ERGONOMICS OF MANUAL MATERIALS HANDLING - Environmentally Conscious Materials Handling - page 34

3. When a load is lifted from the floor, additional stresses are exerted on the
lower back due to the body weight moment when stooping to pick up the
load. Thus, heavy loads should not be stored on the floor, but should be
raised to about standing knuckle height (minimum 50 cm) to avoid the
necessity of stooping over and lifting.
4. The postures used to lift loads from the floor can exert a complex and
relatively unknown effect on the stressors of the lower back during lifting.
Specific instructions as to the safe lifting posture to use will be necessarily
complex, reflecting such factors as leg strengths, load, and load size.
Until such complexities are better researched, it is recommended that
instructions as to lifting postures be avoided.
5. Lifting loads asymmetrically (by one hand or at the side with the torso
twisted) can impart complex and potentially hazardous stresses to the
lumbar column. Such acts should be avoided by instructions and work-
place layouts, which permit the worker to address the load in a symmetric
manner.
6. The dynamic forces imparted by rapid jerking motions can multiply a
load's effect greatly. Instructions to handle even moderate loads in a
smooth and deliberate manner are recommended.

2.1.3 Physiological Approach
Tasks that involve relatively lighter loads handled relatively frequently are gen-
erally considered to be physiologically limiting. The physiological approach gen-
erally utilizes energy expenditure, oxygen consumption, or heart rate to describe
the demands of the materials handling task. However, when examining physio-
logical fatigue, both static as well as dynamic tasks must be considered. Static
(or isometric) tasks might be considered to be infrequently occurring in manual
materials handling until it is recognized that activities such as carrying, holding,
or maintaining fixed postures have significant static components to them. For
static tasks, endurance limitations can be as low as a few (3 to 5) seconds for
efforts demanding 100 percent of a worker's maximum voluntary contraction
(MVC) or strength, to very long endurance times for tasks demanding less than
15 percent of MVC.
For dynamic activities where relatively large muscle masses are utilized, physi-
ological demand can be represented by energy expenditure, oxygen consumption,
or heart rate. Generally, a linear relationship has been assumed between heart rate
and oxygen uptake ( VO 2 ) . Assuming that maximum heart rate can be approx-
imated by HR max Age (in years), a worker's physiological capacity

( VO 2max ) can be determined through submaximal testing and determining the
linear relationship between heart rate ( HR ) and oxygen consumption ( VO 2 ) . Tech-
niques for determining VO 2max can be found in references such as Tayyari and
Smith (1997). A conclusion of the physiological approach in the NIOSH (1981)

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