Civil Engineering Reference
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Training work sessions in a particular wrist position should be made after the wrist neutral position
has been determined. Owing to the fact that static load is an important factor for musculoskeletal dis-
orders development, even after the safe margins for wrist deviation are known (neutral zone), wrist pos-
itions should be alternated within its limits. Serina et al. (1999), studying the typists' posture when using
a standard flat QWERTY keyboard, noted that typing on a keyboard in an adjusted workstation, forces
the users to spend about 75% of the working time with the wrist in greater than 15
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extension and 28%
and 9% of their time with a wrist extension greater than 30
for the left and right hand, respectively.
Ergonomic assessment of hazardous postures should precede the design and introduction of alternate
keyboards. Otherwise, elevated CTS prevalence and complaints will follow.
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39.4.1.1 Wrist Extreme Postures
CTP, an important factor for CTS' pathogenesis (Phalen and Kendrick, 1957; Szabo, 1989a, b; Seradge
et al., 1995; Keir et al.,1998, 1999), is lowest when wrist is in neutral position, hand is relaxed with
fingers flexed at 30
and forearm in a semipronated position (Werner et al., 1997). The aforementioned
postures are rarely reached during a typing task. The typing posture is usually that in which the arm is
abducted and pronated, wrist extended, ulnar deviated and fingers extended in order to fit the keyboard.
All these working positions determine an elevated CTP (Werner et al., 1997).
Wrist extension has a greater effect than ulnar deviation on carpal tunnel pressure (Marklin et al.,
1999). During typing the hand and wrist adopt awkward postures that increased CTP exceeding the
upper safe limit. CTP is increased by the following factors: movement of lumbrical muscles into the
carpal tunnel, changes in cross-sectional area (affected by wrist position), and folding of skin at
the distal palm. Werner and Armstrong (1997) noted that wrist extension stretches flexor tendons and
median nerve, exerting pressure on their dorsal face. During wrist flexion, the flexor digitorum
tendons are pushed against the palmar side of the carpal tunnel, causing pressure on both the tendons
and the flexor retinaculum. Owing to its close relationship with flexor retinaculum and the flexor
tendons, the pressure exerted on the median nerve will rise (De Krom et al., 1990; Szabo, 1998). Overload
of the flexor muscles due to lack of rest, leads to an imbalance between flexor and extensor muscles
causing elevated pressure on the palmar surface of the carpal tunnel (Ostrem, 1995). This increased
pressure exaggerates the already existing elevated CTP, exposing the tissues to greater risk. When com-
pared to wrist flexion, wrist extension is encountered more often during data entry tasks. It causes the
tendons to be displaced against the dorsal side of the carpal tunnel and the head of the radius, leading
to high pressure on the tendons. When the wrist adopts extreme postures, the resultant high pressure
leads to in endoneurial edema and microscopic pathological changes (Cullum and Molloy, 1994).
The flexion of the fingers will lead to an increase in the CTP (Keir et al., 1998). Finger flexion is very
important for CTP. The fingers are constantly fitted to the keyboard during typing. Straight postures and
elevates CTP compared with the relaxed finger posture (Keir et al., 1998) are present. Besides the presence
of elevated CTP in patients with CTS, Szabo (1989b) noted that postexercise, the increased CTP inertia in
CTS patients leads to elevated risk of nerve injury. Werner et al. (1983) and Braun (1988) noted the same
comportment for CTP, indicating elevated CTP and increased sensory impairment in patients with CTP
postactive motion of the wrist.
The CTP is not uniformly distributed in the carpal tunnel. It is higher in the distal portion of the carpal
tunnel and that is why the sensory conduction velocity action potential amplitude is affected more in this
portion (Keir et al., 1998).
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39.4.1.2 Repetition
Extreme postures and high-repetitive actions (38-40 per minute per finger) are frequently required
during computer tasks. This value exceeds the highest acceptable frequency in a repetitive motion (fre-
quency of 30 per minute) (Bergamasco et al., 1998). Cumulative load is a risk factor for causation of mus-
culoskeletal injuries (Kumar, 1990, 2001). The adjacent tendons are sliding one against the other with the
friction force being proportional to the tension in the tendon and inversely proportional to the radius
curvature (Hadler, 1987). Serina et al. (1999) noted that velocities during typing in flexion
extension
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