Effects of 5 Hz subthreshold magnetic stimulation of primary motor cortex on fast finger movements in normal subjects

We evaluated the effects of high-frequency repetitive transcranial magnetic stimulation (rTMS) on motor performance and motor learning of a rapid index finger movement. Two groups of healthy right-handed subjects underwent either "real" rTMS (1800 stimuli over the first dorsal interosseous (FDI) muscle hot spot given at 5 Hz and intensity of 90% of resting motor threshold-RMT) or "sham" stimulation. Both groups performed 60 rapid abductions of the right index finger before and after rTMS. The kinematic variables measured were amplitude, duration, peak velocity and peak acceleration. We also evaluated RMT and motor-evoked potential (MEP) amplitude before, 5 and 30 min after rTMS. In both groups practice significantly increased peak velocity, peak acceleration and amplitude and decreased movement duration independently from the type of intervention ("real" and "sham"). "Real" rTMS significantly increased cortical excitability as measured by MEP amplitude whereas "sham" rTMS did not. In our study, 5 Hz rTMS failed to improve either the motor performance or the motor learning of a rapid index-finger abduction despite the increase in cortical excitability of the primary motor cortex. Since motor behaviour engages a distributed cortical and subcortical neuronal network, excitatory conditioning of the primary motor cortex is probably not sufficient to influence the behavioural output.