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assessment is to quantitatively describe the musculoskeletal loading that occurs during work so that one
can derive an appreciation for the degree of risk associated with an occupationally related task.
The characteristic that distinguishes occupational biomechanics analyses from other types of ergonomic
analyses is that the comparison is quantitative in nature. The quantitative nature of occupational biome-
chanics permits ergonomists to address the question of “how much exposure to the occupational risk
factors is too much exposure?”
The portion of biomechanics dealing with ergonomics issues is often labeled industrial or occupational
biomechanics. Chaffin et al. (1999) have defined occupational biomechanics as “the study of the physical
interaction of workers with their tools, machines, and materials so as to enhance the worker's perform-
ance while minimizing the risk of musculoskeletal disorders.” This chapter will address occupational bio-
mechanical issues exclusively in this ergonomics framework.
11.1.2 Occupational Biomechanics Approach
The approach to biomechanical assessment is to characterize the human-work system situation through
a mathematical representation or model. The idea behind such models is to represent the various under-
lying biomechanical concepts through a series of rules or equations in a “system” or model that helps us
understand how the human would be affected by exposure to work. One can think of a biomechanical
model as the “glue” that holds our logic together when considering the various factors that would affect
risk in a specific work situation.
An advantage of representing the worker in a biomechanical model is that the model permits one to
quantitatively consider the trade-offs associated with risk to various parts of the body in the design of a
workplace. When one considers biomechanical rationale, one finds that it is difficult to accommodate all
parts of the body in an ideal biomechanical environment. It is often the case that in attempting to accom-
modate one part of the body, the biomechanical situation at another body site is compromised. There-
fore, the key to the proper application of biomechanical principles is to consider the appropriate
biomechanical trade-offs associated with various parts of the body as a function of the work requirements
and the various workplace design options. For this reason, this chapter will focus upon the information
required to develop proper biomechanical reasoning when considering a workplace. The chapter will first
present and explain a series of key concepts that constitute the underpinning of biomechanical reasoning.
Next, these concepts will be applied to different parts of the body. Once this reasoning is developed an
attempt will be made to examine how the various biomechanical concepts must be considered collec-
tively in terms of trade-off, when designing a workplace from an ergonomic perspective under realistic
conditions. This chapter will demonstrate that one cannot successfully practice ergonomics by simply
memorizing a set of “ergonomic rules” (e.g., keep the wrist straight or don't bend from the waist
when lifting). These types of rule-based design strategies ultimately result in sub-optimizing the work-
place ergonomic conditions.
11.2 Biomechanical Concepts
11.2.1 The Load — Tolerance Construct
The fundamental concept in the application of occupational biomechanics to ergonomics is that one
should design workplaces so that the load imposed upon a structure does not exceed the tolerance of
the structure. This basic concept is illustrated in Figure 11.1. The figure illustrates the traditional
concept of biomechanical risk in occupational biomechanics (McGill, 1997). A loading pattern is devel-
oped on a body structure that is repeated as the work cycles recur during a job. The structure tolerance is
also shown in this figure. If the magnitude of the load imposed on a structure is far less than tissue tol-
erance, then the task is considered safe and the magnitude of the difference between the load and the
tolerance is considered the safety margin. Also implicit in this figure, is the idea that risk occurs when
the imposed load exceeds the tissue tolerance. While tissue tolerance is defined as the force that
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