Rectification of EMG improves detection of motor unit coherence in the beta frequency band

Rectification of surface EMG prior to spectral analysis is a well established pre-processing method used in the detection of motor unit firing patterns. A number of recent studies have called into question the need for rectification prior to spectral analysis, pointing out that there is no supporting experimental evidence to justify rectification. We present an analysis of 190 records from 13 subjects consisting of simultaneous recordings of paired single motor units and surface EMG from the extensor digitorum longus muscle during middle finger extension against gravity (unloaded condition) and against gravity plus inertial loading (loaded condition). We examine directly the hypothesis that rectified surface EMG is a better predictor of the frequency components of motor unit synchronization than the unrectified (or raw) EMG in the beta frequency band (15–32Hz). We use multivariate analysis and estimate the partial coherence between the paired single units using both rectified and unrectified surface EMG as a predictor. We use a residual partial correlation measure to quantify the difference between raw and rectified EMG as predictor, and analyse unloaded and loaded conditions separately. The residual correlation for the unloaded condition is 22% with raw EMG and 3.5% with rectified EMG, for the loaded condition it is 5.2% with raw EMG and 1.4% with rectified EMG. We interpret these results as strong supporting experimental evidence in favour of using the pre-processing step of surface EMG rectification prior to spectral analysis. ∗[email protected] †[email protected] 1 Articles in PresS. J Neurophysiol (July 31, 2013). doi:10.1152/jn.00296.2013 Copyright © 2013 by the American Physiological Society.