Variability in the amplitude of skeletal muscle responses to magnetic stimulation of the motor cortex in man.

We have investigated variability in the amplitude of compound motor evoked potentials (cMEPs) in right and left thenar and wrist extensor muscles in response to synchronous bilateral transcranial magnetic stimulation (TMS) of the motor cortices using two figure-of-eight stimulating coils. Trials of 50 stimuli revealed a wide range of variability in cMEP amplitudes in relaxed muscles (coefficient of variation, range 0.22-1.12). The amplitudes of the cMEPs in one muscle correlated positively with those in the others. The r2 values (mean +/- SEM) were 0.27 +/- 0.06 for muscles on the same side of the body and 0.19 +/- 0.04 for muscles on opposite sides. Employing the ECG to trigger TMS, clamping the coil relative to the head or altering the orientation of the coil all failed to affect the variability of cMEPs. We conclude that fluctuations in excitability of the corticospinal pathway give rise to the variability in the response to TMS, that they are wide-ranging with respect to the muscles affected, and include a bilateral component. We argue that the variability reveals fluctuations in excitability mainly at the cortical rather than the spinal level. We suggest that measures of variability might provide an indication of cortical activity in conditions where consciousness and voluntary movement are compromised.