Do Additional Inputs Change Maximal Voluntary Motor Unit Firing Rates After Spinal Cord Injury?

Inge Zijdewind, Katie Gant, Rob Bakels, Christine K. Thomas*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

13 Citations (Scopus)

Abstract

Background. Motor unit firing frequencies are low during maximal voluntary contractions (MVCs) of human thenar muscles impaired by cervical spinal cord injury (SCI). Objective. This study aimed to examine whether thenar motor unit firing frequencies increase when driven by both maximal voluntary drive and other concurrent inputs compared with an MVC alone. Methods. Motor unit firing rates, force, and surface electromyographic activity (EMG) were compared across 2 contractions: (a) MVC alone and (b) MVC combined with another input (combination contraction). Other inputs (conditions) included vibration, heat, or cold applied to the anterior surface of the forearm, electrical stimulation delivered to the anterior surface of the middle finger, a muscle spasm, or a voluntary contraction of the contralateral thenar muscles against resistance. Results. The maximal firing frequency (n = 68 units), force, and electromyographic activity (n = 92 contraction pairs) were all significantly higher during the combined contractions compared with MVCs alone. There was a 3-way interaction between contraction, condition, and subject for maximal motor unit firing rates, force, and EMG. Thus, combined contraction responses were different for conditions across subjects. Some conditions (eg, a muscle spasm) resulted in more effective and more frequent responses (increases in unit firing frequency, force, EMG in > 50% contractions) than others. Recruitment of new units also occurred in combined contractions. Conclusions. Motoneurons are still responsive to additional afferent inputs from various sources when rate modulation from voluntary drive is limited by SCI. Individuals with SCI may be able to combine inputs to control functional tasks they cannot perform with voluntary drive alone.

Original languageEnglish
Pages (from-to)58-67
Number of pages10
JournalNeurorehabilitation and neural repair
Volume26
Issue number1
DOIs
Publication statusPublished - Jan-2012

Keywords

  • motor unit
  • firing frequency
  • recruitment
  • spinal cord injury
  • afferent input
  • persistent inward current
  • HUMAN THENAR MUSCLES
  • PERSISTENT INWARD CURRENTS
  • CONTRACTILE PROPERTIES
  • HUMAN MOTONEURONS
  • PLATEAU POTENTIALS
  • AXON STIMULATION
  • CALCIUM CURRENTS
  • FORCE-FREQUENCY
  • PATTERNS
  • RECRUITMENT

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