title: A comparison of two methods for estimating 50% of the maximal motor evoked potential.
abstract: OBJECTIVE
  Two commonly-used methods for setting stimulus intensities in transcranial magnetic brain stimulation studies were compared to determine which best approximated a motor evoked potential (MEP) of 50% of the maximal MEP amplitude (SI50); a suprathreshold intensity relative to resting motor threshold (rMT) or adjusting the intensity to evoke an MEP amplitude of 1mV.
  
  
  METHODS
  Corticomotor stimulus-response curves and rMT for the right first dorsal interosseous (FDI) muscle of 176 subjects (aged 10-74 years) were retrospectively analysed.
  
  
  RESULTS
  Regardless of subject age or sex, SI50 occurred at 127.5 ± 11.3% rMT. Except in young children, MEPs of 1 mV were significantly smaller than those evoked at SI50.
  
  
  CONCLUSIONS
  In the inactive FDI muscle, a stimulus intensity of 127-128% rMT consistently gives the best approximation of SI50 in most subjects, except perhaps young children.
  
  
  CONCLUSIONS
  Setting TMS stimulus intensities relative to rMT provides a less variable inter-subject comparator, with respect to individual differences in corticomotor input-output characteristics, than adjusting the stimulator output to give an absolute MEP magnitude.
authors: Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia. Electronic address: julia.pitcher@adelaide.edu.au.
authors: Julia B
authors: JB
authors: Pitcher
authors: Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia.
authors: Sebastian H
authors: SH
authors: Doeltgen
authors: Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia.
authors: Mitchell R
authors: MR
authors: Goldsworthy
authors: Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia.
authors: Luke A
authors: LA
authors: Schneider
authors: Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia.
authors: Ann-Maree
authors: AM
authors: Vallence
authors: Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia.
authors: Ashleigh E
authors: AE
authors: Smith
authors: Discipline of Physiology, School of Medical Sciences, University of Adelaide, Adelaide 5005, Australia.
authors: John G
authors: JG
authors: Semmler
authors: School of Health Sciences, University of South Australia, Adelaide 5001, Australia.
authors: Michelle N
authors: MN
authors: McDonnell
authors: Robinson Research Institute, School of Paediatrics & Reproductive Health, University of Adelaide, Adelaide 5005, Australia.
authors: Michael C
authors: MC
authors: Ridding
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult
keywords: Ageing; Children; Corticomotor stimulus–response curves; First dorsal interosseous; Motor evoked potential; Resting motor threshold