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1 Department of Medical Physiology, University of Groningen, Groningen, The Netherlands
2 The Miami Project to Cure Paralysis, Departments of Neurological Surgery, Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida, USA
* To whom correspondence should be addressed. E-mail: cthomas{at}miami.edu.
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 5 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 4-fold increase in integrated surface EMG 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 0.3-7.2 % 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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