title: An enhanced level of motor cortical excitability during the control of human standing.
abstract: OBJECTIVE
  The study examined the role of the motor cortex in the control of human standing.
  
  
  METHODS
  Subjects (n = 15) stood quietly with or without body support. The supported standing condition enabled subjects to stand with a reduced amount of postural sway. Peripheral electrical stimulation, transcranial magnetic stimulation (TMS) or transcranial electrical stimulation (TES) was applied to elicit a soleus (SOL) H-reflex, or motor-evoked potentials (MEPs) in the SOL and the tibialis anterior (TA). Trials were grouped based on the standing condition (i.e. supported vs. normal) as well as sway direction (i.e. forward and backward) while subjects were standing normally.
  
  
  RESULTS
  During normal when compared to supported standing, the SOL H-reflex was depressed (-11 +/- 4%), while the TMS-evoked MEPs from the SOL and TA were facilitated (35 +/- 11% for the SOL, 51 +/- 15% for the TA). TES-evoked SOL and TA MEPs were, however, not different between the normal and supported standing conditions. The data based on sway direction indicated that the SOL H-reflex, as well as the SOL TMS- and TES-evoked MEPs were all greater during forward when compared to backward sway. In contrast, the TMS- and TES-evoked MEPs from the TA were smaller when swaying forward as compared to backward.
  
  
  CONCLUSIONS
  The results indicated the presence of an enhanced cortical excitability because of the need to control for postural sway during normal standing. The increased cortical excitability was, however, unlikely to be involved in an on-going control of postural sway, suggesting that postural sway is controlled at the spinal and/or subcortical levels.
authors.affiliation: Karolinska Institutet, Department of Neuroscience, Stockholm, Sweden.
authors.forename: C D
authors.initials: CD
authors.lastname: Tokuno
authors.forename: W
authors.initials: W
authors.lastname: Taube
authors.forename: A G
authors.initials: AG
authors.lastname: Cresswell
mesh_terms: D000328:Adult; D004558:Electric Stimulation / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006181:H-Reflex / Q000502:physiology; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology; D050781:Transcranial Magnetic Stimulation / Q000379:methods; D055815:Young Adult