Civil Engineering Reference
In-Depth Information
an upright posture prefer to look down rather than look up or look straight ahead. In
particular, looking up with the head bent back is a common cause of strain and muscle
fatigue in the neck.
THIGH CLEARANCE AND LOW-PROFILE KEYBOARDS
A person sitting at a desk has limited space for the keyboard and the table top (see Figure
14.2). In 1981, the early German DIN 66234 standard mandated the use of low-profile
keyboards and thin table tops (Deutsches Institut für Normung, 1981). The assumption
behind this standard was that operators prefer to type with horizontal underarms and 90°
elbow angles. If so, the available vertical space between the hands and upper legs can be
calculated from anthropometric measures in Table 8.3, such as sitting elbow height minus
thigh clearance. For a small 5th percentile female operator this is 7.5 cm, barely enough
to fit a 3-cm keyboard and a 3-cm table top. There will definitely not be space for a
keyboard tray under the table top.
The German DIN 66234 standard had a pervasive effect. All computer manufacturers
complied with the standard and manufactured low-profile keyboards (Helander and
Rupp, 1984). But there were many protests because the German requirements were
perceived as excessive. A later German investigation proved that the 90° assumption was
indeed excessive. In this investigation, muscle activity in the shoulders and the neck of
operators was recorded by measuring electromyographic activity (EMG) (Zipp et al.,
1981). This showed that for elbow angles of 70-90° there was a flat minimum in EMG
activity. Thus, it does not seem to matter if the arms are raised to a 70° elbow angle. This
makes quite a difference in design, since there is no longer a strong argument for a low-
profile keyboard. Nonetheless a thin keyboard is a good design feature because it
provides greater flexibility in adjusting a computer workstation to the appropriate height
for the individual operator.
CHAIR DESIGN
Most modern office chairs have design features that are adjustable. The BSR/HFES 100
standard mandates adjustability of the seat height over a minimum range of 11.4 cm, with
a recommended range of 38-56 cm (Human Factors Society and Ergonomics Society,
2004). This is the most important adjustability feature.
The second most important factor is adjustability of the seatback angle. A seatback
angle of greater than 110° reduces the pressure on the spine (Michel and Helander, 1994).
As a person moves from a straight standing posture to a straight sitting posture, the hip
joint angle goes from 180° to about 90°. The last 30° of movement from 120° to 90° are
absorbed by the pelvis, which rotates forward. This biomechanical change reduces the
length of the leverage arm from erector spinae muscles (back muscles) to the spine. As a
result the disk pressure is about 30% greater while sitting as compared with standing
(Andersson and Ortengren, 1974).
The third most important adjustability factor is the lumbar support. This design feature
may have been oversold. Lumbar supports are often not used since chair users do not sit
straight and usually do not press their back all the way into the backrest. In fact, many
chair users prefer a more relaxed sitting posture (Grandjean, 1986). However, individuals
