title: Comparison of the power spectral changes of the voluntary surface electromyogram and M wave during intermittent maximal voluntary contractions.
abstract: BACKGROUND
  To compare the power spectral changes of the voluntary surface electromyogram (sEMG) and of the compound action potential (M wave) in the vastus medialis and vastus lateralis muscles during fatiguing contractions.
  
  
  METHODS
  Interference sEMG and force were recorded during 48 intermittent 3-s isometric maximal voluntary contractions (MVC) from 13 young, healthy subjects. M waves and twitches were evoked using supramaximal femoral nerve stimulation between the successive MVCs. Mean frequency (F mean), and median frequency were calculated from the sEMG and M waves. Muscle fiber conduction velocity (MFCV) was computed by cross-correlation.
  
  
  RESULTS
  The power spectral shift to lower frequencies was significantly greater for the voluntary sEMG than for the M waves (P < 0.05). Over the fatiguing protocol, the overall average decrease in MFCV (~25%) was comparable to that of sEMG F mean (~22%), but significantly greater than that of M-wave F mean (~9%) (P < 0.001). The mean decline in MFCV was highly correlated with the mean decreases in both sEMG and M-wave F mean.
  
  
  CONCLUSIONS
  The present findings indicated that, as fatigue progressed, central mechanisms could enhance the relative weight of the low-frequency components of the voluntary sEMG power spectrum, and/or the end-of-fiber (non-propagating) components could reduce the sensitivity of the M-wave spectrum to changes in conduction velocity.
authors.affiliation: Department of Electrical and Electronical Engineering, Public University of Navarra, Campus de Arrosadía s/n, 31006, Pamplona, Spain, javier.rodriguez.falces@gmail.com.
authors.forename: Javier
authors.initials: J
authors.lastname: Rodriguez-Falces
authors.forename: Mikel
authors.initials: M
authors.lastname: Izquierdo
authors.forename: Miriam
authors.initials: M
authors.lastname: González-Izal
authors.forename: Nicolas
authors.initials: N
authors.lastname: Place
mesh_terms: D000200:Action Potentials; D000328:Adult; D004576:Electromyography; D005260:Female; D005267:Femoral Nerve / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D018763:Muscle Fatigue; D018485:Muscle Fibers, Skeletal / Q000502:physiology*