Civil Engineering Reference
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multiple body parts, the time required to perform instrumented analyses on each subject may limit their
applicability to epidemiologic research (Kilbom, 1994). Another practical concern is the potential
invasiveness that may interfere with job performance, alter work practices, or reduce worker cooperation.
Thus, there is a trade-off between the precision of bioinstrumentation and the time efficiency and flexi-
bility of visual observation and worker self-report. As discussed in Chapter 6, gross categorical exposure
measures (e.g.,
10 kg) used in epidemiologic studies may limit the possibility of observing
an exposure-risk relationship; a continuous measure based on bioinstrumentation might make such a
relationship more apparent. Thus, their high accuracy (for the period of measurement) gives these
methods utility for validating other methods on population subsets and added value when they can
be applied in epidemiologic studies.
A large number of observational methods for ergonomic job analysis have been proposed in the last
two decades (see Kilbom, 1994). These include checklists and similar qualitative approaches to identify
peak stressors (e.g., Keyserling et al., 1993; Stetson et al., 1991). The limitation with checklists is that they
provide little information beyond the presence or absence of an exposure, with a possibly curde estimate
of the exposure duration. The qualitative approaches are not likely to provide sufficient detail to effec-
tively assess exposure for epidemiologic studies.
The most common observational techniques used to characterize ergonomic exposures are based on
either time study or work sampling. Both of these techniques require a trained observer to characterize
the ergonomic stressors. Methods based on time study (e.g., Armstrong et al., 1982; Keyserling, 1986) are
usually used to create a continuous or semi-continuous description of posture and, occasionally, force
level. Therefore, changes in the exposure level, as well as the proportion of time a worker is at a given
level, may be estimated. Because methods based on time study tend to be very time intensive, they are
better suited to work with fairly short and easily definable work cycles. A different approach, work
sampling, involves observation of worker(s) at either random or fixed, usually infrequent, time intervals
and is more appropriate for nonrepetitive work (e.g., Karhu et al., 1977; Buchholz et al., 1996). Obser-
vations during work sampling provide estimates of the proportion of time that workers are exposed to
various stressors, although the sequence of events is lost. Though less time intensive than time study,
work sampling still requires too much time for use in an epidemiologic study, especially one that
employs individual measures of exposure.
There are also a few highly detailed, easily used observational analyses for use as an exposure assess-
ment tool in an epidemiologic study. These methods employ subjective ratings made by expert observers.
For example, Rodgers (1988, 1992) has developed methods based on physiological limits of exposure that
rate effort level, duration, and frequency. The method developed by Moore and Garg (1995) employs
ratings similar to those of Rodgers and adds posture and speed of work ratings. Moore and Garg's
strain index is designed to estimate strain for the distal upper extremity. It is the weighted product of
six factors placed on a common five-point scale (subjective ratings of force, hand
10 kg vs.
.
,
wrist posture, and
speed of work and measurement of duration of exertion, frequency of exertion, and duration of task
per day). The strain index is a single priority score designed to represent risk for upper extremity mus-
culoskeletal disorders and is conceptually similar to the lift index for low back disorders. The lift index
was developed as part of the revised NIOSH lifting equation (Waters et al., 1993) and is the ratio of the
load lifted and the recommended weight limit.
Recently, Latko et al. (1997) developed a method employing visual analog scales for expert rating of
hand activity level (called HAL). The method has also been generalized to assess other physical stressors,
including force, posture, and contact stress (Latko et al., 1997, 1999). The HAL employs five verbal
anchors, so that observers can rate the stressors reliably. In an evaluation, a team of expert observers
comes to a consensus on ratings for individual jobs. These ratings correlated well with two quantitative
measures, recovery time
/
second, and are found to be reliable when compared with
ratings of the same jobs 1.5-2 yr later (Latko et al., 1997).
In sum, there are many methods for assessment of ergonomic exposures. The challenge for ergo-
nomists and epidemiologists is to determine a method for characterizing the level of exposure that is
efficient enough to permit analysis of intersubject and intrasubject variability across hundreds of subjects
cycle and exertions
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