title: Power spectral changes of the superimposed M wave during isometric voluntary contractions of increasing strength.
abstract: BACKGROUND
  We examined the power spectral changes of the compound muscle action potential (M wave) evoked during isometric contractions of increasing strength.
  
  
  METHODS
  Surface electromyography (sEMG) of the vastus lateralis and medialis was recorded from 20 volunteers who performed 4-s step-wise isometric contractions of different intensities. A maximal M wave was elicited by a single stimulus to the femoral nerve and superimposed on the voluntary contractions. The spectral characteristics (Fmean and Fmedian) of sEMG and M-wave signals were calculated.
  
  
  RESULTS
  M-wave spectral indicators increased systematically with contraction intensity up to 60% MVC and then leveled off at higher forces. Over the 10-60% MVC range, the increase in spectral indicators was 3 times higher for M waves (36%) than for sEMG (12%).
  
  
  CONCLUSIONS
  The consistent increase in M-wave spectral characteristics with force is due to the fact that the number of motor units recruited by the superimposed supramaximal stimulus is approximately stable.
authors.affiliation: Universidad Pública de Navarra D.I.E.E., Department of Electrical and Electronical Engineering, Campus de Arrosadía s/n., 31006, Pamplona, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Rodriguez-Falces
authors.forename: Nicolas
authors.initials: N
authors.lastname: Place
mesh_terms: D000293:Adolescent; D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005267:Femoral Nerve / Q000502:physiology*; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D055815:Young Adult
keywords: femoral nerve stimulation; median frequency; step-wise contractions; superimposed M-wave; surface EMG spectral analysis