Civil Engineering Reference
In-Depth Information
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FIGURE 13.14 Cumulative spine compression forces from the work of Godin et al. (2003) for 2 h of at home
activities. The horizontal bars represent the pain groups' (cases) cumulative compression shift exposure averages
for industrial workers.
work environment resulted in substantial peak and cumulative loading (Godin et al., 2003; Lauder et al.,
2002). In following individuals for a 2-h period around their homes, the magnitude of peak spinal
compression forces exceeded the NIOSH AL in 60% of the subjects (Godin et al., 2003). The cumulative
compression over this same 2-h period was also substantial (Figure 13.14). In fact if the 2 h of exposure is
extrapolated out to represent a shift dose, the values would surpass the cumulative exposure reported for
back pain cases in industrial workers (Kumar, 1990; Norman et al., 1998). Also noteworthy is the varia-
bility of the cumulative loading that was present in the ten subjects. This is not surprising when differ-
ences in activity levels and lifestyles are considered. However, it does raise the interesting question of
whether this loading, when combined with workplace exposure, would explain any of the variance in
which workers report an injury from a group experiencing homogeneous external task demands.
While a shift dose limit will likely prove to be the desirable and most useful standard for workplace ergo-
nomic assessments, a daily exposure may be more insightful into risk of developing an injury or pain
given the significant magnitudes of cumulative loads documented outside of work.
13.8 Challenges of Documenting Cumulative Exposure
When examining peak loading, there is consensus on the factors that lead to the most demanding instant
of a task: greatest horizontal moment arm, lowest vertical height, highest external load, etc. While there
may be some small variance in selecting the instant to examine, the joint modeling approach and
assumptions employed, the calculation of the mechanical loading on the spine for a single instance in
time would have relatively small variance between different observers.
Unlike identifying the peak or highest instance of loading, cumulative loading presents the difficulty of
documenting the variation of the spinal loads with respect to time. This is compounded by both the
approach used to document exposure and the method to transform postural and external force exposures
to spinal kinetics. Given the current state of technology and being unable to directly measure spine forces
in the human body, the criterion approach for assessing spine forces is a biomechanical rigid link model
to calculate net joint moments and reaction forces in conjunction with a suitable lumbar spine joint
model (typically L4
S1) to yield joint compression and shear forces. Many secondary
approaches have been employed such as regression equations, heart rate and EMG to simplify this
process and more importantly reduce the large processing time inherent in the biomechanical modeling
approach. At the heart of addressing this issue is what cumulative loading really represents. To faithfully
document cumulative loading of a single worker, even for a single shift, would require measurement, at a
L5 or L5
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