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Voluntary activation does not differ when using two different methods to determine transcranial magnetic stimulator

Christina D Bruce1, Justine R Magnuson1, Chris J McNeil1

  • 1Integrated Neuromuscular Physiology Laboratory, Centre for Heart, Lung and Vascular Health, School of Health and Exercise Sciences, University of British Columbia, Kelowna, British Columbia, Canada.

Journal of Neurophysiology
|September 6, 2023
PubMed
Summary

Current methods for measuring voluntary activation (VA) with transcranial magnetic stimulation (TMS) may underestimate results. A new method, SOSIT, provides similar VA measurements with simplified procedures, reducing participant fatigue.

Keywords:
elbow flexorsestimated resting twitchstimulator intensitytranscranial magnetic stimulationvoluntary activation

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Area of Science:

  • Neuromuscular Physiology
  • Motor Control
  • Human Movement Science

Background:

  • Current guidelines for voluntary activation (VA) measurement using transcranial magnetic stimulation (TMS) recommend limiting stimulator output (SO) based on antagonist muscle motor evoked potential (MEP) size.
  • This limitation, intended to prevent antagonist muscle contamination, may lead to submaximal agonist responses (superimposed twitch [SIT] and estimated resting twitch [ERT]), potentially underestimating VA.
  • The accuracy of VA assessment in elbow flexors is crucial for understanding muscle function and fatigue.

Purpose of the Study:

  • To compare elbow flexor VA calculated using the standard method (SOMEP, intensity based on antagonist MEP size) versus a novel method (SOSIT, intensity based solely on maximal SIT amplitude at 50% MVC torque).
  • To determine if the SOSIT method yields different VA scores compared to the SOMEP method.
  • To evaluate the practical advantages of the SOSIT method in terms of equipment and procedure.

Main Methods:

  • Fifteen healthy young adults participated in the study.
  • Participants performed maximal voluntary contractions (MVCs) at 100%, 75%, and 50% of their MVC torque.
  • Transcranial magnetic stimulation (TMS) was delivered at two stimulator outputs: SOMEP (based on antagonist MEP size) and SOSIT (based on maximal SIT amplitude), with five sets each.

Main Results:

  • While the estimated resting twitch (ERT) torque was significantly greater with SOSIT (15.2 Nm) compared to SOMEP (13.0 Nm), the superimposed twitch (SIT) amplitude at 100% MVC torque did not differ between methods (SOSIT: 0.74 Nm vs. SOMEP: 0.69 Nm).
  • Despite differences in ERT, voluntary activation (VA) scores were not significantly different between the SOMEP (94.6%) and SOSIT (95.0%) methods.
  • The SOSIT method yielded comparable VA results to the SOMEP method.

Conclusions:

  • The SOSIT method, which determines stimulator output based on maximal superimposed twitch amplitude, provides similar voluntary activation (VA) measurements to the standard SOMEP method.
  • This finding suggests that current TMS protocols for VA may not necessarily underestimate activation.
  • The SOSIT method offers practical benefits by simplifying the procedure, eliminating the need for antagonist electromyography (EMG) recordings and maximal voluntary contractions (MVCs) for stimulator output determination, thereby reducing potential fatigue.