LONG-LATENCY INTRACORTICAL INHIBITION DURING UNILATERAL MUSCLE ACTIVITY

The aim of the present study was to investigate the effect of co-activation of antagonist muscles on long-latency intracortical inhibition (LICI) in comparison to isometric index finger abduction. EMG signals were recorded from the first dorsal interosseous muscle (FDI) in response to single-pulse and paired-pulse transcranial magnetic stimulation (TMS).In 10 healthy right-handed volunteers, TMS was used to estimate LICI at 3 different interstimulus intervals (ISIs) - 50, 100 and 150 ms. The intensity of the conditioning and test stimuli was 130 % of the motor threshold in relax. The stimulation procedure was repeated at rest and during tonic isometric index finger abduction and co-activation of antagonist muscles.At rest, LICI was significant at ISIs of 100 and 150 ms and not evident at ISI of 50 ms. During isometric index finger abduction and co-activation of antagonist muscles, LICI was evident at all used ISIs and was even better pronounced at 50 ms. At ISIs of 50 and 100 ms during abduction, LICI was significantly stronger in comparison to co-activation.During abduction and paired-pulse TMS, the mean values of cSP at all used ISIs were significantly shorter compared to single-pulse TMS. The shortening of cSP recorded in response to paired-pulse TMS was gradual, with increasing of ISI from 50 to 150 ms. In contrast, during co-activation, the duration of cSP was almost independent of the value of ISI and similar to the duration of cSP in response to single-pulse TMS.