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motions requiring a cycle time of less than 30 sec is considered a candidate for cumulative trauma dis-
order risk.
Third, the force applied by the hands and fingers during a work cycle has been identified as a risk
factor. In general, the greater the force required by the work the greater the risk of CTD. Greater hand
forces result in greater tension within the tendons and result in greater tendon friction and tendon
travel. Another factor related to force is wrist acceleration. Industrial surveillance studies have reported
that repetitive jobs resulting in greater wrist acceleration are associated with greater CTD incident rates
(Marras and Schoenmarklin, 1993; Schoenmarklin et al., 1994). Since force is a product of mass and
acceleration, jobs that increase the angular acceleration of the wrist joint result in greater tension and
force transmitted through the tendons. Thus, wrist acceleration can be another mechanism of imposing
force on the wrist structures.
Fourth, as shown in Figure 11.20, the anatomy of the hand is such that the median nerve becomes very
superficial at the palm. Direct impact to the palm of the hand through pounding or striking an object
with the palm, as is done often in assembly work, can directly stimulate the median nerve and initiate
symptoms of cumulative trauma even though the work may not be repetitive.
11.3.1.5.3 Grip Design
The design of a tool's gripping surface can dramatically affect the activity of the internal force trans-
mission system (tendon travel and tension). The grip opening and shape have a major influence on
the available grip strength. Figure 11.22 shows how grip strength capacity changes as a function of the
separation distance of the grip opening. This figure indicates that maximum grip strength occurs
within a very narrow range of grip openings. If the grip opening deviates from this ideal range by as
little an inch (a couple of centimeters), then grip strength is dramatically reduced. This change in
strength is also due to the length-strength relationship of the forearm muscles. Also indicted in
Figure 11.22 are the effects of hand anthropometry. The workers hand size as well as hand preference
kg
lb
54
120
50th percentile male
45
100
35
Grip axis
80
27
60
5th percentile male
18
40
50th percentile female
9
20
5th percentile female
0
0
in
0
1
2
3
4
5
0
25
51
76
101
127 mm
Grip axis opening
Grip strength
FIGURE 11.22 Grip strength as a function of grip opening and hand anthropometry. (Adapted from Sanders, M.S.
and McCormick, E.J., Human Factors in Engineering and Design, McGraw-Hill Inc., New York, 1993. With
permission.)
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