Aim: 

The effects of different levels of volitional contraction on the short interval intracortical inhibition (SICI) and short interval intracortical facilitation (SICF) were evaluated with a range of stimulus intensities.

Methods: 

SICI and SICF were assessed in the cortical motor area of the first dorsal interosseous muscle (FDI) of 16 subjects. Paired pulse TMS was delivered to the left hemisphere at interstimulus intervals (ISIs) of 1-5 ms for SICI as well as for SICF. Motor evoked potentials (MEP) were recorded from the resting and active right FDI. The effects exerted on SICI 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 by two intensities (90% and 70% of AMT) of the test stimulus (S2) and the same levels of muscle contraction were also evaluated.

Results: 

During muscle activation at 10% of MVC, the optimal S1 intensity inducing the largest SICI was 70% AMT. The degree of SICI obtained with a S1 of 70% AMT was not significantly different in the resting and active states. SICI progressively decreased with increasing muscle contraction level. During slight muscle activation SICF was facilitated while it progressively disappeared at increasing levels of muscular contraction.

Conclusion: 

Intracortical excitability during voluntary contraction is modulated by a balance between SICI and SICF systems when the S1 intensity is higher than 70% AMT. By contrast, the use of a low S1 intensity activates only the SICI system inducing a pure inhibitory effect.