Cumulative Spine Loading - Fundamentals and Assessment Tools for Occupational Ergonomics - page 302

Injury
Tolerance
Value
Force
Tissue Load
Time
FIGURE 13.1 Acute injury scenario where a single load application exceeds the tolerance of a tissue resulting in an
injury.
and injury reporting. In other words the individual methods used by different workers performing the
same job would result in similar risk assessments with many acute tools that only incorporate task
demands (i.e., rate of lifting, height of lift, etc.) or examine maximum demands (postures or forces).
In contrast by examining the full work cycle of a task the individual differences between workers
would be revealed in loading and postural demands both within a single cycle and over an extended
period, typically expressed as shift exposure.
While cumulative loading is a relatively new terminology in spine biomechanics or low back ergo-
nomics it has a relation to cumulative trauma disorders (CTD) of the upper extremity that warrants
a brief examination. CTDs can be traced back in the ergonomics literature to the 1980s. It appears
that Armstrong et al. (1982) were among the first to use the specific terminology in the peer
reviewed literature. While the underlying rationale between CTD and cumulative loading are
similar, the two terms have some very clear differences. CTD is defined as pathology to soft
tissues as a result of exposure to excessively frequent use of biological tissue, compounded by exces-
sive loads, awkward joint positions and inadequate recovery (Stramler, 1993). In contrast cumulative
loading is a direct quantification of the magnitude of loading that a joint experiences over a given
period of time, which is influenced by posture, repetition, duration and force. Evidently, both CTD
and cumulative loading are tied to injury through common risk factors such as force, posture and
rate of repetition or duration. However, the important difference between the two terms is that
cumulative loading is a direct quantification of loading exposure in response to these risk factors
whereas CTD is an injury definition. The origin of the terminology “cumulative loading” in the
spine can be traced back to the 1990s when it first appeared in the research literature (Kumar,
1990).
The ideas of cumulative loading, repetition rate and duration of exposure, while being related, have
very different meanings, which will be brought to light in this chapter. Since the publication of the
phrase “cumulative loading” the idea has been quickly adopted by the industrial community, including
ergonomists, health care providers, workers and management. The most frequent question asked by these
groups is invariably “How much cumulative exposure is too much?” Unfortunately the question is not
readily answerable with the currently available knowledge base.
This chapter will focus exclusively on cumulative loading of the lumbar spine and associated issues.
Evidence will be presented in support of a cumulative injury mechanism and the hurdles that need to
be addressed before cumulative loading will be usable as an injury prevention approach in industry
will be discussed.