A comparison of cross-correlation and surface EMG techniques used to quantify motor unit synchronization in humans

Two methods used to estimate the strength of motor unit (MU) synchronizatio n in a muscle are the direct cross-correlation of MU discharge times, and a veraging of the surface electromyogram (SEMG) with respect to discharge of a reference MU. Although indirect, the latter approach has the advantage th at a global estimate of MU synchrony can be obtained quickly and easily. Th e two methods are generally regarded as providing equivalent information on the extent of MU synchronization in a muscle, but this proposition has not previously been tested quantitatively. In the present study, we used both the SEMG technique (189 MUs) and cross-correlation of MU discharge (498 MU pairs) to estimate MU synchrony in 28 first dorsal interosseus (FDI) muscle s from 16 subjects. Despite considerable overlap in the identity of MUs use d to quantify synchrony with each method, linear regression revealed no sig nificant correlation between the estimates of MU synchronization in FDI mus cles obtained with the two techniques (r(2) = 0.04, n = 28). This discrepan cy was not due to insufficient sampling of the MU population with the cross -correlation method, although we found evidence for a non-uniform tendency for synchronous discharge in two of 13 motor units providing sufficient dat a for the analysis. The most likely explanation for the discrepancy between the estimates of MU synchrony is that methodological problems with the SEM G technique limit its accuracy. These problems are difficult to avoid under normal experimental conditions, and we conclude that the SEMG method is no t reliable for quantitative comparisons of MU synchrony between muscles and subjects. (C) 1999 Elsevier Science B.V. All rights reserved.