Modulation of short- and long-interval intracortical inhibition with increasing motor evoked potential amplitude in a human hand muscle.

OBJECTIVE: The aim of the current study was to investigate the effect of increasing test motor evoked potential (MEP) amplitude on short- (SICI) and long-interval intracortical inhibition (LICI) at rest and during activation of the first dorsal interosseous (FDI) muscle.

METHODS: In 22 young subjects, a conditioning-test transcranial magnetic stimulation (TMS) paradigm was used to assess SICI and LICI at 5 different test TMS intensities (110-150% motor threshold) in resting and active FDI. In 9 additional subjects, SICI and LICI data were quantified when the test MEP amplitude represented specific proportions of the maximal compound muscle action potential (Mmax) in each subject.

RESULTS: Test TMS intensity influenced SICI and LICI in rest and active FDI muscle. The normalised test MEP amplitude (%Mmax) did not influence SICI at rest, whereas there was a decrease in LICI at rest and an increase in SICI in active FDI with an increased normalised test MEP amplitude (%Mmax).

CONCLUSIONS: Our results demonstrate differential effects of normalised test MEP amplitude (%Mmax) on SICI and LICI in resting and active FDI muscle.

SIGNIFICANCE: Estimation of SICI and LICI under some circumstances may be influenced by the normalised test MEP amplitude in subject populations with different Mmax characteristics.