Application of Ergonomics to the Low Back - Fundamentals and Assessment Tools for Occupational Ergonomics - page 575

is linked to actual injuries (Marras et al., 1993, 1995, 2000a). The LMM has also been validated with
respect to posture in three dimensions (Marras et al., 1992). Thus, the LMM is a potentially powerful
tool with respect to measuring the three-dimensional posture of the trunk but requires significant exper-
tise and monetary resources.
28.5 Assessment of Trunk Motion
Another important risk factor that is related to trunk posture is trunk motion — the rate of change in
trunk position or velocity. Three-dimensional trunk velocities may be important risk factors in highly
dynamic work conditions, even more than just posture itself. Two major research initiatives have
found trunk velocities to be significant risk factors of low back injuries and pain (Marras et al., 1993,
1995; Norman et al., 1998). The general findings have been that complex motion (e.g., motion in mul-
tiple planes) is more detrimental to the low back (Fathallah et al., 1998). The problem with complex
motions is that they increase loads on the spine (Freivalds et al., 1983; Jager and Luttmann, 1989;
Marras and Sommerich, 1991; Granata and Marras, 1995; Davis et al., 1998; Davis and Marras, 2000b).
Generally, the two most viable assessment techniques to evaluate trunk motion are goniometers and
video motion analyses. However, both of these methods require significant resources and expertise. One
potential low-level assessment of trunk velocity might be estimating the total flexion angle by question-
naire or checklist combined with the use of a stopwatch to measure the time it takes to complete the
motion. While by no means would this assessment accurately measure the velocity to the same accuracy
of goniometers or video assessment, this quick and easy method may provide a “ballpark” estimate that
takes into account the dynamics of the task. A major drawback to these less-quantitative methods is that
they have not been adequately developed or validated.
Many exposure measures use a surrogate variable — lift rate or frequency — to account for the
dynamics of the lift. While this factor potentially accounts for trunk motion (in an indirect way), it
fails to account for the influence of other factors such as weight, lift distance, task asymmetry, and
mental demands. As the level of load (weight) being lifted is increased, trunk motions have been
found to decrease (Buseck et al., 1988; Ferguson et al., 1992; Davis and Marras, 2000a,b). Lift rate
also accounts for the number of actual items being lifted, which would be related to the amount of
effort required over a specific time. In other words, lift rate is a composite variable that has limited use-
fulness in accounting for trunk motions. Thus, assessments of trunk motion should rely on more sound
and objective techniques such as video and goniometeric systems.
28.6 Assessment of Other Manual Material Handling Modes
While most low back assessments focus on lifting and lowering, other modes of moving objects may also
be risk factors for low back injuries. Some of these tasks would be pushing, pulling, carrying, and pro-
longed standing. Pushing and pulling have been identified as risk factors in many recent epidemiological
studies (Snook et al., 1978; Garg and Moore, 1992; Hoozemans et al., 1998). Examples of tasks that typi-
cally require pushing or pulling are: moving a cart; transporting patients on a stretcher; using a broom,
mop, rake, or hoe; and moving lift assist devices (e.g., hydraulic lift to move boxes). One potential reason
for increased risk for these two modes of handling may be the nature of the loads on the spine — not as
much compressive force but more shear loading. The actual nature of the force will depend upon
the height of the handle and whether pushing or pulling is being performed (Gagnon et al., 1992;
De Looze et al., 1995; Resnick and Chaffin, 1996).
The typical assessment method for push or pull forces is using a strain gauge. There are usually two
forces that are of interest during these assessments: (1) initial force — force required to start (or accel-
erate) the object in motion; (2) sustained force — force required to keep the object moving (Hoozemans
et al., 1998). Currently, the only method of evaluating these measured forces is to compare it to
maximum acceptable forces that have been established by Snook and associates (Ciriello et al., 1990;