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J Neurophysiol (April 1, 2003). 10.1152/jn.00492.2002
Submitted on Submitted 2 July 2002; accepted in final form 26 November 2002
1Department of Medical Physiology, University of Groningen, The Netherlands; and 2The Miami Project to Cure Paralysis, Departments of Neurological Surgery, Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33136
Zijdewind, Inge and
Christine K. Thomas.
Motor Unit Firing During and After Voluntary Contractions of
Human Thenar Muscles Weakened by Spinal Cord Injury. J. Neurophysiol. 89: 2065-2071, 2003. Spinal cord
injury may change both the distribution and the strength of the
synaptic input within a motoneuron pool and therefore alter force
gradation. Here, we have studied the relative contributions of motor
unit recruitment and rate modulation to force gradation during
voluntary contractions of thenar muscles performed by five individuals
with chronic (>1 yr) cervical spinal cord injury. Mean ± SD
thenar unit firing rates were low during both steady-level 25%
(8.3 ± 2.2 Hz, n = 27 units) and 100%
maximal voluntary contractions (MVCs, 9.2 ± 3.1 Hz,
n = 23 units). Thus modest rate modulation, or a
lack of it in some units, was seen despite an average fourfold increase
in integrated surface electromyographic activity and force. During ramp
contractions, units were recruited at 5.7 ± 2.5 Hz, but still
only reached maximal firing rates of 12.8 ± 4.9 Hz. Motor units
were recruited up to 85% of the maximal force achieved (14.6 ± 5.6 N). In contrast, unit recruitment in control hand muscles is
largely complete by 30% MVC. Thus, during voluntary contractions of
thenar muscles weakened by cervical spinal cord injury, motor unit rate
modulation was limited and recruitment occurred over a wider than usual
force range. Those motor units that were stopped voluntarily had
significantly lower derecruitment versus recruitment thresholds.
However, 8 units (24%) continued to fire long after the signal to end
the voluntary contraction at a mean frequency of 5.9 ± 0.8 Hz.
The forces generated by this prolonged unit activity ranged from 0.3 to
7.2% maximum. Subjects were unable to stop this involuntary unit
activity even with the help of feedback. The mechanisms that underlie
this prolonged motor unit firing need to be explored further.
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