title: Influence of inter-electrode distance, contraction type, and muscle on the relationship between the sEMG power spectrum and contraction force.
abstract: BACKGROUND
  Spectral frequencies of the surface electromyogram (sEMG) increase with contraction force, but debate still exists on whether this increase is affected by various methodological and anatomical factors. This study aimed to investigate the influence of inter-electrode distance (IED) and contraction modality (step-wise vs. ramp) on the changes in spectral frequencies with increasing contraction strength for the vastus lateralis (VL) and vastus medialis (VM) muscles.
  
  
  METHODS
  Twenty healthy male volunteers were assessed for isometric sEMG activity of the VM and VL, with the knee at 90° flexion. Subjects performed isometric ramp contractions in knee extension (6-s duration) with the force gradually increasing from 0 to 80% MVC. Also, subjects performed 4-s step-wise isometric contractions at 10, 20, 30, 40, 50, 60, 70, and 80% MVC. Interference sEMG signals were recorded simultaneously at different IEDs: 10, 20, 30, and 50 mm. The mean (F mean) and median (F median) frequencies and root mean square (RMS) of sEMG signals were calculated.
  
  
  RESULTS
  For all IEDs, contraction modalities, and muscles tested, spectral frequencies increased significantly with increasing level of force up to 50-60 % MVC force. Spectral indexes increased systematically as IED was decreased. The sensitivity of spectral frequencies to changes in contraction force was independent of IED. The behaviour of spectral indexes with increasing contraction force was similar for step-wise and ramp contractions.
  
  
  CONCLUSIONS
  In the VL and VM muscles, it is highly unlikely that a particular inter-electrode distance or contraction modality could have prevented the observation of the full extent of the increase in spectral frequencies with increasing force level.
authors.affiliation: Department of Electrical and Electronical Engineering, Universidad Pública de Navarra DIEE, 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: Daria
authors.initials: D
authors.lastname: Neyroud
authors.forename: Nicolas
authors.initials: N
authors.lastname: Place
mesh_terms: D000328:Adult; D004566:Electrodes; D004576:Electromyography / Q000379:methods*; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000502:physiology*