Civil Engineering Reference
In-Depth Information
TABLE 50.1 Table with Cell Names
Anatomical
Landmarks
Start Height of
Lift (Use of
anatomical
landmarks from
column one is
preferred)
Close Lifts: origin
,
Near Lifts: origin
30-60 cm from
mid-point between
inner ankle bones
Extend Lifts: origin
30 cm from
mid-point between
inner ankle bones
60-80 cm from
mid-point between
inner ankle bones
From overhead
reach limit to 8 cm
below shoulder
height [Ht] (Axilla)
132-183 cm
Cell A
Cell B
No known safe limit
for repetitive lifting
Knuckle Ht to below
shoulder Ht
.
82 to 132 cm
Cell C
Cell D
Cell E
Middle shin Ht to
knuckle Ht
.
30 to 82 cm
Cell F
Cell G
Cell H
Floor to middle
shin Ht
0-30 cm
Cell I
No known safe limit for
repetitive lifting
No known safe limit
for repetitive lifting
into three zones and were defined relative to the horizontal distance of the object lifted from L5
S1. These
horizontal zones were defined as: (1) close lifts — less than 30 cm from the spine, (2) intermediate lifts —
30-60 cm from the spine, and (3) extended lifts whose origins were between 60 and 80 cm from the spine.
Once the lift origin was defined, the maximum weight of lift was determined when lifting within each
of these zones. In order to arrive at an acceptable weight of lift from each of these 12 zones information
based upon biomechanical, psychophysical, epidemiological, and historical surveillance data were com-
pared so that patterns of risk could be established. Although previous National Institute of Occupational
Safety and Health (NIOSH) efforts have attempted to look at risk consistency between the different
approaches, none have assessed the patterns of risk using modern approaches.
The logic behind this effort assumes that several workplace factors influence the risk of low back dis-
order (LBD). These factors include: (1) exposure to high moments, (2) lifting from extreme postures,
and (3) repetitive lifting. Biomechanical reasoning dictates that risk occurs when an imposed load on
a structure exceeds the tolerance level of the structure (McGill, 1997). The workplace factors acknowl-
edged by the TLV, such as moments, posture, and repetition impose loads on the spine. These loads
are compared to biomechanical, physiological, and psychophysical tolerances. A recent review of the lit-
erature has indicated that the better defined the conditions of a lifting task, the stronger the association
with risk (NRC, 2001). The TLV assessment incorporates the most powerful of these defining factors.
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50.2 TLV Development Background
50.2.1 Spine Load Estimates
Accurate estimation of spinal loads depends on predicting the internal loads (muscle activities and
passive tissue contributions) that are needed to support or counterbalance the external loads (load
that is lifted or moved). The sum of the internal and external forces define spinal loading. Early
approaches paved the way for current methods (Chaffin, 1975; Garg, Chaffin & Frievalds, 1982;
Schultz, Andersson, Ortengren, Haderspeck & Nachemson, 1982a; Schultz, Andersson, Ortengren,
Bjork & Nordin, 1982b; Schultz, Haderspeck, Warwick & Portillo, 1983; Schultz & Andersson, 1981).
These early models predicted internal forces based upon the minimum required activity to balance
the external load. However, these methods often found it difficult to explain the coactivity of the
muscles often observed during realistic dynamic exertions. A method to measure indirectly spinal
loads was to employ biologically assisted models. With these models, one monitors the biological
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