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The Journal of Neurophysiology Vol. 82 No. 2 August 1999, pp. 515-525
Copyright ©1999 by the American Physiological Society
1Rehabilitation Research and Development
Center,
Raasch, Christine C. and
Felix E. Zajac.
Locomotor Strategy for Pedaling: Muscle Groups and Biomechanical
Functions. J. Neurophysiol. 82: 515-525, 1999. A group of coexcited muscles alternating with
another group is a common element of motor control, including locomotor
pattern generation. This study used computer simulation to investigate human pedaling with each muscle assigned at times to a group. Simulations were generated by applying patterns of muscle excitations to a musculoskeletal model that includes the dynamic properties of the
muscles, the limb segments, and the crank load. Raasch et al. showed
that electromyograms, pedal reaction forces, and limb and crank
kinematics recorded during maximum-speed start-up pedaling could be
replicated with two signals controlling the excitation of four muscle
groups (1 group alternating with another to form a pair). Here a
four-muscle-group control also is shown to replicate steady pedaling.
However, simulations show that three signals controlling six muscle
groups (i.e., 3 pairs) is much more biomechanically robust, such that a
wide variety of forward and backward pedaling tasks can be executed
well. We found the biomechanical functions necessary for pedaling, and
how these functions can be executed by the muscle groups. Specifically, the phasing of two pairs with respect to limb extension and flexion and
the transitions between extension and flexion do not change with
pedaling direction. One pair of groups (uniarticular hip and knee
extensors alternating with their anatomic antagonists) generates the
energy required for limb and crank propulsion during limb extension and
flexion, respectively. In the second pair, the ankle plantarflexors
transfer the energy from the limb inertia to the crank during the
latter part of limb extension and the subsequent limb
extension-to-flexion transition. The dorsiflexors alternate with the
plantarflexors. The phasing of the third pair (the biarticular thigh
muscles) reverses with pedaling direction. In forward pedaling, the
hamstring is excited during the extension-to-flexion transition and in
backward pedaling during the opposite transition. In both cases
hamstrings propel the crank posteriorly through the transition. Rectus
femoris alternates with hamstrings and propels the crank anteriorly
through the transitions. With three control signals, one for each pair
of groups, different cadences (or power outputs) can be achieved by
adjusting the overall excitatory drive to the pattern generating
elements, and different pedaling goals (e.g., smooth, or
energy-efficient pedaling; 1- or 2-legged pedaling) by adjusting the
relative excitation levels among the muscle groups. These six muscle
groups are suggested to be elements of a general strategy for pedaling
control, which may be generally applicable to other human locomotor tasks.
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