title: Plantar flexion force induced by amplitude-modulated tendon vibration and associated soleus V/F-waves as an evidence of a centrally-mediated mechanism contributing to extra torque generation in humans.
abstract: BACKGROUND
  High-frequency trains of electrical stimulation applied over the human muscles can generate forces higher than would be expected by direct activation of motor axons, as evidenced by an unexpected relation between the stimuli and the evoked contractions, originating what has been called "extra forces". This phenomenon has been thought to reflect nonlinear input/output neural properties such as plateau potential activation in motoneurons. However, more recent evidence has indicated that extra forces generated during electrical stimulation are mediated primarily, if not exclusively, by an intrinsic muscle property, and not from a central mechanism as previously thought. Given the inherent differences between electrical and vibratory stimuli, this study aimed to investigate: (a) whether the generation of vibration-induced muscle forces results in an unexpected relation between the stimuli and the evoked contractions (i.e. extra forces generation) and (b) whether these extra forces are accompanied by signs of a centrally-mediated mechanism or whether intrinsic muscle properties are the predominant mechanisms.
  
  
  METHODS
  Six subjects had their Achilles tendon stimulated by 100 Hz vibratory stimuli that linearly increased in amplitude (with a peak-to-peak displacement varying from 0 to 5 mm) for 10 seconds and then linearly decreased to zero for the next 10 seconds. As a measure of motoneuron excitability taken at different times during the vibratory stimulation, short-latency compound muscle action potentials (V/F-waves) were recorded in the soleus muscle in response to supramaximal nerve stimulation.
  
  
  RESULTS
  Plantar flexion torque and soleus V/F-wave amplitudes were increased in the second half of the stimulation in comparison with the first half.
  
  
  CONCLUSIONS
  The present findings provide evidence that vibratory stimuli may trigger a centrally-mediated mechanism that contributes to the generation of extra torques. The vibration-induced increased motoneuron excitability (leading to increased torque generation) presumably activates spinal motoneurons following the size principle, which is a desirable feature for stimulation paradigms involved in rehabilitation programs and exercise training.
authors.affiliation: Neuroscience Program and Biomedical Engineering Laboratory, Universidade de São Paulo, EPUSP, Avenida Professor Luciano Gualberto, Travessa 3, n,158, São Paulo, SP, Brazil. fhmagalhaes@leb.usp.br
authors.forename: Fernando Henrique
authors.initials: FH
authors.lastname: Magalhães
authors.forename: Diana Rezende
authors.initials: DR
authors.lastname: de Toledo
authors.forename: André Fabio
authors.initials: AF
authors.lastname: Kohn
mesh_terms: D000125:Achilles Tendon / Q000502:physiology*; D000200:Action Potentials / Q000502:physiology; D000328:Adult; D001696:Biomechanical Phenomena; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D005528:Foot / Q000502:physiology*; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D019415:Torque; D014732:Vibration