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reported occupationally related LBD. Occurrences of reported LBDs were considered regardless of the
amount of restricted or lost time associated with the incident.
The dependent variable in this study consisted of two levels of job-related LBD rates. Low-rate group
jobs were defined at those with at least 3 yr of records showing zero low-back injuries and zero turnover.
Turnover was defined as the average number of employees leaving a job per year. High-rate group jobs
were those associated with at least 12 injuries per 200,000 h of work exposure. (The average rate for this
group was 26.0 injuries per 200,000 h.) The incidence rate for the high LBD group category corresponds
to the 75th percentile value of risk for the 403 jobs examined. Of these jobs, 124 were categorized in the
low injury rate group, and 111 were in the high-rate group. The remaining jobs (totaling 168) were in the
medium injury rate group and were not used in this particular analysis.
The independent variables in this study consisted of workplace, individual, and trunk motion charac-
teristics that were indicative of each job. The workplace and individual characteristics consisted of vari-
ables typically considered in current workplace guidelines for MMH (NIOSH, 1981; Putz-Anderson and
Waters, 1991). Specifically, these variables were: (1) the maximum external horizontal distance of the
load from the spine; (2) the weight of the object lifted; (3) the vertical height of the load at both
the origin and destination of the lift; (4) the frequency of lifting (e.g., lift rate); (5) the asymmetric
angle of the lift (as defined by NIOSH, 1981); (6) 12 measures of employee anthropometry; (7) employee
injury history; (8) employee satisfaction; and (9) trunk motion. Trunk motions were obtained using the
iLMM. These variables consisted of the trunk angular position, velocity, and acceleration characteristics
(i.e., means, ranges, maximums, minimums, etc.) in each of the cardinal planes of movement. Selected
trunk motion factors, along with selected workplace factors, were used to develop a quantitative model of
occupational LBD risk.
49.3.3 Data Collection
Data about employee health, employment history, and anthropometry were collected. Next, the
employee was fitted with the iLMM. A baseline reading from the monitor was then taken as the individ-
ual stood erect and rigid. The employee was asked to return to the job, wearing the iLMM for at least ten
job cycles. Thus, the length of time the employee wore the monitor depended upon the cycle time of the
job. Monitoring of back motions was initiated as the employee began the MMH task and concluded when
the task was completed. Extraneous activities not involving MMH were not monitored. Signals from the
iLMM were sampled at 60 Hz via an analog-to-digital converter and stored on a portable microcom-
puter. The data were further processed in the laboratory, to determine position, velocity, and acceleration
of the trunk for each of the job cycles, as a function of time spent in the sagittal, lateral, and axial twisting
planes of motion.
49.3.4 Analysis
The data were examined initially to determine whether the trunk motions gathered were repeatable. This
analysis indicated that task-to-task variation was much larger than the variability due either to multiple
cycles performed within a task or to different employees performing the same task (Allread et al., 2000).
Hence, trunk motions were dictated largely by the design of the task, not by the individual, and repetitive
task cycles resulted in motions that were fairly similar.
The various personal, environmental, and workplace factors from the database were analyzed using
logistic regression techniques, to determine if any single factor could distinguish jobs associated with
high LBD rates from those with low rates. The most powerful single variable was maximum external
moment, which yielded an odds ratio of 5.17. Overall, however, the odds ratios were low, indicating
that few of the individual variables discriminated well between the two injury rate groups. Of the
trunk motion factors, the velocity variables generally produced greater odds ratios than maximum or
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