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Abstract
The objective of this work was to increase our understanding of how motor patterns
are produced during movement tasks by quantifying adaptations in muscle coordination
in response to altered task mechanics. We used pedaling as our movement paradigm because
it is a constrained cyclical movement that allows for a controlled investigation of
test conditions such as movement speed and effort. Altered task mechanics were introduced
using an elliptical chainring. The kinematics of the crank were changed from a relatively
constant angular velocity using a circular chainring to a widely varying angular velocity
using an elliptical chainring. Kinetic, kinematic and muscle activity data were collected
from eight competitive cyclists using three different chainrings--one circular and
two different orientations of an elliptical chainring. We tested the hypotheses that
muscle coordination patterns (EMG timing and magnitude), specifically the regions
of active muscle force production, would shift towards regions in the crank cycle
in which the crank angular velocity, and hence muscle contraction speeds, were favorable
to produce muscle power as defined by the skeletal muscle power-velocity relationship.
The results showed that our hypothesis with regards to timing was not supported. Although
there were statistically significant shifts in muscle timing, the shifts were minor
in absolute terms and appeared to be the result of the muscles accounting for the
activation dynamics associated with muscle force development (i.e. the delay in muscle
force rise and decay). But, significant changes in the magnitude of muscle EMG during
regions of slow crank angular velocity for the tibialis anterior and rectus femoris
were observed. Thus, the nervous system used adaptations to the muscle EMG magnitude,
rather than the timing, to adapt to the altered task mechanics. The results also suggested
that cyclists might work on the descending limb of the power-velocity relationship
when pedaling at 90 rpm and sub-maximal power output. This finding might have important
implications for preferred pedaling rate selection.