Jg. Semmler et Ma. Nordstrom, A comparison of cross-correlation and surface EMG techniques used to quantify motor unit synchronization in humans, J NEUROSC M, 90(1), 1999, pp. 47-55
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.