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similar pattern with ECR being the most active (9.2-11.1% of MVC), followed by ECU, FCR, and FCU
with normalized average EMG values varying between 7.7-9.3%, 6.9-8.4%, and 4.8-8.5% of MVC,
respectively (Figure 39.2). Hence, a drop of up to 75% in the muscle activity in the neutral zone
when compared to normalized average EMG values in wrist-deviated postures (16.5-38.3% of MVC)
was found. Although not significant, females presented higher %MVC EMG values for all muscles.
While keeping the wrist in ulnar deviation, the maximum activity was observed for ECU (26.9-35.7%
of MVC) and FCU (16.5-29.1% of MVC). ECR was the most active muscle in both wrist radial deviation
(25.5-36.8% of MVC) and extension (29.4-38.3% of MVC). FCR (19.9-26.8% of MVC) and FCU
(18.3-23.6% of MVC) were the most active muscles while the wrist was maintained in flexion. FCR
in wrist radial deviation (19.8-24.2% of MVC) and ECU in wrist extension (17.3-34.1% of MVC)
were the second most active muscles after FCU.
The proven coactivation of wrist muscles was also noted by Hoffman and Strick (1999). This coactiva-
tion included both synergists and antagonist muscles. Since wrist extensors have smaller moment arms
compared to flexors, larger forces will be required by extensors to maintain the wrist posture (Keir et al.,
1996) posing this group of muscles to elevated risk of injury while performing with in wrist flexion
posture. Passive muscle forces in antagonist muscles may further increase the risk. The deviated joints
cause muscle overstretch, thus pose a greater risk for musculoskeletal injury.
EMG activity levels between 8% and 17% of MVC were recorded for muscles acting as secondary
effectors (FCU in extension and radial deviation, FCR in extension and ulnar deviation, ECR in
flexion and ulnar deviation, and ECU in flexion and radial deviation). These levels demonstrate their
concomitant dual role in wrist stabilization and force exertion. Prolonged muscle loading promoted
fatigue. As a result, due to lack of rest, the risk of musculoskeletal injury is increased (Kumar, 2001).
Also, Drury et al. (1985) noted an important increase in EMG at extreme wrist deviations, whereas
the muscle activity for wrist angles between 5
8
radial deviation and 10
8
ulnar deviation was low and
almost constant.
ECR was the most active muscle in both radial deviation (25.5-36.8%) and extension (29.4-38.3%)
making it at risk in activities that require this wrist deviation concomitantly (e.g., use of computer
mouse). The higher prevalence of epicondylitis on the extensor side can be explained by the ECR's
role as wrist stabilizer and primary effector in wrist extension and radial deviation. These values were
obtained in passively deviated wrist postures and any active contractions would require significantly
greater muscle activity, increasing the risk for musculoskeletal injuries even more.
Simultaneous recordings of forearm muscle activity and CTP are needed in order to see if the selected
wrist posture corresponds to the lowest values for both EMG and CTP. Although during rest, forearm
muscle activity and CTP are low, some office tasks require awkward upper extremity postures, signifi-
cantly changing the required muscle activity.
Males
Females
30
30
FCR
FCR
20
20
ECR
ECR
10
10
0
0
Left
Right
Left
Right
FIGURE 39.2 The forearm muscles normalized average EMG (% isometric MVC) for both genders in the self-
selected neutral position (FCU, flexor carpi ulnaris; FCR, flexor carpi radialis; ECR, extensor carpi radialis; ECU,
extensor carpi ulnaris).
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