title: Effects of non-target leg activation, TMS coil orientation, and limb dominance on lower limb motor cortex excitability.
abstract: Transcranial magnetic stimulation (TMS) is used to examine corticospinal tract integrity after stroke, however, generating motor-evoked potentials (MEPs) in the lower limb (LL) can be difficult. Previous studies have used activation of the target leg to facilitate MEPs in the LL but this may not be possible after stroke due to hemiplegia. The dominance of the target limb may also be important, however the neurophysiological effects of LL dominance are not known. We investigated whether voluntary activation of the non-target leg combined with optimal TMS coil orientation increases corticomotor excitability in healthy adults, and whether limb dominance influences these results. TMS was delivered to induce a posterior-anterior (PA) and a medial-lateral (ML) cortical current in 22 healthy adults. MEPs were recorded in tibialis anterior (TA) with the participant at rest and when activating the non-target leg. We found that non-target leg activation increased corticomotor excitability in the target leg (reduced rest motor threshold (RMT) and MEP latency, and increased recruitment curve slope). ML cortical current also reduced RMT and MEP latency. The degree of footedness correlated with the degree of RMT asymmetry, with a PA but not ML cortical current direction. In summary, cross-facilitation by activating the non-target leg in a task requiring postural stabilisation and inducing ML current increase corticomotor excitability regardless of limb dominance. This protocol may have practical application in testing CST integrity after stroke when paretic limb thresholds are high, by increasing the likelihood of eliciting a MEP.
authors.affiliation: Department of Medicine, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
authors.forename: Marie-Claire
authors.initials: MC
authors.lastname: Smith
authors.affiliation: Department of Exercise Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
authors.forename: James W
authors.initials: JW
authors.lastname: Stinear
authors.affiliation: Department of Medicine, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand; Centre for Brain Research, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
authors.forename: P
authors.initials: P
authors.lastname: Alan Barber
authors.affiliation: Department of Medicine, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand; Centre for Brain Research, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand. Electronic address: c.stinear@auckland.ac.nz.
authors.forename: Cathy M
authors.initials: CM
authors.lastname: Stinear
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D016014:Linear Models; D035002:Lower Extremity / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D012146:Rest; D000071939:Stroke Rehabilitation; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation / Q000295:instrumentation* / Q000379:methods*; D055815:Young Adult
keywords: Corticomotor excitability; Footedness; Lower limb; Stroke; Transcranial magnetic stimulation