Absence of facilitation or depression of motor evoked potentials after contralateral homologous muscle activation

We have previously described post-exercise facilitation and post-exercise depression of motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS). To determine the presence of post-exercise facilitation after exercise of a contralateral muscle, MEPs were recorded from the resting right extensor carpi radialis (ECR) muscle while the left ECR muscle was activated, then immediately after brief left ECR activation, and, finally, immediately after brief right ECR activation. We repeated the experiment using the first dorsal interosseous (FDI) muscle. To determine the presence of post-exercise depression after exercise of a contralateral muscle, MEPs were recorded from the right ECR after prolonged exercise of the left ECR, followed by right ECR recording after its fatigue. The mean MEP amplitudes from the right ECR and the right FDI after brief activation were 187% and 266% of their pre-exercise values, respectively. There were no significant changes in MEPs recorded from the right ECR or FDI muscles during or immediately after brief activation of their left counterparts. The mean amplitude of MEPs recorded from the right ECR after it fatigued was approximately half the pre-exercise value, but there was no significant change in MEPs recorded from the right ECR after prolonged exercise of the left ECR. Therefore, neither post-exercise facilitation nor post-exercise depression occurred after contralateral homologous muscle exercise.