Civil Engineering Reference
In-Depth Information
this type of work are often minimal. Thus, shoulder position represents a higher priority in the hierarchy
of task needs in this situation. In this situation, ideal neck posture is typically sacrificed in favor of more
favorable shoulder and arm postures. For this reason, heavy work is performed at a height of 70 to 90 cm
above floor level. With the work set at this height, the position wherein the elbows are close to 90
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maxi-
mizes strength (Figure 11.6). In addition, the shoulders are close to 30
of abduction, which minimizes
fatigue. In this situation, the neck is not in an optimal position but the hierarchy logic dictates that the
visual demands of a heavy task would not be substantial and thus the neck would not be flexed for pro-
longed periods of time and, therefore, do not pose much of a risk.
The third work height situation involves light work. Light work is a mix of moderate visual demands
with moderate strength requirements. In this situation, work is a compromise between shoulder position
and visual accommodation and neither visual nor strength demands dominate the hierarchy of work
needs. Thus, the height of the work is set at a height between those of the precision work height level
and the heavy work height level. In this manner, a compromise between the benefits and costs associated
with accommodating the neck versus the shoulder is resolved. This situation dictates that the work is
performed at a level of between 85 and 95 cm off the floor under light work conditions.
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11.3.1.4 The Back
Low back disorders (LBD) have been identified as one of the most common and significant musculos-
keletal problems in the U.S. that results in substantial amounts of morbidity, disability, and economic
loss (Hollbrook et al., 1984; Praemer et al., 1992). LBD are one of the most common reasons for
workers to miss work. Back disorders were responsible for the loss of over 100 million lost workdays
in 1988 with 22 million cases reported that year (Guo, 1993; Guo et al., 1999). Among those under 45
years of age, LBD is the leading cause of activity limitations and can affect upto 47% of workers with
physically demanding jobs (Andersson, 1997). The prevalence of LBD has also been observed to increase
by 2700% since 1980 (Pope, 1993). The costs associated with LBD are significant with health care expen-
ditures incurred by individuals with back pain in the U.S. exceeding $90 billion in 1998 (Luo et al., 2004).
It is clear that the risk of LBD can be associated with industrial work (NRC, 1999, 2001). Thirty percent
of occupation injuries in the U.S. are caused by overexertion, lifting, throwing, holding, carrying,
pushing, and or pulling objects that weigh 50 lb or less. Twenty percent of all workplace injuries and ill-
nesses are back injuries, which account for upto 40% of compensation costs. Estimates of occupational
LBD prevalence vary from 1 to 15% annually depending upon occupation and, over a career, can
seriously affect 56% of workers.
Manual materials handling (MMH) activities, specifically lifting, dominate occupationally related LBD
risk. It has been estimated that lifting and MMH account for upto two-thirds of work-related back injuries
(NRC, 2001). From a biomechanical standpoint, we assume that most serious and costly back pain is disco-
genic in nature and has a mechanical origin (Nachemson, 1975). Studies have found increased degeneration
in the spines of cadaver specimens who had previously been exposed to physically heavy work (Videman,
et al., 1990). This suggests that occupationally related LBDs are closely associated with spine loading.
11.3.1.4.1 Significance of Moments
The most important concept associated with occupationally related LBD risk is that of the external
moments imposed about the spine (Marras et al., 1993, 1995). As with most structures, the loading of
the trunk is influenced greatly by the external moment imposed about the spine. However, because of
the geometric arrangement of the trunk musculature relative to the trunk fulcrum during lifting, very
large loads can be generated by the muscles and imposed upon the spine. Figure 11.18 shows this bio-
mechanical arrangement of lever system. As indicated here, the back musculature is at a severe biome-
chanical disadvantage in many manual materials handling situations. Supporting an external load of
222 N (about 50 lb) at a distance of 1 m from the spine imposes a 222 Nm external moment about
the spine. However, since the spine supporting musculature are at a relatively close proximity relative
to the external load, the trunk musculature must exert extremely large forces (4440 N or 998 lb) to
simply hold the external
load in equilibrium. These internal
loads can be far greater if dynamic
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