Muscle coordination is habitual rather than optimal
Journal of Neuroscience. 2012-05-23; 32(21): 7384-7391
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1. J Neurosci. 2012 May 23;32(21):7384-91. doi: 10.1523/JNEUROSCI.5792-11.2012.
Muscle coordination is habitual rather than optimal.
de Rugy A(1), Loeb GE, Carroll TJ.
(1)Centre for Sensorimotor Neuroscience, School of Human Movement Studies, The
University of Queensland, Brisbane, St Lucia, QLD 4072, Australia.
When sharing load among multiple muscles, humans appear to select an optimal
pattern of activation that minimizes costs such as the effort or variability of
movement. How the nervous system achieves this behavior, however, is unknown.
Here we show that contrary to predictions from optimal control theory, habitual
muscle activation patterns are surprisingly robust to changes in limb
biomechanics. We first developed a method to simulate joint forces in real time
from electromyographic recordings of the wrist muscles. When the model was
altered to simulate the effects of paralyzing a muscle, the subjects simply
increased the recruitment of all muscles to accomplish the task, rather than
recruiting only the useful muscles. When the model was altered to make the force
output of one muscle unusually noisy, the subjects again persisted in recruiting
all muscles rather than eliminating the noisy one. Such habitual coordination
patterns were also unaffected by real modifications of biomechanics produced by
selectively damaging a muscle without affecting sensory feedback. Subjects
naturally use different patterns of muscle contraction to produce the same forces
in different pronation-supination postures, but when the simulation was based on
a posture different from the actual posture, the recruitment patterns tended to
agree with the actual rather than the simulated posture. The results appear
inconsistent with computation of motor programs by an optimal controller in the
brain. Rather, the brain may learn and recall command programs that result in
muscle coordination patterns generated by lower sensorimotor circuitry that are
functionally « good-enough. »
PMID: 22623684 [Indexed for MEDLINE]