Biomechanical Basis for Ergonomics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 263

POSTURAL DATA FROM
PHOTOGRAPH,

VIDEO STOPPED-FRAME,
OR FILM

− 81 °

25 °
125 °

− 51 °

V ORIGIN
38 cm

78 ° − 90 °

200N

H ORIGIN
53 cm

FIGURE 11.30 The 2D-static strength prediction model. (Adapted from Chaffin, D.B. and Andersson, G.B.,
Occupational Biomechanics, John Wiley & Sons, Inc., New York, 1991. With permission.)

11.4.5 Multiple Muscle System Models
One of the significant simplifying assumptions inherent in most static models is that the coactivation
of the trunk musculature during a lift is negligible. The trunk is truly a multiple muscle system with
many major muscle groups supporting and loading the spine (Schultz and Andersson, 1981). This can
be seen in the cross-section of the trunk shown in Figure 11.31. Studies have shown that there is sig-
nificant coactivation occurring in many of the major muscle groups in the trunk during realistic
dynamic lifting (Marras and Mirka, 1993). This coactivation is important because all the trunk
muscles have the ability to load the spine since antagonist muscles can oppose each other during occu-
pational tasks and increase the total load on the spine. Thus, assumptions regarding single-equivalent
muscles within the trunk can lead to erroneous conclusions about spine loading during a task. Studies
have indicated that ignoring the coactivation of the trunk muscles during dynamic lifting can misre-
present spine loading by 45 to 70% (Granata and Marras, 1995a; Thelen et al., 1995). In an effort to
more accurately estimate the loads on the lumbar spine especially under complex, changing (dynamic)
postures multiple muscle system models of the trunk have been developed. Much of the recent
research has been focused upon predicting how the multiple trunk muscles coactivate during
dynamic lifting.

11.4.5.1 EMG-Assisted Multiple Muscle System Models
People recruit their muscles in various manners when moving dynamically. For example, when moving
slowly the agonist muscle may dominate the muscles activities during a lift. However, when moving
cautiously, asymmetrically, or rapidly there may be a great deal of antagonistic coactivation present.
During occupational lifting tasks these latter dynamic conditions are typically the rule rather
than the exception during lifting. As line speeds increase, highly dynamic motions are becoming
more common and it is becoming more important to understand the role of muscle coactivation