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Introduction: Recent epidemiological papers suggest
that conscious control of the neuromuscular system may lessen
the risk of sustaining a low back disorder for manual materials
handlers. Sudden unexpected loading of the back appears to cause
the neuromuscular systems to respond in a manner significantly
different from sudden but expected load handling. This study
is concerned primarily with the activity of the trunk muscles.
It may be the case that during unexpected conditions (accidents)
the trunk muscles contract to a maximum regardless of the magnitude
of the external load. The objective of this study was to determine
the degree to which the spine is overloaded during unexpected
loading in order to quantify this over response.
Method: This study recruited twelve male subjects,
ages twenty-one to thirty-two years, in good physical shape without
any history of lower back disorder. Independent variables consisted
of the expectation condition and the weight of the load. Under
each condition, expected or unexpected loading, a load was suddenly
imposed upon the subject by dropping a weight into a box which
the subject was holding. The subject was allowed to view the
dropping of the weight under the expected condition. Under the
unexpected condition the subject was not allowed to see or hear
the weight being dropped.
The dependent variables consisted of the electromyolgraphic (EMG)
activity of the trunk muscles. Prior studies have shown that
the latissimus dorsi, erector spinae and rectus abdominus muscles
play an active role in supporting the trunk during lifting and
are also primarily responsible for loading the trunk.
Results: Several components of muscle force were evaluated
in this experiment. Two-thirds of the muscles exhibited different
responses between expected and unexpected conditions, and in
each the unexpected condition resulted in greater relative muscle
force. The unexpected condition produced muscle force increases
on average (over static conditions) of 2.7 times greater than
in the expected conditions. Fifty percent of the muscles also
demonstrated greater muscle force as the weight condition increased.
Peak trunk muscle activity was evaluated. Peak muscle activity
during unexpected conditions exceeded the expected condition
by an average of 1.7 times. Eighty-three percent of the muscles
showed significant peak activity as the weight condition increased.
The rate of onset of muscular force development was also evaluated.
The onset rate indicates how rapidly the trunk is loaded during
exertion and may be related to the risk of back injury. Greater
rates of onset were observed during the unexpected condition.
The duration of force required to control the load was investigated.
Under the unexpected condition the muscle force duration increased
by an average of 12%. The duration of the muscle force also increased
significantly when the weight of the load increased.
Discussion: Results stress the importance of dynamic
interpretations in biomechanical models. Static models used for
occupational workplace design consider only the mean muscle force
activity, it appears that such models may underestimate the loading
due to dynamic events. Peak components of muscular force were
significantly affected by all independent variables in this experiment.
Trunk muscles over react during unexpected loading by a factor
of two compared with expected loading. The rate of muscular force
onset is exaggerated in an unexpected exertion which may lead
to a particularly dangerous situation since greater trunk forces
are occurring very rapidly.
Study data indicate that differences to expected and unexpected
loading are quantifiable over a small range of loading weights,
[5, 10 and 20lb]. Future research using weights throughout the
range of safe materials handling guidelines could lead to guidelines
for safe loading limits applicable to work situations where unexpected
loading is common. This information may also provide a useful
starting point for a biomechanical model which describes the
cognitive-biasing effects of expectation upon the musculo-skeletal
system. Refinements of such a model could quantify the risks
associated with unexpected events such as sudden load handling,
slips and falls.
Quote: "Peak components of muscular force were
significantly affected independently by both expectation and
weight for all muscle reactions compared. This comparison indicates
that the effect of unexpected loading makes the muscles respond
as if the weight of an expected load were doubled."
Published in Ergonomics, 1987, Vol. 30, No. 3, 551-562.
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