Engineering Anthropometry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 202

at the lower back. However, these bending and unbending lumbar motions (in the medial plane) are
rather limited, and often lead to overexertions, especially if combined with lateral twisting of the
torso; low back pain has been reported throughout the history of humans (Snook, 2000). Wrist problems
have been associated with excessive motion requirements since the early 1700s (Kroemer, 2001). Our
head and neck have limited mobility in bending and twisting. Our thumbs and fingers, as well as the
eyes, have limited but finely controlled motion capability.
Ranges of motion (also called mobility or flexibility) depend much on age, health, fitness, training, and
skill. Diverse measuring instructions and techniques have been applied to dissimilar groups of people to
assess their mobility; hence, there is much diversity in reported ranges of motion. However, one set of
mobility measurements has been taken on groups of 100 females and of 100 males each by the same
researchers using the same techniques (Houy, 1983; Staff, 1983). Excerpts from these data appear in
Table 9.11. Note that the differences in mobility between males and females are generally negligible.
Designing to fit motion ranges, instead of fixed postures, is not difficult. The articulations in the
human body have varying degrees of freedom for movement. These are shown in Figure 9.5 for major
body joints and the motion ranges are listed in Table 9.11. These maximal ranges were measured on
students of physical education; hence, many people will have slightly less mobility than shown. “Con-
venient” mobility is within the range of maximal values shown in Table 9.11, but not always in the
middle of the ranges. Occasionally, convenient motions are near the limits of mobility. Habits and
skill as well as strength requirements may make different ranges preferred.
Design for motions starts by establishing the actual movement ranges. Convenient motions may
cluster around the mean of mobility in a body joint or may be close to the limits of flexibility. For
example, a person walking about, or standing, has the knees most of the time nearly extended, that is,
the knee angle — in the sagittal view — is close to its extreme value of about 180 8 . The sagittal hip

angle (between trunk and thigh) is also in the neighborhood of 180 8 . Of course, when sitting, both

hip and knee angles change and cluster to about 90 when sitting — see Table 9.12. While sitting or

moving about, the trunk is normally nearly erect, as are the neck and head. In most work situations,
the upper arm hangs from the shoulder while the elbow angle tends to be near 90

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8 ; but the wrist is

best held straight i.e., at about 180 8 .

D ESIGN FOR M OTION IN T HESE S TEPS
Step 1: Determine which major body joints are involved.
Step 2: Adjust body dimensions reported for standardized postures (see Tables 10.4 through
10.9) to accommodate the real work conditions. Use Table 9.3 for guidance.
Step 3: Select appropriate motion ranges in the body joints. The range is between two positions
such as knee angles ranging between 60 and 105 for a comfortably sitting person; or it is a motion

envelope, such as circumscribed by combined hand-and-armmovement, or by the clearance envel-
ope under (through, within, beyond) which body parts must fit. Use Table 9.12 for guidance.
Basic work space design faults should be avoided. These include:

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1. Avoid twisted body positions, especially of the trunk and neck; often caused by bad
locations of work objects, controls, and displays
2. Avoid forward bending of trunk, neck, and head; frequently provoked by improperly posi-
tioned controls and visual targets, and work surfaces that are too low.
3. Avoid postures that must be maintained for a long time, especially at the extreme limits of
the range of motion. This is particularly important for the wrist, neck, and the back.
4. Avoid holding the arms raised. This results commonly from locating controls or objects too
high, higher than the elbow when the upper arm hangs down. The upper limit for regular
manipulation tasks is about chest height.