title: Do additional inputs change maximal voluntary motor unit firing rates after spinal cord injury?
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.
authors: University of Groningen, UMCG, Groningen, Netherlands.
authors: Inge
authors: I
authors: Zijdewind
authors: Katie
authors: K
authors: Gant
authors: Rob
authors: R
authors: Bakels
authors: Christine K
authors: CK
authors: Thomas
mesh_terms: D000328:Adult; D002574:Cervical Vertebrae; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D006225:Hand / Q000503:physiopathology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D011999:Recruitment, Neurophysiological / Q000502:physiology*; D013035:Spasm / Q000503:physiopathology; D013119:Spinal Cord Injuries / Q000503:physiopathology*