The differences of human muscle responses to magnetic nerve stimulation with different coil current waveforms

The differences of muscle responses to magnetic nerve stimulation with different coil current waveforms were predicted using cable theory and investigated on the median nerve in a human subject. The mono- and biphasic coil current whose time derivatives were bi- and triphasic with the equivalent 1st phase were compared concerning (1) the excitation threshold, (2) latency and (3) amplitude of compound muscle action potentials (CMAPs). The effect of different coil current directions, proximal or distal, at the intersection of a figure-eight coil were also considered. The combinations of the coil current waveforms and directions were termed MONO-PROX, MONO-DIST, BI-PROX and BI-DIST, where MONO- and BI-corresponded to monophasic and biphasic, and PROX and DIST to proximal and distal, respectively. The results in man were that (1) the excitation threshold was the lowest with BI-DTST, (2) the latencies with MONO-PROX and BI-PROX were equivalent to each other, and (3) the amplitudes of CMAPs with BIPROX and BI-DIST were almost equivalent and larger than with the others. The results in man were generally consistent with predictions, except for the evaluation of "cathode distal" effect.