The estimation of short intra-cortical inhibition depends on the proportion of spinal motoneurones activated by corticospinal inputs

ObjectiveThe high variability of SICI limits its utility and by extension that of TMS in clinical neurophysiology. Non-linear summation of descending volleys due to heterogeneous motoneurone properties, on which MEP size depends, has not previously been implicated as an issue in SICI evaluation.MethodsMEP size and SICI were normalised to the test MEP (mV), and as a percentage of Mmax to take account of the proportion of motoneurone pool activated by TMS. Two EMG systems, producing large and small MEPs, were used to determine how the normalisation affected MEPs of different amplitude.ResultsMmax normalisation (i) counteracted the influence of recording conditions on the MEP size, (ii) revealed a significant influence of the test size on SICI (between medium and large MEPs), and of test size on the conditioning intensity (the larger the MEP, the stronger the SICI), and (iii) decreased the variability.ConclusionsData normalised to Mmax better reflect the motoneurone recruitment after SICI. To enhance reproducibility, MEP should be normalised to Mmax. This adjusts for some of the non-linear properties at the spinal, and possibly, at cortical levels.SignificanceTo reduce variability is important because TMS is becoming widely adopted and is being used in patients.