ERGONOMICS OF MANUAL MATERIALS HANDLING - Environmentally Conscious Materials Handling - page 33

3. Anthropometry . The selection of workers based on anthropometry (espe-
cially height or weight) is not justified.
4. Lift technique . No controlled epidemiological studies validate any specific
lifting techniques.
5. Attitude . There is no clear evidence relating attitude to injury risk.
6. Training . This is generally accepted as positive, although epidemiological
support is lacking.
7. Strength . There is epidemiological support that risk is increased when
strength capacity is less than job demand.
NIOSH (1981, p. 22) concluded that “heavy load lifting contributes to

increased frequency and severity rates for low back pain.”

2.1.2 Biomechanical Approach
Tasks that involve relatively heavy loads handled relatively infrequently are gen-
erally considered to be biomechanically limiting. The biomechanical approach
examines the body as a system of levers (bones) and forces (muscles) and exam-
ines the static and dynamic forces on body segments and joints as the worker
performs manual materials handling tasks. One of the issues faced in the biome-
chanical approach involves determining the limitations of the body segments and
tissues when subjected to loading.
Probably the system receiving the most attention has been the lower back
area, specifically the L5/S1 area and its corresponding disc. Obviously, the only
source of data relating to compressive disc failure has been that obtained from
cadaver studies. The large variability of disc failures, the validity of cadaver
data, and the limited samples have made it difficult to establish biomechanical
failure points for the lower back. The NIOSH (1981, p. 36) conclusion regarding
biomechanical limitations was, “Jobs that place more than 650 kg compressive
force on the low-back are hazardous to all but the healthiest of workers. In
terms of a specification for design, a much lower level of 350 kg or lower should
be viewed as an upper limit.” As a side note, the 1981 NIOSH guide expressed
forces in kilograms, when the proper units for forces should have been newtons, a
change that was recognized and corrected in later NIOSH publications regarding
manual materials handling recommendations.
The 1981 NIOSH guide listed six conclusions for the biomechanics of lifting:
1. Lifting a 5 kg compact load (wherein the mass CG of the load is within
50 cm of the ankles) could create compressive forces sufficient to cause
damage to older lumbar vertebral disks.
2. As the load mass center of gravity is moved horizontally away from
the body, a proportional increase in the compressive force on the lower
back is created. Thus, even light loads need to be handled close to
the body.