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The Journal of Neurophysiology Vol. 83 No. 3 March 2000, pp. 1469-1479
Copyright ©2000 by the American Physiological Society
Rehabilitation Research and Development Center, Veterans Affairs Palo Alto Health Care System, Palo Alto, California 94304-1200; and Neuromuscular Biomechanics Laboratory, Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853
Valero-Cuevas, Francisco J.
Predictive Modulation of Muscle Coordination Pattern Magnitude
Scales Fingertip Force Magnitude Over the Voluntary Range. J. Neurophysiol. 83: 1469-1479, 2000. Human
fingers have sufficiently more muscles than joints such that every
fingertip force of submaximal magnitude can be produced by an infinite
number of muscle coordination patterns. Nevertheless, the nervous
system seems to effortlessly select muscle coordination patterns when
sequentially producing fingertip forces of low, moderate, and maximal
magnitude. The hypothesis of this study is that the selection of
coordination patterns to produce submaximal forces is simplified by the
appropriate modulation of the magnitude of a muscle coordination
pattern capable of producing the largest expected fingertip force. In
each of three directions, eight subjects were asked to sequentially
produce fingertip forces of low, moderate, and maximal magnitude with
their dominant forefinger. Muscle activity was described by fine-wire
electromyograms (EMGs) simultaneously collected from all muscles of the
forefinger. A muscle coordination pattern was defined as the vector
list of the EMG activity of each muscle. For all force directions,
statistically significant muscle coordination patterns similar to those
previously reported for 100% of maximal fingertip forces were found
for 50% of maximal voluntary force. Furthermore the coordination
pattern and fingertip force vector magnitudes were highly correlated
(r > 0.88). Average coordination pattern vectors
at 50 and 100% of maximal force were highly correlated with each
other, as well as with individual coordination pattern vectors in the
ramp transitions preceding them. In contrast to this consistency of EMG
coordination patterns, predictions using a musculoskeletal computer
model of the forefinger show that force magnitudes 
50% of maximal
fingertip force can be produced by coordination patterns drastically
different from those needed for maximal force. Thus when modulating
fingertip force magnitude across the voluntary range, the number of
contributing muscles and the relative activity among them was not
changed. Rather, the production of low and moderate forces seems to be simplified by appropriately scaling the magnitude of a coordination pattern capable of producing the highest force expected.
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