Rehabilitating Low Back Disorders - Fundamentals and Assessment Tools for Occupational Ergonomics - page 452

profiles, resultant spine loads, etc., can be found in the authors' review chapters and original papers listed
at www.ahs.uwaterloo.ca mcgill.html or in my recent textbooks. “Low Back Disorders:

Evidence Based Prevention and Rehabilitation” and “Ultimate Backfitness and Performance.” 35
In most traditional approaches to designing low back exercise, an emphasis has been placed on
the immediate restoration, or enhancement, of spine range of motion and muscle strength. Generally,
this approach has not been sufficiently efficacious in reducing back troubles, in fact a review of the
evidence suggests only a weak link with improving back symptoms while some studies suggest a
link with negative outcome in significant numbers of people. 31 It appears that the emphasis on
early restoration of spine range of motion continues to be driven by legislative definitions of low
back disability — namely loss of range of motion (ROM). Thus, therapeutic success is often judged
on motion restored. Most recent work suggests little correlation between ROM and work versatility
ratings. 39 The underlying theme of this chapter, and in fact topic, reflects the developing philosophy
based on mechanisms of injury and stability — that a spine must first be stable before moments
and forces are produced to enhance performance but to do so in a way that spares the spine from
potentially injurious load. Preliminary field evidence (although not yet definitive) suggests that the
approach has promise.

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20.1 The Injury Process — Tissues Damage

There is a tendency among those reporting or describing the back injury to identify a single specific
event as the cause of the damage, such as lifting a box and twisting. This description of low back
injury is common, particularly among the occupational medical community who are often required

to identify a single event when filling out injury reporting forms. However, relatively few low back
injuries occur from a single event. Rather, the culminating injury event was preceded by a history of
excessive loading, which gradually, but progressively, reduced the level of tolerance to tissue
failure. 30 Thus other scenarios where sub-failure loads can result in injury are probably more import-
ant. For example, the ultimate failure of a tissue (i.e., injury) can result from accumulated trauma
produced by either repeated application of load (and failure from fatigue) or of a sustained load
that is applied for long duration or repetitively applied (and failure from deformation and strain).
Thus, the injury process may not always be associated with loads of high magnitude. Finally, it goes
without saying that loss of mechanical integrity in any load-bearing tissue of the spine will result in
stiffness losses and an increased risk of unstable behavior. Thus, documenting the injury process is a
necessary foundation for understanding, formulating and utilizing the concepts of spine instability
and stability.
While excellent progress has been made in the laboratory documenting specific instabilities in flexion-
extension, lateral bend and axial rotation modes in animal preparations, 38 understanding the injury
process in humans (the cause of back troubles in real life) has perhaps been hampered by the focus
on exposure to a single variable — namely acute, or single maximum exposure to, lumbar compression.
A few studies have suggested that higher levels of compression exposure increased the risk of LBD 24
although the correlation was low. Further some studies show that higher rates of LBDs occur when
levels of lumbar compression are reasonably low. Are there other mechanical variables that modulate
the risk of LBDs?
There are many tissues in the lower back and many different modes of loading that occur when per-
forming work and exercise. Apart from joint compression, joint shear has been shown to be very
important as a metric for injury risk in the study of Norman et al., 37 particularly cumulative shear
over a work day. Shear is an interesting variable because while most studies report reaction shear
(that is the action of gravity and load in the hands to shear forward the ribcage on the pelvis
through the lumbar spine), this is not the form of shear load that is experienced by the lumbar
joints. In a series of work, the Waterloo group 25,29,41 has shown that if the spine maintains a neutral
curvature (the torso is flexed forward about the hips, neither flexing nor extending the spine itself)

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