Civil Engineering Reference
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different heights or different locations), (2) precise manual control actions are not required, (3) leg room
is not available (when leg room is not available, the moment arm distance between the external load and
the back is increased and thus greater internal back muscle force and spinal load result), and (4) heavy
weights are handled or large forces are applied. When jobs must accommodate both sitting and standing,
it is important to ensure that the positions and orientations of the body, especially the upper extremity,
are in the same location under both standing and sitting conditions.
11.3.1.5 Wrists
The wrist has been of increased interest to ergonomists in the past three decades. The Bureau of Labor
Statistics reports that repetitive trauma has increase from 18% of occupational illnesses in 1981 to 63% of
occupational illnesses in 1993. Based upon these figures, repetitive trauma has been described as the
fastest growing occupational problem. Even though these numbers and statements appear alarming
one must acknowledge that occupational illnesses represent 6% of all occupational injuries and illnesses.
Furthermore, these figures for illness include illnesses unrelated to musculoskeletal disorders such as
noise-induced hearing loss. Thus, the magnitude of the cumulative trauma problem must not be over-
stated. Nonetheless, there are specific industries (i.e., meat packing, poultry processing, etc.) where
cumulative trauma to the wrist is a major problem and this problem has reached epidemic proportions
within these industries.
11.3.1.5.1 Wrist Anatomy and Loading
In order to understand the biomechanics of the wrist and how cumulative trauma occurs in this structure
one must appreciate the anatomy of the upper extremity. Figure 11.20 shows a simplified anatomical
drawing of the wrist. This figure shows that few power-producing muscles reside in the hand itself.
The thenar muscle, which activates the thumb is one of the few power producing muscles in the
hand. The vast majority of the hand's power-producing muscles are located in the forearm. Force is trans-
mitted from these forearm muscles to the fingers through a network of tendons (tendons attach muscles
to bone). These tendons originate at the muscles in the forearm traverse the wrist (with many of them
passing through the carpal canal), pass through the hand, and culminate at the fingers. These tendons are
secured or “strapped down” at various points along this path with ligaments that keep the tendons in
close proximity to the bones forming a sort of pulley system. This system results in a hand that is
very small and compact, yet capable of generating large amounts of force. The price the musculoskeletal
system pays for this design is friction. The forearm muscles must transmit force over a very long distance
in order to supply internal forces to the fingers. Thus, a great deal of tendon travel must occur and this
tendon travel can result in significant tendon friction under repetitive motion conditions thereby initi-
ating the events outlined in Figure 11.3. Thus, the key to controlling wrist cumulative trauma is rooted in
an understanding of those workplace factors that adversely affect the internal force generating (muscles)
and transmitting (tendons) structures.
11.3.1.5.2 Biomechanical Risk Factors for the Wrist
A number of risk factors for wrist cumulative trauma have been documented in the literature. First,
deviated wrist postures are known to reduce the volume of the carpal tunnel and, thus, increase
tendon friction. In addition, grip strength is dramatically reduced by deviations in the wrist posture.
Figure 11.21 indicates that any deviation from the wrist's neutral position significantly decreases the
grip strength of the hand. This reduction in strength is caused by a change in the length-strength
relationship (Figure 11.5) of the forearm muscles once the wrist is bent. Hence, the muscles are
working at a level that is greater than necessary. This reduced strength potential associated with deviated
wrist positions can, therefore, more easily initiate the sequence of events associated with cumulative
trauma (Figure 11.3). Thus, deviated wrist postures not only increase tendon travel and friction, but
also increase the amount of muscle strength necessary to perform the gripping task.
Second, increased frequency or repetition of the work cycle has been identified as a risk factor for
cumulative trauma disorders (CTD; Silverstein et al., 1996, 1997). Studies have indicated that increased
frequency of wrist motions increases the risk of developing a cumulative trauma disorder. Repeated
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