| Summary: | Repetitive stress injuries are common in the workplace where workers perform
repetitive tasks continuously throughout the day. Muscle fatigue may lead to injury
either directly through muscle damage or indirectly through changes in coordination,
development of muscle imbalances, kinematic and muscle activation variability, and/or
movement instability. To better understand the role of muscle fatigue in changes in
movement parameters, we studied how muscle fatigue and muscle imbalances affected
the control of movement timing, variability, and stability during a repetitive upper
extremity sawing task.
Since muscle fatigue leads to delayed muscle and cognitive response times, we
might expect the ability to maintain movement timing would decline with muscle fatigue.
We compared timing errors pre- and post-fatigue as subjects performed this repetitive
sawing task synchronized with a metronome using standard techniques and a goalequivalent
manifold (GEM) approach. No differences in basic performance parameters
were found. Significant decreases in the temporal correlations of the timing errors and velocities indicated that subjects made more frequent corrections to their movements
post-fatigue.
Muscle fatigue may lead to movement instability through a variety of mechanisms
including delayed muscle response times and muscle imbalances. To measure movement
stability, we must first define a state space that describes the movement. We compared a
variety of different state space definitions and found that state spaces composed of angles
and velocities with little redundant information provide the most consistent results. We
then studied the affect of fatigue on the shoulder flexor muscles and general fatigue of the
arm on movement stability. Subjects were able to maintain stability in spite of muscle
fatigue, shoulder strength imbalance and decreased muscle cocontraction.
Little is known about the time course for adaptations in response to fatigue. We
studied the effect of muscle fatigue on movement coordination, kinematic variability and
movement stability while subjects performed the same sawing task at two work heights.
Increasing the height of the task caused subjects to make more adjustments to their
movement patterns in response to muscle fatigue. Subjects also exhibited some increases
in kinematic variability at the shoulder but no changes in movement stability. These
findings suggest that people alter their kinematic patterns in response to fatigue possibly
to maintain stability at the expense of increased variability. === text
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