Standardization of facilitation of compound muscle action potentials using a modified myometer during magnetic stimulation in healthy volunteers.

In 20 normal subjects motor evoked potentials (MEPs) from first dorsal interosseous (FDI) were obtained following transcranial magnetic stimulation during relaxation and voluntary contraction of 2-20% of maximum. A new method to standardize facilitation of MEP using surface EMG and a modified myometer was used. The MEP amplitudes increased significantly at contraction levels of 2%, 10% and 15% of maximum. The latencies decreased significantly at contraction levels of 2%, 5% and 15%. Coefficients of variation of the amplitude decreased from 6.4% during relaxation to approximately 2% during facilitation. In contrast, coefficients of variation of take-off latencies (T-lat) increased significantly from 1.7% during relaxation to approximately 3% during facilitation. The variability of amplitudes and latencies was mainly due to inter-individual variation. The observations in the present study indicate that the actual contraction level should be precisely defined during transcranial magnetic stimulation.