Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects

Inge Zijdewind, Rob Bakels, Christine K. Thomas

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


Involuntary contractions of paralyzed muscles (spasms) commonly disrupt daily activities and rehabilitation after human spinal cord injury (SCI). Our aim was to examine the recruitment, firing rate modulation, and derecruitment of motor units that underlie spasms of thenar muscles after cervical SCI. Intramuscular electromyographic activity (EMG), surface EMG, and force were recorded during thenar muscle spasms that occurred spontaneously or that were triggered by movement of a shoulder or leg. Most spasms were submaximal (mean: 39%, SD: 33 of the force evoked by median nerve stimulation at 50 Hz) with strong relationships between EMG and force (n = 12) rate-rate correlations, and derecruitment reversals (21 pairs, 29%). Later recruited units had higher or lower maximal firing rates than lower threshold units. These discrepant data show that coactive motoneurons are drive both by common inputs and by synaptic inputs from different sources during muscle spasms. Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

Original languageEnglish (US)
Article number922
JournalFrontiers in Human Neuroscience
Issue numberNovember
StatePublished - Nov 14 2014
Externally publishedYes


  • Afferent input
  • Motoneuron
  • Motor unit derecruitment
  • Motor unit firing rate modulation
  • Motor unit recruitment
  • Persistent inward current

ASJC Scopus subject areas

  • Neuropsychology and Physiological Psychology
  • Neurology
  • Psychiatry and Mental health
  • Biological Psychiatry
  • Behavioral Neuroscience


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