Evaluation of the low-frequency components in surface electromyography.

The surface electromyogram (SEMG) is a signal noninvasively (through electrodes located on the body surface) acquired for evaluating the electrical activity produced by skeletal muscles. In thoracic acquisitions, SEMG is typically affected by the electrocardiographic (ECG) signal, representing the electrical activity of the heart. SEMG and ECG power spectra mainly fall within 5-450 Hz and 0.05-50 Hz, respectively. Consequently, SEMG and ECG components overlap in the 5-50 Hz range. Although removing SEMG low spectral components, high-pass linear filtering (LF) with a cut-off frequency of 20 Hz remains the standard technique to clean SEMG from ECG. Thus, the aim of the present study was to propose the Segmented Beat Modulation Method (SBMM) as a tool to clean SEMG from ECG without removing SEMG components below 20 Hz. A SEMG recording was first acquired from the left rectus abdominis of a subject, and then filtered using both SBMM and LF. Filtered SEMGs obtained with the two techniques were compared. Results indicate that SBMM eliminates ECG interference from SEMG better than LF, since the latter procedure maintains ECG components between 20 and 50 Hz. In addition, after ECG removal by SBMM, SEMG showed a significant amount of spectral components (up to 20%) in the low-frequency range. Maintaining such low-frequency components, which go lost when applying LF, is desirable since they likely associate to the firing rates of the active motor units. In conclusion, SBMM represents a useful tool to clean SEMG from ECG without loss of frequency components.