![contraction intensity scale contraction intensity scale](https://i.pinimg.com/originals/a5/39/b3/a539b344b8f79e98557e49664becd5b7.jpg)
An issue with this normalization is that the size of the MEP test is known to influence SICI outcome. The amount of SICI is conventionally quantified by expressing the mean MEP cond amplitude as a percentage of the mean MEP test amplitude or the difference between the mean MEP cond and the mean MEP test then expressed as a percentage of the mean MEP test amplitude (SICI MEPtest). It is thought that the CS activates low-threshold inhibitory interneurons that employ γ-aminobutyric acid type A receptor (GABA A) and reduce the excitatory inputs activated by the TS. With optimal settings, the CS exerts a decrease in the conditioned MEP (MEP cond) amplitude in comparison with the unconditioned MEP (MEP test) induced by a TS alone. The high-intensity suprathreshold TS leads to a corticospinal output called motor-evoked potential (MEP), measured peripherally by means of electromyographic (EMG) recording over the target muscle. SICI results from a subthreshold conditioning stimulus (CS) followed 1 to 6 ms later by a suprathreshold test stimulus (TS) applied through the same coil over M1. Short-interval intracortical inhibition (SICI) is a well-established paired-pulse measure to evaluate inhibitory circuits within the M1 area. Paired-pulse transcranial magnetic stimulation (TMS) enables the non-invasive examination of these intracortical excitatory and inhibitory circuits at the time of the stimulation. For example, the reduction in intracortical inhibition within M1 is a crucial part of neural adaptation following acute and multi-session challenging strength training programs. Under similar experimental conditions, it is thus possible to predict SICI individually from a specific level of corticospinal excitability in healthy subjects.Ĭortical inhibitory and excitatory circuits within the primary motor cortex (M1) play an important role in the fine tuning of descending motor commands and in the neural adaptation following motor training. However, when normalized to Mmax, we provided the first evidence of a strong individual relationship between SICI and MEP test, which was ultimately independent from experimental conditions (muscle states and TMS intensities). When normalized to MEP test, SICI remained unchanged by stimulation intensity and only decreased at 30% of MVC when compared with rest. We probed SICI changes according to various MEP test amplitudes that were modulated actively (four levels of muscle contraction: rest, 10%, 20% and 30% of maximal voluntary contraction (MVC)) and passively (two intensities of test transcranial magnetic stimulation (TMS): 120 and 130% of motor thresholds). In this study, we underpin the importance of taking into account the proportion of spinal motoneurones that are activated or not when investigating the SICI of the right flexor carpi radialis (normalization with maximal M-wave (Mmax) and MEP test, respectively), in 15 healthy subjects. However, SICI estimation is based on the amplitude of a motor-evoked potential (MEP), which depends on the discharge of spinal motoneurones and the generation of compound muscle action potential (M-wave). Short-interval intracortical inhibition (SICI) represents an inhibitory phenomenon acting at the cortical level.