Improving localization and measurements of M-waves using high-density surface electromyography.

Abstract

Surface electromyography (sEMG) is useful for studying muscle function and controlling prosthetics, but crosstalk from nearby muscles often limits its effectiveness. High-density surface EMG (HD-sEMG) improves spatial resolution, allowing for the isolation of M-waves in the densely packed forearm muscles. This study assessed HD-sEMG for localizing M-waves and evaluated the impact of spatial filters on crosstalk reduction. We administered peripheral nerve stimulation to activate forearm muscles in five participants. We analyzed crosstalk by correlating the shape of M-waves between electrodes and used ultrasound to confirm muscle identity and location. At low stimulation intensities, we successfully isolated M-waves with minimal crosstalk without spatial filtering. Higher recruitment levels produced significant crosstalk, which was reduced by applying bipolar or tripolar spatial filters. M-waves from the monopolar HD-sEMG montage showed high correlations between electrodes (r = 0.97 transversely; r = 0.95 longitudinally), while bipolar and tripolar montages showed lower correlations (bipolar: r = 0.41 transversely; r = 0.19 longitudinally; tripolar: r = 0.17 transversely; r = 0.01 longitudinally). The tripolar filter significantly reduced crosstalk (51.10% amplitude decay one electrode away) compared to no filter (10.32% amplitude decay one electrode away), effectively reducing crosstalk to negligible levels at distances ≥ 2.55 cm. Ultrasound was crucial for distinguishing true activation from artifacts caused by converging signals along muscle boundaries. Spatially-filtered HD-sEMG accurately detects and isolates M-waves in the forearm, and ultrasound imaging is useful for verifying the location and identity of the muscles underlying the HD-sEMG grids.