Voluntary activation of knee extensor muscles with transcranial magnetic stimulation
Document Type
Journal Article
Publication Title
Journal of Applied Physiology
Volume
130
Issue
3
First Page
589
Last Page
604
PubMed ID
33270515
Publisher
American Physiological Society
School
School of Medical and Health Sciences
RAS ID
35664
Funders
National Health and Medical Research Council of Australia
Grant Number
NHMRC Number : 1055084
Abstract
We examined if transcranial magnetic stimulation (TMS) is a valid tool for assessment of voluntary activation of the knee extensors in healthy individuals. Maximal M-waves (Mmax) of vastus lateralis (VL) were evoked with electrical stimulation of femoral nerve (FNS); Mmax of medial hamstrings (HS) was evoked with electrical stimulation of sciatic nerve branches; motor evoked potentials (MEPs) of VL and HS were evoked with TMS; superimposed twitches (SIT) of knee extensors were evoked with FNS and TMS. In study 1, TMS intensity [69% output (SD: 5)] was optimized for MEP sizes, but guidelines for test validity could not be met. Agonist VL MEPs were too small [51.4% Mmax (SD: 11.9); guideline ≥ 70% Mmax] and antagonist HS MEPs were too big [16.5% Mmax (SD: 10.3); guideline < 10% Mmax]. Consequently, the TMS estimated resting twitch [99.1 N (SD: 37.2)] and FNS resting twitch [142.4 N (SD: 41.8)] were different. In study 2, SITs at 90% maximal voluntary contraction (MVC) were similar between TMS [16.1 N (SD: 10.3)] and FNS [20.9 N (SD: 16.7)], when TMS intensity was optimized for this purpose, suggesting a procedure that combines TMS SITs with FNS resting twitches could be valid. In study 3, which tested the TMS intensity [56% output (SD: 18)] that evoked the largest SIT at 90% MVC, voluntary activation from TMS [87.3% (SD: 7.1)] and FNS [84.5% (SD: 7.6)] was different. In sum, the contemporary procedure for TMS-based voluntary activation of the knee extensors is invalid. A modified procedure improves validity but only in individuals who meet rigorous inclusion criteria for SITs and MEPs.NEW & NOTEWORTHY We discovered that the contemporary procedure for assessing voluntary activation of the knee extensor muscles with transcranial magnetic stimulation (TMS) is invalid. TMS activates too few agonist quadriceps motoneurons and too many antagonist hamstrings motoneurons to estimate the resting twitch accurately. A modified procedure, in which TMS-evoked superimposed twitches are considered together with the resting twitch from femoral nerve stimulation, is valid but only in select individuals who meet rigorous eligibility criteria.
DOI
10.1152/japplphysiol.00717.2020
Access Rights
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Comments
Nuzzo, J. L., Kennedy, D. S., Finn, H. T., & Taylor, J. L. (2021). Voluntary activation of knee extensor muscles with transcranial magnetic stimulation. Journal of Applied Physiology, 130(3), 589-604. https://doi.org/10.1152/japplphysiol.00717.2020