title: Firing duration of masseter motor units during prolonged low-level contractions.
abstract: OBJECTIVE
  The study aimed to determine whether motor units (MUs) of the masseter muscle can be continuously active during a prolonged low-level sustained contraction.
  
  
  METHODS
  Intramuscular fine-wire EMG activity was recorded unilaterally from the masseter muscles of 13 pain-free volunteers (mean age ± SD = 26.7 ± 7.7 years), during low-level biting tasks performed for 30 min. The stored intramuscular EMG signals were decomposed into individual MU action-potential trains using long-term decomposition software. Depending on relative duration of the MU activity, MUs were classified as sporadically (<50% of the time), intermittently (50% ≤ time ≤ 95%), or continuously active (>95% of the time).
  
  
  RESULTS
  The overall number of MUs identified during the low-level biting task was 203. Of these, nine continuously active MUs (4.4%) were found in six of the 13 subjects investigated, whereas 50 intermittent MUs (24.6%) were found in 12 subjects. The remaining MUs (71.0%) were sporadically active and were found in all of the subjects investigated.
  
  
  CONCLUSIONS
  The majority of masseter MUs is sporadically or intermittently active during prolonged low-level contractions, but some of them can also be continuously active.
  
  
  CONCLUSIONS
  Sustained clenching efforts may be accompanied by continuous activity of a few selected muscle fibres.
authors.affiliation: Clinic for Masticatory Disorders, Removable Prosthodontics, and Special Care Dentistry, University of Zürich, Zürich, Switzerland. mauro.farella@otago.ac.nz
authors.forename: M
authors.initials: M
authors.lastname: Farella
authors.forename: M E
authors.initials: ME
authors.lastname: De Oliveira
authors.forename: L M
authors.initials: LM
authors.lastname: Gallo
authors.forename: T
authors.initials: T
authors.lastname: Läubli
authors.forename: L
authors.initials: L
authors.lastname: Tomatis
authors.forename: C
authors.initials: C
authors.lastname: Müller
authors.forename: R
authors.initials: R
authors.lastname: Obrenovic
authors.forename: S
authors.initials: S
authors.lastname: Palla
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000328:Adult; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology; D012984:Software; D055815:Young Adult