The surface electrode placement determines the magnitude of motor potential evoked by transcranial magnetic stimulation

Objective: Different electrode montages have been adopted to record the surface electromyographic signal (sEMG) from which motor potentials (MEP amplitude) evoked by transcranial magnetic stimulation (TMS) are extracted. It can lead to divergences in comparing treatment outcomes or comparisons across them. This study aimed to evaluate the effect of three different sEMG electrode montages on the MEP amplitude. Methods: Eight healthy right-handed participants (6 women; 24–28 years) without sensorimotor disorders participated in the present study, which the local ethical committee approved. Surface EMG signals were recorded from both upper limbs’ biceps brachii (BB) muscles. Three sEMG electrode montages (E1-E2 differential) were placed according to the following protocols: Protocol by Garcia et al. [E1 – Innervation zone]–[E2 – epicondylus lateralis] (Garcia⊢_IZ-BP); Protocol by IFCN [E1 – Muscle belly]–[E2 – epicondylus medialis] (IFCN⊢_MB-BP); and Protocol by Munneke et al. [E1-E2 over the muscle belly, but with an inter-electrode distance between 40 and 90% of the total muscle length] (Munneke⊢_40-90%). Thirty single TMS pulses were applied on the BB hotspot with a figure of eight coil at 120% of the resting motor threshold (rMT). Results: The rMTs were significantly higher (p < 0.05) for Munneke⊢_40-90% protocol than for the other two, and IFCN⊢_MB-BP protocol was the lowest. IFCN⊢_MB-BP protocol also provided MEP amplitudes about 2.0 to 4.0 times greater (p < 0.05) than the other two. Conclusion: Different electrode montages can provide contrasting MEP amplitudes. Significance: It sounds imperative to create standard recommendations on electrode placement for MEP recordings.