On the electrode positioning for bipolar EMG recording of forearm extensor and flexor muscle activity after transcranial magnetic stimulation

After stroke, motor pathways are often affected, leading to paresis. It remains difficult to reliably predict motor recovery of the upper extremity, for which transcranial magnetic stimulation (TMS) may add to clinical examination. Placement of the surface electromyography (sEMG) electrodes in TMS is essential for information about specific muscle groups and corticospinal pathways. This study primarily aimed to determine the optimal sEMG electrode positions for recording activity of forearm flexor and extensor muscles. The first goal was to optimize sensitivity in measuring any motor evoked potentials (MEP), because they may be reduced or absent in stroke patients. The second goal was adequate distinction between forearm flexor and extensor muscle groups. For optimal flexibility in choosing montages, a multichannel sEMG set-up with 37 electrodes encircled the forearm. The determination of optimal pairs was based upon electrical peripheral nerve stimulation. We found pairs with the highest compound nerve action potential (CMAP) amplitudes and pairs that optimally distinguished between the flexor and extensor muscles. Large interelectrode distances lead to responses with larger amplitudes and therefore sensitively measure any remaining corticomuscular connections. As a follow-up, specific muscle group responses can be targeted with smaller interelectrode distances. In conclusion, this study helps to identify better electrode locations for the use of clinical TMS studies.