Civil Engineering Reference
In-Depth Information
20
Average Time
(Min) for Young
Males to Reach
Significant Muscle
Fatigue (Severe
Pain)
MIDDLE
15
CLOSE-IN
10
EXTENDED
5
0
0
40
80
20
60
100
Weight Held in Hand (N)
Arm Positions Studied
Close-in = 55° Elbow Angle
Middle = 90° Elbow Angle
Extended = 140° Elbow Angle
Forearm is held horizontal
Elbow
Supports
FIGURE 11.15 Expected time to reach significant shoulder and armmuscle fatigue for different armpostures and hand
loads with the elbow supported. The greater the reach, the shorter the endurance time. (Adapted fromChaffin, D.B., and
Andersson, G.B., Occupational Biomechanics, John Wiley & Sons, Inc., New York, 1991. With permission.)
disadvantageous for another part of the body. Thus, ergonomic design of the workplace requires one to
consider the various trade-offs and rationales for various design options.
One common trade-off encountered in ergonomic design is the trade-off between accommodating the
shoulders and accommodating the neck. This trade-off is resolved by considering the hierarchy of needs
required by the task. Figure 11.17 illustrates this reasoning. The recommended height of the work is a
function of the type of work that is to be performed. Precision work requires a high level of visual
acuity, which becomes the greatest need in order to perform the work task. However, if the work is per-
formed at too low of a level the head must be flexed in order to accommodate the visual requirements of
the job and this becomes a problem for the neck. Therefore, in this circumstance, visual accommodation
is at the top of the hierarchy of task needs, so that the work is raised to a relatively high level (95 to 110 cm
above the floor) in order to accommodate vision and the neck posture. This posture accommodates the
neck but creates a problem for the shoulders since they must be abducted when the work level is high.
Thus, a trade-off should be considered. In this instance, ideal shoulder posture is sacrificed in order to
accommodate the neck since the visual requirements of the job represent the greater priority for work
performance, whereas, the minimal shoulder strength is required for precision work and, thus, represents
a lower priority. Thus, visual accommodation is given a higher priority in the hierarchy of task needs and
this criterion must be given priority over any other criteria. Besides, the shoulder problems can be mini-
mized by providing wrist or elbow supports at the workplace.
The other extreme example of the working height situation involves heavy work. The greatest demand
on the worker during heavy work involves a high degree of arm strength, whereas, visual requirements in
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