Biomechanical Basis for Ergonomics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 250

F

5 cm 1 m

222 N

Internal Muscle Force (F)
F • 5 cm = 222 N • 1 m
F = 222 Nm
0.05 m

F = 4,440 N (998 lbs)

FIGURE 11.18 Internal muscle force required to counterbalance an external load during lifting.

motion of the body is considered (since force is a product of mass and acceleration). Thus, the most
important concept to consider in workplace design from a back protection standpoint is to keep the
moment arm at a minimum.

11.3.1.4.2 Lifting Style
The external moment concept has major implications for lifting styles or the best “way” to lift. Since the
externally applied moment significantly influences the internal loading, the lifting style is of far less
concern compared to the magnitude of the applied moment. Some have suggested that proper lifting
involves lifting by “using the legs” as opposed to “stoop” lifting (bending from the waist). However,
spine loading has also been found to be a function of anthropometry as well as lifting style. Biomecha-
nical analyses (Park and Chaffin, 1974; van Dieen et al., 1999) have demonstrated that no one lift style is
correct for all body types. For this reason the National Institute of Occupational Safety and Health
(NIOSH, 1981) has concluded that liftstyle need not be a consideration when assessing the risk of occu-
pationally related LBD. Some have suggested that the internal moment of the trunk has a greater mech-
anical advantage when lumbar lordosis is preserved during the lift (NIOSH, 1981; Anderson et al., 1985;
McGill et al., 2000; McGill, 2002a,b). Thus, from a biomechanical standpoint, the primary indicator of
spine loading and, thus, the correct lifting style is whatever style permits the worker to bring the center of
mass of the load as close to the spine as possible.

11.3.1.4.3 Seated vs Standing Workplaces
Seated workplaces have become more prominent of late, especially with the aging of the workforce and
the introduction of service-oriented and data processing jobs. It has been well documented that loads on
the lumbar spine are always greater when one is seated compared to a standing posture (Andersson et al.,
1975). This is due to the tendency for the posterior (bony) elements of the spine to form an active load
path when one is standing. When seated, these elements are disengaged and more of the load passes
through the intervertebral disc. Thus, work performed in a seated position puts the worker at greater
risk of loading and therefore damaging the disc. Given this situation, it is important to consider the
design features of a chair since it may be possible to influence disc loading through chair design.
Figure 11.19 shows the results of pressure measurements made in the intervetebral disc of workers as