Differentiation of motor evoked potentials elicited from multiple forearm muscles: An investigation with high-density surface electromyography

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Transcranial magnetic stimulation (TMS) is a non-invasive method to measure corticospinal excitability of the primary motor cortex. However, motor evoked potentials (MEPs) elicited by TMS in a target muscle are variable; inconsistent MEPs may be due to overlapping cortical muscle representations and/or volume conduction from neighbouring muscles. The source of variable muscle responses may not be apparent using conventional bipolar electromyography (EMG), particularly over areas with several distinct neighbouring muscles (e.g. the forearm). High-density surface EMG (HDsEMG) may provide a useful means to investigate the underlying variability in amplitude and spatial distribution of MEPs. Here, we investigated the spatial distribution of MEPs in the forearm extensors using HDsEMG. HDsEMG consisted of a 16×5 grid of surface electrodes placed on the right (dominant) dorsal forearm over the extensor carpi radialis (ECR), ulnaris (ECU) and extensor digitorum communis finger extensors (EDC). MEP amplitude and distribution were recorded from 100 to 170% of resting (RMT) and active motor threshold (AMT). The distribution of MEPs was correlated to the activity recorded during selective, isometric contractions of the ECR, ECU, middle (EDC-D3) and ring (EDC-D4) finger extensors to determine the spatial distribution of MEPs in the forearm extensors. Although ECR was the hotspot, resting MEP spatial distribution was primarily correlated to that of EDC-D4 and ECU. With background ECR activation, the spatial distribution of MEPs correlated strongly with ECR. Further, while holding a background ECR contraction, EDC-D4 and ECU MEPs increased with greater stimulation intensity. Our results suggest that HDsEMG provides a useful way to differentiate which wrist extensor muscles are activated by TMS.