ERGONOMICS OF MANUAL MATERIALS HANDLING - Environmentally Conscious Materials Handling - page 36

1. The majority of lower back injuries were shown to occur on jobs that were
not “acceptable” to 75 percent of the population. For a female workforce,
this percentile specification should be protective; however, for a male
workforce, the limit would be overly restrictive.
2. For low-frequency lifting, capacities are limited by strength rather than
endurance.
3. Psychophysical criteria are most appropriate for occasional moderate to
higher frequency lifting.

2.2 NIOSH 1981 Recommendations

The committee arrived at recommendations for evaluating manual materials han-
dling tasks and published those recommendations in the Work Practices Guide
for Manual Lifting in 1981. The recommendations were based on the following
set of assumptions or limitations and apply only to lifting tasks:
1. Smooth lifting (no jerking or erratic patterns)
2. Two-handed, symmetric lifting in the sagittal plane (directly in front of
the body; no twisting during lift)
3. Moderate width [e.g., 70 cm (30 inches) or less]
4. Unrestricted lifting posture
5. Good couplings (handles, shoes, floor surfaces)
6. Favorable ambient conditions
It was further stated that the guide does not include any “safety factors”
commonly used by engineers to assure that unpredicted conditions are ac-
commodated.
The recommendations of the 1981 guide resulted in the establishment (calcula-
tion) of two limits: an action limit (AL) and a maximum permissible limit (MPL).
For tasks with demands below the AL, those tasks should represent minimal risk
to workers and should be monitored but not require further action. For those
tasks where task demand exceeded the MPL, significant risks are present and
the tasks should be redesigned to reduce task demand to a more acceptable level
(engineering controls were recommended). Task demands falling between the
AL and MPL indicate some risk and should be modified to be more acceptable
(administrative or engineering controls recommended).
The four research approaches were related to the AL and MPL as follows
(NIOSH 1981):
At the Maximum Permissible Limit (MPL)
• Epidemiological . Musculoskeletal injury incidence and severity rates have
been shown to increase significantly in populations when work is performed
above the MPL.