Psychophysical Approach to Task Analysis - Fundamentals and Assessment Tools for Occupational Ergonomics - page 918

limited headroom, asymmetrical lifting, load asymmetry, couplings, load placement clearance, and heat
stress.
Mital 28 found that psychophysical data collected in short periods (i.e., 20-25 min) assuming a longer
work period (8-12 h) should be reduced. Subsequently, Mital 22 presented psychophysical data for males
and females performing lifting tasks for eight-hour work shifts based on the adjustments determined in
the earlier study. The data were collected from 37 males and 37 females experienced in manual lifting.
Mital 22 also presented a modified database representing the combined data from his study, Snook's 1
data, and data collected by Ayoub et al. 21 Although the modified database only accommodates lifting
tasks, the combined sample size is considerable. Similarly, Mital 23 presented a psychophysical database
for lifting tasks for males and females working 12-h shifts. The database represents values valid for 12 h
based on adjustments of 8-h data.

47.2.1.1.2 Data for Nonstandard MMH Tasks
One advantage of the psychophysical approach is that it allows for the realistic simulation of many types
of materials handling tasks. Several such examples will be provided to illustrate how psychophysics has
been used to develop guidelines for specific applications.
Smith et al. 29 presented a psychophysical database for evaluating MMH tasks performed in unusual
postures. Maximum acceptable weight data for 99 different tasks were presented, including data for
one- and two-handed lifting and lowering tasks performed in postures such as lifting on one knee,
lifting on two knees, lifting while lying down, etc. These data are particularly appropriate for occasional
maintenance tasks which impose postural constraints on the operator.
Like maintenance, mining is comprised of many activities for which standard psychophysical data
are not applicable to. Mining tasks are often performed under postural constraints, such as limited head-
room. Gallagher 30 collected psychophysical data to address tasks performed under restricted head-
room conditions and provided guidelines for tasks performed in low-seam coal mines requiring
lifting while kneeling. Mining also requires handling of a variety of materials. Gallagher and
Hamrick 31 provided psychophysical guidelines for the handling of rock dust bags, ventilation stopping
blocks, and crib blocks.

47.2.1.1.3 Data for Return to Work from Low-Back Disability
Snook 3 suggested that a future direction for psychophysical research is including symptomatic
subjects. Previous research and databases typically reflect values generated by healthy subjects, in
some cases with no history of musculoskeletal disorders. Since low-back pain will never be completely
prevented, data that indicate the loads and forces that can be handled by persons with musculoskeletal
disorders can be used to prevent the associated disability. This information could be used to design
workplaces that allow workers to work at sufficient levels of productivity in spite of low-back pain or
similar disorders.

47.2.1.2 Assessment of Multiple Component MMH Tasks
More often than not, workers that perform MMH tasks perform a number of tasks, often in sequence.
For example, a common combination task in industry is where a worker lifts material, carries it for some
distance, then lowers the material. In such situations, all of the tasks should be evaluated.
Snook and Ciriello 20 recommend using the weight or force limit for the task with lowest percentage of
the population accommodated as the design criterion for multiple component tasks. This recommen-
dation was based upon the findings of Ciriello et al. 18,19 Thus, for a combination where the worker
lifts materials, carries materials, and then lowers material, the limiting component task would be the
lift. Snook and Ciriello 20 do caution that this method of analysis may result in violation of recommended
energy expenditure criteria for some multiple component tasks.
Straker et al. 32 disagree with the multiple component methodology described in the preceding para-
graph, stating that “combination tasks should probably be assessed as whole entities and not separated
into components for analysis.” However, Straker et al. 32 recommend no alternative methodology to