allows a relatively high open leg room when thin support stand surfaces and keyboards are used. Different operators will select different postures and adjust- ments of workstation components, and the latter are dependent on each other. Furthermore, experience has shown that the ease of adjustment will determine whether the workplace components will be adapted to each other and whether individual comfort and postural satisfaction can be achieved. It is apparent that much adjustability in all major workstation components is necessary to allow individual accommodation according to body size, postural preferences, and work habits.