Civil Engineering Reference
In-Depth Information
a significant impact. Unfortunately most chair users don't put their back all the way back
into the chair and can therefore not take advantage of the extra reduction in pressure. This
is, in a way, expected! There are no nerve endings or pressure sensors in the disks, so
chair users do not get any feedback from the increased pressure, unless they happen to
have a bad back, in which case there may be feedback from pressure on adjacent nerves.
The implication for design of chairs is that an adjustable back rest angle is important so
that users can fold the chair back.
SUPPORTS FOR THE HANDS, ARMS, AND FEET
A footrest can be helpful for short operators, so that they can support their feet. However,
footrests should not be used out of convention. In Figure 14.2 the footrest is unnecessary
since the operator can put her feet on the floor.
Arm rests should not interfere with the table or desk top. For a keying task, where the
operator must pull the chair close to the table, short arm rests (elbow rests) are often
preferred over long arm rests. Today, many chairs come with height adjustable arm rests
that can be used in many different ways.
Wrist rests are optional. Because typing habits are different, some operators prefer
wrist rests and some do not. Soft wrist rests (rather than hard) are supposed to put less
pressure on the wrist and reduce the risk of developing carpal tunnel syndrome. However,
research has not been able to prove the significant benefits of wrist rests, and whether soft
wrist rests really make a difference. But footrests, armrests and wrist rests are
inexpensive, and should be readily available to operators who require them (Sauter et al.,
1985).
14.3
VIEWING DISTANCE AND EYE GLASSES
There are a few controversial issues concerning recommendations for viewing distance.
Some researchers claim that the viewing distances to the screen, to the documents on the
document holder, and to the keyboard should all be the same, so that it is not necessary to
refocus the eyes. Refocusing takes time and is unproductive (Cakir et al., 1980). In
addition, for older operators with presbyopia (inflexible lens in the eye) the uniform
distance is helpful since it is easier to focus.
Other researchers claim that it is important to keep exercising the focusing mechanism
(accommodation) of the eye, and that thereby visual fatigue and temporary myopia can be
avoided (National Research Council, 1983). The term temporary myopia implies that the
accommodation or focusing of the eyes adjusts to the somewhat closer viewing distance
which is imposed by a close working task. Thereby the range of clear vision is moved
closer to the eye, and it is difficult to focus on distant objects. This phenomenon is not
unique to computer work. Every close work task may cause temporary myopia, which
typically disappears an hour after work. Nonetheless, many individuals notice these
effects and are overly concerned. For example, when driving home after work during
darkness, temporary myopia, combined with dilated eye pupils, makes it difficult to read
traffic signs. Some individuals may misinterpret this and obtain eye glasses to correct a
condition which hardly needs any correction.
