ERGONOMICS OF MANUAL MATERIALS HANDLING - Environmentally Conscious Materials Handling - page 35

guide was that “33 percent of aerobic capacity will be assumed for 8-hour work
duration.”
The 1981 NIOSH guide arrived at the following physiological design criteria:
1. For occasional lifting (for one hour or less) metabolic energy expenditure
rates should not exceed 9 Kcal/min for physically fit males or 6.5 Kcal/min
for physically fit females.
2. Likewise, continuous (8-hour) limits should not exceed 33 percent of aer-
obic capacity or 5 Kcal/min and 3 Kcal/min, respectively. These guideline
limits do not reflect the increased metabolic rates, which would be asso-
ciated with overweight or deconditioned workers.
3. Personal attributes of age, gender, body weight, and so on are insufficient
to accurately predict work capacity for any particular individual, although
such data are sufficient for making predictions of group averages.
4. The primary task variables which influence metabolic rate during lifting
are: (a) load handled, (b) vertical location at beginning of lift, (c) vertical
travel distance, and (d) frequency of lift.
2.1.4 Psychophysical Approach
Psychophysics is based on a research methodology that states that the body
integrates stresses (physical, mental, and environmental) and responds with an
output that reflects the worker's “acceptable level of response to the stress.”
Typically, worker capacity data are collected through experimentation in which
the worker determines his or her maximum acceptable response to a set of task
conditions. For example, in determining the psychophysical lifting capacity of
an individual, that individual would adjust weights in a container until the maxi-
mum amount of weight that could be handled under the given task conditions is
determined. The procedure attempts to eliminate factors such as visual clues by
placing false bottoms in the container, with various amounts of weights hidden
in the false bottom to eliminate a visual clue as to how much weight the person
was selecting. Also, individuals are given no feedback regarding their weight
selection or performance and are tested in isolation in an attempt to eliminate
competitive factors. If conducted properly, researchers can usually expect people
to give responses within 10 percent variability from day to day. Psychophysical
responses are often summarized as levels acceptable to various percentages of
the population. For example, a 30-pound load lifted once a minute from floor to
knuckle height might be determined to be acceptable to 25 percent of the male
population and less than 5 percent of the female population. An excellent sum-
mary of psychophysical materials handling capabilities can be found in Snook
(1978) and Ayoub et al. (1978).
Some of the psychophysical approach conclusions of the 1981 NIOSH guide
include these three: