Rehabilitating Low Back Disorders - Fundamentals and Assessment Tools for Occupational Ergonomics - page 454

cadavers. 15 This paragraph suggests that personal work technique, or more specifically, spine motion
can effect the risk of spine damage. Recent work by Yingling et al. 46 on pig spines has shown that load
rate is not a major modulator of shear tolerance unless the load is very ballistic such as what might
occur during a slip and fall. Summarizing the lumbar sagittal motion and shear issue, evidence
from tissue-specific injury studies generally supports the notion of avoiding full lumbar flexion
when performing loading tasks to minimize the risk of low back injury. There is no evidence to
support a conscious effort to perform “pelvic tilts” (i.e., hyperlordosis or lumbar flexion) during
lifting or exertion.
While twisting has been named in several studies as a risk factor for low back injury, the literature
appears confused by not making the distinction between the kinematic variable of twisting and the
kinetic variable of generating twisting torque. While many epidemiological surveillance studies link a
higher risk of LBD with twisting, twisting with low twist moment results in a relatively low muscle
activity and correspondingly low spine load. 26,27 Further, passive tissue loading is not substantial until
the end of the twist range of motion. 17 However, developing twisting moment places very large compres-
sive loads on the spine due to the enormous coactivation of the spine musculature 27 and this can occur
when the spine is not twisted, but in a neutral posture where the ability to tolerate loads is higher. It
would appear that either single variable (the kinematic act of twisting or generating the kinetic variable
of twist torque while not twisting) is less dangerous than may be suggested by epidemiological surveys.
However, it would appear that elevated risk from very high tissue loading occurs when the spine is fully
twisted at the same time where there is a need to generate high twisting torque. 27
There are many personal factors, which appear to affect spine tissue tolerance, for example, age and
gender. Jager et al. 22 compiled the available literature that passed their inclusion criteria on the tolerance
of lumbar motion units to bear compressive load. Their results revealed that if males and females are
matched for age, females were able to sustain only approximately two-thirds of the compressive loads
of males. Furthermore, Jager et al.'s data showed that within a given gender, the 60-yr-old spine was
able to tolerate only about two-thirds of that tolerated by a 20-yr-old. There are other personal
factors such as motor control system fitness where it appears that a motor control error can lead to a
back injury during very benign tasks such as picking up a pencil from the floor. (This will be explained
in a subsequent section.)
Many factors appear to modulate the risk of specific low back tissues damage other than load magni-
tude and loading mode. While disc herniations have been produced under controlled conditions, 19
Callaghan and McGill 9 have been able to consistently produce disc herniations by mimicking spine
motion and load patterns seen in workers and in replicating the motion and loads of some lumber exten-
sion exercise machines. Specifically, it appears that only a very modest amount of spine compression
force is required (only 800-1000 N) but the spine specimen must be repeatedly flexed — mimicking
repeated torso-spine flexion from continual bending to a fully flexed posture. In these experiments,
the progressive tracking of disc nucleus material travelling posteriorly through the annulus of the disc
was documented with sequestration of the nucleus material around 18,000-25,000 cycles of flexion
(fewer cycles were required for herniation with higher simultaneous compressive loads). This study
included the utilization of a pig degenerative trauma model, which on one hand was an animal model
but on the other, control over age, diet, physical activity provided a unique opportunity. Spines and
discs obtained from humans are typically older and have lost sufficient disc hydration to match the
hydration levels, and potential for herniation, seen in the age groups of workers at risk for this specific
type of event (typically 30-50-yr-olds). But the important point here is that herniation appears to be
more strongly linked to repeated flexion motion rather than load.
Another modulator for tissue damage appears to be the posture of the joint resulting from the curva-
ture of the spine in vivo. For example, Adams et al. 1 showed that a fully flexed spine is weaker than the one
that is moderately flexed. In a most recent study, Gunning and McGill 20 have shown that a fully flexed
spine (using a controlled porcine spine model) is 20 to 40% weaker than if it were in a neutral posture,
and that hydration levels matched to the changes seen in peoples' discs throughout the work day also
modulate the tolerance. For example, the spinal discs are more easily damaged first thing in the