Civil Engineering Reference
In-Depth Information
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Spine Compression (N)
FIGURE 35.5 Distribution of probability of high-risk group membership values for framing carpenters.
required trunk posture are two important variables to consider when evaluating these work activities.
Further, as stooping period continues, the passive tissues in the lumbar spine lose their stiffness and
change their original dimensions, and micro-damage can occur in these tissues. These responses can
result in the increase of the laxity and range of motion of the lumbar spine, and consequently,
degrade the spinal stability.
A number of injury mechanisms have been proposed to understand the relationship between stooped
postures and low back injury risk. A mechanism of lumbar spine instability has been suggested by in vivo
experiments and biomechanical modeling studies. Cholewicki and McGill (1996) studied the lumbar
spine stability during three-dimensional dynamic tasks and observed that there was a sufficient stability
safety margin during tasks that demand a high muscular effort, whereas lighter tasks had a potential
hazard of spine buckling and the risk increased if passive tissues lose their stiffness, which is the response
of prolonged stooping. Another mechanism for low back disorder related to prolonged stooping has been
recently introduced by Solomonow et al. (2003). These authors examined muscle activity patterns of
multifidus and micro-damage in the L4
L5 supraspinous ligaments of in vivo feline lumbar spine
during 20 min constant creep loading and 7 h recovery period. In the creep loading period, the multi-
fidus showed exponential decrease over time and random spasms, suggesting possible decrease in the
stability of the lumbar spine due to reduced muscle force and the development of inflammation in liga-
ments. The damage and reduced stiffness in the ligaments were not fully recovered even after 24 h. This
indicates that the lumbar spine requires greater activation of multifidus muscles to maintain the stability
and protect the damaged tissues even after a full day's rest. The concern might be that a similar task per-
formed on the following day may cause continued creep deformation and severe tissue damage in the
ligaments because of cumulative exposures to creep loading.
Risks associated with prolonged stooping include decreasing stability in lumbar spine structure,
increasing muscle exertion level, and micro-damage in passive tissues, and these are time-dependent
(stooping time and recovery time) and mainly driven by changes in physical characteristics of the pos-
terior spinal ligaments. Assessment of these risks is quite essential to understand and prevent low back
disorder from work-related prolonged stooping. No risk assessment tool of prolonged stooping has yet
been developed but is the subject of on-going development research in our laboratory. The approach
being pursued is to use finite element analysis (FEA) to develop a time-dependent biomechanical evalu-
ation of the system (Figure 35.6). Mechanical changes of passive tissues in the lumbar spine under creep
loading (prolonged stooping and recovery) can be simulated by modeling the lumbar spine using three-
dimensional FEA technique. Nonlinear and viscoelastic material properties of ligaments and disc com-
ponents have been investigated in in vitro experiments, and those data can be input into the FEA model.
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