Effects of volitional contraction on intracortical inhibition and facilitation in the human motor cortex

Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF) and short-interval intracortical facilitation (SICF) were assessed in the cortical motor area of the first dorsal interosseous muscle (FDI) of 16 healthy subjects. Paired-pulse TMS was delivered to the left hemisphere at the following interstimulus intervals (ISIs): 2 and 3 ms for SICI, 10 and 15 ms for ICF and 1–5 ms for SICF. Motor-evoked potentials were recorded from the resting and active right FDI. The effects exerted on SICI and ICF by four intensities (60–90% of active motor threshold, AMT) of the conditioning stimulus (S1) and by three levels of muscle contraction (10%, 25%, 50% of maximal voluntary contraction, MVC) were evaluated. The effects exerted on SICF were evaluatedwith two intensities (90% and 70% ofAMT) of the test stimulus (S2) and with the same levels ofmuscle contraction. Results showed that: (i) during 10%MVC, maximum SICI was observed with S1=70% AMT; (ii) the amount of SICI obtained with S1=70% AMT was the same at rest as during 10%MVC, but decreased at higher contraction levels; (iii) ICF was observed only at rest with S1=90% AMT; (iv) SICF was facilitated at 10% and 25% MVC, but not at 50% MVC. We conclude that during muscle activation, intracortical excitability reflects a balance between activation of SICI and SICF systems. Part of the reduction in SICI during contraction is due to superimposed recruitment of SICF. Lowintensity (70%AMT) conditioning