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The Journal of Neurophysiology Vol. 81 No. 2 February 1999, pp. 544-551
Copyright ©1999 by the American Physiological Society
1Rehabilitation Research and
Development Center (153), Veterans Affairs Palo Alto Health Care
System, Palo Alto, California 94304;
2Department of Mechanical
Engineering,
Phase reversal of biomechanical functions and muscle activity in
backward pedaling. Computer simulations of pedaling have shown
that a wide range of pedaling tasks can be performed if each limb has
the capability of executing six biomechanical functions, which are
arranged into three pairs of alternating antagonistic functions. An
Ext/Flex pair accelerates the limb into extension or flexion, a
Plant/Dorsi pair accelerates the foot into plantarflexion or
dorsiflexion, and an Ant/Post pair accelerates the foot anteriorly or
posteriorly relative to the pelvis. Because each biomechanical function
(i.e., Ext, Flex, Plant, Dorsi, Ant, or Post) contributes to crank
propulsion during a specific region in the cycle, phasing of a muscle
is hypothesized to be a consequence of its ability to contribute to one
or more of the biomechanical functions. Analysis of electromyogram
(EMG) patterns has shown that this biomechanical framework assists in
the interpretation of muscle activity in healthy and hemiparetic
subjects during forward pedaling. Simulations show that backward
pedaling can be produced with a phase shift of 180° in the Ant/Post
pair. No phase shifts in the Ext/Flex and Plant/Dorsi pairs are then
necessary. To further test whether this simple yet biomechanically
viable strategy may be used by the nervous system, EMGs from 7 muscles
in 16 subjects were measured during backward as well as forward
pedaling. As predicted, phasing in vastus medialis (VM), tibialis
anterior (TA), medial gastrocnemius (MG), and soleus (SL) were
unaffected by pedaling direction, with VM and SL contributing to Ext,
MG to Plant, and TA to Dorsi. In contrast, phasing in biceps femoris
(BF) and semimembranosus (SM) were affected by pedaling direction, as
predicted, compatible with their contribution to the directionally
sensitive Post function. Phasing of rectus femoris (RF) was also
affected by pedaling direction; however, its ability to contribute to
the directionally sensitive Ant function may only be expressed in
forward pedaling. RF also contributed significantly to the
directionally insensitive Ext function in both forward and backward
pedaling. Other muscles also appear to have contributed to more than
one function, which was especially evident in backward pedaling (i.e.,
BF, SM, MG, and TA to Flex). We conclude that the phasing of only the
Ant and Post biomechanical functions are directionally sensitive. Further, we suggest that task-dependent modulation of the expression of
the functions in the motor output provides this biomechanics-based neural control scheme with the capability to execute a variety of lower
limb tasks, including walking.
0022-3077/99
$5.00
Copyright © 1999 The American Physiological Society
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